Appreciation of Loads and Roof Truss Design What s in this

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Appreciation of Loads and Roof Truss Design

• Whats in this presentation
• Basic 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 loads
• Truss 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 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
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Structural Loading of Truss Members

• 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.

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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).

Note Specific load capacities of members depend
on the timber grade and potentially other 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 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).

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

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

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Roof Load Width of Trusses

• 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).

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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 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

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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

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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.
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Wind loads

• Wind 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

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

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• 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

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• 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.

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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.

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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

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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.

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Putting Principles into Practise

• 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.

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An Example

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

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• 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
  manufacturers drawing.

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