Building Construction Handbook, Eighth Edition

Bowstring Truss ~ a type of lattice truss formed with a curved
upper edge. Bows and strings may be formed in pairs of laminated
timber sections that are separated by solid web timber sections of
struts and ties.

Spacing ~ 4.000 to 6.000 m apart depending on sizes of timber
sections used and span.

Purlins ~ to coincide with web section meeting points and at about
1.000 m interim intervals.

Decking ~ sheet material suitably weathered or profiled metal
sheeting. Thermally insulated relative to application.

Top bow radius ~ generally taken as between three-quarters of the
span and the whole span.

Application ~ manufacturing assembly areas, factories, aircraft
hangers, exhibition centres, sports arenas and other situations
requiring a very large open span with featured timbers. Standard
steel sections may also be used in this profile where appearance is
less important, eg. railway termini.

Variation ~ the Belfast truss that pre-dates the standard
bowstring shown. It has much smaller interlaced struts and ties
therefore it is more complicated in terms of assembly and for
calculation of stress distribution. See next page.

web sections

span

(in excess of 50m possible)

bow in pairs of laminated sections

depth, generally

0.125 to 0.167 span

string in

laminated timber

pairs often

cambered

10mm in

every 2.4m

Long Span Roofs