Civil Engineering Construction

Construction Technology

CRN : 31888
Credit Rating : 20 Credits
Level 4, Semesters 1 and 2

Date

Version

Nov-11

1.0

Revision
Initial print for 2011-12

Module Leader:
Neil Currie B.Eng(Hons), FIStructE, MICE, CEng
Newton Building, LG8, University of Salford, Greater Manchester, M5 4WT
Email: n.g.r.currie@salford.ac.uk
Civil Engineering website: www.cse.salford.ac.uk/civilengineering
Blackboard website: vle.salford.ac.uk

Directorate of Engineering & Physical Sciences

School of Computing, Science & Engineering
Faculty of Science, Engineering and Environment

Forward

This text is intended to give an introduction to construction techniques,

considerations and general materials for civil engineering students, although it
may prove useful to students of other engineering disciplines, surveyors and
architects; and practicing structural engineers who require an everyday
reference or introduction to the topic.

The entire module will be subdivided into 5 smaller sub-sections, with construction
techniques being one of these smaller sections. The other components are
engineering contracts, sustainability, health and safety and contract law.

Construction techniques and technology is an ever-growing area of research

and great care should be taken by students and practitioners to maintain a solid
understanding of the subject through constant CPD.

It is by no means exhaustive, as the topic of constructability is a vast area of study

and changes as new materials and processes are determined and defined.
Instead the student should use this as a springing point for their learning about
construction design and technologies.

Module Requirements

Engineering construction is a two semester, 20 credit module taken by all year 1

civil engineers. This means that students are expected to devote 200 hours of
study over two semesters.

The aims of the module are:

To develop knowledge and understanding in the field of civil engineering
construction.
The learning outcomes of the module are:
Upon successful completion of the module, students should be able to:

Understand the formation, nature and types of contract used in civil

engineering practice.
Understand the principles of specification of construction works.
Understand the nature, purpose, field of operation, legal requirements and
use of accounting.
Prepare and read; a balance sheet, a profit and loss account, a cash flow
statement.
Understand costing and budgeting theory.
Embrace a culture of sustainable development.
Be aware of different techniques of constructing a structure.
Be familiar with items of plant and their capability.
Be aware of health and safety issues
Understand the role of the client, engineer and contractor in conventional,
DBFO and partnering

The learning modes are:

Students will use lectures and case study appraisal, supported by tutorials and
group work. Visual aids and demonstrations will be used as appropriate. This
module handbook contains essential theory and will be cross referenced with
blackboard resources which will include other relevant material and tutorials.

Face-to-face lecturing will be the main vehicle of acquiring knowledge. Lectures

will introduce each topic and explain the important concepts which underpin it.
Tutorial sessions offer a chance to work through examples under guidance.
Worked Examples and real world examples may be used throughout the notes
and lectures to assist in understanding of the theory and application to real life.
Self Assessment Exercises are provided at the end of each topic to allow students
to judge whether they have understood the theory and application. Completed
solutions are available to allow rating of understanding (these are posted on
Blackboard).
The assessment regime:
The object of studying structures is to develop a useful capability in civil
engineering construction topics. Since understanding how civil engineering
structures are procured and constructed is a key requisite for being a civil
engineer, it is also necessary to ensure that sufficient knowledge has been
acquired by the student (to ensure the possibility of success at years 2 and 3), so
the assessment regime constitutes one end-of-semester 2 examination and a
component of the design project that is common across various modules in this
year.
The Examination is a two and half hour opportunity to demonstrate how much
has been learnt during the course. The results of the examination form 80% of the
assessment.
The Design Exercise is continually assessed group work and constitutes a
comprehensive design process. The results of the coursework form 20% of the
assessment for this specific module.
Students must pass all assessments to pass the module. A student who does not
attempt the last element of assessment of a module (in this case the
examination) must be expelled from the course under the Universitys current
rules.
The University of Salford provides students with access to the Blackboard Virtual
Learning Environment (vle.salford.ac.uk). This is an on-line aid to learning where
you will find this handbook, answers to tutorial questions, email access and
discussion boards. You are encouraged to use the Discussion Board to ask any
questions you have about the course material, this way all students who have the
same question can see the answer provided.
Students are also provided with an ATHENS login and password, which can be
used to access the websites of many technical publishing houses and

www.info4education.com where electronic copies of many publications can be

found.
All civil engineering students are encouraged to join The Institution of Civil
Engineers and Institution of Structural Engineers in year 1. Student membership is
free of charge and gives access to regular magazine publications, the Institution
libraries and evening meetings. Join on-line (web addresses are in the
References section of this handbook).

All students are granted access to www.info4education.co.uk where you will be

able to log in using your Athens log in details that are provided for you by the
library. There are various guides available from the CIS component of the
website that you can use to support your learning. You are expected to read
beyond this handbook and your lecture notes to promote deep learning, the
Clifford Whitworth library also has various texts relating to construction that you
will find useful. You should also consider attending ICE and IStructE seminars in
the evenings, specifically those that talk about case study projects. They are free
to everyone and you will find them good opportunities to meet experienced
engineers in the area.

Recommended Reading
In order to acquire an understanding of construction design and technology you
must read wider than just this module handbook. This is because you will need to
experience a diversity of opinion and methods to fully understand the subject.
The Clifford Whitworth library (first floor) retains a large amount of high quality
material on this subject. Students should refer to texts listed in the references
section at the end of this handbook, many of which are held in the Clifford
Whitworth Library, there are also likely to be various blogs, wikis and online
guidance documents available, but care should be taken with regards verifying
the appropriateness of these sources.

Contents

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1.1 Blackboard.
1.2 Weighting of the assignment
1.3 Key dates
Construction Techniques.
2.1 Order of construction
2.2 Permanent Works
2.3 Temporary Works
2.4 Types of activity
2.5 Temporary propping
2.6 Adjacent foundations
2.7 Highway construction
2.8 Basement construction.
What is superstructure?
What is substructure?
4.1 Substructure
Steel
5.1.1 What is steel?
5.1.2 Process
5.1.3 Simple Frames.
5.1.4 Portal frames
5.1.5 Trusses
5.1.6 Long span floors
5.1.7 Composite decking
5.1.8 Bi-steel
5.1.9 Modular construction
5.1.10 Example
5.2 Cranage
5.3 Lots
5.4 Phases
Concrete
6.1.1 What is concrete?
6.1.2 Types of concrete?
6.1.3 Designated
6.1.4 Designed
6.1.5 Self compacting
6.1.6 Super high strength
6.1.7 Pour sequence
6.1.8 Identity testing
6.1.9 Jointing
6.2 Process
6.2.1 Design
6.2.2 Drawings
6.2.3 RC Placement drawings
6.2.4 Bar bending schedules
6.2.5 Placement

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6.2.6 Vibrating
6.3 Common defects
6.4 Formwork
6.4.1 Timber shuttering
6.4.2 Table forms
6.4.3 Steel forms
6.4.4 Cardboard
6.5 Tremie pipe
6.6 Concrete pump.
6.7 Tall buildings.
6.7.1 Table forms.
6.7.2 Cantilevers
6.7.3 Jump forming
6.7.4 Slip forming
7 Concrete (Alternate forms)
7.1 Edge protection
7.2 Debonding agent
7.3 Sealant
8 Masonry
8.1 What is masonry?
8.2 How are they made?
8.3 Common units.
8.4 Clay bricks
8.5 Facing bricks
8.6 Calcium Silicate Brick
8.7 Concrete block
8.8 Fair finished block
8.9 Terminology
8.10 Ties
8.11 Bond
8.12 How is it built?
8.13 Common defects
9 What is timber?
9.1 Forms of timber.
9.2 Origins of timber.
9.3 Softwoods
9.4 Considerations.
9.4.1 Hygroscopy
9.4.2 Hygroscopy
9.4.3 Propensity to creep under sustained load
9.5 Strength
9.6 Deflection
9.7 Creep relationships.
9.8 The anisotropic nature of timber strengths.
9.9 Processing timber.
9.10 Engineered Timbers.
9.11 Kerto

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9.12 Plywood
9.13 Cross Laminated Timber (CLT)
9.14 Drying of timber
9.15 Air Drying
9.16 Kiln Drying
9.17 Grading of timber
9.18 Strength classes.
9.19 Sourcing timber.
9.20 Defects
10 Self based exercise.
10.1 Civil Structures
10.2 Buildings
10.3 Infrastructure

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Table of figures.
Figure 2-1 Temporary propping to a concrete slab.
Figure 2-2 Section through an indicative highway.
Figure 2-3 Excavation with batter along the right hand edge.
Figure 3-1 Bridge superstructure
Figure 3-2 Skyscraper height comparison including Burj Khalifa
Figure 4-1 Leaning Tower of Pisa.
Figure 4-2 Cofferdam being used to construct bridge pier.
Figure 5-1 Steel sections
Figure 5-2 Plate Girder
Figure 5-3 Compound sections
Figure 5-4 Cell beams
Figure 5-5 Castellated Curved Rafters
Figure 5-6 A truss for No 1 Deansgate.
Figure 5-7 Weight restriction sign for a weak bridge.
Figure 5-8 Composite beam with fire protection.
Figure 5-9 Cross-section through a bi-steel panel.
Figure 5-10 Photograph of MoHo in Castlefield.
Figure 5-11 Mobile crane.
Figure 5-12 Eiffel tower construction sequence.
Figure 6-1 Reinforcement placement drawing from design and detailed.
Figure 6-2 Example of Shape Code 13 for RC Detailing.
Figure 6-3 Steel reinforcement being concreted.
Figure 6-4 Forwork for a column.
Figure 6-5 Formwork
Figure 6-6 Backpropping load distribution.
Figure 6-7 Carboard Formwork
Figure 6-8 Tremie pipe used in piled foundations.
Figure 6-9 Concrete pumping truck.
Figure 6-10 Concrete skip used on tall buildings.
Figure 6-11 Pier Luigi Nervi hangar roof using downstand beams.
Figure 6-12 Stability core at Media City formed using slip form.
Figure 7-1 Bubbledeck
Figure 7-2 Formwork for a viaduct.
Figure 8-1 Normalised block strengths from a manufacturer.
Figure 8-2 Coursing dimensions for brickwork.
Figure 8-3 Types of masonry units.
Figure 8-4 Forms of masonry wall construction.
Figure 8-5 Common Wall Ties
Figure 8-6 Common UK bond patterns.
Figure 9-1 Deflection of a timber beam.
Figure 9-2 Common sawing patterns.
Figure 9-3 Timber gridshell using engineered timber
Figure 9-4 Structure of ply.

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Figure 9-5 Grading stamp.

Figure 9-6 Timber seasoning defects.

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Structure of the module

1.1 Blackboard.
Each section of the course will be co-ordinated with Blackboard
and will require you to regularly log in and co-ordinate with the
reading programme. Within BlackBoard there will be a series of ongoing self-tests and self-based learning activities. These may form
part of your overall mark and will require you to achieve a minimum
mark in each section.
These tests will be taken within controlled conditions and each test
will be randomly taken from a pool of questions maintained within
the Blackboard database.
Once the self-based tests have been completed for all students, the
same pool of questions will be thrown open to allow you to study
and revise for your exams.

1.2 Weighting of the assignment

The coursework for this module will be integrated into the larger
design project for all first years and will form 20% of your overall mark
for this module. You should ensure that sustainability, construction
technology, construction sequencing, financial matters, legal and
contracts are specifically identified within your design reports.

1.3 Key dates

You will be examined on this subject at the end of semester 2.

