INTRODUCTION

The need to study reinforced concrete slabs, beams, columns, steel structures, and timber

structures cannot be overemphasized owing to its relevance in building development, its stands

in the delivery of durable and functional buildings to the taste of the occupants or owners with

regards to cost, maintenance, environment factors and above all the satisfaction of the owners.

Etymologically, the used or the employment of reinforced concrete slab, beam, column,

steel structure, and timber structure in building have gained a greater priority in the built

environment for many centuries now in developed countries and some of these elements likewise

have gained a greater priority in the developing countries like Nigeria. Although, the

employment of steel structures and timber structures in the built environment in Nigeria are at an

advancing level. However, the used of these elements in Nigeria will in no distant time be

advanced due to the taste of the citizens.

Reinforced concrete slab is a structural element in a building that provides a platform or

floor or ceiling in a building.

Reinforced concrete beams are horizontal or inclined structural element that resist load

applied to it laterally and transfer same to the columns while columns while columns are vertical

or inclined structural element that transfer loads to the foundation.

In addition, timber is a structural material similar to steel that is capable of acting as

tension, compression and bending members. The details of these elements are succinctly

discussed in the main units.

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 MAIN UNITS

A reinforced concrete slab is a crucial structural element and is used to provide flat

surfaces (floors and ceilings) in buildings. According to Oyenuga (2008), a slab is a reinforced

concrete structure which more often than not is subjected to bending (tension or compression)

but in rare cases, (such as in bridge deck) subjected to shear. Slab is a structural element through

which loads from the occupant (live load) and dead load are transferred to the foundation

through beams, and columns or through columns as the case may be.

Reinforced concrete slabs are in most instances horizontal structural members but can

also be used as vertical members such as walls to infill panels, side walls to drains and sewer

appurtenances, etc.

Reinforced concrete slabs are classified into one way and two-way slabs based on the

reinforcement provided, beam support and the ratio of the spans. The one-way slab is one

supported on two sides and the ratio of long to short span is greater than two. However, the two-

way slab is supported on four sides and the ratio of long to short span is smaller than two.

According to Ettu (2016), slabs may be classified as either one-way spanning slabs or (one-way

slabs) or two-way spanning slabs (or two-way slabs), depending on whether their structural

behavior is in one direction or in two perpendicular directions respectively.

Types of Slabs

Slabs according to BS 8110 (1997) classified reinforced concrete slabs into three

according to their cross-sections and / or whether they are supported by beams or directly by

columns.

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 In addition, slabs are of types and their preference depends on (i) the span of the slab (ii)

the use of the space which may determine the span (iii) the load to be carried and (iv)

architectural aesthetics that are required. The types of slabs are as follows:

1. Solid Slabs (cantilever, simply supported, continuous and two ways) (Cl. 3.5, BS 8110)

2. Ribbed floor slabs (Cl. 3.6, BS 8110).

3. Flat slab (although solid, but of different construction) (Cl. 3.7, BS 8110).

4. Waffle slab (with beams, or mushroom waffle)

5. Flat plates

6. Two-way slabs on beams

7. Hollow core slab

8. Hardy slab

9. Bubble deck slab

10. Composite slab

11. Precast slab

12. Slab on grade i). slab on ground. Ii) stiffened raft slab. Iii) Waffle raft slab

To succinctly discuss on these types of slabs are as follows:

SOLID SLABS:

Solid slab is one that has a uniform thickness and is usually constructed of the same material

throughout its cross-section as shown in figure 1.1 below (Ettu, 2016). Solid slabs are the most

common especially in residential areas and offices and are generally employed when the span

does not exceed 6.0m. If the span exceeds 6.0m, Victor Oyenuga opined that based on

experience, deflection may be problematic or unnecessarily heavy slab thickness result. Solid

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slabs due to structural configuration may present itself as a cantilever slab, a simply supported

slab, a two-way spanning slab or a flat slab.

RIBBED SLABS

Ribbed slabs are those slabs that are ribbed at certain portions (usually at their tension

zones) in order to achieve a reduction in their self-weights. Ribbed slabs are like flat slabs used

in offices and buildings where large spans are expected. Ribbed slab can be whole concrete

ribbed floor or ribbed floor with hollow pots in-fill. The floor consists of series of T-beams

closely spaced, in most cases, between 400 and 600mm. the ribbed slab is more or less designed

as T-beams with thin concrete as flanges. See figure 1.2 for ribbed slab.

