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Re: Dynamic base shears, is it considered?

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Bill Sherman wrote:
> 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.
Bill, We all know that response spectrum analysis is a more rational way
of analysis. The reason why dynamic base shears( typical Upper level
earthquake, 475 years event) are higher than Code static base shears
because Code allows the structure to undergo inelastic deformation, i.e.
ductility of the system.  If you calculate the dynamic(475-year event)
demand to capacity( ultimate) ratios of the members, they will usually
be more than 1.0 or commonly known as IDR.

> 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.
Code static forces have to be factored (e.g. 1.4 E for concrete etc.) to
be checked against ultimate capacity level, whereas for wood, masonry
etc. the Code static forces are checked against allowable stress.

> >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.)
Most of the highrises were steel structures and were checked v.s.
allowable stresses at Code static force level. Whereas, for concrete
structure (the tallest one that I designed back in 1983, was 30 story
tall in downtown LA), Load factors were applied and checked v.s.
ultimate capacities.  We also performed site-specific response spectrum
analysis and calculated ductility ratios of all the members to ensure
that inelastic deformations will not be excessive.
I agree that R-values can be reduced for important structure such as
hospital or school buildings. As far as I know, it's rarely done for
highrises in LA. We have cheap developers here as compared to other
parts of the country.

Tom Chiu, SE
Thomas Engineering