Environmental Engineering Reference
In-Depth Information
Vertical
Yawing
Pitching
Rocking
FIGURE 11.42
Six modes, or degrees of freedom,
of foundation vibration.
Translation modes: vertical,
longitudinal, lateral. Rotational
modes: rocking, pitching, yawing.
Lateral
Longitudinal
Conventional or Simple Approach
The conventional or simple approach considers only values for acceleration g, which are
obtained from provisions in the International Building Code (IBC) or local building codes.
(In 2000, the Uniform Building Code [UBC], the Building Officials Code Administrators
[BOCA], and the Standard Building Code [SBC] were combined to become the
International Building Code [IBC]). These codes often refer to seismic probability maps
such as
Figure 11.14,
or simply give values for acceleration in terms of
g
for various loca-
tions. This approach, at times referred to as the
seismic coefficient method
, does not take into
account all of the significant dynamic properties of either structures or earthquakes.
Comprehensive Approach
All ground response factors (see
Section 11.4.3
)
are considered in the comprehensive
approach. The design earthquake (see
Section 11.4.6)
is considered in terms of peak accel-
eration, frequency content, and duration by statistical analysis of recorded events; infor-
mation obtained by strong motion seismographs; and geologic conditions. In terms of the
structure, the design spectrum, damping, and allowable design stresses are specified. This
approach is becoming standard practice for all high-risk structures such as nuclear power
plants, 50-story buildings, large dams, long suspension bridges, and offshore drilling plat-
forms, regardless of location.
11.4.3
Site Ground-Response Factors
Design Bases
Seismic design criteria are based on ground-motion characteristics, including acceleration,
frequency content, and shaking duration, which are normally given for excitation of rock
or strong soils. Soil conditions have an effect on these values.
Maximum Acceleration
Peak horizontal acceleration
(PGHA) is considered to be closely related to the lateral forces
imposed on a structure, and is the value normally used for approximating earthquake
forces.
Peak vertical acceleration
(PGVA) has been generally accepted as about one half of the
mean horizontal acceleration, but close to dip-slip faulting the fraction may be substan-
tially higher. In Managua (1972), for example, recorded near the epicenter were values of