Civil Engineering Construction

2 Construction Techniques.
One of the key requirements often overlooked by engineers when
designing a structure or a piece of civil engineering infrastructure is
how these components will be built, maintained and even
operated. This section of the module is intended to take the student
through some of the common techniques utilised within construction
within the UK.
This handbook will make reference to various articles linked into the
corresponding area on blackboard and the student will need to
ensure that these are also read in conjunction with reading this text,
the associated texts, attending lectures and tutorials.

2.1 Order of construction

Construction is an ordered and logical process if undertaken
correctly. For example, its often difficult to put a roof on the
building until youve constructed the supporting frame - and to build
the supporting frame you need to first construct the corresponding
foundations.
There are various permutations on these sequences though and
unusual techniques, which whilst not frequently used in industry, do
have a position and can be the most optimum solution for given
criteria.

2.2 Permanent Works

When a structure is designed it will be designed to support a given
load, for a set of deflection criteria that will result in the structure
safely supporting the loads and ultimately fulfilling the design brief.
This analogy has been simplified greatly, but ultimately the end result
of the design process is a structure that is safe, economical and fulfils
the brief.
This structure is said to be the permanent works, ultimately being the
end result of the design process. As the designers if this structure
deflects too much, or perhaps is unable to support the full design
load then the designer will be liable for these deficiencies.

2.3 Temporary Works

One of the key concepts to understand when designing structures is
that frequently they are unstable during construction and require
additional support to allow them to be built safely.
This additional support could be simple propping, or formwork for a
concrete beam perhaps, but the key element to consider is that
without the use of these temporary propping sequences and
requirements is that the structure could not be built.
As designers you will have a requirement to ensure that your
structure is not only safe in its final state, but also that it can be
constructed safely.

2.4 Types of activity

Civil engineering is a very broad and wide ranging topic, which
encompasses a large number of activities. Within this module you
will be given an overview of different forms of common construction
methods, including how they can be applied to various different
forms of civil engineering structures.

This handbook is sub-divided into two key components of

construction, the substructure and superstructure.
There is also a section relating to site organisation and planning.

2.5 Temporary propping

Temporary propping is frequently used to aid with the stability of a
structure during construction. Whilst the structure should be strong
enough to support itself and any imposed loadings once
completed, it may require additional structure to enable it being
constructed. Common examples include the use of scaffold,
concrete formwork, temporary steel bracing and wailings.

Figure 2-1 Temporary propping to a concrete slab.

2.6 Adjacent foundations

One of the key considerations when excavating is will the
excavation be stable and will it influence any adjacent areas. This is
particularly important when other buildings, structures or properties
are along adjacent boundaries.

These properties are protected by a robust piece of legislation

called the Party Wall Act 19961.
The Party Wall Act provides a framework for preventing and
resolving disputes in relation to party walls, boundary walls and
excavations near neighbouring buildings. It is based on some tried
and tested provisions of the London Building Acts, which applied in
inner London for many decades before the Act came into force.
Anyone intending to carry out work (anywhere in England and
Wales) of the kinds described in the Act must give Adjoining Owners
notice of their intentions.

2.7 Highway construction

The UK has an estimated 225,000 miles of road all of which must be
maintained and relief roads constructed, this doesnt include other
types of highway and hard standing construction such as dock
yards, runways and private areas of hard standing at large
distribution warehouses.

Figure 2-2 Section through an indicative highway.

Highways must be constructed systematically, firstly, by being

excavated down to suitable strata; secondly, the sub base and

http://www.communities.gov.uk/publications/planningandbuilding/partywall

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corresponding levels are laid, rolled, and compacted using suitable

stone; thirdly, the finer upper layers are constructed; finally, the
wearing course or blacktop is installed on the top.

2.8 Basement construction.

When constructing a basement there will commonly be a need to
integrate some temporary support around the perimeter whilst the
hole is being excavated. This can be provided by carefully
designing the permanent works to support the earth during the
temporary condition, through the provision of additional support by
using sheet piles, or where space permits by digging the perimeter
of the excavation with an appropriate batter (angle) around the
perimeter.

Figure 2-3 Excavation with batter along the right hand edge.

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Superstructure

3 What is superstructure?
Superstructure is a term used in civil engineering to identify a specific
area of a building or development. The phrase superstructure is
used in many other disciplines and is not exclusive to engineering,
ranging from psychology through to describing parts of Marxist
theory. They all share a common notion though in that it is used to
describe a thing that is supported by another.
In simple buildings, the superstructure can simply be thought of the
area of the building that sits above the foundations or basement
level. For bridges the superstructure is classed as the section that sits
upon the piers and abutments.

Figure 3-1 Bridge superstructure

Superstructure can be built a wide variety of forms, materials and

types and the purpose of this module to identify various techniques
and processes involved when constructing different types of
structure.

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One of the key challenges for an engineer is to identify the

boundaries where the superstructure begins and how this can affect
the design of the building.
For a skyscraper the superstructure can change material at various
points in the building. The Burj Khalifa for example the lower stories
are constructed from a combination of insitu concrete and posttensioned concrete floors, but several of the upper floors are
constructed from steel. Whilst the steel component of the building is
clearly supported by the lower concrete stories, it could perhaps be
argued that this is the reason that the steel elements are classed as
superstructure, but in reality both the concrete and steel elements all
belong to the skyscraper as its entirety. The whole of the skyscraper
frame above ground is then classed as superstructure, with the
foundations being classed as substructure, which will be the subject
of our next chapter.

Figure 3-2 Skyscraper height comparison including Burj Khalifa

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Substructure

4 What is substructure?
Substructure is a term used in civil engineering to identify a specific
area of a building or development. In its simplest form the
substructure can be imagined as something that provides support to
something above and is thus a critical component.
A common example of substructure is the humble foundation, the
first component typically constructed on a building site and
frequently one given the least amount of acknowledgement when
looking at large exotic structures, but without solid foundations,
buildings would sink into the ground or dangerously topple.

Figure 4-1 Leaning Tower of Pisa.

4.1 Substructure
The substructure is a key component within civil engineering, it is
typically the element of works that either sits in or beneath the

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ground and provides primarily a supporting function to the primary

structure.
There are lots of different types of foundations for example,
including:

Pad foundations.
Strip footings.
Combined foundations.
Raft foundations.
Piled foundations.
Secant piles
CFA piles
CHD piles
Bottom driven piles
Top driven piles
Sheet Piles
Timber Piles
Nestled timber planks

But each of these types of foundations can be used as a permanent

foundation to hold up a structure long term, or equally they could be
used to provide support in the temporary condition.
Some substructure works is specifically targeted to enable other works
to proceed, for example, a piling mat is commonly required to support
the heavy weights of the piling rigs. A piling rig can be of the order of
45 tonnes and if it were to become unstable it could easily topple whilst
the auger is at its highest position. To combat this a piling mat is formed
using stone, sometimes including geotextiles, which allow the weight of
the rig to be evenly dispersed over a wider area and reduce the risk of
the rig toppling over from the soft spots.
Another example of temporary works used to enable construction is
that of a cofferdam. A cofferdam is typically a steel box that is
constructed to exclude water; either in a river or stream; the
introduction of a dam completely excludes water and allows for
construction to proceed in a safe and efficient manner.

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Figure 4-2 Cofferdam being used to construct bridge pier.

Sometimes, there may be large amounts of groundwater present

and a cofferdam may not work as the groundwater can still come
up through the floor. Where this is a concern, then it is common to
require additional pumps to continually remove water from the
excavation during construction until the permanent water exclusion
measure is constructed. Even so, the effects of buoyancy are
critical as once the water has returned to the ground, if there is not
enough mass or ballast within the new structure it can be susceptible
to floating and thus buoyancy checks will need to be carried out
during the design process.

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Steel

5 Steel

5.1.1 What is steel?

In its simplest form steel is an alloy between iron and varying
amounts of carbon, anywhere between 0.2% and 2.1% typically.
Other elements can be added to the composition to give differing
mechanical and chemically resistant properties, creating variations
such as stainless steel.
Structural steel is commonly provided within the industry at various
common grades, with the two most commonly used grades in the
UK being S275 and S355 although higher grades are available for
specialist uses.
The naming convention for these steels is logical with S275 steels
commonly having a minimum strength of 275N/mm2 and S355 steels
having a minimum strength of 355N/mm2.
Question?
What is the minimum diameter of S355 steel wire that would be
needed to support a male African Elephant weighing 6 tonnes?
Answer: 14.53mm
Steel frames are common in industry, being one of the two common
construction materials in the UK, with the other being reinforced
concrete.
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There are fundamentally two different types of steel sections used to

construct bridges and buildings, the first being pre-rolled steel
sections such as those supplied by CORUS which are manufactured
to high tolerances for a variety of shapes.

Figure 5-1 Steel sections

Sometimes though, the sections provided through the standard

tables may not be large enough for the loading regime being
specified and a customised section may need to fabricated. These
sections can either be fabricated from stock size steel plates, or
through the amalgamation or adaptation of existing steel sections.
It is not unusual for plates to be welded into steel sections for high
loading regimes, or for long spans, such as bridges or transfer beams
in tall buildings. These plates can be welded into a variety of shapes
including box-girders, plate girders or fish belly girders for example.

Figure 5-2 Plate Girder

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Where a single standard steel section perhaps doesnt have the

required section properties, but a plated section would prove too
expensive, then the steel sections can be modified to increase the
geometrical properties. This can be accomplished through fixing
too sections together either with welds or bolts, these types of
sections are called compound sections.

Figure 5-3 Compound sections

Other common adaptations of standard steel sections include the

splitting of the section along its length in a saw tooth pattern and
then re-welding the section to create a castellated beam.

Figure 5-4 Cell beams

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These can be quite labour intensive, but there are various

companies that create equivalent standardised stock sections such
as Westok and Fabsec.

Figure 5-5 Castellated Curved Rafters

Castellated beams and cellular beams have distinct advantages

when compared to stock sections in that building services can be
fitted to run through the openings and typically there can be a
weight saving within the design. The downside though is that these
beams will require a deeper overall structural zone to sit within the
floor zone and this cannot always be accommodated within the
building as it may affect the overall building height.

5.1.2 Process
As with all elements of design, the design cannot start properly until
a brief has been determined. The brief can take different forms
depending on the nature of the design problem, for a simple beam
to form an aperture through a masonry wall this could simply be the
performance criteria for the beam (i.e. its span, limiting deflection
and load it must support).
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For more complex buildings this could be simply how many square
metres of floor are required and that certain areas are not allowed
to have internal columns. It is at this point that the brief will start to
drive the potential solutions and start to attract costs for the design
of the scheme.
From the conception of the structural form, its critical that the
designers start to consider how these structures can and will be
constructed. Many schemes have never made it through to
construction stage due to them being impractical or the associated
costs of temporary propping or the shear number of cranes required
to lift them.
There are numerous families of structural forms that can be
constructed in steel and below are a selection of some of the more
common forms that engineers will be expected to understand,
identify and present the construction sequence for.

5.1.3 Simple Frames.

A simple frame in the context of steel construction is a common
multi-storey frame as used in office buildings, hotels, student
accommodation etc. The building maintains stability either through
the use of bracing or shear walls. These stability elements are
typically contained in blank walls, around lift shafts or stair cores and
prevent the building from falling over on windy days
The floor beams in this form of construction are often simply
supported beams that span between columns or other beams to
form a grillage. This grillage is in turn held up by columns that
support the floors, because these columns dont resist the moments
from the end of the beams, they are often referred to as simple
columns within the Eurocodes although they need to be designed
to resist bending moments due to the eccentricity of the beam
loads placed upon them from the end reactions. [This is something
that you will be taught how to design in future years of your degree]

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5.1.4 Portal frames

Portal frames are used widely in construction, particularly in the
construction of large commercial units. If youve ever been to a
supermarket or a warehouse then the chances are that youve
been within a portal frame. The frames themselves provide the
stability in one direction, but typically have bracing along the
longitudinal axis which means that the temporary stability of the
building during construction needs to be considered quite carefully.
Question: What sort of building commonly use portal frames?

