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

when to use Special Seismic Load from ASCE7-02?

when to use Special Seismic Load from ASCE7-02?

(OP)
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.

RE: when to use Special Seismic Load from ASCE7-02?

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.

RE: when to use Special Seismic Load from ASCE7-02?

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

RE: when to use Special Seismic Load from ASCE7-02?

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.

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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.

RE: when to use Special Seismic Load from ASCE7-02?

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.

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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...

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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.

RE: when to use Special Seismic Load from ASCE7-02?

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

RE: when to use Special Seismic Load from ASCE7-02?

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

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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.

RE: when to use Special Seismic Load from ASCE7-02?

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

RE: when to use Special Seismic Load from ASCE7-02?

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.

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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.

RE: when to use Special Seismic Load from ASCE7-02?

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

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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.

RE: when to use Special Seismic Load from ASCE7-02?

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.

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
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?

RE: when to use Special Seismic Load from ASCE7-02?

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??

RE: when to use Special Seismic Load from ASCE7-02?

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.  

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
Oh, okay...I think I see...so you define your system as "Structural Steel Systems Not Specifcally Detailed for Seismic Resistance" which is permitted for SDC A, B and C.  Then you use R=3, omega=3 and Cd=3...alright.  Yes?

thanks

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
....and the Detail Reference Standard for Structural Steel Systems Not Specifcally Detailed for Seismic Resistance is simply the AISC regular manual, and you don't have to use 341...?...thanks

RE: when to use Special Seismic Load from ASCE7-02?

that's how I see it.

RE: when to use Special Seismic Load from ASCE7-02?

As I interpret the code, the braces themselves are designed for 1.1 Ry Ag and the connections are designed for the omega loads. The reason being that you want your brace to yield/buckle prior to your connection failure. This also enable energy dissipation under cyclic loading.

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
which code, what yr?

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
So we have E=rho*Qe+-0.2SdsD
           Em=omega*Qe+-0.2SdsD


My next question...say you are in SDC D, with an OCBF structure, where you've designed your members for Em, your column base plate connection for Em, your connections for regular E....let's look at the foundation.

ASCE 9.5.2.7 says you can neglect 0.2SdsD for fdn o'turning.
IBC 1801 says you can take 75% of your quake load for fdn o'turning.

All that being said....now, for the foundation, do you do your strength design for Em or E?  


How about when you check that under service-level loads you are < qa recommended by the geotech, and check that you have no net uplift...E or Em?

Thanks!

RE: when to use Special Seismic Load from ASCE7-02?

as for the foundation, in chapter 8 of AISC 341-02 it discusses this, it will kick you back to you building code for the soil (looks like you've already got that far).

When you say "strength design", I am assuming you're talking about reinforcing the foundation.  IN MY INTERPRETATION all the omega factors and such are there to create a certain amount of pre-determined behavior/ductility in the "structural steel".  With that being said, I ditch the omega for concrete design, but I've always retained Qe for overturning (i've never seen that exclusion in IBC, but I will look for it next time).

RE: when to use Special Seismic Load from ASCE7-02?

(OP)
But since the column and braces are being designed for Em, wouldn't it make sense to design the column base plate connection for Em, and therefore the foundation itself for Em, since Em is the force being transferred to the foundation by the column base plate connection?  What's the point of designing your braces and columns for Em if your foundation will fail in flexure or bearing at Em anyway?

It just seems like you would include Em * 0.75 (IBC 1801) for your foundation reinforcing design, and include 0.7Em for your service-level pressure bearing check < qa.

No?

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