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196 Barry’s Advanced Construction of Buildings
Lightweight roof covering on
225 × 50 timber purlins
20 diameter bolt
30° slope
Laminations taper
Radius 2.5
Cramp
Cavity
wall
Span 14.5
Foot of portal
frame inside
cast iron shoe
Floor slab
Shoe bolted to concrete base
Figure 4.39 Glue-laminated timber portal frame.
portals are spaced fairly widely apart to support timber or steel purlins, which can be
covered with sheet cladding materials, slates or tiles. Timber decking is usually used to
provide a soffit of natural timber. For buildings that require heating, the thermal insulation
is placed above the timber soffit. The laminations of the timber from which the portal is
made are arranged to taper so that the depth is greatest at the knee, where the frame tends
to spread under load and where the depth is most needed. The portal is more slender at
the apex and at the base of the post where the least section is required for strength and
rigidity. The maximum radius of curve for shaped members is governed by the thickness
of the laminates. A maximum radius of 5625 mm is recommended for 45 and 2500 mm for
20 mm thick laminae. Because of the labour involved in the assembly of curved members,
they are appreciably more expensive than straight members.