5.1.5 Trusses
Trusses are used to span long distances and are commonly used in
large span structures such as exhibition arenas, concert halls, large
lecture theatres and similar type buildings. As the trusses themselves
are typically longer than a standard trailer can transport, they are
brought to site in several sections and then bolted together to
create a complete truss. The positions where the trusses are bolted
back together are called splices and the number of sections that a
truss is subdivided into can be as a result of many governing factors
including the size of the fabrication shop, the length of the vehicles
delivering the components, low bridges en route, etc

Figure 5-6 A truss for No 1 Deansgate.

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Typically vehicles and their loads that are no wider than 2.9m and
no longer than 18.75m require no special measures to be
undertaken such as police escorts. However the planning of the
route needs careful consideration so as to avoid low bridges and
other sections of road that may not be suitable to support the axle
load from the wagons.

Figure 5-7 Weight restriction sign for a weak bridge.

Clearly until the entire truss is bolted together it has no ability to span,
its incomplete and has no structural integrity. This may mean that
the truss is assembled on the floor and lifted into place as one piece,
or that the truss is assembled insitu using some form of crash decking
or temporary support decks.
Question: Where on a house would you see a truss?
Question: And how large are they and would they require special
permission and/or access requirements to be delivered?

5.1.6 Long span floors

Long span floors are commonly used to span between steel beams
to create open areas to increase the spacing between columns.
This type of arrangement is beneficial when constructing column
free environments, such as car parks for example.

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Typically these kinds of flooring systems will make use of prestressed

concrete planks, which can create their own issues particularly
during erection where they may not be stable. As a plank is added
to one side of the beam it can create a twisting load which is called
a torsion. If this torsional load is too great then the beam can rotate
and in extreme cases this can result in the planks slipping from the
top flange and several accidents have happened over the years
from just this type of phenomenon.

Question: Why would you use a long span floor in a car park?

5.1.7 Composite decking

Composite decking is also used widely for creating steel buildings,
with the composite decking acting as a permanent shutter for the
concrete. By attaching the slab to the beam with a positive
connection such as shear studs, it is possible to enhance the load
carrying capacity of the beam.
There are several risks associated with the installation of composite
decking installation as the decking must be affixed to the beam to
prevent it from sliding off.

Figure 5-8 Composite beam with fire protection.

From the general arrangement drawings, the supplier of the

composite decking is able to take the slab drawings and create a
layout drawing which shows how many sheets of composite decking
24

are needed and their accompanying layout including how large

the sheets are, how many shear studs are needed and where
additional support is required. This sequencing will need to be
aligned with deliveries onto site and co-ordinated with the steel
frame erection, the sequencing of the frame construction may well
require that there are floors constructed to allow the cherry pickers
to erect the steel frame.
Question: Do floors constructed from composite decking require
temporary propping?

5.1.8 Bi-steel
Corus produced a product called Bi-steel which is essentially two
sheets of steel that are connected together using metal studs to
create a sandwiched panel.

Figure 5-9 Cross-section through a bi-steel panel.

The benefit of these panels is that they are comparatively light when
compared to a concrete wall and can be erected quickly to form
shear walls around lift cores and similar types of structures. Once
they have been erected then they can be filled with concrete if
necessary to increase their axial load carrying capacity.

5.1.9 Modular construction

Modular construction is a type of offsite construction, the modules
are constructed in a factory and then delivered to site and simply
stacked on top of each other to create a completed building.
There are clearly limits on how many units can be stacked together
25

whilst retaining structural stability from wind loads and accidental

damage.
Modular construction can take a variety of forms from subcomponents such as prefabricated bathroom pods that are
delivered to site complete with showers, toilets, baths, tiled surfaces
and other secondary fittings right down to including a toilet roll
holder. Other modular forms include prison cells pods, which are
designed to single stackable units formed entirely out of precast
concrete. They have integral toilets, beds and washing facilities all
cast from concrete to prevent inmates from damaging them and
the doors are designed with hinges that can open both ways to
prevent doors being barricaded in the event of a riot.
Larger modular units can be achieved; some are large enough to
encompass entire flats. The key to an efficient design for a modular
structure is repetition the more of a single type of unit that you can
manufacture, the more efficient the factory becomes. There are a
variety of manufacturers that make residential units and they all
come complete with integrated kitchen appliances and services
that are connected together on site as the modules are
interconnected. Large developments are possible using this form of
construction and in Manchester a large development was
completed in Castlefield, called Moho which has a central steel
framed skeleton that forms the stability core and the modular units
are connected around the edges.

26

Figure 5-10 Photograph of MoHo in Castlefield.

Question: Would modular construction require any special

requirements at site?

5.1.10

Example

Think how each of these types of building might have been

constructed, specifically identify the following:

What materials are used?

How was the material delivered to site?
How was it built?
Did it use temporary support, permanent support or both?
Draw a sequence of construction sketch to show the process.

27

Building 1: Wembley Stadium.

Building 2: Steel framed shed.

28

5.2 Cranage
Another of the key considerations of constructing steel frames is the
amount, positioning and location of any cranage that is necessary to
lift the steel elements into position. As steel elements are delivered to
site on the back of a trailer, they are lifted off and stored in sequence
to allow the frame to be assembled in a logical and systematic fashion.

Figure 5-11 Mobile crane.

5.3 Lots
The order that the steelwork is delivered is referred to as lots. Each
batch (or lot) of steel that is delivered to site is carefully planned so that
it maintains stability for itself whilst the rest of the frame is erected and is
frequently co-ordinated with the access for cranage and other site
access requirements. For taller buildings though, the allocation of lots is
determined by the logical construction sequence.

Figure 5-12 Eiffel tower construction sequence.

29

5.4 Phases
Larger steel frames are further sub-divided into phases or regions, with
each phase being an independent frame. Examples of this include
large shopping centres such as the Trafford Centre or the Lowry Outlet
Mall, the latter of which is subdivided into three sub-regions or phases.

30

Concrete

6 Concrete
6.1.1 What is concrete?
Concrete in its simplest form is a combination between cement,
coarse aggregate, fine aggregate and water. By varying the
different forms and ratios of these elements allows for concrete with
differing properties to be manufactured. Further amendments to
the mix such as the inclusion of fly ash, use of recycled aggregate or
various chemical admixtures can also reduce the amount of carbon
required to manufacture concrete.
The design requirements of concrete will vary dramatically
depending on its intended end use, the environment to which it is
intended, the method of construction, placement and climatic
conditions to name a few.
The following sections will cover some of the key criteria used when
designing and constructing in concrete and are intended to aid the
student develop an understanding of modern concrete
construction. It is fully intended that the student however will also
research the ideas contained within under their own initiative and
research the topic and construction techniques further.

6.1.2 Types of concrete?

Concrete can be provided in a variety of forms, ranging from
historical lime based mixes through to highly technical and complex
31

ultra high strength mixes. The intention of this section is not to teach
teach you about the finer points of concrete and mix design, but to
instruct you on some of the basic principles associated with
concrete specification and construction to provide you with
information to aid with your future designs.

6.1.3 Designated
Designated mixes are typically the most common type of concrete
used in modern building and civil engineering construction. The
methodology behind designated mixes is that they are standardised
both in terms of their specification but also in their production and
provision from concrete suppliers.
In theory this should mean that each batch of concrete is uniform
and has been through the same quality control procedure and thus
will have the same mechanical properties.
Each batch of concrete will have a ticket when it leaves the factory
and should the concrete be amended in any way (including the
addition of water) between it leaving the factory and being placed
then the driver should note this on the batch ticket before the
concrete is placed. If the batch has been amended then the
customer does not necessarily have to accept delivery of the
concrete.
Not recording any alterations to the mix on the ticket can often
result in the person responsible being dismissed such is the risk to the
mechanical and chemical properties2 of the concrete mix.

One of the reasons toilets are often set up at regular intervals through the floors of tall

buildings to prevent workers urinating in the formwork. There have been cases where the
ammonia from the urine has adversely affected the performance of the concrete.

32

Consequently it is advisable wherever possible to use standardised

designated mixes to enable economical designs and to increase
the quality control of the overall construction.

6.1.4 Designed
Every building is designed to respond to a certain set of criteria and
occasionally this will require the specification of a concrete mix that
is beyond the capability of a standard designated mix.
This may be because the client requires the inclusion of a specific
admixture, such as a colour to the concrete or a specific high
quality finish. These are outside the scope of a generic concrete
specification and mix design and will require the mix for the
concrete to be designed specifically.
High strength concretes above 60 N/mm often require the use of
designed mixes.

6.1.5 Self compacting

Self compacting concrete has the ability to self compact with no
vibrating of the formwork required after the initial pour. This can be
valuable where complex forms and intricate shapes are required
within the structural element that would prevent the use of a
vibrating poker to aid the compaction of the concrete.
Self compacting concrete is very different from high slump concrete
and the two should not be confused or exchanged during the
construction process.
High slump concrete whilst more workable than standard mixes does
not possess the same level of self-compaction as self compacting
concrete and if used instead will frequently leaving voids within the
concrete depending on the complexity of the forms.

33

This has been noted on the exchange between the two mixes when
constructing hybrid structures such as twin wall, where voids have
been uncovered at the ends of panels.

6.1.6 Super high strength

Typically concrete within civil and structural engineering are used
between a strength range of between 20 N/mm and 60 N/mm
depending on their location and intended design life. However,
given the ongoing race for taller and taller buildings there are
developments of much stronger concretes with the use of 100
N/mm and 120 N/mm concretes becoming more commonplace
within skyscraper design and construction.
The use of these new concretes can be outside the scope of the
design standards though and special consideration should be given
to the design and construction when using high strength concretes.
This is due to the high amount of energy and heat released when
these high strength concretes are placed as they can result in the
element being subjected to significant shrinkages and thermal
strains.
The increased use of stronger concretes is also gaining momentum
in the use of precast construction concrete design where the turn
around time on the forms is often reduced to allow the production
rate of the factory to be increased.

6.1.7 Pour sequence

When concrete is placed on a project the contractor will make a
note of where the concrete was placed on a drawing including the
details of that particular batch. The pour sequence for the concrete
is an important part of the construction process, this will determine
the short and long term behaviour of the structure and consequently
the contractor should provide details of the pour sequence ideally

34

as part of the tender process, but certainly no later than 2 weeks

from commencement on site.
The size and nature of the concrete pours can significantly affect
the programme and this information should be defined by the
design team at the earliest opportunity and confirmed with the
contractor as part of the negotiation and tender award process.
In order for the contractor to physically place the concrete using a
pump there may be a need for a plasticiser to be added to the mix
or for the aggregate size to be reduced to allow the pumps to run at
a reduced pressure or to increase the distance which concrete can
be pumped.
This may require a departure from the specified mix (designated or
designed) and will require the contractor to submit proposals for the
engineers approval.

6.1.8 Identity testing

Whilst the use of designated mixes in theory is intended to remove
the need for contractors to undertake additional mechanical testing
of their concrete cubes, it is still sometimes advisable to take
additional cubes for identity testing during the construction works.
These tests can allow you to identify areas where the concrete is
non-compliant with the construction specification, such as it could
be slow in attaining the required 7 or 28 day design strength during
the curing and placement process.
By having these additional tests, the engineer can correlate them
with the pour sequence drawings to identify which areas may be
affected by the non-compliant concrete.
Additional identity testing of the concrete is often resisted by the
contractors, who will note that identity testing is not required as the
concrete is being supplied by an accredited supplier and the
necessary paperwork will be provided with the delivery.

