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258 Barry’s Advanced Construction of Buildings
provide a level surface on which the wall panels can be cast. A bond breaker/cure-coat is
then applied to the concrete slab and the panels cast around reinforcement inside steel (or
timber) edge shuttering, which is placed as near as possible to the final position of the wall
panel. Lifting lugs and other fittings are usually cast into the upper face of the panels (which
will be covered by insulation and internal finishes). Wall panels may be cast individually
or as a continuous strip. If the panels are cast as a continuous strip, they are cut to size
once the concrete has gone off but during the early stages of the concrete’s maturity (one
or two days). Panels may also be cast as a stack, one on top of the other, separated by a
bond breaker. Once cured, the hardened panels are then gently lifted or tilted into position
and propped or braced ready to receive the roof deck. The panels are tilted up and positioned
on the levelled foundations against a rebate in the concrete, or up to timber runners
or on to a sheathing angle and then set level on steel levelling shims. A mechanical connection
between the foot of the slabs to the foundation and/or floor slab is usually employed.
Cast in metal, dowels projecting from the foot of the panels are set into slots or holes in
the foundations and grouted in position. Alternatively, a plate welded to studs or bar
anchors, cast into the foot of the panel, provides a means of welded connection to rods cast
into the site slab as illustrated in Figure 4.98. The roof deck serves as a diaphragm to give
support to the top of the wall panels and to transmit lateral wind forces back to the foundation.
Lattice beam roof decks are welded to seat angles, welded to a plate and cast in studs
as shown in Figure 4.98. A continuous chord angle is welded to the top of the lattice beams
and to bolts cast or fixed in the panel. The chord angle serves as a transverse tie across the
panels and is secured to them with bolts set into slots in the angle to allow for shrinkage
movements of the panels.
4.5 Shell structures
A shell structure is a thin, curved membrane or slab, usually of reinforced concrete, that
functions both as a structure and covering, the structure deriving its strength and rigidity
from the curved shell form (see Photograph 4.24 and Photograph 4.25). The term ‘shell’ is
used to describe these structures by reference to the considerable strength and rigidity of
thin, natural, curved forms such as the shell of an egg. The material most suited to the
construction of a shell structure is concrete, which is a highly plastic material when wet
and which can take up any shape inside formwork (also known as centring). Small section
reinforcing bars can readily be bent to follow the curvature of shells. Wet concrete is spread
over the centring and around the reinforcement, and compacted to the required thickness
with the stiffness of the concrete mix and the reinforcement preventing the concrete from
running down the slope of the curvature of the shell while the concrete is wet. Once the
concrete has hardened, the reinforced concrete membrane or slab acts as a strong, rigid
shell, which serves as both structure and covering to the building. The strength and rigidity
of curved shell structures make it possible to construct single curved barrel vaults 60 mm
thick and double curved hyperbolic paraboloids 40 mm thick in reinforced concrete for
clear spans up to 30 m.
The attraction of shell structures lies in the elegant simplicity of the curved shell form
that utilises the natural strength and stiffness of shell forms with great economy in the use
of material. The main disadvantages relate to their cost and poor thermal insulation proper-