Multi-storey buildings may be built of standardised construction with column spaces based on some modular grid (say 6 m × 9 m or 6 m × 6 m). Based on dimensions, multi-storey buildings are erected by means of tower or derrick cranes placed on one or both sides of the frame (Fig. 18.2).
Figure 18.2 Diagram of locating crane for erecting high-rise building
When two cranes are put into use, they are to be arranged such that there is no dead area (i.e., areas not covered by the cranes). Further, another requirement is that the booms or loads they hoist should not interfere with one another. This is achieved by positioning the cranes such that they are spaced more than two radii of reach apart so as to provide a space sufficient for their safe operation. Sometimes the cranes operate in sequence with one lagging behind the other.
When cranes are placed outside the building, erection should be carried out one storey after another. Based on this procedure, the erection of a storey is started only after the completion of all the structures of the preceding storey have been completed. Further, the erection braces are placed so as to ensure longitudinal stability of the building. When cranes are located within the building cross-section, the vertical break between adjacent cells (units) of the framework should not exceed one-tier.
Columns at the ground floor are placed on heads of foundation columns or in foundation pockets. Columns of subsequent storey are mounted using group jigs which are intended for the erection of four or six columns (Fig. 18.3). A group jig consists of a box-type metal structure with collars for securing columns and a wooden working platform for the erectors. Further, a jig carries three collars for each column. The bottom collar is attached to the jig projecting caps of the underlying storey columns. The jig is aligned in the centre lines with the aid of a special frame. Further, it is secured to erection parts of intermediate floors of bracing, using screw jacks and the jig is levelled.
After the jig has been placed and secured to the caps of columns of all underlying storeys, all four new columns are installed, secured and trued by adjacent screws.
Figure 18.3 Single-tier jig for trueing four columns
Using a theodolite, the verticality of the columns are checked. The admissible deviation of longitudinal axes of columns is ±10 mm for columns less than 4.5 m and ±15 mm for columns 4.5–15 m high. Once columns are trued and secured to jigs, their joints are then grouted with concrete (Fig. 18.3).
Crossbars of the ground floor are placed then. Once the crossbar has been positioned correctly, its cast-in fittings are then welded to column brackets. After securing the crossbar throughout the width of the building, reinforcement projections are welded. The cast-in fittings of crossbars and column brackets are permanently welded. The joints are then grouted with concrete.
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