35

Experience has shown that the additional testing often provides an

additional element of re-assurance to the clients that the concrete
supplied is of appropriate quality and issues with batches are
typically identified early in the construction process which allow for
remediation strategies to be employed whilst still cost-effective and
also prevent any shortcomings from re-occurring.
Testing can be done insitu, but will typically require cores to be
taken from the existing structure to allow these tests to be calibrated
and compared between the various parts of the structure. It should
be noted that these tests are expensive, time consuming and are no
replacement for good quality record keeping.

6.1.9 Jointing
Where large areas of slab are required, these cannot be readily be
poured in a single pour for several reasons. Primarily there will be a
logistic limitation as to how much concrete can be delivered to site
and continually poured before the site must close for the day and
this must be factored into the construction sequence. Where pours
stop and start for the day are frequently called day joints.
Even if the amount of concrete required to pour a large floor slab
could all be delivered and placed in one day, the amount of
concrete involved may result in high quantities of heat being
generated within the concrete which can have an adverse affect
on its curing times and sequences. This is because concrete is
exothermic and consequently gives up large amounts of heat as it
cures, this effect is exacerbated for deep and large volume pours
such as mass concrete retaining walls and dams, where the amount
of heat generated can be quite considerable.
As concrete cures; it slowly gives up a portion of its water content as
it dries out and this giving up of water results in shrinkage of the
concrete. This can be problematic for large areas of slabs,
depending on their restraint conditions near large stability cores and
other stiff elements. The shrinkage of concrete slabs can be a
36

significant limitation on the amount of concrete poured in a day

and one method to prevent these early thermal shrinkage effects
from limiting the structure is to integrate a control strip around stiff
elements such as cores, with the control strips being poured much
later than the rest of the slab, ideally once the early thermal
shrinkage has occurred.

6.2 Process
The design, detailing and construction of insitu concrete buildings
passes through several key stages during the process. Some of the
key stages are outlined below but do not make reference to the
early conceptual design stages, merely in the production of
construction information through to the physical construction of the
building.

6.2.1 Design
The design of the structural element to be constructed will
determine the shape, form, strength of concrete, amount and
arrangement of the reinforcement. A good designer will always be
developing how the element can be constructed safely and
economically as well as ensuring that the design is safe and will not
deflect excessively in the permanent condition.

6.2.2 Drawings
To identify the overall shape of the element and its location within
the building a well drawn set of plans, sections, elevations and
details are likely to be required. These drawings will be produced by
the engineer at the appropriate design stage and will typically be
co-ordinated with the other disciplines such as the architect and
building services engineers to allow for a co-ordinated design to be
produced.

37

6.2.3 RC Placement drawings

Reinforcement placement drawings differ from general
arrangement drawings as they are not intended to show how the
building is arranged, but instead show where and in what
configuration individual or groups of steel reinforcement should be
position within the structural elements.

Figure 6-1 Reinforcement placement drawing from design and detailed.

These drawings will give each piece of steel reinforcement their own
unique number for the project, this number is called the bar mark
and will be accompanied with a bending schedule.

38

6.2.4 Bar bending schedules

These standardised shapes are called shape codes and enable

reinforcement suppliers to automate their factories and also enable
the creation of simple repeatable
patterns of reinforcement that Neither
are
(B)
A nor B
governed by a common set of rules and parameters.
in Table 2 nor l

13

A + 0.57B + (C)
(C)

Licensed copy:, 24/11/2011, Uncontrolled Copy, BSI

12

Neither A nor B

(B)

Whilst reinforcing bar can be bent into any practical shape withinin Table 2
the limitations of its diameter and associated diameter, it is typically
A + (B) 0.43R
bent into predefined standardised shapes that are governed by a
British Standard3.

B shall not be l
Neither A nor C
in Table 2 nor l
See Note 3.

A
Key
Figure 6-2 Example of Shape Code 13 for RC Detailing.
1 Semi-circular

14

These rules also prevent reinforcing bars being bent to radii that are
A + (C) 4d
too tight and that might otherwise affect the strength of the
reinforcement through cold working of the steel reinforcement.

6.2.5 Placement
B

The reinforcement is placed on the formwork as described in

previous sections. Reinforcement should be placed on clean, dry
formwork within a temperature range as identified within the
engineering specification. The reinforcement within a beam or
column element is tied together using the shear links to create a

Neither A nor (
in Table 2. See

(C)

12

BS8666

39

cage that can be lifted up into place and position directly into the
formwork.
Reinforcing bar lengths are typically provided in either 6m or 12m
lengths, depending on the diameter and the designer should give
consideration as to how these bars will be physically picked up and
positioned on site whilst they are preparing their designs. If a
reinforcing bar cannot be position by the labourers then this can
frequently tie up valuable resources and crane time to lift and
position the reinforcing bar into place, plus it may not present a safe
and economical solution.
If a structural element is longer than the typical length of a
reinforcing bar, then this will require a lap in the reinforcement, this
lap can be formed by lapping the reinforcing bars next to each
other for a minimum length as described within the relevant
standards, by the inclusion of mechanical couplers or by positioning
the lap in a location where the reinforcement is no longer required
to form a beam design utilising lapless construction. Mechanical
couplers are frequently avoided in commercial applications given
the increase in costs and standard laps instead are prepared, in
nuclear construction though the bar diameters are typically large
and congested and in these instances couplers provide an
economical alternative to lapped reinforcement.
All debris should be removed from the bottom of the formwork prior
to concrete being placed, there is often clippings of tying wire
around the bottom of the formwork that can cause unsightly
staining to the underside of the concrete slab and any foreign
matter left on the bottom of the formwork can result in the cover to
the reinforcement being compromised.

6.2.6 Vibrating
Once the reinforcement is in place within the formwork and all
debris has been removed from the bottom of the forms, then the
concrete can be placed.
40

The concrete can be placed in a variety of methods, from being

hand barrowed into place or it can be pumped or skipped into
place. Frequently contractors are using concrete pumps to place
concrete into position with new pumps being able to pump
concrete over appreciable distances and heights before running
into difficulties.

Figure 6-3 Steel reinforcement being concreted.

Careful design and consideration should be given when designing

the concrete itself so as to prevent the mix from segregating during
the placement process as this can adversely affect the quality and
workmanship of the concrete.
Unless self compacting concrete is being used however, there will
be a need for the labourers to use a vibrating poker to vibrate out
any air pockets within the concrete so as to increase the
consistency of the concrete within the forms and to prevent the
inclusion of any voids whilst the concrete cures.

41

6.3 Common defects

Within the construction of concrete structures there may be several
defects that are encountered whilst they are being built, some of
which are listed below: Ill fitted formwork allowing the wets from the concrete to bleed
out at the joints.
Poor compaction of the concrete.
Incorrect spacers being used, providing too much or too little
cover.
The use of paving slabs as spacers for the reinforcement.
Lack of containment on the reinforcement.
Hydrostatic pressure causing the formwork to burst and fail.
Each of these defects may require different forms of remediation
depending on the types of concrete elements being constructed,
but the best form of control is to ensure that a pre-pour inspection is
undertaken by the engineer prior to construction beginning.

6.4 Formwork
Formwork is a key component of construction using concrete and
can take a variety of forms from the traditional timber formwork,
through to the introduction of permanent formwork through the use
of composite steel decking.
The costs associated with formwork and the complexity involved is
often overlooked by engineers during the design process, but
ultimately if you cant make the formwork, then you cant make the
structure.

42

Figure 6-4 Forwork for a column.

Various materials can be used to make the more complex shapes

seen in concrete construction including GRP, Polystyrene and even
inflatable formwork has been used to make domes and other
historically expensive forms.
The purpose of formwork is to provide a clean, rigid mould that
allows the wet concrete to develop its long term strength and
subsequently become a concrete frame. To achieve this the
formwork must be designed to resist the wet weight of the concrete
and the associated hydrostatic head that the wet concrete
develops whilst in the concrete.
If the formwork is not strong enough or if the formwork deflects
beyond acceptable limits then this can lead to the concrete
element not being within allowable limits and subsequently being
condemned. Once an element is condemned then it may either be
remediated or in extreme cases demolished and rebuilt which may

43

have financial and time costs to the person or persons responsible

for the creation of the defect.

6.4.1 Timber shuttering

Traditional formwork is constructed from timber and carpenters will
frequently specialise in formwork joinery to allow them to construct
formwork for construction sites.
Timber formwork design is both a skill and an art within its design and
construction, with the ability of the formwork being able to resist the
hydrostatic pressures and self weight of the concrete whilst not
excessively deflecting or the joints prising apart and leaking being
fundamental to the success of the project and the element design.
Timber formwork is used widely on construction sites and within
precast factories, although through the use of more sophisticated
construction processes the use of timber formwork is not as widely
used as it was perhaps 20 years ago.
For highly complex areas of construction where intricate forms are
required the use of timber formwork still possesses various
advantages as it can be adapted and adjusted on site with relative
ease and can have high performance materials and mouldings
integrated into its design to increase its performance.
One of the significant advantages of timber formwork is the ability
for it to adopt highly complex and organic shapes and
consequently it was the formwork material of choice of one of the
great engineers Heinz Isler who was famous for creating complex
concrete shell structures in a pre-computer age.

6.4.2 Table forms

Where flat soffited slabs are required, the use of table forms can
accelerate the construction of slabs and also allow for safer
construction with higher quality finishes.
Table forms particularly used in multi-storey buildings, where there
are high levels of repeatability within the floor plates. Effectively the
44

table element of the formwork is constructed from high quality steel

or timber with adjustable trestles or supports beneath it.

Figure 6-5 Formwork

The formwork is propped on the floor below (which may be

subsequently propped to preceding floors) to allow the weight of
the wet concrete to be supported. The number of floors able to
contribute to the support of the wet floor is defined within various
industry guides but for ease of reference the load distribution tbale
below is replicated from BCA document Early Striking and improved
backpropping. (ISBN 07210 1556 5)

45

ds careful consideration.
is responsible for
striking process safely.
bility of the Temporary
ator (TWC) to manage
striking. This will
ng detailed procedures
ements, which should
the safety officer and
ks Designer (PWD).

is to provide design
hat the construction
adversely affect the
the structure.
d managerial staff
ware of the implications
and procedures adopted.
striking and backures are given in Ref. 1.

on load
ons

Table 3: Load distribution by backpropping

Location

Load No backprops
On slab

On slab

New slab being cast

total

100%

100%

Falsework/formwork

wp

100%

In backprops

On slab

100%
65% w p

30% w p
30% w p

In props

100%

70% w p

w b1

On lower slab (2)

In props

Two levels of
backprops

100%

100% w p

On supporting slab (1)

In backprops

One level of
backprops

35% w p
23% w p

w b2

12% w p
12% w p

On lower slab (3)

Notes

1. Assumes lower and supporting floors have been struck, have taken up their deflected
shape and are carrying their self-weight

n-situ concrete
e critical loading
lab is not necessarily
and becomes self-

2. Floor loading from imposed loads and self-weight is not considered

3. The strength of particular slabs to carry applied loads will have to be
considered separately
4. All floors are suspended floors
5. Figure 2 gives location of loads w p, w b1, and w b2

Figure 6-6 Backpropping load distribution.

As table forms will typically be brought out of the sides of tall

buildings to then be lifted up to floors above, it is often beneficial to
not include edge beams and to use flat slabs where possible to
simplify the removal of the formwork at the edges.
Modern table forms will often project beyond the edges of the
building to provide a safe working platform around the perimeter of
the slabs, these projections will have integral hand railing. This
additional working space is essential for post-tensioned slabs where
space is required around the perimeter to allow the tendons to be
jacked4.

