Civil Engineering Reference
In-Depth Information
more accurate component simply because of the equipment, level floor area, dry condi-
tions and specialist operatives used in the construction.
Accuracy
Most bolt and connector trusses for domestic roof construction will be assembled using
sawn timber. The tolerance on sawn timber in terms of over and under sizing, particularly
in the larger dimension, is significant and for this reason the full size setting-out of the
truss has been recommended in the assembly procedures described. The precise location
for the bolt centres is essential to maintain the design edge and end distance for the timber
connectors themselves. Insufficient end distance may result in a timber connector shearing
out a section of timber thus allowing the truss joint to fail. Great care must therefore be
taken to follow the precise dimensions detailed on the engineer's drawing. Reference
should be made in this respect to the dimensions indicated on the design illustrated in Fig.
4.3.
Standard Designs.
The TRADA designs as stated earlier are no longer available, but the design principles are
sound and all relevant metalwork and connectors are still available. Figure 4.3 is contin-
ued in this third edition by way of illustrating this typical bolt and connector truss but it
must be emphasised that the illustration is
not
a current TRADA design.
Many homes built in the fifties, sixties and seventies used this type of construction and it
has proved to be a rugged durable roof structure. Similar roof trusses today would have to
be engineered, but that done, purlins, common rafters and ceiling joists can all be 'de-
signed' from building regulation span tables.
The Roof Construction
The bolt and connector truss is generally used as a principal truss with common rafters
and ceiling ties supported from it and on the wall plate. The designs referred to above are
spaced at 1.8 m centres with the design of the purlin, binders, plate and ridge being given
on each individual sheet. A study should be made of the design sheet illustrated in Fig.
4.3.
