Fig. 4.49 Steel shearheads, which are cross-shaped
arrangements of steel plates joined by welding, are an
alternative method to column caps or drop panels for
increasing the shear strength of flat-slabs in the vicinity of
columns.
The maximum size of column grid of this
type of structure can be increased to around
18 m for reinforced slabs and 20 m for pre-
stressed slabs if a coffered system is used to
reduce the weight of concrete present in the
underside of the slab. The span/depth ratios
for coffered slabs are around 25 for
reinforced slabs and 30 for pre-stressed
slabs.
The two-way-spanning slab system is de-
pendent, to a large extent, on structural conti-
nuity for its strength. It performs best (i.e.
allows the highest ratios of span-to-depth)
with column grids which have at least three
bays in each direction and in which the varia-
tion in the sizes of the bays is kept to a
minimum. The efficiency of the system is such,
however, that slab thicknesses are approxi-
mately the same as those in frame structures
of equivalent span, i.e. a system in which the
slab is supported on downstand beams.
Fig. 4.50 Typical arrangements of vertical-plane bracing
for flat-slab structures, which should be as symmetrical as
possible, are shown here.Where a thin, solid slab is used its stiffness
can be insufficient to allow rigid-frame action
to develop with the columns in response to
lateral load. Extra bracing must therefore be
provided and in situ concrete walls are usually
incorporated into the structure for this
purpose. These must be arranged in two
mutually perpendicular directions and can
usually be accommodated around lift or stair
towers (Fig. 4.50). In the long-span part of the
range, that is for spans greater than around
10 m, the depth of the coffered slab which is
required will be greater than 350 mm and the
slab is usually sufficiently stiff to allow rigid-
frame action to develop between the floor and
columns, in response to lateral load; in these
cases no additional bracing is required.
The high degree of statical indeterminacy
which is associated with the two-way-spanning
slab allows greater flexibility in the column grid
than is possible with frame structures. The small
construction depth of flat-slab structures,
compared to frame structures, also facilitates the
easy accommodation of a services zone and will
usually result in a lower overall storey height
than is possible with a beam-column frame. 137Reinforced concrete structures