Topic1 4 Structural Supporting 104
Topic1 4 Structural Supporting 104
Topic1 4 Structural Supporting 104
Flooring systems
There are two main types of flooring system:
1. Those in contact with the ground
2. Suspended floors.
The choice of floor system depends upon a number of factors; the structural system of the
building, the types of floor finish required, the thermal performance required, the sound
insulation required, the ease of erection and economy.
Cut in floor
Unlike floors on the ground in which the load bearing capacity of the ground provides
structural support, in suspended floors the system has a support structure that carries and
transfers the load to the footings.
The structural support for suspended slabs for the ground floor is usually dwarf masonry
walls. For timber floors the sub-structure generally consists of joists, bearers and engaged
piers along the external walls and isolated piers internally as shown in the figure below.
To prevent dampness causing timber deterioration the sub-floor area must be ventilated. To
prevent rising damp affecting the timber the piers must have a damp proof course or ant cap.
Minimum sizes and spacing for timber members for the floor can be found in AS 1684 –
Residential Timber Framed Construction. Acceptable Standards of Construction has tables
derived from AS1684 that provide a quick reference for bearer and joist size and spacing
combinations for various timber grades.
Timber flooring may be either solid timber floor boards or engineered timber floor sheeting.
(from http://hebel.com.au/sites/default/files/PowerFloorDIGuide.pdf)
(from http://hebel.com.au/sites/default/files/PowerFloorDIGuide.pdf)
Wall Construction
Most walls are a composite construction, that is, they are made of more than one type of
material. Most of these walls have a structural component and a protective component which
has the purpose of resisting weather, thermal extremes, noise transfer, insect attack and fire.
The principal wall systems used in houses are:
• framed wall systems—hardwood, softwood, engineered timber or lightweight steel
• solid wall systems made from masonry - cavity brick or solid brick; concrete or
AAC block; stone; straw bale; etc.
Other systems that are used include:
• post and beam – timber, steel or reinforced concrete structure with infill walls
• portal frame – timber or steel with infill walls (commonly used for industrial
buildings)
• tilt-slab—vertical concrete slabs (commonly used for industrial buildings
The areas applying to the walls of buildings that the drafter or building designer needs to pay
particular attention to as part of the building design are:
• water and moisture entry into the interior of the building
• moisture entering timber plates from slab floors, masonry supports and wet areas
• fixing of member to member for stability of the structure in relation to bracing and
tie-down requirements
Framing Methods
Framed walls are the most common wall system used in residential construction in Australia.
This system uses thin vertical members (called studs) fixed top and bottom to long horizontal
(sometimes the top will be sloping) members (called plates).
Framed walls generally consist of a basic frame which bears the loads with cladding on the
external face of the frame and lining on the internal faces of the external wall, and both faces
of the internal walls. Additionally there is insulating sheeting between the frame and the
lining.
Timber is the most commonly used framing material; however steel is gaining a share of the
marketplace as the cost of timber increases.
Timber wall frames are easy to build and alter on site with a minimum amount of easily
transportable equipment. Factory built frames are also commonly used, especially in project
homes.
Steel frames can be built on-site, but often require special tools for fixing. More usually, steel
frames are built in a factory on special jigs then delivered to site and then erected.
There are two types of bracing visible in this residential building project: double diagonal
metal strap bracing and compressed board sheet bracing.
These are not widely used but are more common now than a few years ago. Frame systems
in steel and aluminum are light, strong, stable and quick to erect. The principal of metal
framing is the same as for timber although generally the components vary in profile. The
diagram below shows the arrangement of a metal frame.
Steel framing is versatile, light and strong and has time and labour savings. It is not
attacked by termites.
Complete, integrated metal framing systems are available which include the floor, wall and
roof framing. The framing systems are engineered and result in structurally sound
construction with economic use of materials. The spacing of members is similar to that of
conventional timber framing.
Steel framing can be used with a wide range of architectural materials and styles including
timber floor and concrete slab-on-ground construction, timber roofs with tiles, steel or other
roof coverings, brick veneer, weatherboard, metal or any other sheet cladding.
The individual members of a steel frame are channels of various shapes and depths. Refer
to figure for a summary of these. Different manufacturers may produce different profiles.
