Simplicity and symmetry

Earthquakes repeatedly demonstrate that the simplest structures have the greatest chance of survival. There are three main reasons for this. First, our ability to understand the overall behaviour of a simple structure is markedly greater than it is for a complex one - for example, torsional effects are particularly hard to predict on an irregular structure. Secondly, our ability to understand simple structural details is considerably greater than it is for complicated ones. Thirdly, simple structures are likely to be more buildable than complex ones.

Symmetry is desirable for much the same reasons. It is worth pointing out that symmetry is important in both directions in plan (Figure 8.5), and helps in elevation as well. Lack of symmetry produces torsional effects which are sometimes difficult to assess, and can be very destructive.

The introduction of deep re-entrant angles into the facades of buildings introduces complexities into the analysis which makes them potentially less reliable than simple forms. Buildings of H-, L-, T- and Y-shape in plan have often been severely damaged in earthquakes, such as the Hanga Roa Building in Vifla del Mar in the 1985 San Antonio, Chile, earthquake. This 1970, 15-storey, Y-shaped reinforced concrete building failed at the junction between one of the wings and central core area. Such plan forms should only be adopted if an appropriate three-dimensional earthquake analysis is used in the design.

An asymmetrical shape that can be readily made to work in strong earthquakes is where there are structural walls on three perimeter facades (i.e. in a U-shape) which is common in shops. The torsional moments in the horizontal plane are resisted by the pair of parallel walls. Many such buildings have been subjected to strong shaking in New Zealand earthquakes without collapse (see Section 13.2).

An asymmetrical structural form not shown in Figure 8.5 is that with structural walls on only two (adjacent) perimeter facades. These buildings are referred to as 'corner buildings' because they are usually built on street corners. This form of asymmetry is

Plans

Don't

Comments Ideal for behaviour and analysis

Good symmetry, analysis less easy

Beware of differential behaviour at opposite ends of long buildings

Bad for asymmetrical effects

Although symmetrical, long wings give behaviour prediction problems

Projecting access towers. Problems with analysis and detailing

Asymmetry of members resisting horizontal shear. Analysis and torsion problems.

Figure 8.5 Simple rules for plan layouts of seismic buildings (only with dynamic analysis and careful detailing should these rules be broken)

to be avoided in high-rise buildings, some of which have collapsed (e.g. in Mexico City in 1985). However, low-rise corner buildings seem not to be especially vulnerable, as all such buildings of up to three storeys have performed well in strong shaking in New Zealand earthquakes (Dowrick and Rhoades, 2000).

External lifts and stairwells provide similar dangers, and should be used with the appropriate attention to analysis and design. In the 1971 San Fernando, California, earthquake external access towers at the Olive View Hospital were not tied into the buildings they were meant to serve, and either collapsed or rotated so far as to be useless.

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