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Chapter 4

Earthquake loading

Andreas J.Kappos

4.1 INTRODUCTION

Earthquakes give rise to dynamic loads that have a high potential for disastrous consequences
for structures, as well as humans. There are different ways in which structures are affected by
earthquakes, the vibration of the ground being the most common, but not the only one. Other
earthquake effects, not specifically addressed in this chapter, are ground failures such as
liquefaction (loss of strength in silt or sand layers due to build-up of pore water pressure),
landslides and mudflows (usually triggered by liquefaction); further effects include sea waves
(tsunamis) and lake waves (seiches). By far, most of the damage due to earthquakes is caused
by the ground motion, but other effects can also be quite devastating, as shown, for instance,
by the July 1998 tsunami that hit the coast of Papua—New Guinea, causing over 2,000 deaths
and complete destruction of the villages near the coast.
In the remainder of this chapter, following a brief description of the earthquake
phenomenon and the methods of assessing seismic hazard, the focus will be on the different
ways the seismic actions (loads) can be defined in a design project, which strongly depend on
the type of analysis chosen, and range from simple sets of horizontal forces to response
spectra (deterministic or probabilistic) or acceleration time histories. The chapter will
conclude with a brief discussion of the principles governing the design of structures to resist
earthquakes, touching on issues beyond the seismic loading itself (structural configuration,
hierarchy of member strength, systems for response control).

4.2 EARTHQUAKES AND SEISMIC HAZARD

4.2.1 Generation of earthquakes

Earthquakes are generated wherever the accumulation of strain at geological faults
(discontinuities of the rock) leads to their rupture and to slip along the fault, until a new stable
state is reached. Fault rupture gives rise to waves propagating in all