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396 Barry’s Advanced Construction of Buildings
Secondary beams
span between
main beams
Concrete columns
Main beams
span between
columns
Thin slab
between
beams
Figure 6.40 Square grid beam and slab floor.
monolithically with reinforced concrete floors, it is logical to design the floor slabs to span
in both directions so that all the beams around a floor slab can bear part of the load. This
two-way span of floor slabs effects some reduction in the overall depth of floors as compared
with a one-way spanning floor slab construction. Since the most economical shape
for a two-way spanning slab is square, the best column grid for a reinforced concrete frame
with monolithically cast floors is a square one, as illustrated in Figure 6.40.
The in situ cast reinforced concrete floor illustrated in Figure 6.40 combines main and
secondary beams as a grid to provide the least thickness of slab for economy in the mass
of concrete in construction, and comparatively widely spaced columns. This square grid
results in the minimum thickness of floor slab and minimum depth of beams, and therefore
the minimum deadweight of construction. Departure from the square column grid, because
of user requirements and circulation needs in a building, will increase the overall depth,
weight and therefore cost of construction of a reinforced concrete frame.
The rectangular column grid, illustrated in Figure 6.41, supports main beams between
columns that support one-way spanning floors with the beams between columns. The floor
slab can be cast in situ on centring and falsework, or precast concrete floor beams or planks
may be used. This arrangement involves closely spaced columns and the least mass of
concrete in floors.
In a steel frame, the skeleton of columns and beams is designed to carry the total weight
of the building. The floors, which span between beams, act independently of the frame.
With an in situ cast reinforced concrete frame and floor construction, columns, beams and
floors are cast and act monolithically. The floor construction, therefore, acts with and affects
the frame and should be considered as part of it.