Multi-storey car parks

Precast systems of construction can have benefits in terms of speed of erection and quality control. For savings to be made however, the concrete must usually be capable of being struck from the moulds on a 24-hour cycle. The addition of chlorides was one method of achieving this production rate, although the practice was ceased in the late 1970s.

There are several different configurations of precast:

• Frame clear span: a precast portal frame is erected having a clear span of 16m, with flooring spanning onto each frame in the longitudinal direction. Since the frames are cast as single assemblies they possess their own rigidity in one direction only.
• Hollow core clear span: 400mm depth hollow cored units spanning the width of the car park. The flooring units are relatively heavy. Water entrapment within the voids is possible and so they must be drained to prevent risks of disruption through freezing.
• Double tee clear span: usually 500-600mm depth and able to give a 16m clear span and usually the planks are covered with a concrete topping.
• Beam clear spans: in which the beams (either in-situ or precast) span 16m and support shorter spans (possibly 6m) of flooring.

Thermal movements, particularly in clear span structures, can be severe and must be accommodated in joint design. Sliding joints are often problematic and can lead to weeping problems. Therefore, split columns are desirable, although from an aesthetic point of view they may be less welcome.