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FLAT SLABS

According to Cl. 3.7, BS 8110, flat slabs are those slabs supported directly by columns

and not by beams or wall. One of the greatest problems of such slabs is that heavy imposed loads

on them result in high shear stresses at the supports and this may cause failure as the columns

tend to punch through the slab. In order to avoid this, shear stresses in the slab can be reduced by

one or more of the following techniques:

a. Using columns with large diameters and / or slab with increased thickness.

b. Enlarging or flaring column heads

c. Local thickening of the slab at the column points (by providing drop panels).

Flat slabs are used where large spans and / or heavy live loads are required or where their use

would be aesthetically pleasing. Flat slabs span between columns with no beams at all. The may

however, have drop panels around the columns. Flat slabs are common in banking halls, markets

or shopping plazas and libraries. Flat slab can withstand live loads in excess of 5KN/m2.

Flat slabs according to Draycott 1990, are designed to act in conjunction with columns as

a structural frame without the necessity for beams and hence have flat soffit as shown in figure

1.3.1. Flat slabs may be solid with a drop step to rest on the column as in figure 1.3.2 or may

have recesses formed in the soffit to give a series of two directional ribs in which case they are

often referred to as waffle or coffered slabs as in figure 1.3.3.

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WAFFLE SLABS (GRID SLAB)

Waffle slabs are ordinary ribbed slabs which are two-way spanning and are constructed

with ribs in both directions of span. Waffle slab is a type of reinforced concrete slab that contains

square grids with deep sides. Waffle slab construction process includes fixing forms, placement

of pods on shuttering, installation of steel mesh on top of pods, and pouring of concrete.

Waffle or grid slabs are suitable for spans of 9 – 15m and live loads of 4 – 7KN/m 2.

Formwork, including the use of pans, is quite expensive. See figure 4.1 for the waffle slab.

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FLAT PLATES:

Flat plates can be constructed as one-way or two-way slabs and it is directly supported by

columns or walls. It is easy to construct and requires simple formworks. Flat plates are most

suitable for spans of 6 – 8m, and live loads between 3 and 5KN/m 2. Added to that, the range of

spans for prestressed flat plates is between 8 – 12m, and it can also be constructed as post-

tensioned slabs. The advantages of adopting flat plates include low-cost formwork, exposed flat

ceilings, and faster construction. Flat plates have low shear capacity and relatively low stiffness,

which may cause noticeable deflection. See figure 5.1 for flat plate.

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TWO-WAY SLABS ON BEAMS:

The construct of this type of slab is suitable is similar to that of one-way slab on beams,

but it may need more formworks since two-way slabs are supported on all sides. Slabs on beams

are suitable for spans between 6 and live loads of 3 – 6KN/m 2. The beams increase the stiffness

of the slabs, producing relatively low deflection.

HOLLOW CORE SLAB:

It is a type of precast slab through which cores are run. Not only do these decline slab

self-weight and increase structural efficiency but also act as service ducts. It is suitable for cases

where fast constructions are desired.

There is no restriction on the hollow core slab units, and their standard width is 120mm

and depth ranges from 110mm to 400mm. it has been observed that hollow core slab can support
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2.5KN/m2 over a 16m span. It is suitable for offices, retail or car park developments. See figure

7.1 for hollow core slab.

HARDY SLAB:

It is constructed using hardy bricks which significantly decline the amount of concrete

and eventually the slab self-weight. The thickness of slab is commonly greater than conventional

slab and around 270mm.

Hardy slab is constructed through the installation of formwork, hardy block placement,

placement of reinforcement into the gaps between blocks, placement of steel mesh on the blocks

and finally pouring of concrete.

It is economical for spans o length up to 5m and it reduces the quantity of concrete below

neural axis and moderate live loads shall be imposed. It is constructed at locations are very high.

This type of slab can be seen in Dubai and China. See figure 8.1 for Hardy block below:

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BUBBLE DECK SLABS

It is constructed by placing plastic bubbles which ae prefabricated and the reinforcement

is then placed between and over plastic bubbles and finally, fresh concrete is poured. The plastic

bubbles replace he ineffective concrete at the center of the slab.

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 This type of slab reduces weight, increase strength, larger spans can be provided, fewer

columns needed, no beams or ribs under the ceiling are required. It is environmentally friendly

since it reduces the amount of concrete and also decline the total cost of construction. See figure

9.1 below:

PRECAST SLAB

Precast concrete slabs are cast and cured in manufacturing plants, and then delivered to

the construction site to be erected. The most outstanding advantage of the slabs in manufacturing

plants is the increase in efficiency and higher quality control which may not be achieved on site.

