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Re: Dynamic base shears, is it considered?

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Tom Chiu wrote:

>The response spectrum base shears are often higher than the Code static
base
>shears. I think you got confused here, Dynamic base shears should be
checked
at the >ultimate stress level with ductility taken into account (i.e. the R
factor), while >the Code static base shears should be checked at the
allowable
stress level.

Tom, I would disagree that the dynamic base shears are necessarily at
ultimate
stress level.  In the code formula, Z and C are intended to represent an
approximation of the response spectrum curve (with Z as the ground
acceleration and C as the period dependent response).  Thus a site specific
response spectrum and dynamic analysis is simply a more accurate
representation of the seismic response, regardless of whether you use
allowable stress design or ultimate stress design.

Also, note that the 1997 UBC seismic formulas represent ultimate stress
levels
rather than allowable stress levels.  The Z-values did not change since Z
represents "actual" ground acceleration, but the R-values were reduced to
magnify the response to ultimate levels.  Thus it is in the choice of
R-values
that determines the stress level being designed at, not whether it is a
dynamic analysis.  However, your response spectrum should be based on a
seismic event with the same recurrence interval as used by UBC (i.e. 10%
probability of being exceeded in 50-years) - unless you have special design
requirements for the project, where a "maximum credible earthquake" may be
determined.

>As far as I know, when I was working for a firm designing highrises here in
LA, it >is standard practice to scale the dynamic results to Code base
shears
and checked >v.s. allowable stresses.  Sometimes we also check the dynamic
results v.s.
>ultimate( yield)capacity level and calculate the D/C ratios to see whether
they are >acceptable. I believe the standard of practice hasn't changed that
much.

Since the code allows scaling down dynamic results, this method is often
used
as project criteria to account for the higher than code base shears.  But if
you have a project where operability is critical after the seismic event,
such
as critical utility structures, it would be more appropriate to use the
site-specific response spectrum as a direct substitute for the code
formulas.
Then R-values and load factors should be applied as applicable to the design
method (allowable stress vs ultimate stress).  (R-values could be reduced
even
more than code-specified values if reduced damage levels are required.)