Civil Engineering Reference
In-Depth Information
3
Earthquake Input Motion
3.1 General
Earthquake response of structures and their foundations is an outcome of the complex interaction
between the random input ground motion and the continuously changing dynamic characteristics of the
system subjected to the ground motion. Therefore, to arrive at reliable assessment of assets, a complete
understanding of both input motion and structural system, and their interaction, is required. Following
the structurally oriented Chapter 2, in this present chapter, a simple but comprehensive outline of
earthquake strong motion is given. Selection of return periods and probabilities of a certain ground-
motion parameter being exceeded during the lifetime of an asset is discussed. Ground- motion models
(or attenuation relationships) relating the intensity of ground shaking to the distance from the source
are reviewed and their regional characteristics studied. Different commonly used forms of input motion
representations, as outlined in Figure 3.1, and their ranges of applicability are discussed. Both time and
frequency domain representations are addressed. The input characterizations presented are suitable for
the whole range of applications, from simple code design to inelastic response history analysis. The
material presented in this chapter provides the 'demand' side of the earthquake engineering design and
assessment, while the next chapter provides, along with Chapter 2, the ' supply ' or capacity side. Finally,
the strong-motion characterization provided in this chapter maps onto the methods of structural analysis
in Chapter 4 .
3.2 Earthquake Occurrence and Return Period
It is of importance to estimate the frequency of occurrence of earthquakes that are likely to occur in an
area that may infl uence the construction site during the lifetime of the intended facility. Account should
be taken of the uncertainty in the demand imposed by the earthquake, as well as the uncertainty in the
capacity of the constructed facility. Current seismic design approaches deal with uncertainties associated
with structural demand and capacity by utilizing probabilistic analysis (e.g. Cornell
et al.
, 2002 ).
Earthquakes are usually modelled in probabilistic seismic hazard assessment as a Poisson process.
The Poisson model is a continuous time, integer-value counting process with stationary independent
increments. This means that the number of events occurring in an interval of time depends only on the
length of the interval and does not change in time. Recent developments in hazard analysis employ
time-dependent models that account for the occurrence of an earthquake in estimating the probability
of occurrence of subsequent earthquakes (e.g. Lee
et al.
, 2003, among others). In the conventional
approach described therein, the probability of an event occurring in the interval is independent of the