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 He most commonly used precast slabs are he channel and double – T types. They can be

used for spans up to 1m. see figure 10.1 and 10.2 for the channel and double – T types.

SLAB ON GRADE:

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 The slab which is cast on the surface of the earth is called a ground slab. Generally, slab

on grade is classified into three types: viz 1. Slab on ground – it is the simplest type of slab o

grade which is a composite o stiffening beams constructed from concrete around perimeter of the

slab and has a slab thickness of 100mm. it is suitable for stable ground which is mostly

composed of sand and rock and not influenced by moisture, and soils that undergo slight

movement due to moisture. 2. Stiffened raft slab – it is similar to slab o ground apart from

stiffening beams which are set in channels through the middle of the slab. Consequently, it

creates a kind of supporting grid of concrete on the base of the slab. Soil with moderate, high

amount, and severe movement due to moisture. 3. Waffle raft slab – it is constructed entirely

above the ground by pouring concrete over a grid of polystyrene blocks known as ‘void forms.

This type of slabs are generally suitable for sites with less reactive soil, use about 30% less

concrete and 20% less steel, than a stiffened raft slab, and are generally cheaper and easier to

install than other types. These types of slabs ae suitable only for very flat ground.

REINFORCED CONCRETE BEAMS

Reinforced concrete beams are structural elements that resist loads applied laterally to

their axis. They typically transfer loads imposed along their length to their end points where the

loads are transferred to walls, columns, foundations and so on. Beams may be

1. Simply supported; that is, they are supported at both ends but are free to rotate.

2. Fixed; supported a both ends and fixed to resist rotation.

3. Overhanging; overhanging their supports at one or both ends.

4. Continuous; extending over more than two support

5. Cantilevered; supported only at one end.

conditions or they can be statically indeterminate. Beams are of various shapes ranging from

square, rectangle, circular, I-shaped, T-Shaped, H-Shaped, C-Shaped, tubular shaped, etc. It may

be straight, curved or tapered.

According to Draycott (1990), beams are classified as:

1. Simple beam

2. Continuous beam

3. Semi-continuous beam

4. Cantilevered beam

5. T – beam

SIMPLE BEAM:

Simple concrete beam refers to the beam having a single span supported at its end

without a restraint at the support. Simple beam is sometimes called simply supported beam. See

figure 2.1. below.

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CONTINUOUS BEAM:

Continuous beam is a beam that rest on more than two supports. It can be a single beam

provided for long span between columns or walls with intermediate supports of smaller beams or

a single continuous beam for entire length of the structure with intermediate column or wall

supports. See figure 2.2 below:

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SEMI-CONTINUOUS BEAM

This refers to a beam with two spans with or without restraint at the two extreme ends.

See figure 2.3 below:

CANTILEVER BEAM

Cantilever beams are supported on one end and the other end projecting beyond the

support or wall. See figure 2.4 below:

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T – BEAM:

When floor slabs and beams are poured simultaneously producing a monolithic structure

where the portion of the slab at both sides of the beam serves as flanges of the T – beam. The

beam below the slab serves as the web member and is sometime called stem. See figure 2.5

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 REINFORCED CONCRETE COLUMNS

Columns are vertical or inclined compression members used for transferring

superstructure loads to the foundation. The structural design of reinforced concrete (R.C)

columns involves the provision of adequate compression reinforcement and member size to

guaranty the stability of the structure. In typical case, columns are usually rectangular, square, or

circular in shape. Other sections such as elliptical, octagonal, etc. are also possible.

Columns are usually classified as short or slender depending on their slenderness ratio,

and this in turn influences their mode of failure. In framed structures, columns are either

subjected to axial, uniaxial, or biaxial loads depending on their location and / or loading

condition. Eurocode 2 demands that we include the effects of imperfections in the structural

design of columns. The structural design of reinforced concrete columns or covered in section

5.8 of EC2.

Columns can fail by the following ways when not properly designed:

1. Crushing

2. Buckling

3. Shear, or

4. By the combination of any of the above

The steps in the design of reinforced concrete columns are;

a. Determine design life

b. Assess actions on the column

c. Determine which combinations of actions apply

d. Assess durability requirements and determine concrete strength

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 e. Check cover requirements for appropriate fire resistance period

f. Calculate minimum cover for durability, fire and bond requirement

g. Analyze structure to obtain critical moments and axial forces

h. Check slenderness

i. Determine area of reinforcement required

j. Check spacing of bars.