6.4.3 Steel forms

Steel forms are expensive to produce and are frequently utilised
where repetition of common elements have been identified.
Through their continual re-use through the building the cost of the
formwork can realise significant savings through their repeated use.

Visit blackboard to see a video of tendons being jacked around the

perimeter of the slab.
4

46

Also worth noting is that the finished quality of the concrete and
accuracy of the formwork can be increased when compared to
timber formwork and steel forms are frequently used for columns
that are exposed and consist of a visual element of concrete.
Steel shuttering may also be more economical for tall complex pours
where large hydrostatic pressures are anticipated due to their
increased strength compared to timber forms.

6.4.4 Cardboard
The use of modern engineered cardboard formwork is a common
sight on UK construction sites due to its robustness and economical
benefits. Disposable cardboard formwork is bought in predefined
sizes and is more commonly used for columns where it can be
bought for circular, rectangular and square column profiles. The
formwork can only be used once and is designed to be cut from the
columns with a knife once the formwork has been struck.

Figure 6-7 Carboard Formwork

47

The inside of the cardboard tubes is lined with high quality polished
cardboard which enables the formwork to give a high quality
surface finish with even standard designated mixes.
Whilst the formwork itself allows for high quality construction to be
completed, the nature that the tube will slide over the bars can
sometimes result in the inside of the formwork becoming snagged
and ripping on the reinforcement of the spacers.
To overcome this, the column is designed with a slightly greater
cover to accommodate tolerances within the cage construction
and special spacers can be used that spin to facilitate the
placement of the concrete tubes over the top of the reinforcement.
Another benefit is that once the column has been struck and
inspected, then the cardboard formwork can be placed back onto
the column and taped back on to offer some element of protection
to the concrete columns during the rest of the construction stages.

6.5 Tremie pipe

All methods of placing concrete under water are designed to
prevent cement washout and the consequent formation of weakly
cemented sand and gravel pockets. In the tremie process, concrete
is placed through a vertical steel pipe with an open, funnel-shaped
upper end. The lower end of the tremie is kept immersed in plastic
concrete so that freshly placed concrete doesn't come into contact
with the water. This process is not just reserved for pouring concrete
in rivers, lakes, and streams and is frequently used for concreting
piles where groundwater is present.

48

Figure 6-8 Tremie pipe used in piled foundations.

Tremie pipe diameter usually ranges from 200-300mm. End plates or

plugs are used when a dry pipe technique is employed for starting
the tremie pour. As the pipe is lowered to rest on the bottom, water
pressure seals the gasket and the pipe is kept dry. In very deep
placements, an open-ended pipe can be set and a go-devil or
traveling plug inserted to keep water from penetrating the first
concrete placed in the pipe. All vertical movements of the tremie
pipe must be done slowly and carefully to prevent a loss of seal. If
loss of seal does occur, the tremie must be brought back to the
surface, the end plate must be replaced, and flow restarted. A godevil must not be used when restarting a tremie after a loss of seal.
Water pushed out by the go-devil will wash cement out of the
previously placed concrete. Concrete placement should be as
continuous as possible through each tremie. Longer delays must be
treated by removing, resealing and restarting the tremie.

6.6 Concrete pump.

As concrete is a fluid, it is possible to pump it from a discrete point to
another point under pressure. This is typically achieved by using a
truck mounted pump with a large mounted boom arm attached.

49

Figure 6-9 Concrete pumping truck.

To allow the concrete to be pumped large distances, the concrete

mix will require some form of modification, typically through the
introduction of plasticisers which help the concrete to remain more
fluid like for longer periods of time. Some of the larger more
powerful pumps are able to pump and place concrete up to a
maximum distance of 70 metres. Clearly due to the inclusion of
plasticisers to the concrete mix, this can increase the curing time
required for the concrete to achieve its 28 day strength.

6.7 Tall buildings.

Tall buildings present their own unique challenges compared to their
shorter; vertically challenged brethren. As the height of the building
increases, so do the logistical challenges of moving materials to the
construction interface of the building. Similarly as the weight of the
building above increases, so does the load within the columns and
this can cause the columns to compress and shorten. This process is
called axial shortening and can be significant when designing
skyscrapers as it can cause cladding to not fit as the distance
between floors shortens, this results in the floor to floor height for the

50

lower floors being smaller than the upper floors due to the increase
in axial load
As the construction work reaches higher and higher levels, it will
extend beyond the reach of a traditional concrete pump and a
different approach will be needed. Typically the concrete is lifted in
a skip via the tower cranes up to higher floors and either deposited
into a hopper for pouring or placed directly onto the relevant floor
slab.

Figure 6-10 Concrete skip used on tall buildings.

6.7.1 Table forms.

Table forms are so called because they resemble large tables and
because of this they are typically used to cast large areas of flat
soffited concrete such as slabs.
The legs on the table forms are designed to enable them to be
folded underneath them, and this is how they are collapsed and
removed from the building once the concrete has achieved
adequate strength and is able to support itself. There is division
between contractors with regards the integration of edge beams
and the use of table forms. Most contractors prefer that downstand
beams are used in the design, as this allows for the concrete to be
poured in a single pour because upstands require additional
formwork and are typically poured after the slab has gained
51

strength. An upstand sits above the top of a slab, like a small wall
whereas a downstand sits below the slab like a beam.

Figure 6-11 Pier Luigi Nervi hangar roof using downstand beams.

6.7.2 Cantilevers
Cantilevers require special attention when assessing their
constructability, particularly in concrete, as concrete has no
inherent strength until it has cured for sufficient time. The formwork
therefore, provides all of the support to the cantilever until the
strength requirements have been met and it is this requirement that
can introduce great cost and complexity to the construction of
significant cantilevers, rather than just the fixed lengths of the
cantilever in the permanent condition.

6.7.3 Jump forming

Generally, jump form systems comprise the formwork and working
platforms for cleaning/fixing of the formwork, steel fixing and
concreting. The formwork supports itself on the concrete cast earlier
so does not rely on support or access from other parts of the building
or permanent works.

52

Jump form, here taken to include systems often described as

climbing form, is suitable for construction of multi-storey vertical
concrete elements in high-rise structures, such as:
Shear walls
Core walls
Lift shafts
Stair shafts
Bridge pylons
These are constructed in a staged process. It is a highly productive
system designed to increase speed and efficiency while minimising
labour and crane time. Systems are normally modular and can be
joined to form long lengths to suit varying construction
geometries. Three types of jump forms are in general use:
Normal jump/climbing form units are individually lifted off the
structure and relocated at the next construction level using a
crane. Crane availability is crucial.
Guided-climbing jump form also uses a crane but offers greater
safety and control during lifting as units remain anchored/guided
by the structure.
Self-climbing jump form does not require a crane as it climbs on
rails up the building by means of hydraulic jacks.
Jump forming offers various benefits over traditional shuttering,
including:
Fast construction can be achieved by careful planning of the
construction process.
Self-climbing formwork cuts down the requirement for crane time
considerably. By allowing the crane to be used for other
construction work this may reduce the total number of cranes
needed on site.

53

 The formwork is independently supported, so the shear walls and

core walls can be completed ahead of the rest of the main
building structure.
High quality surface finishes can be achieved.
Climbing forms can be designed to operate in high winds.
Highly engineered nature of jump form systems allows quick and
precise adjustment of the formwork in all planes.
Some formwork systems can be used at an inclined angle.
A small but skilled workforce is required on site.
It is easier to plan construction activities due to the repetitive
nature of the work.
As the formwork system is self-enclosed, it offers a wide range of
safety benefits, including:
Working platforms, guard rails, and ladders are built into the
completed units of market-leading formwork systems.
Self-climbing formwork systems are provided with integral free-fall
breaking devices.
The completed formwork assembly is robust.
The reduced use of scaffolding and temporary work platforms
results in less congestion on site.
The setting rate of concrete in those parts of the structure
supporting the form is critical in determining the rate at which
construction can safely proceed.
The repetitive nature of the works means site operatives are
quickly familiar with health and safety aspects of their job.
Jump form is typically used on buildings of five storeys or more; fully
self-climbing systems are generally used on structures with more than
20 floor levels.
Trailing and suspended platforms are used for concrete finishing and
retrieving cast-in anchor components from previous pours.
54

6.7.4 Slip forming

Slip form is similar in nature and application to jump form, but the
formwork is raised vertically in a continuous process. It is a method of
vertically extruding a reinforced concrete section and is suitable for
construction of core walls in high-rise structures lift shafts, stair
shafts, towers, etc. It is a self-contained formwork system and can
require little crane time during construction.

Figure 6-12 Stability core at Media City formed using slip form.

This is a formwork system which can be used to form any regular

shape or core. The formwork rises continuously, at a rate of about
300mm per hour, supporting itself on the core and not relying on
55

support or access from other parts of the building or permanent

works.
Commonly, the formwork has three platforms. The upper platform
acts as a storage and distribution area while the middle platform,
which is the main working platform, is at the top of the poured
concrete level. The lower platform provides access for concrete
finishing.
Benefits
Careful planning of construction process can achieve high
production rates
Slip form does not require the crane to move upwards, minimising
crane use.
Since the formwork operates independently, formation of the
core in advance of the rest of the structure takes it off the critical
path enhancing main structure stability.
Availability of the different working platforms in the formwork
system allows the exposed concrete at the bottom of the rising
formwork to be finished, making it an integral part of the
construction process.
Certain formwork systems permit construction of tapered cores
and towers.
Slip form systems require a small but highly skilled workforce on
site.
Safety
Working platforms, guard rails, ladders and wind shields are
normally built into the completed system.
Less congested construction site due to minimal scaffolding and
temporary works.
Completed formwork assembly is robust.
Strength of concrete in the wall below must be closely controlled
to achieve stability during operation.
56

 Site operatives can quickly become familiar with health and

safety aspects of their job
High levels of planning and control mean that health and safety
are normally addressed from the beginning of the work.
Other considerations
This formwork is more economical for buildings more than seven
storeys high.
Little flexibility for change once continuous concreting has begun
therefore extensive planning and special detailing are needed.
Setting rate of the concrete had to be constantly monitored to
ensure that it is matched with the speed at which the forms are
raised.
The structure being slipformed should have significant dimensions
in both major axes to ensure stability of the system.
Standby plant and equipment should be available though cold
jointing may occasionally be necessary.

57

Concrete (Alternate forms)

7 Concrete (Alternate forms)

Hybrid forms of construction are becoming increasingly common in
industry with the development of thin high strength concrete being
used to form hollow tubular structures that can be filled with insitu
concrete to give them their final load carrying capacity.
Various different forms of hybrid structure are available for forming
walls within buildings including TwinWall5 systems, with each system
being a different manufacturers variation on a common theme.
The ability for these systems to have integral formwork are wide
ranging and include:

high quality finish,

accurate tolerances,
precast window apertures,
integrated insulation,
lighter weights for craning into position
increased speed of construction.

Similar hybrid systems (such as Omnia Deck6) exist for concrete slabs
and follow a similar principle with a thin high strength concrete

http://www.precaststructures.com/TwinWall.asp

http://www.heidelbergcement.com

58

biscuit providing the load carrying capacity within the slab to allow
the support of the structural element in the temporary condition with
the additional weight of the wet concrete, but once the concrete
has cured then the slab will attain its permanent load carrying
ability. This method typically requires no additional reinforcement to
be included as longitudinal steel is included within the concrete
biscuit, however additional reinforcement can be included where
required on an ad hoc basis.
The weights of these panels can be considerably reduced through
the introduction of biaxial void formers such as Cobiax7 or
Bubbledeck8.

