Civil Engineering Reference
In-Depth Information
Most trussed rafter manufacturers use terminology not yet covered in previous chapters
and the reader should refer to Fig. 5.2 for familiarisation with terms to be used throughout
this chapter. The illustration shows a 'fink' truss, the most common configuration in use
today. The geometry of this configuration can be found in Fig. 2.5. The standard spacing
for trussed rafters is 600 mm, although 400 mm and 450 mm are not uncommon. Timber
sizes are standardised throughout the United Kingdom, the timber being machined on all
surfaces for accuracy in accordance with the standards set out in BS EN 1313/1. The tim-
ber is usually stress graded in accordance with BS 4978 and should be stamped with a
grade mark. Whilst so called 'nominal' sizes are often quoted, i.e. 75, 87, 100, 125 and 150
× 38 mm, the finished section will be 72, 84, 97, 122 and 147 × 35 mm. Timber of 47 mm
finished thickness is frequently used for attic trussed rafters with depths going up to 222
mm for heavily loaded rafters and floor joists. Trussed rafters in excess of 11 m span must
use this thicker timber, or be made of multiple trusses of minimum 35 mm thickness, per-
manently fixed together by the truss manufacturer at works.
An understanding of the function of the trussed rafter is essential if good roof construction
is to be achieved. The trussed rafter is designed to carry only the vertical loads imposed
upon it, no lateral loads are catered for. The design assumes that the trussed rafter is main-
tained in its truly upright position by the various bracing and
