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264 Barry’s Advanced Construction of Buildings
stiffening member between the columns supporting the shell. The downstand reinforced
concrete beam, which is usually 150 or 225 mm thick, is the most efficient of the four
because of its depth. To avoid the interruption of the line of the soffit of the vaults caused
by a downstand beam, an upstand beam is sometimes used. The disadvantage of an upstand
beam is that it breaks up the line of the roof and also needs protection against the weather.
Arch ribs are sometimes used because they follow the curve of the shell and therefore do
not interrupt the line of the vault; however, these are less efficient structurally because they
have less depth than beams.
Edge and valley beams
Reinforced concrete edge beams are cast between columns as an integral part of the shell
to resist the tendency of the thin shell to spread and its curvature to flatten out due to selfweight
and imposed loads. The edge beams may be cast as dropped beams, upstand beams,
or partly upstand or partly dropped beams, as illustrated in Figure 4.103. Between multibay
vaults, the loads on the vaults are largely transmitted to adjacent shells and then to the
edge beams, thus allowing the use of comparatively slender featheredge beams.
Rooflights
Natural light through the shell structure can be provided by decklights formed in the crown
of the vault, as illustrated in Figure 4.101, or by domelights. Rooflights are fixed to an
upstand curb cast integrally with the shell, as illustrated in Figure 4.101. Care is required
to avoid overheating and glare. One way of providing natural light and avoiding glare and
overheating is to use a system of north light barrel vaults, as illustrated in Figure 4.104 and
Dropped
valley beam
Featheredge
valley beam
Upstand
edge beam
Dropped
edge beam
Figure 4.103 Edge and valley beams for reinforced concrete barrel vaults.