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Re: Questions on UBC-97 Seismic code

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										November 13, 1997
Hi:

Thanks Sasha, Bill, Stan and Chang for taking the time to address the
question posed last week.

Sasha said:

"I just attended the SEAONC seminar on UBC'97. I have to say that I also
share your confusion. Unfortunately, your question has not been addressed
there.  One thing that I've noticed is that the Ca coeff. is for short
period of vibration part of the spectrum (flat area of constant
acceleration).  Could it be that the soft soils are not affecting stiff
buildings as much? But then again it should be the same for all seismic
zones, but it is not?

Any other opinions? "

Exactly....look at the Cv coefficient which controls the base shear for
less stiff buildings (relatively high period). You will notice that this
coefficient (also referred to as the "velocity-related acceleration")
indeed increasers with soil type. It assumes a value of 0.96 for Soil SE
and 0.64 for Soil SD, 0.32 for soil SA. 

What this means is that for less stiff buildings, the design base shear
under SE could be 1.5 times that of Soil SD; However, for stiffer
buildings, the design base shear could be as low as 80% of that of Sd soil
condition.

Again, this leaves one with the question:

* Why does the non-linear effect reduce only Ca and not Cv, especially,
since according to the sources quoted, soil loses its strength under high
seismicity?

Could some one attending the upcoming SEAONC seminar ask the presenters
this question, please? 

regards,

Samuel Kassegne, Ph.D.