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Re: Dynamic base shears, is it considered?

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Tom Chiu wrote:

>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.

Tom, I agree that the code allows ductility to reduce loads to design
levels.
My point is that you can do this by applying Rw factors to the dynamic
analysis as well as to a static analysis.  The type of analysis does not
dictate whether you should use allowable stress or ultimate stress - the
type
of ductility factors and load factors you use do matter.  Whether to go
beyond
the code minimum requirements is a judgement matter which depends on the
type
of project and should involve input from the owner.  For example, on
critical
utility projects (electric power plants, water treatment plants, etc), the
owner often wants the facility to remain operational after a given seismic
event (e.g. with a given recurrence interval).

>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.

This situation has been reversed in the 1997 UBC - since the seismic loads
are
now developed for ultimate strength, a load factor of 1.0 is used for
seismic
loads when using load factor design, per Section 1612; but for allowable
stress design, the seismic loads are divided by 1.4 per Section 1612.3.