Stiffness to suit required vibrational characteristics

With regard to vibrational characteristics, we note first that it would be desirable in general to avoid resonance of the structure with the dominant period of the site as indicated by the peak in the response spectrum (Figure 3.3). This is particularly true for flexible longer-period structures, while shorter-period structures with ample structural walls can be made to work on any kind of site.

In the case of sites where the soil is soft and deep enough to amplify the lower frequencies, resonance with longer-period structures may occur, and high frequencies may be largely filtered out. The extreme case of this is in the Lake Bed Zone of Mexico City, where in the powerful 1957 and 1985 earthquakes old low-rise unreinforced masonry buildings were undamaged, although adjacent to heavily damaged modern high-rise buildings. A survey of buildings in central Mexico City conducted by the National University (UNAM, 1985; Scawthorn et al., 1986) after the 1985 earthquake found for example that 1% of one- to five-storey buildings were damaged, compared with 14% of nine- to twelve-storey buildings. The partial inverse of this situation is that taller, more flexible structures are suited to rock sites.

Unfortunately, in terms of conventional construction, often it will not be possible to arrange the structure to benefit in this respect. In industrial installations it may be necessary to have very stiff structures for functional reasons, or to suit the equipment mounted thereon, and this will of course override any preference for seismic performance.

However, if we turn to unconventional techniques, notably the use of base isolation (Section 8.5), it is often possible to greatly modify the horizontal vibrational characteristics of a structure whether it is inherently stiff or flexible above the isolating layer. This not only allows the horizontal seismic responses to be greatly reduced, but does not conflict with some functional needs for high stiffness (e.g. nuclear reactors or containment structures are inherently stiff).

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