Members may be joined by bolting, welding, screwing or clipping together.
Bricks are not moisture resistant and need to be protected from water to stop dampness
entering the dwelling. Methods used to stop moisture entering the dwelling through the wall
is to have a cavity (a gap of 50-60mm) between the two leaves of brickwork to break the
contact and not permit the water to enter the inside face. To stop rising damp from the
foundation a damp course must be placed above the ground level but below the floor level.
The down side of a cavity is that you end up with two thin walls unable to rely on each other
for structural stability; therefore wall ties are introduced to improve stability. The inner wall is
the one on which the load is supported (although in older homes, it was often the outer skin)
and therefore may need strengthening by tying to the outer wall.
Blockwork
Hebel (AAC) Block Wall Systems are loadbearing external wall solutions for homes as an
alternative to traditional double brick construction. The systems consist of a single skin
external wall, typically constructed from 200 mm Hebel Blocks and internal walls using a
single skin of 100 – 125 mm Hebel Blocks or 150 mm Hebel Blocks for loadbearing walls.
Hebel Lintels are also available to negate the need for steel over window and door openings.
For detailed information and technical specifications on AAC Hebel blockwork construction,
go to the following website:
http://www.hebelaustralia.com.au
Portal frame
The portal frame is a combined wall and roof system. It is made up of a frame composed of
vertical side columns fixed to either a single run rafter or two pitched rafters. The area
between the portal frames is in-filled with a non-loadbearing wall.
Portal frames (uncommon in houses) are usually made from laminated timber beams, ply
box beams or more commonly from structural steel sections. Portal frames are used
extensively in industrial buildings, prefabricated metal sheds and garages or where large
open spans are required.
This type of construction requires that the joints are built to be rigid (that is they cannot
rotate) by using a minimum of two bolts per joint, or some alternative form of bracing is used.
The main function of a roof system is to:
• Provide protection from rain and snow
• Provide shade
• Help maintain thermal comfort
• Resist uplift forces from wind
The roof structure of a building supports the roof cladding, the principle barrier against the
elements for the building and the occupants. The roof structure is designed to support the
weight of the roof cladding and resist the forces created by wind and in some areas snow.
Construction techniques include:
• conventionally framed, pitched roof with separate rafters and ceiling joists which
may be joined at the base to form a tied, triangle shape called a coupled roof.
• roof beams (rafters) - either flat, vaulted or skillion (on-the-rake) with roofing and
ceiling lining (if used) supported by the same member.
• trussed roof.
• rafter and purlin—used mainly with portal frames
Roof Terminology
The following terms are used in roof structures and are important to be familiar with:
• A wall plate: is a timber beam bedded in mortar on top of the inner skin of a cavity
wall.
• A common rafter: is a sloping timber member placed from the wall plate to the
ridge to carry the loads from tiles and wind etc. Long rafter need intermediate
support which is purlins.
Roof types
Flat / Skillion Roof
The basic components of flat and skillion roof are rafters and
battens. The rafters bear on the top plate with a birds mouth joint
and are skew nailed. In high wind areas the top plate and rafters
must be strapped down the full depth of the wall. In cavity brick
and brick veneer the roof is usually always supported by the inner
skin leaving the outer skin free to expand and contract without
affecting the roof structure.
Joists have the dual purpose of preventing the rafters from spreading and carrying the
ceiling loads. The load of the ceiling could cause sagging of the joist. To prevent this, a
hanging beam can be added spanning between walls with the joists tied to it.
Hip Roof
Another type of roof is the hip and valley. This consists of a
series of intersecting, equally pitched planes, forming a series of
hips and valleys. These roofs have essentially the same
components as gable roofs.
(jw)
Hip and valley roof that is being conventionally framed (not a trussed roof)
Trussed roofs
Trussed roofs are now often prefabricated and are composite members performing the work
of larger single members. They are fabricated to strict engineered guidelines so materials
used can be minimised. The essential components of a truss are the top chord, the bottom
chord and intermediate members which may be either struts or ties. Trusses require no
support from internal walls and are specifically designed to span to the external walls and
should not be supported by internal walls.
