Page 151 - Dynamic Loading and Design of Structures

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●For many building structures, and also for some ‘small-scale’ civil engineering structures
(such as small bridges, viaducts, etc., and typical geotechnical structures such as retaining
walls), the equivalent lateral force procedure can be used. The procedure is well
documented in most current seismic codes, and will be described in Section 4.3.5 with
specific reference to two major codes, the 1995 Eurocode 8 (EC8) and the 1997 UBC
(American code).
●For buildings with configuration problems (irregular plan and/or elevation), for many types
of medium bridges, and for many of the structures falling beyond the scope of this chapter,
an elastic dynamic analysis has to be carried out, typically in the form of modal response
spectrum analysis. The definition of the elastic spectrum (Section 4.3.2), its modifications
due to site effects (Section 4.3.3), and its reduction to an inelastic design spectrum (Section
4.3.4), are some of the most important issues relating to seismic loading. Specific mention
will be made in the aforementioned sections to the EC8 and the UBC spectra. In
exceptional situations where a probabilistic approach is warranted, power spectra (Section
4.3.8) may be used instead of ‘normal’ response spectra.
●n cases such as the design of very important structures, or structures clearly falling outside
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the limits of the existing codes (e.g. structures with very high fundamental natural periods),
a full time history analysis, typically in the inelastic range may be required. Note that there
is no advantage in using this procedure for an elastic analysis of the structure which can be
conveniently carried out (at essentially the same accuracy) using the modal superposition
approach, the exception being structures where due to highly irregular geometry it is
difficult to combine the modal contributions, or whenever the structural model includes
critical frequency dependent parameters (Clough and Penzien, 1993). An appropriate
selection and scaling of natural and/or artificial records has then to be made; a key point to
be addressed is the correspondence between these records and the (code) design spectrum.
The EC8 and the UBC recommendations will be referred to in Section 4.3.7 and it will be
made clear that this type of procedure is more common in the case of assessment of
existing structures which might not comply with current code requirements.
●Again for some exceptional cases, such as important structures whose construction cost is
particularly high and/or the consequences of their failure particularly severe (a typical
example being nuclear power plants), as well as in the case of construction in areas where a
design spectrum or a code is not available, a site specific seismic hazard assessment study
has to be made, typically using probabilistic techniques. Although normally the civil
engineer will not carry out such a study, it is important that s/he realizes the main
assumptions involved, and, more significantly, is capable of appropriately evaluating the
results of such a study and making use of them for design purposes. A brief coverage of
this procedure has already been given in the previous section (4.2.5).