STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

204

Fig. 6.24 St loseph's Chapel, Ipswich, J. Powlesland,

structural engineer. The timber roof structure here is based

on large cross-section laminated timber elements, which

are used in conjunction with laminated timber secondary

elements and solid timber decking [Photo: Sidney

Baynton]

veneer plywood (Fig. 6.25). Plyweb beams are

normally considerably cheaper than gluelam

equivalents. They are used in similar types of

construction to solid sawn-timber beams but

they allow wider spacings (around 1.3 m) and

longer spans (up to 20 m ) to be achieved.

6.6.3.4 Other forms of built-up-beams

A number of other types of built-up-beam

are produced in timber. One example is the

lattice-type beam, which is illustrated in

Fig. 6.26.

6.6.4 Trussed rafters

Trussed rafters are lightweight timber trusses,

Fig. 6.27; they are usually factory-made assem-

blies of small, sawn-timber sections,

regularised on thickness and jointed with

punched-metal-plate fasteners, but they can be

assembled on site with bolted joints. Typical

individual element sizes are 38 mm X 75 mm

for short-span versions (6 m) and 50 mm X

100 mm for the larger versions (10 m). Most

manufacturers produce a range of sizes in

100 mm increments of span and 2.5° incre-

ments of pitch between 15° and 35°.

Trussed rafters are designed to be used in

pitched roof construction and spaced at close

centres (around 600 mm) so as to carry the

roof cladding without the need for a secondary

structural system, other than tile battens or

sarking boards. They therefore perform a

similar role to that of the rafters in traditional