Earthquakes - Geotechnical Engineering Investigation - page 965

α h(peak) 0.28 g . Hall and Newmark (1977) recommended taking design

motions in the vertical direction as two thirds those in the horizontal direction. These rela-
tionships generally are applicable where recorded strong motion data are not available.
“Effective” peak acceleration (a fraction of the peak) is often selected for design because
very high peaks are frequently of short duration and have little effect on a structure. Low-
magnitude events, of the order of M

0.35 g and α v(peak)

4.5, can have peaks of 0.6 g , but these large acceler-
ations usually occur only as two or three high-frequency peaks, which are probably S
wave arrivals, and carry little energy.
Selection of values is described in Section 11.4.6.

Frequency Content
Significance
Maximum accelerations occur when the ground-motion period (frequency) approaches or
equals the period of the structure and resonance occurs. Ground-motion amplitude
decreases with distance by geometrical spreading and frictional dissipation (attenuation).
The high frequencies (shorter periods) are close to the source; at distances of the order of
100 km it is the longer vibrational periods of the Rayleigh waves (1-3 s) that cause ground
shaking.
Building periods are usually in the low to intermediate frequency ranges, and buildings
are therefore subject to resonance from the long-period waves. Tall buildings are caused to
sway, and, when in close proximity, to beat against each other as in Los Angeles during the
Kern County event of 1952, where the focus was 125 km distant, and in Mexico City in
1957, where the focus was 300 km distant. In both cases, old and weak, but smaller struc-
tures, did not suffer damage.

Design Approach (Hudson, 1972)
In high-frequency systems ( 5 Hz), the relationship between horizontal ground accelera-

tions and lateral forces on a structure governs design.
Intermediate (

0.2 Hz, long periods) systems include most
buildings and engineering works. Ground accelerations alone are not considered a good
approximation of the actual lateral forces. In addition to peak ground accelerations, the
maximum ground velocity for intermediate frequencies and the maximum ground dis-
placement for low frequencies should be specified.

≈ 1 Hz) and low frequency (

Duration
Duration, a measure of the number of cycles, is associated with fatigue in structures and
has a major effect on the amount and degree of damage.
Length of faulting is considered to have a strong effect.
Bracketed durations (Section 11.2.7), prepared from strong ground-motion records, are
often specified for design (see Section 11.4.6) .

Comparisons of Acceleration, Frequency, and Duration

Parkfield (1966, M 5.5): High peak α (0.5 g ) but high frequency and very short

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duration caused little damage.

Mexico City (1957, M (0.01 to 0.1 g ) had lower frequencies

and a longer duration, causing complete collapse of multistory buildings in a
geologic basin with weak soils.

7.5): Lower peak α

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