CHAPTER 2
horizontal distance), the distance from the surface
INTRODUCTION
faulting (this may or may not be the same as the
horizontal distance from the source), the nature of
the geological formations between the source of the
2-1.
General.
earthquake and the building, and the nature of the
soil in the vicinity of the building site (e.g., hard
This chapter provides an introduction to the basic
rock or alluvium).
Although fully accurate
concepts of designing buildings to resist inertia
prediction of ground motion is not possible, the art
forces and related effects caused by earthquakes. An
of ground motion prediction has progressed in recent
earthquake causes vibratory ground motions at the
years to the point that nationally approved design
base of a structure, and the structure actively
criteria have been developed by consensus groups of
responds to these motions.
For the structure
geotechnical and building design professionals.
responding to a moving base there is an equivalent
system: the base is fixed and the structure is acted
b.
Representation of Ground Motion. The
upon by forces (called inertia forces) that cause the
motion at the site can be described by a single
same distortions that occur in the moving-base
system. In design, it is customary to visualize the
single
number,
however,
does
not
give
the
structure as a fixed-base system acted upon by inertia
information on the characteristics (or signature) of
forces. Seismic design involves two distinct steps:
the earthquake.
determining (or estimating) the earthquake forces
that will act on the structure, and designing the
(1) Response spectra. For design purposes, it
structure to provide adequate strength, stiffness, and
would be ideal to forecast the acceleration time
energy dissipation capabilities to withstand these
history of a future earthquake having a given hazard
forces.
of occurrence; however, the complex random nature
of an accelerogram makes it necessary to employ a
more general characterization of ground motion.
2-2.
Ground Motion.
Specifically, the most practical representation is the
earthquake response spectrum.
Although this
a.
General. The response of a given structure
spectrum is used to describe the intensity and
depends on the characteristics of the ground motion;
vibration frequency content of accelerograms, its
therefore, it would be highly desirable to have a
most important advantage is that spectra from
quantitative description of the ground motion that
several records can be normalized, averaged, and
might occur at the site of the building during a major
then scaled according to seismicity to predict future
earthquake. Unfortunately, there is no description
ground motion at a given site.
The physical
that fits all the ground motions that might occur at
definition of an acceleration response spectrum is
any particular site. The characteristics of the ground
shown in Figure 2-1. A set of linear elastic single-
motion are dependent on the magnitude of the
degree-of-freedom
(SDOF)
systems
having
a
earthquake (i.e., the energy released), the distance
common damping ratio, ∃,
from the source of the earthquake (depth, as well as
2-1
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