IBC vs. AISC Seismic - revisited

[JAE]

Some time ago I posted a question here about a funny relationship between the IBC 2000 and AISC Seismic provisions. This was in this thread:

thread172-89757

The subject came up again, so I’m reposting the question to see if anyone else has a take on this. It is essentially the same in IBC 2003 and IBC 2006.

Alright - here's the question - In the IBC 2000, there is Chapter 16 that outlines the seismic loads for buildings - these are based on an R value which is based on the type of structure. The end of the Table 1617.6 has a part 7 - "Steel systems no detailed for seismic" - so far so good.

In Chapter 22 (steel) the section 2212.1.1 calls for the use of the AISC Seismic Provisions to be used for SDC A, B or C IF you use any of the R values from Table 1617.6. It then says that IF you use the part 7, "Steel systems no detailed for seismic", you can use R=3 and then NOT use the AISC Seismic Provisions.

I can follow that, too.

Now in the AISC Seismic Provisions, Section 1.0 "Scope" immediately says that, "These Provisions shall apply to buildings that are classified in the Applicable Building Code as Seismic Design Category D and higher..."

So I guess the IBC was written later than the AISC Seismic and inserted the use of SDC A, B and C buildings into it, with the only OUT being the use of R=3.

Do you agree? Thanks for any responses.

 

[prsconsultant]

JAE:

This issue has been widely discussed in AISC website. Anytime you design a building with R>3, irrespective of the Seismic Design Category, you have to use the seismic detailing provisions of AISC. For example, if you use a OCBF system (R=5, IBC 2000) in a building which falls under SDC "A", you still have to use all the seismic detailing requirements.

Based on several studies, it has been proven that it will be economical to use R=3 for SDC A, B & C.

Hope this helps.

 

[JAE]

Thanks prsconsultant. I understand that. I understand the concept that according to the IBC with R>3 you are directed to follow the AISC requirements.

The odd thing I'm bringing up is that the AISC Seismic Design Specification, for three different editions, has continually stated that "this is only for SDC D and greater".

Yet for the IBC 2000, 2003, and 2006 that code still counters the AISC spec by mandating for SDC A, B, and C that you follow it.

Bottom line issue: I just would have thought that the two entities would have come together on this before now to make the logic flow better.

 

[Taro]

The 2005 AISC Seismic Provisions have been revised to be consistent with the IBC. They now state that the Provisions apply for R greater than 3 instead of using the SDC.

 

[JAE]

Taro,
thanks for that...I'll check that wording out tomorrow. I quickly checked on the new Seismic Manual Spec that I just got and guess I brushed over it too fast.

 

[swearingen]

I just went to the seismic seminar in Birmingham and the presenter said that the R for steel OCBF's was specifically set at 3.25, not because of a numerical need, but to force the use of seismic detailing regardless of SDC. This illustrates that AISC is on board.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[Atomic25]

Maybe in the old seismic code it says that, but it isn't the same in IBC2006. 2006 leads you to seismic provisions 341-05. In section 1.0 "Scope" it now says "These provisions shall apply when the seismic response modification coefficient, R, is taken greater than 3, regardless of the seismic design category".

 

[JAE]

OK - so it looks like for IBC 2006 and the AISC Seismic Spec 341-05 they are now consistent.