Building Design and Construction Handbook - Merritt - Ventech!

3.12 SECTION THREE
directions, both through the earth’s crust and along the surface, and at different
velocities. Consequently, the shock waves arrive at distant points at different times.
As a result, the first sign of the advent of an earthquake at a distant point is
likely to be faint surface vibration of short duration as the first longitudinal waves
arrive at the point. Then, severe shocks of longer duration occur there, as other
waves arrive.
Movement at any point of the earth’s surface during a temblor may be recorded
with seismographs and plotted as seismograms, which show the variation with time
of displacements. Seismograms of past earthquakes indicate that seismic wave
forms are very complex.
Ground accelerations are also very important, because they are related to the
inertial forces that act on building components during an earthquake. Accelerations
are recorded in accelerograms, which are a plot of the variation with time of components
of the ground accelerations. Newton’s law relates acceleration to force:
W
F � Ma � a (3.2)
g
where F � force, lb
M � mass accelerated
a � acceleration of the mass, ft/s 2
W � weight of building component accelerated, lb
g � acceleration due to gravity � 32.2 ft/s 2
3.3.2 Seismic Scales
For study of the behavior of buildings in past earthquakes and application of the
information collected to contemporary aseismic design, it is useful to have some
quantitative means for comparing earthquake severity. Two scales, the Modified
Mercalli and the Richter, are commonly used in the United States.
The Modified Mercalli scale compares earthquake intensity by assigning values
to human perceptions of the severity of oscillations and extent of damage to buildings.
The scale has 12 divisions. The severer the reported oscillations and damage,
the higher is the number assigned to the earthquake intensity (Table 3.1).
The Richter scale assigns numbers M to earthquake intensity in accordance with
the amount of energy released, as measured by the maximum amplitude of ground
motion:
100
M � log A � 1.73 log (3.3)
D
where M � earthquake magnitude 100 km from epicenter
A � maximum amplitude of ground motion, micrometers
D � distance, km, from epicenter to point where A is measured
The larger the ground displacement at a given location, the higher the value of the
number assigned on the Richter scale. A Richter magnitude of 8 corresponds approximately
to a Modified Mercalli intensity of XI, and for smaller intensities,
Richter scale digits are about one unit less than corresponding Mercalli Roman
numerals.