2003 IBC Seismic - troubleshooting
2003 IBC Seismic - troubleshooting
(OP)
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?
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?






RE: 2003 IBC Seismic - troubleshooting
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?
RE: 2003 IBC Seismic - troubleshooting
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
RE: 2003 IBC Seismic - troubleshooting
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?
RE: 2003 IBC Seismic - troubleshooting
I got 0.086.
RE: 2003 IBC Seismic - troubleshooting
RE: 2003 IBC Seismic - troubleshooting
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?
RE: 2003 IBC Seismic - troubleshooting
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?
RE: 2003 IBC Seismic - troubleshooting
I got the same answer, V=.086W.
RE: 2003 IBC Seismic - troubleshooting
RE: 2003 IBC Seismic - troubleshooting
http://www.skghoshassociates.com/sk_publication/Bl...
RE: 2003 IBC Seismic - troubleshooting
RE: 2003 IBC Seismic - troubleshooting
The soils report always gives me the seismic design values. The one time I did use my own value, based on USGS because UBC was inappropriate for the structure, the reviewer sent it back and said to get approval from the soils engineer?
So, I'm just a little confounded about the above discussion. Is it different for buildings? Or maybe other states/counties are more lenient?
RE: 2003 IBC Seismic - troubleshooting
RE: 2003 IBC Seismic - troubleshooting
The admendments are easily reconizable by their being yellow (canary) in color. Since I am not at my workstation and not near my code, I can not give you the exact reference to the Figures in the code for the Ss and S1 values. But if you have a 2003 IBC you can get the amendments and replace the 2003 IBC pages. The International Code Council (ICC) should have the Oregon Admendments for sale.
If you are using an IBC or other design source without the Oregon Amendments, you may be in violation of Oregon law. Be careful as other states have admendments also, such as Washington and California.
RE: 2003 IBC Seismic - troubleshooting
Is this not the case in Oregon? Do all Oregon cities simply adopt the Oregon state code? Or does each city have their own?
RE: 2003 IBC Seismic - troubleshooting
Here in the northeast alls the states have state codes and state ammendments. there are no local codes. the city are the enforcement arm, they do not set the law though. in fact in my state you can appeal any ruling oof the local building official to the state code if need be. i beleive oregon is the same i was surpised that you still have local codes.
RE: 2003 IBC Seismic - troubleshooting
RE: 2003 IBC Seismic - troubleshooting
But I never realized that the states, in many cases, set the code to use. Very foreign concept compared to here in the midwest.