Abstract

Precast columns are the main vertical-load-carrying members in skeletal frames. They may also be used as the horizontal-load-carrying members in sway frames, but their capacity to achieve this in buildings of more than three storeys is very limited. Typically a precast column is manufactured in grey concrete and is square or rectangular in cross sections. However, there are many instances where columns of other shapes are used as part of the external architecture, as shown in Figures 6.1 through 6.3. The 12 m high circular columns in Figure 6.1 were manufactured horizontally in circular moulds; sometimes they are cast vertically in storey-height pieces. Generally, columns are manufactured in the largest length possible to erect on site. In Europe this is typically 12–18 m, but in the United States lengths of 25–30 m are possible. If a column is joined on site, the connection is called a ‘splice’. Figure 6.4 shows two-storey precast columns in a 22-storey frame in Leicester, UK. The splice connections here are made at the lower floor level using projecting threaded bolts into the so-called steel shoes (see also Section 10.4.4). At the intermediate level, a gap is left to form an in situ stitch to either beams or slabs, typically 200 mm deep, allowing tie steel to cross between the beams or slabs. Figure 6.5 shows an option for using projecting rebars in in situ grouted sleeves. The 20 m high columns in Figure 6.6 weigh about seven tonnes, which is acceptable providing the crane is being fully utilised at this lifting capacity, but often it is not. It therefore makes sense to reduce weight commensurate with the weights of other components, beams and slabs each weighing four to five tonnes.