huangyhg

2003 ibc seismic - troubleshooting
i had designed a building in oregon in 1997 ubc, only to have it put on hold. we will try to maintain our original design calculations, but if we have to go to 2003 ibc to submit it, i was just looking at the updated seismic base shear. what i found was the base shear was 30 to 45% lower. this didn't seem quite right, so i just wanted to check one thing.
we took a seminar from the s.k. ghosh associates, and in my notes i had written not to use the usgs website to get ss and s1. nobody else had this written down, and i don't remember why they said not to use it. our office doesn't have the cd anymore (ex-employee took it with him), so i was using the ss and s1 from the usgs website, which gave these somewhat suspect results.
the only other thing that i can think of is that soil profile d in ubc becomes soil profile e in ibc. is this perhaps the case?
check out our whitepaper library.
did you look at the ibc 2003 contour map to check the values you got off the website? the map references usgs.
what are your ibc 2003 ss, s1, seismic use group, and seismic force system parameters?
and what base shear did you get for ibc 2003?
ss = 0.9081
s1 = .3261
site class d
sug = i
ie = 1.0
sdc = d
fa = 1.137 (interpolated)
fv = 1.748
sds = 0.689 (versus 0.90 = 2.5*ca)
sd1 = 0.38 (versus 0.54 = cv (zone 3, sd soil profile)
period in one direction was 0.611 seconds
r = 8 in ibc, 7.5 in ubc
in looking at the map for western oregon, these values for ss and s1 seem to be approximately correct.
seismic dead load was 6777 kips.
my original base shear was 798 kips, which lowered down to 527 kips in 2003 ibc.
i guess my question then becomes does everybody else feel comfortable lowering their base shears down by this substantial of an amount?
it looks like your ibc base shear coefficient was 0.077.
i got 0.086.
when you say you got 0.086, how did you get something different? were you using the cd-rom? or did i make a mistake somewhere, because i did have the 0.077 value.
i just used the same values you posted above in a spreadsheet i use for ibc. i haven't checked the usgs data myself.
i got the same fa, fv, sds, and sd1 as you did, but i think cs would be controlled by sds/(r/i)=0.689/(8/1)=0.086.
correct?
i see what you are talking about.
in asce 7-02, it says cs need not be taken greater than equation 9.5.5.2.1-2 (page 146), which is the equation based on sd1. so that seems to be the maximum value for the base shear, 0.077.
thanks a lot for running those numbers for me.
i would still welcome any thoughts from people on this substantial change in design loads . . . does the fact that you now have to include the vertical component of the seismic force (in asd) take away from this load decrease?
haynewp,
i got the same answer, v=.086w.
i think the 0.077 is right if you limit the value by the period=0.611.
i think this paper will help answer your question

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