when to use Special Seismic Load from ASCE7-02? 2

[hippo11]

The Special Seismic Load of 9.5.2.7.1:

Em=omegaQE +- 0.2SdsD

is to be used "where indicated" in the code...

My question is, where do you use this Special Seismic Load, Em, besides 9.5.2.6.4.2 (collectors and collector connections)?

Do I use Em anywhere else besides for collectors? I hunted through the code couldn't find anywhere else besides 9.5.2.6.4.2...

Thnaks.

 

[blake989]

depending on your SLFRS, it will kick in on your detailing requirements in a few places. for example, in AISC 341-02 it is sprinkled out for some of the SLFRSs, it is also referenced in the members section for columns, and probably other places I have yet to pick up.

in IBC 2000 a diaphram is also considered a "collector", by the way....if you know a way around this, please let me know.

 

[hippo11]

I have SDC D but Ordinary Concentric Braced Frames because I am under 35'...

 

[blake989]

in the 2002 seismic provisions, you are required to design all parts of OCBFs (with the exceptions noted) for the amplified seismic loads. In the 2005 provisions, the "members" requirements are spelled out a little better, and now it would even include anchor bolts and other stuff.

 

[hippo11]

Hey Blake989, I see the part in AISC 341-02 that says the COLUMNS have to be designed for amplified seismic (sec 8.3 pg 12) but are you saying that the braces themselves (I am using diagonal HSS in my OCBF) have to be designed for amplified seismic as well? I thought the braces themselves simply have to be designed for

rho*QE+_0.2SdsD

Maybe I'm wrong...Can you tell me where I can read about the braces needing to be designed for amplified Em? Thanks for the help.

 

[blake989]

in AISC 341-02 section 14 is for OCBF, part 14.2 discusses required strength of members and connections (with the exception of the brace connection....which is much worse). Here it states "including amplified seismic load".

The amplified load is not too bad, its more of a pain in the neck to track than anything. The problem that I can't get over is the req'd strength of the brace connections. I've been using SCBF to try to get around that req'ment when I have a single story bldg. Multy-story and you have to re-configure your braces or you'll end up with some monster beams due to the 13.4a req'ment.

Post back and let me know what you are able to figure out, I'm always trying to learn how to interpret these provisions.

 

[hippo11]

Blake989, I think I agree with your interpretation...section 14 on 341-02 calls for all members to be designed for the Amplified load, and the Glossary defines that as using omega...

My question is, if all members are to be designed for Amplified Em, when do you ever use regular E (rhoQe+_SdsD) in a steel structure? Never it sounds like.

ETABS uses regular E for OCBF bracing design, by the way, not Em...I emailed CSI about this, it may be a mistake in the program...

 

[hippo11]

The 1997 AISC Seismic Provisions (yellow book) doesn't require Amplified for members in an OCBF system, but the new one (341-02) does...weird.

 

[blake989]

structural steel systems not specifically detailed for seismic resistance.....SDC C or less, this will bail you out of the seismic provisions.

 

[blake989]

you reference the "new one", depending on your building authority, some locations have made the move to the 2006IBC, which incorporates an even "newer one"....341-05

 

[hippo11]

Where does it say that SDC C or less doesn't need seismic provisions? That should've been step 1 for me! I am in SDC D but still that is an important section.

I looked at AISC.org and 341-05 takes it back out again for OCBF, so again it's E and not Em for OCBF.

Thanks.

 

[blake989]

hippo11, I am assuming you are working out of the 2000 IBC, if not, well I'm not the guy to be helping here. If so, IBC 2000 Table 1617.6 is where you pull your seismic design coeffs for your LFRS.....the very last row in the table is what I'm talking about. The first column in the table directs you to the applicable detailing reference for the selected LFRS. HTH

 

[blake989]

OK...in ASCE 02 the table I'm refering to above is 9.5.2.2, last row. I'm not familiar with ASCE seismic yet, so take my advice with caution.

 

[hippo11]

More info on OCBF and Em...Even though 341-05 switches from Em back to E for OCBF, apparently it is accompanied by an R reduction of OCBF in ASCE7-05, so it sort of evens out.

Blake, I am using IBC03 for this OCBF. It too has that row at the bottom of table 1617.6.1 that lists systems not required seismic resistance...it references somtheing called AISC335--what is that?! Never heard of AISC335.

Thanks.

 

[blake989]

it's your steel manual....the specs.

 

[hippo11]

Okay...moving along...so where in the AISC specs does it say what SDC's require 341 and which don't?

Man this is a treasure hunt...thanks for your help by the way.

 

[blake989]

in the 2003 IBC, Table 1617.6.2 (i think) the second column is labeled "detailing reference". there will be a letter or something in that column beside your choice of LFRS, at the end of that table you look and see what that letter (or number) correspondds to....it list different provisions for different types of materials (e.g. AISC 341). I'm sure there is probably something somewhere in ASCE 7, but again I'm not familiar with ASCE 7 seismic.

 

[hippo11]

For example, in IBC 03 it says:

D. Ordinary Concentrically Braced Frames...(14)j

Which points me to AISC 341 sect 14...but I don't understand how you know that SDC A, B, and C are exempt from 341...it seems like IBC is saying that ALL SDC's with OCBF must use sect 14 of 341...no?

 

[blake989]

yes, you are correct...all OCBF (when your LFRS is classified as such) must be designed to the provisions. That is why whenever I am in SDC C or less, I may detail an OCBF......BUT I do not classify it as such on the drawings, or design to such requirements....I state on my drawings this:

Basic Structural System = Building Frame System
Basic Seismic Force Resisting System = Structural steel system not specifically detailed for seismic.

I did not mean to confuse you, but you ask when could you not design to the provisions. Confusing huh??

 

[blake989]

let me add this, when you can select "not specifically detailed......", you must use the corresponding response modification factors for the "non-specific system" not an OCBF or whatever, that is a difference.