Seismic Codes, J. V. Tyrrell
CASE S6 -
Unacceptable Seismic Design
Distress in structures; requests for criteria waivers
Collection of Facts: One of the most common mistakes, made in connection with
earthquake design, is the assumption that a structure which can resist code
lateral forces is OK, even though it does not meet the detailed requirements
of the code. These requirements are put in the code on the basis of
experience. Many are necessary to insure ductility. The philosophy of
seismic codes has been to use lateral forces which are considerably less than
the maximum forces that would be associated with the largest earthquake
motions expected to be seen at the site. It is expected that structures so
designed will be undamaged by a moderate earthquake and will not collapse
under the maximum earthquake.
Solution: The detailed provisions of the code are necessary to provide the
ductility to meet performance expectations. Do not attempt to use portions of
a design code selectively. Good details and adequate connections are as
important to earthquake performance as the forces applied as design loads.
Seismic Soft Story J. V. Tyrrell
CASE S7 -
Problem: Earthquake damage.
Damage to columns; discontinuous shear walls; failures.
Collection of Facts: In the late 1950's and 60's, a seismic design concept
was introduced which utilized shear walls except for one story, usually the
first floor, where a moment resisting frame was used. The idea was to absorb
energy by inelastic deformations at this story. In practice, a number of
serious failures have resulted, such as the Olive View hospital which was
damaged in the 1971 San Fernando quake. In other cases, the same sort of
problem has sometimes occurred unintentionally where discontinuities in shear
walls have been used to accommodate architectural purposes.
Solution: It is evident that it is highly desirable to carry shear walls down
to the foundation level.