1 Intro to roof trusses by ed6WhU3


									Introduction to Timber
Roof Trusses
            What’s 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
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

               Because of scale effects, it can
               be quite challenging to span the
               distances encountered in
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.
Trusses - a Relatively New and Efficient
  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
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
 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.

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
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
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.
Joining Triangles to Make Advanced
           Truss Designs
         Triangle doesn’t
         change shape       Advanced truss designs build on the
                            previous principles by adding many
                            small triangles together, to make
                            trusses capable of spanning long
                            Triangles are good shapes because
         Rectangle does     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.
Holding Triangles Together with Nail
                          Even though joints can usually be
               crushing   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.
The timber truss industry as we know it
would not be possible without nail plate
The plates are used in pairs - identical
plates are pressed into each side of the
joint using special equipment in a
                   Truss Terminology

      Top chord


                         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
Bottom Chord
Defines the bottom member of the truss, usually horizontal, and
carrying a combined tension and some bending stress (from gravity
Top Chord
Defines the top members of the truss, usually sloping, and carrying
combined compression and some bending stress (from gravity
Webs are members joining top and bottom chords to form a truss.
They may be in tension or compression depending on the truss
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.
The point on a truss where the undersides of the top and bottom
chords join.
Panel points
The points where web members and chord members meet
The distance between the outer edges of the load bearing walls
supporting the trusses (usually heel to heel)
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.
   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.
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.
         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
Truncated Standard Truss – takes a
standard truss shape but cuts off the
top to suit the slope at the top of a hip

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.
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

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

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
           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
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.
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)
   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 aren’t always available. Large strutting beams must be used
    and as a result, sag and timber availability issues resurface.


                   Strut                                           Strut
                                      Ceiling joist    Strutting
                           Internal                    beam
                                                            Much smaller kiln
 Negligible short and                                       dried plantation
 long term deflection                                       sourced members

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
 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 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.

          Camber                                 Camber

     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

                                                    Click on the
                                                    picture to watch
                                                    a video
 Codes used in Truss installation
Once trusses have been designed and manufactured according to
previous engineering principles, the emphasis is on site installation
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
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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