Figure 7-1 Bubbledeck

http://www.cobiax.ch/en

http://www.bubbledeck.com

59

7.1 Edge protection

The formalisation of the CDM regulations and the education of the
values of Health and Safety through the construction industry has
boosted the development of various technologies and innovations
in the design and construction of buildings.
One of these is the increasing availability that formwork has to
integrate temporary edge protection around its perimeter during
construction and for this philosophy to be extended through cast in
edge protection systems once the formwork has been struck.

Figure 7-2 Formwork for a viaduct.

7.2 Debonding agent

As the formwork is being prepared, the faces that are to be in
contact with the wet concrete are first painted with a de-bonding
agent that aid with the striking of the concrete. This prevents the
concrete from bonding itself to the formwork and can help create a
higher quality finish on the concrete.
If de-bonding agent isnt used, then as the formwork is struck then
pieces of concrete can come away with the formwork that leave
unsightly pocks and marks on the face of the concrete.
60

7.3 Sealant
Concrete structures can produce large quantities of dust, even after
the concrete itself has long since cured and attained its 28 day
strength. This can be problematic in areas that require dust free
environments or even in car parks where the slabs are frequently
trafficked by vehicles.
To limit the amount of dust produced from the building, dust sealant
can be applied to the concrete elements in the form of a clear
paint. This paint suppresses the dust and prevents it leaving the
concrete and is frequently used on areas of exposed concrete such
as core walls and soffits of slabs.
Where additional toppings are to be applied to the tops of concrete
slabs then it may be required to omit the dust sealant so as to
provide a good key between the two surfaces. Each area should
be carefully assessed by its intended use and proposed finishes to
determine if dust sealant will be required. Frequently the sealant is
specified by the architect and included within their finishes
schedules.

61

Masonry

8 Masonry
8.1 What is masonry?
Masonry is one of the oldest materials still used today and has been
used in several of the iconic historic buildings still standing today
including the Great Wall of China and the Great Pyramids.
Masonry is an encompassing name for a variety of units, which
include stone, bricks and various man made blocks when it comes
to structural design and it is not uncommon for different units to be
mixed and matched. For example, on modern houses frequently
the internal skin is constructed using blockwork due to the speed at
which it can be erected, although traditional bricks are still
commonly used for the external skin because of the increased visual
quality that they present and also they perform well in external and
exposed environments.
One of the key things to remember when designing and
constructing masonry structures is that whilst it may have a small
tensile component, it is generally advisable to construct masonry
structures so that they work in compression. If as an engineer you
are ever unsure if a material is suitable to design to resist tension
forces, ask yourself a simple question
If I was in prison, would I make an escape rope from it?
If the answer is no, then the chances are, that its a poor material in
tension.

62

8.2 How are they made?

Bricks traditionally were made through the placing of clay into
moulds to create the brick shape and then these units were fired in
an oven at high temperatures to create a stable unit. Modern
methods also include the creation of long continuous columns
which are then cut into smaller units using a wire cutting device.
These sub units are then dried and then fired in a kiln to create
bricks.
Unit strength
(in N/mm2)
to BS 5628-1

Normalised strengths (in N/mm2) for unit widths of:

75mm

90mm

100mm

140mm

190mm

215mm

2.9

4.1

4.1

4.0

3.8

3.5

3.4

3.6

5.1

5.0

5.0

4.7

4.3

4.2

7.3

10.4

10.2

10.1

9.5

8.8

8.5

10.4

14.9

14.6

14.4

13.5

12.5

12.1

17.5

25.0

24.5

24.2

22.8

21.0

20.3

22.5

32.2

31.5

31.1

29.3

27.0

26.1

30.0

42.9

42.0

41.4

39.0

36.0

34.8

40.0

57.2

56.0

55.2

52.0

48.0

46.4

Figure 8-1 Normalised block strengths from a manufacturer.

Masonry units come in a variety of strengths and each different type

of unit can behave very differently from another unit, however the
strength of the unit is only one factor that can affect the overall
strength and load carrying capacity of masonry structure. The
mortar and quality of the construction will also greatly affect the
overall load carrying capacity of a masonry structure.
The 6 key factors that will affect the compressive strength of masonry
are listed below.

The mortar strength

The unit strength
The relative values of unit and mortar strength
The aspect ratio of the units (ratio of height to least horizontal
dimension)

63

 The orientation of the units in relation to the direction of

applied load
The bed joint thickness

copper, polythene,
8.3 Common units.
courses of bonded
Fig. 5.5 Types of bricks: (a) solid; (b) perforated;
ed on Table 2,
There is(c)
anfrogged.
overwhelming range of masonry products available and

given the amount of time that theyve been used in the UK for
size ofthere
the brick,
i.e.standard
215 102.5
65 mm, plus
an
construction
is no real
size as frequently
specials
allowance
of for
10heritage
mm for and
the conservation
mortar joint work
(Fig.where
5.6).
are used,
especially
iety of materials
Clay bricks are also manufactured in metric moduperhaps the bricks were made by a local artisan originally.

and sand/flint),
lar format having a coordinating size of 200 100
However,
formm.
modern
construction,
commonly
adopted
ese, clay bricks
75
Other
cuboid and
special shapes
arebrick
also
d variety in the
available
(BS 4729).
dimensions
include:

y shaping suitnormally taken

5). Sand facings
ed to the green
ay be perforated
elf-weight of the
fired in kilns to
500 C in order
ctural use. The
ases both the
s.
he coordinating
en to be 225
e actual or work

Figure 8-2 Coursing dimensions for brickwork.

Fig. 5.6 Coordinating and work size of bricks.

8.4 Clay bricks

241

There are a wide range of clay bricks within the UK, each with subtly
different finishes, colours and performance criteria that have been
developed over the years to suit the various uses and intended
locations of the bricks.
11/3/09, 11:11 AM

Clay bricks can be made by hand or within a factory and can have
a density ranging from anywhere between 22.5 to 28kN/m3.

64

Clay bricks have a tendency to expand as a result of water

absorption, whereas engineering bricks are much more stable with
regards water absorption.
Locations where a high strength, high quality brick is required will
usually benefit from the inclusion of engineering brick due to its
excellent durability and structural stability.
Class A Engineering Bricks - 70N/mm2; Water absorption 4.5%
by mass.
Class B Engineering Bricks - 50N/mm2; Water absorption 7.0%
by mass.
There are typically 60 standard bricks per square metre of wall, with
a 2:1 gang (2 bricklayers and 1 labourer) being able to lay
approximately 1,000 bricks in a day.

8.5 Facing bricks

Facing bricks are typically not as strong as engineering bricks and
are used to create a high quality visual appearance in areas of a
building where a certain type of look is required.

8.6 Calcium Silicate Brick

These types of bricks are aimed at the mass produced, low cost/unit
end of the market. They are typically formed from sand and slaked
lime and are rarely used due to their tendency to shrink and crack
which can make their integration into a building structure very
difficult.

8.7 Concrete block

Concrete blocks are widely used in the industry for a variety of
reasons. Whilst they may not be as visually appealing as bricks, their
relative cheapness offers them a significant advantage.
65

Similarly as the units themselves are quite a bit larger than bricks, the
amount of wall that can be constructed in a day is much greater as
a bricklayer can effectively replace multiple bricks with a single
block.
There are a multitude of different types of concrete blocks, ranging
from lightweight blocks with a density of 5kN/m3 through to dense
blocks with a density of 20kN/m3.
Care should be taken when specifying the blocks, as it is possible to
get the same strength of block at a variety of densities and
depending on the use of the block this can affect how it performs
with regards its durability.
For example, hanging large heavy components such as boilers off a
wall constructed in lightweight block can sometimes cause
difficulties as the wall anchors are being installed as they can cause
lighter weight blocks to disintegrate and fall apart.
The solution isnt to specify the heaviest block possible, there are
limits on block weights that you can legally expect a brick layer to lift
continually through the course of the day and these are contained
within the CDM regulations.
Similarly, if the heaviest blocks are used, these can increase the
dead load that a floor needs to support which can have associated
ramifications on the cost of the building if larger floors, columns and
foundations are needed to support the extra weight of the masonry.
Common9 strengths of concrete blocks range between 3.6 to
22.5N/mm2.

http://www.tarmacbuildingproducts.co.uk/products_and_services1/blocks_and_mortar/
blocks/aggregate_blocks/topcrete_standard.aspx

66

8.8 Fair finished block

Blockwork walls typically will either have an external render or leaf of
brickwork and be concealed.
For internal blockwork, the walls are typically plastered or lined but
an alternative is the specification of fair faced blocks which will
allow the wall to receive a coat of paint directly.

8.9 Terminology
Masonry units have a strange terminology which may not be
apparent from the description given and engineers will need to be
familiar with these terms so that they are able to appropriately
communicate with contractors and anyone else that may need to
use 6:their
Eurocode
Designdrawings.
of masonry structures

Solid Unit

Frogged Unit

Vertically Perforated Unit

Vertically perforated Unit

Vertically Perforated Unit

Fig. 10.1 Examples of high density (HD) Figure

clay masonry
unitsof
(Fig.
3, EN units.
771-1).
8-3 Types
masonry

Brick and blockwork is typically constructed in leafs, this can be

either a single lead, cavity wall, or sometimes the leaves can be
reinforced
through
the2555
integration
piers
to help
increase
calcium
silicate units
have between
per cent of A
number
of procedures
for either
conditioning of
voids
and
aggregate
concrete
units
between
2560
masonry
units
prior
to
testing
are
outlined
their vertical or lateral load carrying772-1:
capacity
of the wall as can
bein EN
per cent voids. Table 10.3 gives further details
Methods of test for masonry units: Determination
of the requirements for unit groupings. All UK
bricks currently manufactured to British Standards,
including frogged and perforated bricks, fit into
Group 1 unit specification although the author
understands some Group 2 units are becoming
available. Cellular and hollow blocks fit into
Group 1 or Group 2 unit specification, depending
on the void content. Masonry units which fall within
Groups 3 and 4 have not historically been used in
the UK. The manufacturer will normally declare
the group number appropriate to his unit.

10.8.2 NORMALISED COMPRESSIVE STRENGTH

of compressive strength. This standard advises that

the conditioning factor for air-dried units is 1.0.
Masonry units manufactured to British Standards
are wet strengths in which case the recommended
value of the conditioning factor is 1.2. Values for
the shape factor, , are given in Table A1 of EN
7721, to allow for the height and width of units.
For example, for 102.5 mm 65 mm bricks =
0.85, and for 215 mm (height) 100 mm (width/
67thickness) blocks, = 1.38.

10.8.3 COMPRESSIVE STRENGTH OF MORTAR

Table 10.4 shows the masonry mortar mixes recommended for use in the UK to achieve the appro-

Design in unreinforced masonry to BS 5628

seen in Figure 8-4 below.

Fig. 5.8 Masonry walls.

Figure 8-4 Forms of masonry wall construction.

