appreciation of loads and truss design

8/2/2019 Appreciation of Loads and Truss Design

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Appreciation of Loads and

Roof Truss Design

Whats in this presentationBasic truss requirements

Structural loading of truss members

Examples of bending, tension and compression

Roof load width of trusses

Specific loads - dead, live and wind loads

Combinations of loadsTruss patterns of tension and compression (to resist loads)

Putting the principles into practise

A worked example - calculating loads in truss members

Finalising the truss design


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Basic Truss Requirements

A timber roof truss is a two-dimensional assembly of stickelements that work in a vertical plane and carry roof loads acrossa span between load-bearing walls

The pattern is made up of stable triangles consisting of chordand web members

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

Load bearingwall

Bottom Chord

WebWeb

Web Web

Load bearingwall

Non- loadbearing wall

Gap


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A truss is strong in one direction (the span) because the chord andweb members are arranged to work mostly in tension andcompression 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. Trussmembers loaded in this way can resist more load than in bending.

Structural

Loading of Truss

Members

Tension

CompressionBending


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Example of Timber in Bending

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

While trusses are strong because axial force is the main action inthe members, there is some bending in some elements (particularly

in the bottom chord of girder trusses).

Note: Specific load capacities of members depend on the timber grade and potentiallyother design issues as well. The above example is for demonstration only


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Example of Timber in Tension

In tension (along the grain of the timber) the same piece of70x35 softwood can withstand a weight force of 2000kg beforeit breaks

This is much more than the 180kg it can sustain in bending

(where the load is applied across the grain).



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Example of Timber in Compression

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

Although the piece resists a much greater load in

compression than the 180kg in bending, this is muchless than its 2000kg tension capacity this is because ofbuckling



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Compression and Buckling

Buckling occurs in a slender memberunder compression when the middleof the member suddenly deflectssideways. The tendency to buckle isvery sensitive to unrestrained length

There is not much warning whensomething buckles

The shorter the length betweensupports and the straighter it is, theless likely a member is to buckle

Because many of the slendermembers in a truss feel axialcompression, this effect is veryimportant, so for trusses, the designof the compression members often

dominates.


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Trusses (made of tension or compression members) are set up at

regular intervals to form the shape of the roofEach truss supports loads from a certain contributing area of the roofand this influences the size of the compression and tension members

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

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

Roof Load Width of Trusses


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Specific Loads on Roofs

The most common loads falling within the roof load width are:

Gravity Dead Loads including roof and ceiling materials these arefelt by the structure all of the time

Gravity Live Loads including people working on the roof and stuffstacked 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 thegravity loads above, while uplift works in the opposite directions


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Gravity Dead Load

The weight of the roofing materialcan be expressed as weight (kg) perunit area of roof (square metres), ie.(kg/m2)

The weight of a tiled roof withbattens, a plasterboard ceiling andinsulation is approximately 75 kg/m2

The weight of a sheet metal roof withsoftwood ceiling and insulation is

approximately 20 kg/m2

DEAD LOAD (structure)


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Gravity Live Loads

Live loads result from theoccasional presence of peopleand materials on the roof

For our purposes, we can

assume a live load around25kg/m2

We also must allow for theweight of a large person standinganywhere 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)


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

suctionThe amount of wind load which acts on the roof depends on severalthings - the most important being the speed of the wind

Suction

InternalWind

Suction


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As the wind speed increases so does wind load this load is spreadover the area of the building exposed to the wind


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For different areas in Australia, thewind load standard, AS1170.2,

provides basic wind speeds tocalculate loads on buildings

Roofs in protected areas will besubject to less wind load than thoseon exposed sites

To calculate the wind load that theroof is likely to feel, the basicspeeds are adjusted for factorssuch as height, shielding andterrain type

AS 4055 provides a simplifiedversion of wind speeds (comparedto AS1170.2). It is especially forresidential buildings


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

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

Suction

Internalpressure

Suction (uplift)

Wind


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Combinations of loads

More than one type of load can be acting on a truss at the sametime. The designer must check that the truss is strong enough toresist 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 anduplift occur), therefore acting in the opposite direction to gravity deadand 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 isuseful.

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.


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Compression and Tension Membersfor Downward Loads

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

To help imagine this, assume a tiled roof is being carried by the

truss because tiles assist dead loads compared to lightweight metalroofs

SupportBottom Chord

Web

WebWeb Web

Compression members

Tension members

Support


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The Reverse Pattern Due toSuction and Uplift

In this load combination, assume a light sheet metal roof instead of aheavy tile roof. If the roof is overcome by wind load, the resultingupward loads force the truss members into the reverse pattern oftension and compression (compared to the previous example). Thiscan easily outweigh the downward loads.

SupportBottom Chord

Web

WebWeb Web

Compression members

Tension members

Support


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Given the previous examples, truss members need to have enoughcapacity to cope with either tension or compression (and a smallamount of bending) for upwards and downwards forces in theworst case scenario for each

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

Putting Principles into Practise


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

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

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

Note: Specific load capacities of members depend on the timbergrade and potentially other design issues as well. The aboveexample is for demonstration only

An Example


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For gravity dead loads (using a tiled roof) andlive loads, the maximum compressionincluding safety factors, works out to be510Kgs. Compression members usuallydominate design requirements.

The 510kgs is within the capacity of the 70x35timber as long as it is laterally restrained at nomore 1m intervals

A similar calculation would check uplift fromwind loads

All relevant information goes on themanufacturers drawing.

510kgsmax

Compression

Tension

510 kgs.max

Tile loads, live loadsplus safety factors


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appreciation of loads and truss design

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