Suspended Ground Floor Slabs.

For situations where the ground is unsuitable for the support of the ground floor slab or where the slope of the site makes it economic to leave a void below the ground slab, suspended floors are adopted. Where a void is left below the slab, a precast floor system can prove economic and durable if suitable cover to the reinforcement and adequate ventilation to the void is provided. However, in situations where the existing ground can form a suitable temporary support to act as a shutter, then in situ concrete rather than precast is usually the most suitable and economic. In addition, the slab in a suspended situation is often required to restrain ground beams and an in situ solution can usually achieve this without the need for additional tie beams;  the suspended slabs are designed as simply supported or continuous between the lines of ground beams or support walls (see Fig. 9.46).

Suitable concrete cover to the reinforcement for durability must be provided for either solution. When using standard precast components which have been designed for normal internal use then the cover provision must be checked against the exposure condition. Such components often have secondary steel used in manufacture which is too close to the surface. In addition post-tensioned wires often extend to the ends of the unit. These details create few prob- lems when used on internal floors but in the environment of ground floor slabs could prove problematical.

Fig. 9.46 Suspended ground floor slabs.