Columns according to Ettu (2009), may be classified as braced or unbraced. A column may be

considered braced in a given plane if lateral stability to the structure as a whole is provided by

walls or bracing or buttressing designed to resist all lateral forces in that plane. Otherwise, it

should be considered as unbraced.

In other words, braced columns are prevented from sway (lateral displacement) under the

effect of lateral load (e.g., Wind) whereas unbraced columns are not. It should be noted that a

column may be braced in one plane (direction) and unbraced in a perpendicular plane (direction)

to that in which it is braced and must be treated accordingly.

Effective Height of Columns (Cl. 3.8.1.6):

The effective height of a column depends on the end condition of the column (i.e.

restraint conditions) and whether it is braced or unbraced. BS 8110 has classified the end

conditions into four (4), namely;

a. Condition 1: the end of the column is connected monolithically to beams on either side

which are at least as deep as the overall dimension of the column in the plane considered as

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shown in figure 3.1. below. When the column is connected to a foundation structure, this should

be of a form specifically design to carry moment.

b. Condition 2: the end of the column is connected monolithically to beams or slabs on

either side which are shallower than the overall dimension of the column in the plane considered,

as shown in figure 3.2 below:

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c. Condition 3: the end of the column is connected to members which, while not

specifically designed to provide restraint to rotation of the column will, nevertheless, provide

some nominal restraint e.g a non-monolithic column – beam construction where the beam is

simply supported on the column as shown in figures 3.3.1 and 3.3.2 below:

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d. Condition 4: the end of the column is unrestrained against both lateral movement and

rotation (e.g the free end of a cantilever column in an unbraced structure) as shown in figure 3.4

below:

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The effective height of a column is given as:

Lc = βlo, where

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Lc = effective height of the column in the plane of bending considered

Lo = clear height between end restraints

β = coefficient which depends on the end conditions and whether column is braced or unbraced

columns respectively.

Slenderness Limits for Columns (Cl. 3.8.1.7)

Slenderness limits for columns are required to prevent excessive buckling and deflection

in very slender columns. The code specifies that generally the clear distance, L o, between end

restraints should not exceed sixty times the minimum thickness of the column, i.e.,

Lo ≤ 60 b

For unbraced cantilever columns, the limit is given as:

Lo ≤ 100b2 ≤ 60 b,
h
where h and b are respectively the larger and smaller dimensions of the column. For unbraced

columns, the consideration of deflection may further affect the permissible limits.

Moments and Forces in Columns (Cl. 3.8.2 and 3.8.3)

The moments and forces in columns may be calculated by the methods and techniques of

structural analysis earlier discussed. Alternatively, the axial force in columns that are of column-

and-beam construction or in monolithic braced structural frames may be calculated by assuming

that the beams and slabs transmitting force into it are simply supported.

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 For axially loaded column (i.e., column with zero bending moment) and columns with

dominant axial forces such as those supporting symmetrical arrangement of beams, only the

design ultimate axial force need be considered in design together with a nominal moment due to

eccentricity. This minimum eccentricity moment is given as: Mmin = Nemin, ………. Equ. 1

Where N = design ultimate axial load, and emin = Minimum eccentricity.

The code requires the emin should be equal to 0.05 times the overall dimension of the

column in the plane of bending considered but not more than 20mm, i.e., e min = smaller of (i) 0.05

b, if b is the dimension in the plane of bending, and (ii) 20mm.

A column must never be designed for a moment less than the minimum eccentricity

moment as defined in equation 1.

The deflection of slender columns under axial load introduces an additional moment that

should be taken into consideration in the design of such slender columns.

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 STEEL STRUCTURES

According to MacGinley (1998), steel structure is a metal structure which is made of

structural steel components connected with each other to carry loads and provide full rigidity.

Because of the high strength grade of steel, this structure is reliable and requires less raw

materials than other types of structure like concrete structure and timber structure.

In modern construction, steel structure is used for almost every type of structure

including heavy industrial building, high-rise building, equipment support system infrastructure,

bridge, tower, airport terminal, heavy industrial plan, pipe rack, etc.

