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RE: Questions on UBC-97 and ASCE-95 Seismic Codes

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Hi Samuel,

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 Ca coeff. is for short period of vibration part of the spectrum (flat area of constant acceleration).  Could that 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?

Regards,


Sasha Itsekson, P.E.

 ----------
From: 	Samuel K Kassegne, Ph.D.[:ramusa(--nospam--at)ramanalysis.com]
Sent: 	Thursday, November 06, 1997 5:55 AM
To: 	seaoc
Cc: 	ITSEKSON SASHA
Subject: 	Questions on UBC-97 and ASCE-95 Seismic Codes

Hi:

This is a question regarding the recently released UBC-97 and ASCE-95 codes
for seismic loads. Both codes use what are called Ca and Cv seismic
coefficients which basicaly depend on the soil type and the zone. The
values are given in Table 16-Q (page 2-34) of UBC-97 and Tables 9.1.4.2.4A
and 9.1.4.2.4B (page 55) of ASCE-95.

We have noticed that, in these tables, for soil type SE, the Ca factor
drops to a value of 0.36Na compared to a Ca value of 0.44Na for soil type
SD. If one looks closely at the table, it is easy to notice that as the
soil gets worse, Ca increases in almost all the cases as expected except
for soil type SE in zone 4. The Ca coefficient directly affects the maximum
and minimum bound on the base shear these codes subscribe. For,
particularly, drift calculations and certain products of Rw*Period, these
bounds will govern.

We had suspected that there is some discrepancy in this and had approached
the ICBO folks for some advice. Their initial response is that they feel
the numbers are right. But they have promised to get back to us with their
final findings.

In the mean time, though, we run some numbers to see how this could
potentially affect the base shears and other response parameters in
building analysis and design. Here is what we found out.

Input Data:
A 4 story hypothetical building.
Period = 0.62 seconds.
Drift calcs.
Weight of building = 3725 Kips
Rw = 4.0
Height of building = 55.84 ft
Zone 4
Importance Factor = 1.0
Soil types - SA, SB, SC, SD and SE considered in separate runs.
code used - UBC-97


Summary of Results
========================================================
Soil Type ----- Base Shear ------Drift at center of mass
========================================================
SA.............480.50 kips.........4.90 inches
SB.............600.65 kips.........6.12 inches
SC.............840.90 kips.........8.57 inches
SD.............961.03 kips.........9.80 inches
SE.............837.90 kips.........8.54 inches


Clearly, for soil type SE, lower base shears are given. This is very
suspicious, to say the least. Not only the base shear for soil type SE is
less than that of SD but also SC. Furthermore, it is also possible that it
could be lower than base shears corresponding to Soil type SB also.  Even
though, we didn't run the numbers for ASCE-95 seismic code, similar results
are expected.

Has anyone else encountered the same problem? Any thoughts why the base
shears for Soil type SE could be lower than that of SD, SC and SB?  Is it
possible that this is indeed a mistake and needs correction before people
use it widely.

with best wishes,

Samuel K Kassegne, Ph.D.
Ram Analysis,
Carlsbad, CA