2003 IBC Seismic - troubleshooting

[DEL2000]

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?

 

[haynewp]

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?

 

[DEL2000]

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

 

[DEL2000]

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?

 

[haynewp]

It looks like your IBC base shear coefficient was 0.077.

I got 0.086.

 

[DEL2000]

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.

 

[haynewp]

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?

 

[DEL2000]

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?

 

[jhall]

haynewp,

I got the same answer, V=.086W.

 

[haynewp]

I think the 0.077 is right if you limit the value by the period=0.611.

 

[haynewp]

I think this paper will help answer your question

http://www.skghoshassociates.com/sk_publication/Bldg_stds_IBC_Str_Prov_Alterna.PDF

 

[DEL2000]

Thank you . . . that was exactly what I was looking for (although right now I've just glanced through it).

 

[yack]

Hey just a general question on this topic. I'm not a "structural" engineer, but end up doing civil-type structures on a semi-regular basis.

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?

 

[kenzda]

I can’t imagine why they would have told you not to use the USGS website for your values. But I can give you a warning about doing this. I often use the zip code section and recently I had an incorrect result. There was a typo on their part. I emailed them and they agreed and fixed the problem but they advised me to use the latitude and longitude section from now on. When I first started using the website I always verified that the numbers made sense with the map in the code but I had gotten lazy about that. I won’t get lazy again.

 

[cadpe]

If you are providing professional structural design services for a client for a building in Oregon, you should be using 2003 Oregon Structural Specialty Code (OSSC) with Admendments. The OSSC is the 2003 IBC with admendments that have been adopted into law by the State of Oregon Legislature.

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.

 

[JAE]

cadpe - Out here in the midwest, there are most states that adopt a specific code, and then possibly add amendments to it. In most cases within a city, the city itself mandates the code to use and the state code is only utilized if you are building in an area without a city jurisdiction.

Is this not the case in Oregon? Do all Oregon cities simply adopt the Oregon state code? Or does each city have their own?

 

[TFL]

jae,

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.

 

[UcfSE]

Florida has its own state code. Local municipalities make their own amendments to the code to be used in that area and south Florida has it's own complete area of each code chapter.

 

[JAE]

I've done work all over the US and in each case I've called the local city to ask for the applicable code. Each time they responded....I guess whether they (the city) mandates the code or whether the state mandates it, the city still responds with "Are code is ____".

But I never realized that the states, in many cases, set the code to use. Very foreign concept compared to here in the midwest.