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176 Barry’s Advanced Construction of Buildings
Purlins fixed across
beams support roof
covering
Lattice beam
Column
Sheeting rails
Figure 4.16 Single-bay symmetrical pitch lattice beam and column frame.
Purlins fixed across roof
frames support roof covering
Lattice steel
roof frames
Lattice girder in
valley supports roof
frames between
internal columns
Steel
column
Figure 4.17 Two-bay lattice beam roof on steel columns.
construction where the space is to be heated. The beams are fabricated from tubular and
hollow rectangular section steel, which is cut and welded together with bolted site connections
at mid-span to facilitate the transportation of half-lengths to the site. The top and
bottom chords of the beams are usually of hollow rectangular section for ease of fabrication.
End plates, welded to the lattice beams, are bolted to the flanges of I-section columns.
Service pipes and small ducts may be run through the lattice frames, and larger ducts
suspended below the beams inside the roof space.
Multi-bay symmetrical pitch lattice steel beam roof construction
For multi-bay symmetrical pitch lattice beam roofs, it is usual to fabricate a form of valley
beam roof as illustrated in Figure 4.17. The valley beam is designed to be the same depth
as the beams to prevent any increase in the unwanted volume of roof space. To provide the
maximum free floor space, a form of butterfly roof with deep valley beams is used, as