A trussed frame is composed of a series of triangles welded, bolted, riveted or nailed
together to form an integral unit that:
• Need only be supported on columns or perimeter walls,
• Will carry the combined weight of itself and the roof covering,
• Will span across openings
Trussed roofs can be covered by any of the conventional roof coverings such as tiles and
metal sheets, however, the type of covering will govern the spacing of the trusses. Often
ceiling battens are fixed directly to the underside of the bottom chord.
In domestic trussed roof construction only the perimeter walls need to be designed as load-
bearing walls when roof trusses are used. Internal walls become simple partitions and can
be arranged without the need to provide supports for propping beams, hanging beams, etc.
The sub-floor structure is simplified as stumps and bearers don’t need to be arranged under
internal non load-bearing walls and where concrete slab floors are used, the arrangement of
internal beams is simplified.
Cyclones
© Vlado – freedigitalphotos.net
Tropical cyclones are giant whirlwinds of air and dense cloud spiralling at over 120 km/h
around a central 'eye' of extreme low pressure. They often produce winds in excess of 200
km/h and gusts can exceed 280 km/h. This can cause extensive damage to property and
turn debris into dangerous missiles. Most deaths from cyclones occur as a result of
drowning, collapsed buildings, or flying debris which becomes lethal in high winds.
As a building surveyor, it is important to be aware of the threats posed by cyclones. It is
important to be able to evaluate and apply cyclone-resistant construction practices to
buildings. In this topic we will be looking at cyclone categories, methods of tie down and
bracing, and the requirements of the BCA for cyclonic regions.
©istock
Severity Categories
These range from:
1 for weak tropical cyclones (strongest wind gusts less than 125 km/h); minimal house
damage, but damage to crops, trees, etc.
2 winds are destructive at 125-164 km/h; minor house damage, significant damage to crops,
trees, signs, etc.
3 for severe tropical cyclones with very destructive winds of 165-224 km/h.
4 for severe cyclones causing significant roofing and structural damage, and dangerous
airborne debris. Winds to 279 km/h
5 for the most severe cyclones (wind gusts more than 280 km/h). Extremely dangerous with
widespread destruction.
Part 3.10.1.0 of the BCA
As indicated the explanatory information supporting Part 3.10.1 of the BCA, prepared by the
ABCB,
Geographical location
For the determination of wind classification, Australia is divided into four regions, according
to recorded average wind speeds. The Australian/New Zealand Standard 1170.2-2011 sets
out the wind speeds to be used for design purposes.
Regions B, C and D cover the coastal regions north of the 30 degree latitude which are
prone to tropical cyclones. Above the 30 degree latitude, the regions are designated as:
• Region A: from a distance greater than 150 km from a smoothed coastline;
• Region B: within the distance of 100 to 150 km from a smoothed coastline;
• Region C: within the distance of 100 to 50 km from a smoothed coastline; and
• Region D: within the distance of 0 to 50 km from a smoothed coastline.
Terrain
The site of a house is given a terrain category TC1, TC2, TC2.5 or TC3. Terrain category
takes account of the number of obstructions within a radius of 500 metres, such as trees and
houses, which can substantially reduce wind speeds.
A terrain category of TC3 applies to sites having numerous obstructions that are close
together, such as is generally the case for urban areas. A site located in open plains with few
trees would have a category of TC1.
Shielding
Shielding also affects wind speed. It is also due to the number of obstructions but is a more
local effect than is considered when assessing the terrain category. A classification of FS (or
full shielding) would apply where a site is surrounded by at least two rows of houses (or
permanent obstructions of similar size). Sites may also be classified as having partial
shielding (PS) or no shielding (NS).
Wind Loads
Wind typically applies a force over an area. The whole of the side of a building experiences
the force due to the wind and so does the whole of the roof.
Mostly wind forces are expressed as a pressure, a force over an area, in units of pressure
which is the pascal, although typical wind loads are usually large enough to be in the order
of kilopascals (kPa).
The roof of a house can be subjected to external suction wind pressures or to positive
pressures. During a severe cyclone event, a typical truss or rafter support needs to be able
to resist pulling forces upon it.
The diagram below indicates the pressures acting on a house showing the suction pressures
that can occur over the roof and on the leeward side.