Field of Application of Steel Structure

1. Structural frame work of buildings, multi-storey structures and large span building

2. Off-shore building platform

3. Television mast

4. Exhibition pavilion

5. Tanks and reservoirs (for storage of liquid and gas gases)

6. Oil and gas pipeline, etc.

7. Arch structures

8. Equipment support structures

9. Infrastructures

10. Pipe racks

11. Product and services

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 Structural steel is steel construction material which is fabricated with a specific shape and

chemical composition to suit a project’s applicable specifications. Depending on each project’s

specifications, the steel sections might have various shapes, sizes and gauges made by hot or

cold rolling, others are made by welding together flat or bent plates. Common shapes include;

STRUCTURAL STEEL SECTIONS:

A. Hot Rolled Sections

a. Rolled Steel Joist (RSJ) or British Standard beams (BSB). See figure 5.1

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Under a, we have (i) Universal beam with minimum section of D = 914mm; B = 419mm. weight

= 388Kg/m; this is usually written as 914 X 419 X 388. (ii) Universal column. (iii) Universal

bearing piles. (iv) Joists

b. Rolled Steel Channel. See figure 5.2 below

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Main Structural Types:

1. Frame structures: beams and columns

2. Grids structures: lattices structure or dome

3. Prestressed structures

4. Truss structures: bar or truss members

5. Arch bridge

6. Arch structure

7. Beam bridge

8. Cable-stayed bridge

9. Suspension bridge

10. Truss bridge: truss members

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 TIMBER STRUCTURE

Timber as structure – timber as a structural material is similar to steel and both materials

are available in similar shapes and even jointing of timber or steel members respectively, is often

comparable.

Timber structures have the following properties:

1. Timber members are particularly capable of acting as tension, compression and bending

members.

2. The modulus of elasticity is low compared to steel.

3. The texture and appearance of timber makes it very suitable for use in visually exposed

structures.

4. The combination of steel and timber often produces light and competitive structures with

timber as compression and steel as tension members.

5. Most timbers are found in building having a simple rectangular form used, for example,

in floor joists, rafters and other roof components or for walls in timber framed housing, large

structures can be built economically in other forms such as domes spanning over 100 metres.

6. Timber construction elements can be either vertical or horizontal like in tie beam, post

beam, wall plate, etc.

7. Timber transmit load

8. Timber resist lateral forces

9. Timber structure support beams through compression (equilibrium, instability and loads)

10. Timber carries axial load which is a force administered along the lines of an axis

(Mdzainal, 2013).

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 CONCLUSION

Reinforced concrete slab is one of the structural elements in a building that provides a

platform or floor or ceiling in a building. Its classification is based on the reinforcement

provided, beam support and the ratio of the spans.

Similarly, slabs are categorized based on its cross-sections and or whether they are

supported by beams or directly by columns. The type of slab employed also depend on the span

of the space, the load to be carried, the architectural aesthetics that are required and the span of

the slab.

Reinforced concrete beams are horizontal or inclined structural element that resist loads

applied laterally to their axis. Beams transferred loads applied to it to the columns or wall down

to the foundation.

Columns are vertical or inclined structural element used in transferring superstructure

loads from the beam, roof, slab down to the foundation.

Steel structure which has gain popularity in recent years is a metal structure which consist

of structural steel components connected to each other to carry loads and provide full rigidity.

In synopsis, timber is a natural material of which essential properties vary considerably.

Timber is a lightweight material with a high strength to weight ratio. The strength and stiffness

properties of timber are highly dependent on the angle between load and grain. Timber is strong

and stiff parallel to the grain whereas it is prone to cleavage along the grain if tension stresses

perpendicular to the grain occur. Hence, shrinkage and swelling have to be considered during the

design life of timber structures.

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 REFERENCES

BS. 8110: Part 1: 1997

Clause 3.5 of BS. 8110: Part 1: 1997

Clause 3.6 of BS. 8110: Part 1: 1997

Clause 3.7 of BS. 8110: Part 1: 1997

Clause 3.8.1.7 of BS. 8110: Part 1: 1997

Clause 3.8.2 of BS. 8110: Part 1: 1997

Clause 3.8.3 of BS. 8110: Part 1: 1997

DrayCott, T. (1990) Structural Elements Design Manual. ISBN 0750603135

Ettu, L. O. (2016) Design of Reinforced Concrete Element to BS 8110 (1997) (2nd ed.) Imo
State. Charismatic Forum Publication Ltd

MacGinley, T. J. (1998) Practical Design Studies (2nd ed.)

Mdzainal, M. (2013) AAR553 Structural Theories and Application. Building Structural System
(timber)

Oyenuga, V. O. (2008) Reinforced Concrete Design

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