8.10 Ties
COMPRESSION:
Gk
Qk
Wk
f
m
fk
N
NR

state philosophy (Chapter 1). This code states that

Masonry
leaves are knitted togetherthethrough
the
introduction
primary aim
of design
is to ensureof
an wall
adequate
characteristic dead load
margin of safety against the ultimate limit state
tiescharacteristic
that can imposed
take aload
variety of shapes
and
formed.
As ofbuilding
being
reached.
In the case
vertically loaded walls
characteristic wind load
this
is
achieved
by
ensuring
that the to
ultimate
efficiencies
and the requirement
to increase the cavity
partial safetyincrease
factor for load
design load (N ) does not exceed the design load
partial safety factor for materials
resistance
of thegreater
wall (NR ):and greater,
help
raise thecompressive
thermalstrength
performance
becomes
characteristic
of
masonry
N NR stronger (5.1)
so the
size of the cavity too will increase and this requires
ultimate design vertical load
ultimate
designwall
vertical
load resistance of
and
stronger
ties.
wall

Materials

FLEXURE:
fkx par
fkx perp

M
MR
Mpar
Mperp
Mk par
Mk perp

characteristic flexural strength of masonry

with plane of failure parallel to bed joint
characteristic flexural strength of masonry
with plane of failure perpendicular to bed
joint
bending moment coefficient
The ultimate design load is a function of the actual
orthogonal ratio
loads bearing down on the wall. The design load
ultimate design moment
resistance is related to the design strength of the
design moment of resistance
masonry wall. The following sub-sections discuss
design moment with plane of failure
the procedures for estimating the:
parallel to bed joint
design moment with plane of failure
1. ultimate design load,
perpendicular to bed joint
2. design strength of masonry walls and
design moment of resistance with plane
3. design load resistance of masonry walls.
of failure parallel to bed joint
design moment of resistance with plane
5.5.1 ULTIMATE DESIGN LOADS, N
of failure perpendicular to bedFigure
joint 8-5 Common
Wall Ties in section 2.3, the loads acting on
As discussed

Fig. 5.3 Wall ties to BS EN 845-1: (a) butterfly tie; (b) double triangle tie; (c) vertical twist tie.

a structure can principally be divided into three

basic types,
deaddouble
loads, imposed
(or live)
the above
figure type
(a) are butterfly
ties,namely
(b) are
triangle
5.5InDesign
of vertically
loaded
loads and wind loads. Generally, the ultimate design
ties
and (c)walls
are half twist ties which load
are istypically
used
for the
obtained by
multiplying
thelarger
characteristic
masonry
(dead/imposed/wind) loads (Fk) by the appropriate
In common
with most modern codes of practice
cavities.
partial safety factor for loads ( )
dealing with structural design, BS 5628: Code of
Practice for Use of Masonry is based on the limit

N = fFk

(5.2)

246
Fig. 5.4 Damp proof courses: (a) lead, copper, polythene,
bitumen polymer, mastic asphalt; (b) two courses of bonded
slate; (c) two courses of d.p.c. bricks (based on Table 2,
9780415467193_C05
246
BS 5628: Part 3).

5.2.1 BRICKS

Bricks are manufactured from a variety of materials

such as clay, calcium silicate (lime and sand/flint),
concrete and natural stone. Of these, clay bricks
are by far the most commonly used variety in the
UK.

Fig. 5.5 Types of bricks: (a) solid; (b) perforated;

(c) frogged. 9/3/09, 2:10 PM

68

size of the brick, i.e. 215 102.5 65 mm, plus an

allowance of 10 mm for the mortar joint (Fig. 5.6).
Clay bricks are also manufactured in metric modular format having a coordinating size of 200 100
75 mm. Other cuboid and special shapes are also
available (BS 4729).

8.11 Bond
The pattern that the bricks are laid is called the bond pattern and in
the UK there are five commonly used bond patterns, although many
more patterns are available.

Figure 8-6 Common UK bond patterns.

69

8.12 How is it built?

As the mortar is a key component of masonry construction, it can
also be a limiting factor. The amount of masonry that can be
constructed in a single day is not just dictated by the rate at which
the bricklayer can construct the wall.
Indeed if too much masonry is constructed in a day, then the weight
of the bricks above the wall can cause the mortar to squeeze out
between the bricks like toothpaste and vastly reduce the quality of
the wall.
Similarly if walls are constructed in temperatures that are too cold,
this can affect how the mortar cures and can result in serious
defects with the masonry resulting in the wall needing to be
reconstructed at a later date.
Another key consideration is that whilst the mortar is still curing it will
be very weak and present no appreciable strength characteristics
and so the weather conditions will need to be considered along
with the need for temporary propping. This could be to prevent the
wind from blowing it over during construction, or just to support the
load of the masonry itself before it has obtained the ability to
support its own weight on something like a masonry dome.

8.13 Common defects

Common defects associated with the construction of masonry are
commonly linked with geometric imperfections or material
deficiencies. The geometric imperfections can take various forms
including misalignment of the bricks resulting in the bricks not being
coursed effectively, walls and columns may not necessarily be built
truly vertical and the lean induced into the element can induce
additional overturning or flexural components into the wall.
The capacity for tolerance and potential quality control is reflected
in the large factors of safety typically included within masonry
designs. Although surprisingly masonry is quite a resilient material,
with the peak district being a prime example of this resilience with

70

various walls damaged through vehicle impact, but still free

standing.

71

Timber

9 What is timber?
Timber is one of the oldest construction materials. It is biologically
produced by nature and further processed by man.
Timber is a commonly used construction material, with the ability for
taller and taller timber buildings continuing to develop with the
recently completed 9 storey Stadthaus10.
If appropriately sourced, timber can be a sustainable material and
there are a variety of forestry stewardship programmes that ensure
that as trees are harvested that a minimum number of trees are
planted for replacement.
There are numerous timber products available:
Solid timber (traditional sawn timber)
Glue Laminated timber (laminates planks of wood glued
together)
Laminated Veneered Lumber (2mm thick laminates glued
together)
Plywood (panel product of thin laminates glued together)

10

http://www.woodawards.com/the-stadthaus/

72

 Particleboard (panel product formed by compacting chippings

with resin)

9.1 Forms of timber.

There are numerous timber products available:
Solid timber (traditional sawn timber)
Glue Laminated timber (laminates planks of wood glued
together)
Laminated Veneered Lumber (2mm thick laminates glued
together)
Plywood (panel product of thin laminates glued together)
Particleboard (panel product formed by compacting chippings
with resin)

9.2 Origins of timber.

Whilst timber comes from trees, there are two fundamental different
types of timber that are used in construction, softwood and
hardwood. Hardwood and softwood are broad biological terms
used to describe species of wood. The terms have nothing to do
with the physical hardness of the wood. Hardwoods come from
broad-leaved trees and softwood species from coniferous,
evergreen trees.
Timber is manufactured from the trunk part of the tree. When the
tree is felled, it is taken to the timber mill where it is sawn into sections
of the desired size and shape. Primarily, there are two sawing
techniques adopted plane-sawing and quarter-sawing. Planesawing is quicker and easier to undertake, but more of the tree
material is wasted. Quarter-sawing is more time consuming, but
generates less waste.

9.3 Softwoods
Various softwoods are commonly used within the UK for construction
purposes and for different purposes.
73

Commonly used species include:

Douglas Fir
Scots Pine
European Spruce
Sitka Spruce
Pitch Pine
Parana Pine
Western Hemlock
Western Red Cedar
Hardwoods.

Various hardwoods are commonly used within the UK for

construction purposes and for a different purposes.
Commonly used species include:

Beech
Iroko
Mahogony
Oak
Sapele
Teak

9.4 Considerations.
When compared with other structural materials, there are
peculiarities to the use of timber in construction. These are:

9.4.1 Hygroscopy
Propensity to creep under sustained load
Anisotropy

9.4.2 Hygroscopy
Timber is a natural product derived from trees. Since trees need
water to survive during their lifespan, timber, by its nature, is a
hygroscopic material it attracts water. The effect of hygroscopy is
not just true for living timber but also for dead timber. If, for example,
a structural timber element is taken from a dry timber merchant and
74

applied in a damp, moist environment, it will attract and absorb

water from the environment.
The significance of hygroscopy is that water influences timber at the
microscopic level, to such an extent that the absorbed water affects
the strength of the timber. The strength reduces as more water is
absorbed into the microscopic structure.

9.4.3 Propensity to creep under sustained load

When loaded over a sustained period of time, timber experiences:
A significant loss of strength
A loss of stiffness (increase in deflection)

9.5 Strength
A piece of timber has an initial value of strength when a load is first
applied to it. If the load is sustained over a considerable period of
time it can be seen that the load carrying capability is significantly
reduced. Its strength after this time period is a fraction of its initial
strength value. Severe loss of strength could result in failure of the
timber or creep rupture.

9.6 Deflection
When a load is applied to a piece of timber it has an initial elastic
deflection. If the load is sustained the deflection steadily increases
even though there is not any increase in the magnitude of load. In
other words, the piece of timber is subjected to the effect of creep.
The deflection increases as time progresses until creep rupture
occurs.

75

Introduction to Relevant Eurocod

uinst

ufin

ucreep

g. 2.6. Deformation.

Figure 9-1 Deflection of a timber beam.

9.7 Creep relationships.

The effects of creep are more exaggerated if a piece of timber has
moisture
is high
relevant
tocontent.
show the direction of the grain of the timber

Where it
e symbol used
in Figure 2.5.
Anisotropy

viceability

it is defi

As stated previously, timber is a construction product derived from

trees. A tree grows in a way which best suits the demands that are
placed upon it throughout its life. Trees, and therefore structural
limit
(EC5,strength
2.2.3)properties in different directions. It is
timber states
have different
an anisotropic material, as shown below.

EC5 the 9.8

deformation
of a member or structure is required at two stages:
The anisotropic nature of timber strengths.
(i)

(ii)

Eurocode 5 uses the limit state design philosophy. This means that
theloading
strength capacity
of the timber
to withstand
appliedthe
actions
When the
is immediately
applied;
this the
is called
instantaneous
at the ultimate limit state (ULS). Additionally, a check on
mation:isuchecked
inst .
the actual deflection under the applied actions is within an
After allacceptable
time-dependent
displacement (i.e. creep deformation, u creep ) ha
deflection limit is undertaken at the serviceability limit
place; this
is(SLS).
called the final deformation: u fin .
state

9.9 Processing timber.

These deformations
shown
diagrammatically
in Figure
2.6grain
in relation to a
Timber is cutare
down
from whole
trunks and the direction
of the
upported beam
without
pre-camber.
aligns
with theany
longitudinal
axis of the trunk. Below is a diagram
which
shows some common
forms of cutting
timber
down intoon the creep beh
Deformation
is calculated
in two different
ways,
depending
useable planks and timber elements.
the structure:

(a) Structures comprising members, components and connections having th

76
creep behaviour
Creep behaviour in timber and wood-related products is a function of seve

Radial sawing

(b) Tangential and radial sawing

Through conversion
(plain sawing)

Through conversion
(billet sawing)

Quarter sawing (two different radial cuts)

slow procedure requiring large logs

Through conversion with

near quarter sawing

Tangential sawing conversion

with boxed heart

Figure 9-2 Common sawing patterns.

(c) Typical sawing patterns

9.10 Engineered Timbers.

Fig. 1.3. Examples of log breakdown and cutting pattern.
The use of engineered timbers is commonplace in the UK and the
development of high performance materials are continuing to push
the boundaries within their use, the following descriptions are not
intended to be exhaustive but give an indication as to some of the
current uses of these materials.
9.11 Kerto
Deriving its strength from a homogeneous bonded structure Kerto is
produced from 3mm rotary-peeled Spruce veneers glued together
to form a continuous billet.
Available in 3 different types, Kerto-S, Kerto-Q and Kerto-T it is a
product suited to a variety of applications.
Kerto-S: ideally suited to deliver long spans delivering excellent
technical performance with minimal deflection. Suitable for all roof
shapes as well as joists and lintels.

77

Kerto-Q: with roughly 20% of the veneers cross-bonded Kerto-Q is

suited to applications where high shear strengths are a necessity.
Perfect for large floor or roofing panels.
Kerto-T: made from lighter veneers, Kerto-T is ideally suited for use as
a stud in both load-bearing and non load-bearing structures.

