Introduction
- Seismic forces are a particularly important consideration for engineers
working in the Western U.S. where the frequency of earthquake occurrences
is common.
- Seismic building forces are the result of the sudden movement and
rupturing of crustal plates along fault lines.
- There are more than 160 known active faults in California alone.
- New faults continued to be discovered, usually when an unexpected earthquake occurs.
- When a fault slip occurs suddenly, it generates seismic shock waves that travel through the ground in a manner unlike that of tossing a pebble onto the surface of calm water.
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These seismic waves cause the ground to shake.
- The effect of this dynamic ground motion can be simply modeled using
a cereal box standing upon a piece of sand paper.
- Upon yanking the paper, the box topples in the direction opposite of the yank, as if a pushing force had been applied to the box.
- The heavier the box, the greater the apparent applied force which is called an inertia force.
- As the ground moves suddenly, the building attempts to remain stationary,
generating the inertia induced seismic forces that are approximated
by the static lateral force procedure covered here.
- This procedure is introduced in UBC '97 1629.8.3 and discussed
in detail in UBC '97 1630.
- This procedure is introduced in UBC '97 1629.8.3 and discussed
in detail in UBC '97 1630.
- The static force procedure is limited to use with regular structures less than 240 feet in height.
- And, also to irregular structures £
65 feet or 5 stories in height.
- See UBC '97 1629.8.3 for exact definition of limitations.
- Regular structures are symmetric, without discontinuities in plan
or elevation.
- The building plan is generally rectangular.
- The mass is reasonably uniform throughout the building's height.
- The shear walls line up from story to story.
- Irregular structures include both vertical irregularities (UBC
Table 16-L) or plan irregularities (UBC Table 16-M). These irregular
features include:
- Reentrant corners.
- Large openings in diaphragms.
- Non-uniform distribution of mass or stiffness over building height (e.g. soft story).
- Basic premise of seismic code provisions:
Earthquake Damage to Structure Minor None Moderate Some damage to non-structural elements Major Maybe severe damage, but not collapse.
- Seismic zones in U.S. (UBC '97 Figure No. 16-2):
*MMI = Modified Mercalli Intensity scale of 1933.Zones Damage to Structure MMI* Scale 0 No Damage ----- 1 Minor V, VI 2 Moderate VII 3 Major ³ VII 4 Major -----
