283757893275

Structural Steel Frames 333
Floor beams supported
by top hat beam
Top hat section
beam
Figure 5.55 Top hat section beam.
Reinforced concrete
topping to plank units
Solid plank
floor unit
Reinforcement
Steel
beam
Plank units bear
on top of beam
Figure 5.56 Prestressed solid plank floor unit.
particularly suited to multi-storey residential flats where the comparatively small imposed
loads on floors facilitate a combination of overall beam depth and floor units to minimise
construction depth. A screed is spread over the floor for lightly loaded floors and roofs,
and a reinforced concrete constructional topping for more heavily loaded floors.
Precast prestressed concrete floor units
These comparatively thin, prestressed solid plank, concrete floor units are designed as
permanent centring (shuttering) for composite action with structural reinforced concrete
topping, as illustrated in Figure 5.56. The units are 400 and 1200 mm wide, 65,
75 or 100 mm thick and up to 9.5 m long for floors and 10 m for roofs. It may be necessary
to provide some temporary propping to the underside of these planks until the
concrete topping has gained sufficient strength. A disadvantage of this construction is
that as the planks are laid on top of the beams so that the floor spans continuously over
beams, there is increase in overall depth of construction from top of floor to underside
of beams.