New Challenges in Seismic Design - Earthquake Engineering for Structural Design - page 17

Figure 1.5 Wind loads versus earthquake loads: (a) Actions; (b) Variation in time;
(c) Maximum recorded values
The seismic design against earthquake loads is based on a completely different
philosophy, due to some very justifiable reasons. First, the design for the largest
credible seismic load, resulting from the strongest expected earthquake in the
structure site, is unreasonable and economically unacceptable. While the maximum
values for static loads can arise frequently, these maximum seismic loads are very
rare during the structure life. Therefore, the design earthquakes are selected at a
given level, which is smaller than the one associated to the largest possible load.
Secondly, the determination of the proper seismic loads is a very difficult task, due
to the complexity of seismic phenomena. Thus, one must accept the fact that, in
some cases, the earthquake actions could exceed the design values. As a
consequence, the structures can occasionally fail to exhibit their expected
performance under these events, which exceed the design values, and,