Appreciation of Loads and Roof Truss Design

What’s in this presentationBasic truss requirementsStructural loading of truss membersExamples of bending, tension and compression Roof load width of trussesSpecific loads - dead, live and wind loadsCombinations of loads Truss patterns of tension and compression (to resist loads)Putting the principles into practiseA worked example - calculating loads in truss members Finalising the truss design

Basic Truss RequirementsA timber roof truss is a two-dimensional assembly of stick elements that work in a vertical plane and carry roof loads across a span between load-bearing walls

The pattern is made up of stable triangles consisting of chord and web members

In a trussed roof, the trusses are the main load-carrying structural elements

Load bearing wall

Top Chord

Bottom Chord

Web Web

Web

Top ChordWeb

Load bearing wall

Non- load bearing wall

Gap

Top Chord

A truss is strong in one direction (the span) because the chord and web members are arranged to work mostly in tension and compression along their long axes

There is some bending in these members but it is the compression and tension loading that does most of the work.

Tension and compression are types of axial loading. Truss members loaded in this way can resist more load than in bending.

Structural Loading of Truss Members

Tension

CompressionBending

Example of Timber in Bending

In bending, a piece of 70x35mm softwood, 1m long can withstand a point load of 180kg applied in the middle

While trusses are strong because axial force is the main action in the members, there is some bending in some elements (particularly in the bottom chord of girder trusses).

Example of Timber in Tension

In tension (along the grain of the timber) the same piece of 70x35 softwood can withstand a weight force of 2000kg before it breaks

This is much more than the 180kg it can sustain in bending (where the load is applied across the grain).

Example of Timber in Compression

In compression (along the grain of the timber) a very straight piece of 70x35 softwood 1m long can withstand a weight force of about 540kg before it buckles

Although the piece resists a much greater load in compression than the 180kg in bending, this is much less than its 2000kg tension capacity – this is because of buckling

Compression and Buckling

Buckling occurs in a slender member under compression when the middle of the member suddenly deflects sideways. The tendency to buckle is very sensitive to unrestrained length

There is not much warning when something buckles

The shorter the length between supports and the straighter it is, the less likely a member is to buckle

Because many of the slender members in a truss feel axial compression, this effect is very important, so for trusses, the design of the compression members often dominates.

Trusses (made of tension or compression members) are set up at regular intervals to form the shape of the roof

Each truss supports loads from a certain contributing area of the roof and this influences the size of the compression and tension members

The contributing area is usually a strip whose width is defined by the mid-lines between adjacent trusses (shown shaded below)

Trusses are commonly spaced 600mm apart but may differ depending on local conditions and the roofing material used (e.g. tiles or sheet metal).

Roof Load Width of Trusses

Specific Loads on Roofs

The most common loads falling within the roof load width are: Gravity Dead Loads including roof and ceiling materials – these are

felt by the structure all of the time Gravity Live Loads including people working on the roof and stuff

stacked on it – these are only felt some of the time by the structure Wind loads including downward pressure or suction that lifts upwards –

these are only felt some of the time but downward pressure adds to the gravity loads above, while uplift works in the opposite directions

Gravity Dead Load

The weight of the roofing material can be expressed as weight (kg) per unit area of roof (square metres), ie. (kg/m2)

The weight of a tiled roof with battens, a plasterboard ceiling and insulation is approximately 75 kg/m2

The weight of a sheet metal roof with softwood ceiling and insulation is approximately 20 kg/m2

DEAD LOAD (structure)

Gravity Live Loads

Live loads result from the occasional presence of people and materials on the roof

For our purposes, we can assume a live load around 25kg/m2

We also must allow for the weight of a large person standing anywhere on the roof.

Did you know weight force is sometimes expressed as kilonewtons - a term commonly used by structural engineers. A kilonewton is the force generated by a mass of about 102kg. Think of a kilonewton as the weight force of a large person.

Live loads (people,)Construction loads

(people, materials)

Wind loadsWind loads push against the roof but can also cause uplift and suction

The amount of wind load which acts on the roof depends on several things - the most important being the speed of the wind

Suction

Internal Wind

Suction

As the wind speed increases so does wind load – this load is spread over the area of the building exposed to the wind

Wind load on a flat surface vs speed

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100

Wind speed (kph)

Loa

d (k

g/m

)

For different areas in Australia, the wind load standard, AS1170.2, provides basic wind speeds to calculate loads on buildings

Roofs in protected areas will be subject to less wind load than those on exposed sites

To calculate the wind load that the roof is likely to feel, the basic speeds are adjusted for factors such as height, shielding and terrain type

AS 4055 provides a simplified version of wind speeds (compared to AS1170.2). It is especially for residential buildings

When the wind passes over a roof it can cause a suction. When it gains access to the interior it can cause an uplift.

The trusses must be strong enough to resist the load developed by suctions and uplift. They must be attached adequately to the rest of the structure so the whole roof is not sucked off.

Suction

Internal pressure

Suction (uplift)

Wind

Combinations of loads

More than one type of load can be acting on a truss at the same time. The designer must check that the truss is strong enough to resist the worst combination of loads possible.

This may be a combination of gravity dead loads plus gravity live load, plus wind loads – all acting downwards.

In other instances wind may be acting upwards (where suction and uplift occur), therefore acting in the opposite direction to gravity dead and live loads.

In high wind areas, wind uplift can easily exceed downward gravity loads. For resisting uplift, the heavy dead load from a tiled roof is useful.

Tip: Did you know that because dead load is there all the time, any

combination of loads the truss can feel, must include dead load.

Compression and Tension Members for Downward Loads

Below is the pattern of tension and compression members that result in trusses from downward loads i.e. dead loads, live loads and downward wind pressure

To help imagine this, assume a tiled roof is being carried by the truss because tiles assist dead loads compared to lightweight metal roofs

SupportBottom Chord

Top Chord with heavy tile roof pushing down

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Top Chord with heavy tile roof pushing down

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Compression members

Tension members

Support

The Reverse Pattern Due to Suction and Uplift

In this load combination, assume a light sheet metal roof instead of a heavy tile roof. If the roof is overcome by wind load, the resulting upward loads force the truss members into the reverse pattern of tension and compression (compared to the previous example). This can easily outweigh the downward loads.

SupportBottom Chord

Top Chord with light metal roof sucking up

Web Web

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Top Chord with light metal roof sucking up

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Compression members

Tension members

Support

Given the previous examples, truss members need to have enough capacity to cope with either tension or compression (and a small amount of bending) for upwards and downwards forces – in the worst case scenario for each

The designer then looks at the structural properties of the timber that will be used and makes sure each member and its connection is strong enough to cope with those loads.

Putting Principles into Practise

Say we want to check the member sizes of a type A truss (as shown previously) to span 8 metres and spaced at 600mm apart

Assume that 70x35 softwood will be used as this is an economical and readily available size. From earlier examples, we also know that this size can take 2000kgs in tension and 540kgs in compression (for a straight length 1m long)

The designer would use structural analysis software to work out forces felt in the truss members, based on a scenario just before the truss would collapse. Safety factors are also incorporated in the loads.

An Example

For gravity dead loads (using a tiled roof) and live loads, the maximum compression including safety factors, works out to be 510Kgs. Compression members usually dominate design requirements.

The 510kgs is within the capacity of the 70x35 timber as long as it is laterally restrained at no more 1m intervals

A similar calculation would check uplift from wind loads

All relevant information goes on the manufacturer’s drawing.

510kgs max

Compression

Tension

510 kgs.max

Tile loads, live loads plus safety factors

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