Introduction to Trussed Roof Framing

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Introduction to Trussed Roof Framing

• Whats in this presentation
• Trusses - a relatively new and efficient approach
• Trusses use axially loaded members
• A basic example of truss design
• Joining triangles to make advanced truss designs
• Making truss joints using nail plates
• Truss terms
• Truss types
• Putting the camber into trusses
• Clear spanning internal walls

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Trusses - a Relatively New and Efficient Approach

• Manufactured timber roof trusses provide a
  structurally efficient alternative to raftered
  roofs
• In structural terms they work differently to
  raftered roofs - they place greater emphasis on
  axial loading of the truss members and less on
  bending

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Trusses Use Axially Loaded Members Instead of
Beams

• Beams (e.g. rafters) are slender members which
  cope with loads - such as the weight of the roof
  - by resisting bending.
• Beams are convenient but not efficient. For
  instance the easiest way to break a beam is to
  bend it in the middle until it snaps, not squash
  or stretch it from end to end. See which is the
  easiest by practicing on a pencil.
• Bending places load across the axis, while
  squashing (compression) and stretching (tension)
  place load along the axis. Axial loading is far
  more efficient than bending,
• Truss members are designed by maximising axial
  loading and minimising bending.

Compression
Bending
Tension
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A Basic Example of Truss Design using axially
loaded members

• Traditional roofing materials such as thatch and
  shingles are not waterproof - they require steep
  pitches to shed water.
• As the pitch of the roof increases, the rafters
  feel more axial load and less bending load. This
  is because the load increasingly runs down the
  rafter (thus compressing it) rather than running
  across it like a beam.
• Roofs of this type were often constructed with a
  load bearing ridge beam

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• Coupled rafters lean on each other at the top and
  obviate the need for a load bearing ridge. At
  the bottom however, the axial thrust down the
  rafters tends to spread the walls outwards. In
  traditional construction, large buttresses were
  used to stop this spread from happening

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• By adding a member tying the bottoms of the
  coupled rafters to prevent them spreading the
  walls apart, a simple triangular truss is formed
  i.e. the rafters are in compression the tie
  member is in tension beam action in all members
  is minimal

• The underlying concepts in the example have since
  been used to more fully utilise axial loading in
  advanced truss design

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Joining Triangles to Make Advanced Truss Designs
Triangle doesnt change shape

• Advanced truss designs build on the previous
  principles by adding many small triangles
  together, to make trusses capable of spanning
  long distances.
• Triangles are good shapes because the joints in
  trusses are thought to act like hinges and
  triangles maintain a stable shape even when
  hinged joints are loaded. In contrast,
  rectangles move out of shape more easily.
• Therefore the patterns of trusses tend to be made
  up of many triangles networked together.

Rectangle does
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Holding Triangles Together with Nail Plates

• Even though joints can usually be thought of as
  hinges, trusses depend a lot on their joints.
• This is challenging because of the different
  three dimensional properties in timber.
• The stress concentrations at single point joints
  such as bolts, cause problems as shown in the top
  sketch.
• Multiple-toothed nail plate connectors used in
  trusses, successfully deal with this by
  distributing the joint loads across a larger area.

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• The timber truss industry as we know it would
  not be possible without nail plate connectors.
• The plates are used in pairs - identical plates
  are pressed in each side of the joint using
  special equipment in a factory.

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Truss Terminology
Top chord
Webs
Bottom chord

• Given the previous discussion, a truss can be
  described as a pre-fabricated, engineered
  building component which functions as a
  structural support member.
• There are different types of trusses but the same
  basic terms apply
• Members are either top chords, bottom chords or
  webs
• Each will be in tension or compression according
  to the type of truss involved

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• Bottom Chord
• Defines the bottom member of the truss, usually
  horizontal, and carrying a combined tension and
  some bending stress (from gravity loads).

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• Top Chord
• Defines the top members of the truss, usually
  sloping, and carrying combined compression and
  some bending stress (from gravity loads)

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Web Webs are members joining top and bottom
chords to form a truss. They may be in tension or
compression depending on the truss design.
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• Apex
• The point where the chords meet. This can be
  either a Top Chord Apex or much less commonly a
  Bottom Chord Apex (not shown). The Top Chord Apex
  of multiple trusses in a row, forms the ridge
  line of the roof.

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Heel The point on a truss where the undersides of
the Top and Bottom Chords join.
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Panel points The points where web members and
chord members meet
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Span The distance between the outer edges of the
load bearing walls supporting the trusses
(usually heel to heel)
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• Overhang Eaves OH
• The part of the Top Chord that extends beyond the
  intersection with the bottom chord. It forms the
  eaves overhang of the roof.

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Using Trusses to Make Three Dimensional Roof
Shapes

• The previous discussion identifies roof trusses
  as two-dimensional assemblies. These are added
  together to make three dimensional roof shapes.
• To this end, trusses are usually spaced at
  regular intervals typically 600mm, 900mm or 1200
  apart, depending on the mass of the roofing
  material involved.

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Truss Types for Gable Roofs
To make gable roof shapes, standard trusses are
simply added together at regular intervals for
the length of the building. This is very
efficient in terms of truss production because
only one truss setup is required
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Truss Types for Hip Roofs

• A wider variety of trusses are required for hip
  roofs
• Truncated trusses have the top of the triangle
  removed to suit the hip shape
• Truncated girders are made stronger to take the
  load of the other hip end trusses
• Hip and jack trusses are like half trusses
  carried by the truncated girder
• Creeper trusses are also like half trusses and
  are carried by the hip truss (in turn supported
  by the truncated girder truss

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Putting the Camber into Trusses

• Irrespective of the type of truss, truss design
  forces an upward bend into the chords of trusses
  referred to as Camber
• Camber helps to resist loads e.g. the amount of
  bend is calculated to resist the load of tiles
  and ceiling lining. The calculations are
  designed to ensure the truss eventually flattens
  out to provide straight chords, once fully loaded.

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Clear Spanning Internal Walls

• Trusses can span long distances in one go.
  External walls are usually used to provide
  support but internal walls are not needed.
• Internal walls cause problems if used for
  support because they change the way the truss
  works. To prevent this
• External load bearing walls are made slightly
  higher than internal walls, leaving a gap between
  the bottom chord and the internal wall
• Special brackets fix the bottom chord to the
  internal wall the brackets maintain a gap and
  allow the bottom chord to move up and down

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