Introduction to Timber Roof Trusses

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Introduction to Timber Roof Trusses

• Whats in this presentation
• Spanning between supports
• Rafters the traditional approach
• 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
• Using trusses to make three dimensional shapes
• Features of trusses that make better use of
  timber
• Codes used in roof truss installation

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Spanning Between Supports

• Since our first attempts to enclose space for
  shelter, the most challenging process in building
  has been the carrying of loads over horizontal
  distances without touching the ground, ie.
  spanning between supports.
• Because of scale effects, it can be quite
  challenging to span the distances encountered in
  buildings.

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Rafters - The Traditional Approach

• Possibly one of the first structures by which our
  species obtained shelter was to support a roof by
  spanning across walls using timber rafters acting
  as simple beams (framing for skillion roofs is
  still done this way).
• As span increases so does beam size.
  Unfortunately, there are practical limits to the
  ability to continually increase beam size.

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Trusses - a Relatively New and Efficient Approach

• Manufactured timber roof trusses provide a
  structurally efficient alternative to timber
  beams. They place greater emphasis on axial
  loading of members and less on bending.
  Associated advantages of trusses include
• Strong but light to erect
• Can be made to suit most roof shapes
• Less onsite fabrication, therefore less site
  labour and less effected by bad weather
• Factory production allows automated production
• Better quality control is possible
• Trusses make maximum structural use of the timber
• Trusses are capable of long spans
• Internal walls are usually non-loadbearing
  therefore lighter weight internal walls are
  possible

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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 practising 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 in advanced truss design, where axial
  loads are used to greater effect.

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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 into 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 top point where two 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 parts of the top chords that extends beyond
  the intersection with the bottom chord (at the
  heel). This forms the eaves overhang of the
  roof.

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Truss Types - Standard Trusses
Standard trusses conform to an outer triangular
shape typically resembling an isosceles triangle.
Many web layouts within the outer triangle are
used to address spanning ability, as follows

• King Post has only one central vertical web.
  Used for small spans e.g. spans up to 5.0m.

 
Queen Post - two additional webs fanning outwards
from the base of the central web and connecting
to the middle of the top chords. Used for spans
up to 6.0m.   
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A-Type - most common truss type but has no
central web. Instead, the truss span is divided
into three equal panel lengths with webs fanning
outwards from each . Spans up to 9.0m.
B-Type compared to the A-Type has 2 extra webs.
The panels points are also equally spaced. Spans
up to 14.0m
 
Standard girder can be based on any of the
previous types but is designed to be stronger to
support other trusses.
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Truss Types for Hip Roofs
Hip end trusses include a variety of types
required to shape a hip end. The basic concept
is shown below then each type is discussed
individually
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Truncated Standard Truss takes a standard truss
shape but cuts off the top to suit the slope at
the top of a hip end.

• Truncated Girder Truss - is the main truss in a
  hip end. It occurs below the standard truncated
  trusses and takes the load of the outer hip
  trusses including the hip, jack and creeper
  trusses. It is made stronger than the standard
  truncated trusses to take these loads.

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Hip Truss - forms the hip line of the roof. It
is similar to a half truss but with an extended
top chord extending over the truncated girder
truss and finishing as the top of the hip. Some
jack and all creeper trusses butt into it.

• Jack Truss runs into the hip truss. It is also
  similar to a half truss but with an extended top
  chord extending over the truncated girder and
  meeting the hip truss.

Creeper Truss - runs into the hip truss with no
extension of the top chord i.e. stops short of
the truncated girder.
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Other Truss Types

• Scissor Truss - are modified standard trusses to
  suit a sloping ceiling. Most scissors have an
  equal pitch ceiling each side of the apex. Other
  ceiling lines are also possible

Bell Truss - a common roof shape for federation
and homestead style houses. The top chord has two
pitches, the lower pitch is usually over a
veranda or patio area.
Bowstring Truss - mostly used as a commercial
truss but becoming more common in the domestic
sector. The top chords are designed to allow a
curved roof
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Variations to Truss Types
Cantilever Truss - can be any type of truss but
the support point on one or both sides is
located inside the span, not at the heel. An
extra web is required at the inner support
location(s).
Cut Off Truss - Can be any type of truss but does
not have a heel. This truss shape is determined
by the location and comparative height of the
pitching lines on either side of the roof area
Half Truss - A half truss is a full truss cut off
at the apex.
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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 and local practise.

• In making different shapes, a range of previous
  truss types may be configured to attain the
  required shapes. This, along with manufacturing
  and installation issues are discussed in greater
  detail under dedicated presentations.

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Features of Trusses that Make Better Use of Timber

• All types of engineered trusses improve upon
  certain short comings of roofs requiring large
  timber rafters and roof beams. Issues include
• Timber roof member sags under bending and keep on
  sagging over the years - the bigger the span, the
  bigger the problem
• Large, seasoned and clear timber sections are
  required to deal with sag issues but are
  expensive and increasingly hard to get

Large rafters Likely to sag (deflect)
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• Traditional roof designs try to reduce rafter
  spans using underpurlins and struts, but these
  may require large sizes and support is reliant on
  internal walls which arent always available.
  Large strutting beams must be used and as a
  result, sag and timber availability issues
  resurface.

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• Trusses are engineered to help in the following
  ways
• Much lighter timber members can be used because
  the predominant actions are tension and
  compression, not bending
• The lighter timber can be predominantly sourced
  from plantations and easily kiln dried so there
  are no surprises as they dry out
• Deflections are much smaller, particularly in the
  long term
• The roof frame can be planned and prefabricated
  off-site, making it more possible to take
  advantage of an engineered design

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Putting the Camber into Trusses

• During fabrication, trusses further improve on
  traditional rafter design by forcing an upward
  bend into the chords of trusses referred to as a
  Camber.
• Camber helps to resist loads e.g. the amount of
  bend is calculated to help 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

• A benefit of trusses is that they 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 allow the bottom
  chord to move up and down in the gap (but not
  sideways)

Click on the picture to watch a video
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Codes used in Truss installation

• Once trusses have been designed and manufactured
  according to previous engineering principles, the
  emphasis is on site installation practices
• Australian Standard AS4440-2004 Installation of
  Nailplated Timber Trusses is the standard applied
• It relates to residential construction (including
  BCA building classification 1,2,3 and 10) and
  light commercial structures.
• It covers a broad variety of issues including
• Terms and definitions
• Installation techniques
• Bracing requirements
• Connection requirements
• Eaves and gables
• Lifting, storage and temporary bracing practices

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• Limits to the application of AS4440 include
• Roofs with a maximum roof pitch of 45O
• Roofs that are essentially rectangular layouts or
  a combination of rectangular elements
• Roofs with a maximum truss span of 16m
• Truss spacings at a maximum of 900mm for tiled
  roofs or 1200mm for metal sheet roofs
• Nail plated trusses only
• Maximum wind speeds - refer to either AS1170.2 or
  AS4055

• Support documents linked to AS4440 include
• AS1684 Residential Timber Framing Code
• Installation manuals produced by individual truss
  manufacturers
• Teaching resources in this package provide
  general information that draws on these sources.
  For specific advice, full detail must be obtained
  from the above documents.

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