Figure 9-3 Timber gridshell using engineered timber

Kerto is commonly used to create advanced forms and geometries

such as timber gridshells, because it is an engineered product it has
a much greater reduced risk from concealed defects within the
timber and has much greater load carrying capacity.

9.12 Plywood
Plywood is a wood product manufactured out of many sheets of
veneer, or plies, pressed together and glued, with their grains going
in opposite directions. Plywood tends to be extremely strong, though
not very attractive, and is treated in many different ways depending
upon its intended application. Because of the way in which plywood
78

1: PAB/RPW
P2: PAB
LUK117-Porteous
6, 2007
19:22
isOctober
constructed,
it also

20

resists cracking, bending, warping, and

shrinkage, depending upon its thickness. Plywood is also referred to
as an engineered wood, although it is made from a composite of
wooden materials, and various forms of it have been made for
Structural
Timber
thousands
ofDesign
years.to Eurocode 5

(b) Five-ply plywood

Face ply

(c) Three-ply blockboard

Grain directions
Cross ply (core)

(d) Five-ply blockboard

Back ply
(a) The structure of a three-ply plywood

(e) Laminboard

Figure
9-4 Structure
of ply.
Fig. 1.9. Examples of plywood and
wood
core plywood.

The plies that form plywood are generally cut on a rotary lathe,
1.7.2 Plywood
which cuts a continuous roll of wood while a log, called a peeler, is
turned against it. Rotary lathing is rapid and makes efficient use of
Plywood is a flat panel made by bonding together, and under pressure, a number of
the
while
turning
out veneers
highly
suitable Plywood
for plywood.
thinwood
layers of
veneer,
often referred
to as plies
(or laminates).
was the first
type oflathes
EWP toare
be invented.
Logsto
areexpose
debarkedmore
and steamed
or heated
in hotofwater
Some
designed
interesting
parts
the for
about
24
hours.
They
are
then
rotary-peeled
into
veneers
of
24
mm
in
thickness
wood grain, although they may be more wasteful of the wood. and

clipped into sheets of some 2 m wide. After kiln-drying and gluing, the veneers are
laid up lathed
with the veneers
grain perpendicular
one
another
and bonded under
pressure in an
Rotary
tend to tobe
dull
in appearance,
although
odd number of laminates (at least three), as shown in Figure 1.9a. The outside plies,
perfectly functional. After the veneers are cut, they are overlaid with
always made of veneer, are referred to as faces (face ply or back ply) and the inner
layers
of glue
together
untilordry
to formwood,
a flat,
laminates,
whichand
couldpressed
be made of
either veneers
sliced/sawn
are even,
called core.
Examples
of
wood
core
plywood
include
blockboards
and
laminboards,
as
shown
tight piece of plywood. Plywood is sturdier than regular sheets
or in
Figures 1.9c1.9e.
panels of wood, because the veneers are laid with their grains
Plywood is produced in many countries from either softwood or hardwood or a
opposing,
wood
product
tocommonly
resist warping
combinationwhich
of both.also
The causes
structuralthe
grade
plywoods
that are
used in the
United Kingdom
are as follows:
because
the grains
pull each other tight.

The!main
types
of Plywood
Sheets plywood
are Shuttering, WBP, Softwood,
American
construction
and industrial
!
CanadianInterior,
softwoodExterior
plywood and
and Douglas
fir plywood
Hardwood,
Marine.
Within these different types
! Finnish birch-faced (combi) plywood, Finnish birch plywood and Finnish conifer
there are then also different grades depending on the type of wood
plywood
that! isSwedish
used for
constructing
softwood
plywood. the wood laminates and the quality of
The plywood sheet sizes available sizes are 1200 mm 2400 mm or 1220 mm
79 with the longer side of the sheet except
2440 mm. The face veneer is generally oriented
for Finnish made plywoods in which face veneers run parallel to the shorter side.
Structural plywood and plywood for exterior use are generally made with waterproof
adhesive that is suitable for severe exposure conditions.

the face of the sheet it can be smooth, sanded and with or

without knots.
One of the common difficulties with the use of plywood on
construction sites is that if it gets wet, it can cause the panel to
disintegrate and become like wet cardboard. To overcome this the
use of marine grade ply is commonly used where there is a risk of the
timber being wetted.

9.13 Cross Laminated Timber (CLT)

Cross laminated timber is being used to create modular buildings
that have the capacity to be prepared offsite, there are a variety of
companies that manufacture different types of walls and floors from
Cross laminated timber including Eurban11 and KLH12.
Cross laminated timber (CLT) panels are produced from
mechanically dried spruce boards which are stacked together at
right angles and glued over the entirety of their surface. Each CLT
panel is produced is between three and seven boards thick
depending on the amount of structural loading required.
Gluing at high pressure reduces the timbers expansion and
shrinkage potential to a negligible level. The result is a rigid structural
timber member that can be used both vertically and horizontally to
construct a buildings frame.
By alternating the grain of the timber through both directions in this
manner, very strong floor panels can be constructed that have bidirectional spanning capabilities that allow thin floor zones to be
constructed.

11 http://www.eurban.co.uk/
12 http://www.klhuk.com/

80

9.14 Drying of timber

Look at any felled tree and the first thing you notice is how wet it is.
When its sawn and is still wet, it will not twist, warp or bend. When
you dry timber, this problem is eliminated. If you manufacture
anything from wet timber, there is a very good chance it will
become distorted, shrink, split and generally be unusable if it dries in
the wrong conditions.
By drying timber in a correct manner, tension within the timber,
which can cause many defects, is relieved, and we create timber
that is ready for the manufacturing process. There are several
advantages to using dry timber. It is stronger and holds nails better
than green timber; it is more stable and also minimises future
warping; it is less subject to stain, decay and insect attack and is
easier to paint and treat with preservatives.

9.15 Air Drying

This is carried out by stacking pieces of timber on top of each other,
separated by lathes or sticks to allow air to circulate. Alternatively, it
can be achieved by placing timbers at right angles to each other,
creating gaps for air circulation. This method of drying can take
some considerable time.

9.16 Kiln Drying

This is the method used to speed up the drying process using a
kilning chamber or de-humidifier.

9.17 Grading of timber

The grading of timber considers the size, quality and condition of a
piece of timber at the time of the original inspection.
It is necessary to distinguish between the two fundamental types of
grading:
Mechanical grading is employed when we try to determine the
strength characteristics of a piece of timber. This is usually done by

81

Western red cedar

GS (C14), SS (C18)

Douglas fir-larch (Canada and USA)

Hem-fir (Canada and USA)
Spruce-pine-fir (Canada and USA)
Sitka spruce (Canada)
Western white woods (USA)
Southern pine (USA)

GS (C16), SS (C24)
GS (C16), SS (C24)
GS (C16), SS (C24)
GS (C14), SS (C18)
GS (C14), SS (C18)
GS (C18), SS (C24)

measuring
the
stiffness of the timber when a load is applied to it by a
Timber
graded in accordance with BS 4978:1996; based on Table 1.2, BS 5268-2:2002.
strength grading machine.
the load to induce a known deflection) is then automatically measured and compared
withto
pre-programmed
which leads
the directof
grading
of the is
timber
section
The need
assess thecriteria,
strength
of atopiece
timber
usually
and marking with the appropriate strength class. An example of the grading marking,
requiredbased
where
timber is of
toBSbe
in a constructional
on the requirements
ENused
14081-1:2005,
is shown in Figure 1.7.or load
In general less material
rejected
machine graded; however,
timbertrusses.
is also visually
bearing capacity,
such isas
the ifmanufacture
of roof
inspected during machine grading to ensure that major, strength-reducing, defects do
not exist.

Visual grading is a judgement of the timbers appearance and

suitability for its end use, considering both the natural characteristics
1.5.3 Strength classes
and manufacturing imperfections of each piece.
The concept of grouping timber into strength classes was introduced into the United

Kingdom with
BSbe
5268-2
in 1984.
Strength
classes offer a number
of advantages
Visual grading
can
and
is used
to determine
grades
for
both to the designer and the supplier of timber. The designer can undertake the design
constructional
it is more
usually
used
toofdetermine
the
without the use,
need toalthough
check on the availability
and price
of a large
number
species and
grades thaton
might
be used. Suppliers can supply any of the species/grade combinations
grade based
appearance.
PRODUCT

NBODY
M

CODE

DRY GRADED

C24

Key:

PRODUCT: producer identification

CODE: Code number of documentation
DRY GRADED: used if appropriate
NBODY: identification of notified body
M: machine graded
C24: strength class or grade and grading

9-5 Grading stamp.

Fig. 1.7. Example of grading Figure
marking.

Timber can have many defects, most of which will be identified

within the grading process, but an awareness of some of the most
common types of defect is important to allow the engineer to
understand and recognise these defects on site.

9.18 Strength classes.

The strength class of the timber is dictated by the wide range of
timber species that are available within the market.
The strength of a timber element is highly dependant upon the
duration of the load and the environmental exposure of the element
in consideration. These elements will need to be considered during
the specific design life and assessment of how the element will
behave, particularly if its envisaged that there is the potential for
these elements to change and become more onerous than the
original conditions that the element was designed for.

82

9.19 Sourcing timber.

The selection of materials is made simpler through various schemes
currently in operation for numerous materials, one of the better
known schemes which extends to sustainably sourced timber within
the UK is the Forestry Stewardship Scheme (FSC13) which provides an
audit trail for the production of timber, from the planting of the
sapling through to its harvesting and subsequent processing. There
are several alternative schemes available throughout Europe,
including the PEFC14 and the SFI15 and whilst timber from other
European countries will require more energy to be delivered to a UK
site, the timber produced particularly from colder Scandinavian
countries can be desirable as the colder climate forces the timber to
grow slower, creating a denser stronger timber.

9.20 Defects
Timber can have many defects, most of which will be identified
within the grading process, but an awareness of some of the most
common types of defect is important to allow the engineer to
understand and recognise these defects on site.

13

http://www.fsc-uk.org/ (Accessed 6th April 2011)

14

http://www.pefc.org/ (Accessed 6th April 2011)

15

http://www.sfiprogram.org/ (Accessed 6th April 2011)

83

10

Structural Timber Design to Eurocode 5

Shake

Diagonal grain

Knot

Cross grain

Wane

Flat grain

(a) Natural and conversion defects

Cupping

Springing

End splitting

Bowing

Honeycombing

Twisting

Figure 9-6 Timber seasoning defects.

(b) Seasoning defects

Fig. 1.6. Defects in timber.

Timber is described as being hygroscopic, which means that it attempts to attain an

equilibrium moisture content with its surrounding environment, resulting in a variable
moisture content. This should always be considered when using timber, particularly
softwoods, which are more susceptible to shrinkage than hardwoods.
As logs vary in cross-section along their
84 length, usually tapering to one end, a board
that is rectangular at one end of its length might not be so at the other end. The
rectangular cross-section may intersect with the outside of the log, the wane of the

Civil engineering structures

10 Self based exercise.

This chapter is all about self based learning, with a group of friends
on your course, form teams to determine what the following
structures are, what materials they may be constructed from and
how they could be built one thing to note is that there may be
several variations on the materials and the construction techniques
for each type of structure.

10.1 Civil Structures

Dams
Tunnels
Bridges
Coastal revetments

10.2 Buildings
Foundations
Shear cores
Jump form
Slip form
Traditionally formed stability cores.
85

Tall buildings and their facades.

Top down construction.

10.3 Infrastructure
Rail
Roads
Drainage
Airports
Ports
Underground tunnels.

86