Setbacks

A setback is where a plan dimension of a storey above a certain level in a multi-storey building reduces (Fig. 9.21). Seismic codes categorize buildings with abrupt setbacks as irregular. Sophisticated structural analyses quantify the ' notch effect' of a setback, but even though structural engineers avoid notches wherever possible because of stress concentrations, setbacks can be designed satisfactorily. The need for 3D modelling of setback buildings can be appreciated from Fig. 9.21(b) ' Although the irregular vertical configuration in the x direction can be designed for, y direction shaking induces torsion due to the way the positions of the CoM and CoR change at every setback.

The podium or plaza and tower form represents a rather severe setback configuration (Fig. 9.21 (a)). Designers are faced with several choices. They can treat the building as one structure. In this case, the podium roof is probably designed as a transfer diaphragm to force the podium framing to contribute to the horizontal force resistance at the bottom storey of the building (Fig. 9.22(a)). Alternatively, designers can provide

Buildings on sloping sites foundations

▲ 9.23 Structural configurations for moment frames on sloping sites.

foundations

▲ 9.23 Structural configurations for moment frames on sloping sites.

the podium with little if any horizontal resistance and tie it strongly to the primary structure of the main tower, which then resists the seismic force of the entire building. Finally, the podium can be seismically separated from the tower. The tower then becomes a regular structure with more predictable seismic performance. Seismic separation joint treatment is discussed in the Chapter 8. A structurally independent podium must be seismically self-reliant so it requires its own seismic structure. The final choice should be made only after discussion between architect and structural engineer. Certainly a situation where the tower is off-set significantly in plan from the podium would encourage separation to avoid the additional complexity of torsion.

Building on a sloping site has already been raised with reference to avoiding short columns (Fig. 9.8(b)) . The structural options for designers are now summarized in Fig. 9.23. In the first two options the ground is either benched and retained, or a strong and stiff built-up platform is formed by a walled retaining structure. Alternatively, the solution of Fig. 9.23(c) can be adopted to provide regular building configuration.

Buildings on sloping sites

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