Page 149 - Dynamic Loading and Design of Structures

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probability of exceedance. By repeating the procedure for an appropriate number of Ai a
complete hazard curve as shown in Figure 4.6(middle) can be derived. The actual procedure is
somewhat more complicated as the scatter in the attenuation relationship is also included in
the analysis.
Results from such procedures are used to construct the hazard maps used as a basis for
seismic codes. An example of such a map is shown in Figure 4.7; it provides the contours of
the effective peak acceleration coefficient Aa for the United States (FEMA, 1995). This map
was derived from similar maps showing the PGA’s with a 10 per cent probability of
exceedance in 50 years, after converting PGA to effective peak acceleration using procedures
based in part on scientific knowledge and in part on judgement and compromise. For the
purpose of defining design seismic actions, hazard maps such as that of Figure 4.7 are further
simplified to include a limited number of seismic zones within which the value of Aa is
considered as constant.
Response spectra for a target annual probability of exceedance P (e.g. 0.2 per cent) can be
T
constructed by calculating the corresponding A=a' from the curve of Figure 4.6 (middle) and
then anchor a fixed spectral shape to a', as shown in Figure 4.6 (bottom), and further
discussed in Section 4.3.2. Alternatively, a more complex procedure may be followed,
whereby the attenuation relationships are developed for spectral ordinates (e.g. the spectral
acceleration S ), rather than for PGA. These period dependent attenuation relationships are
pa
then used to construct the design spectrum period by period; this is called a hazard consistent
or uniform hazard spectrum (EERI, 1989; Reiter, 1991).

4.3 DESIGN SEISMIC ACTIONS AND DETERMINATION OF ACTION
EFFECTS

4.3.1 Design situations
The design seismic action or the design earthquake is a ground motion or a set of ground
motions defined in a way appropriate for the design of engineering structures. Depending on
the type and importance of the structure to be designed, the seismic action can be defined in
different ways, i.e. as:
●a set of (equivalent) lateral forces;
●a response spectrum;
●a power spectrum;
●a set of acceleration time histories.

The foregoing can be defined either on the basis of a seismic code (most common case), or by
carrying out a site specific seismic hazard analysis with due consideration of ground effects
(see Sections 4.2.3–4.2.5). The scope of each procedure can be appreciated by considering the
following four situations that might be faced by an engineer in practical design: