Seismic Trivia

[McEngr]

Which code governs in SDC A-C for Steel ASD Design:

A. IBC 2003

B. AISC 341-02

C. ASCE 7-02

D. LRFD 3rd edition

Which code governs in SDC D-F for Steel ASD Design:

A. IBC 2003

B. AISC 341-02

C. ASCE 7-02

D. LRFD 3rd edition

Oops... my options were totally off bump I fixed them as of 1:37 PM EST.

 

[kevo_55]

McEngr-

Hmm, is this a trick question? <_<

I'd have to say is for the most part the same for both questions.

For SDC A-C, you would use IBC 2003 and that would tell you to choose your seismic reinforcing system. If your building weight/Sds/Sd1 are quite low I'd use a "structural steel system not specifically detailed for seismic resistance" and make sure the seismic loads can get a load path through the system in ASD. If they can't, I'd choose an actual seismic system and then no matter what you'd get kicked into AISC 342-02 for the detailing requirements. I believe that you can use part III of 342-02 for ASD, but it's for very similar to the LRFD portion (part I)

As for SDC D and above, you must choose a seismic system that can actually be within that seismic design category. So, you'll still have to be bunped into AISC 342 for detailing requirements.

So, what controls for ASD? I would have to say IBC 2003. IBC would tell you what to do with a seismic system in ASD and also in LRFD.

It took me 6 months to actually get this stuff straight in my head. :thumbs:

 

[McEngr]

McEngr-
Hmm, is this a trick question? <_<

I'd have to say is for the most part the same for both questions.

For SDC A-C, you would use IBC 2003 and that would tell you to choose your seismic reinforcing system. If your building weight/Sds/Sd1 are quite low I'd use a "structural steel system not specifically detailed for seismic resistance" and make sure the seismic loads can get a load path through the system in ASD. If they can't, I'd choose an actual seismic system and then no matter what you'd get kicked into AISC 342-02 for the detailing requirements. I believe that you can use part III of 342-02 for ASD, but it's for very similar to the LRFD portion (part I)

As for SDC D and above, you must choose a seismic system that can actually be within that seismic design category. So, you'll still have to be bunped into AISC 342 for detailing requirements.

So, what controls for ASD? I would have to say IBC 2003. IBC would tell you what to do with a seismic system in ASD and also in LRFD.

It took me 6 months to actually get this stuff straight in my head. :thumbs:

kevo, I agree with you completely! The reason I posed this question is because I sat in an interview with a structural department manager in a Portland, OR consulting firm ( http://www.kpff.com ), and the guy said that he fooled SK Ghosh in a formal seminar on seismic design for structural steel in high seismic driven areas. This guy said he "stumped" Ghosh when he said that IBC 2003 forces the designer to design based on LRFD design. I told him that there's nothing explicitly in the code that says such things. He went on to say that Part III of the AISC 341-02 code requires it even in part III. I, still, don't agree with him. Because I didn't want to detract from the good feel of the interview, I said, "Well, I wasn't aware of that... and neither is anyone else at my company!" I said this, of course, with a smile and he grinned with the Ghosh comment.

Study on my friend!!! :laugh:

 

[McEngr]

kevo,

I should qualify my statement above: He said that SDC D-F requires LRFD for structural steel design. I don't agree. Part III of AISC 341-02 gives different qualifications for each LFRS. Just read on Concentrically Braced Frames and Special Moment frames and you'll see what I mean.

McEngr

 

[kevo_55]

McEngr,

Yeah, you're right. You can use parts I or III. Honestly, I've heard the same thing about LFRD detailing myself and I don't really see it in the codes. <_<

As for studying, I'll be keeping at it. You just be sure to do the same!! :laugh:

 

[rdbse]

kevo,
I should qualify my statement above: He said that SDC D-F requires LRFD for structural steel design. I don't agree. Part III of AISC 341-02 gives different qualifications for each LFRS. Just read on Concentrically Braced Frames and Special Moment frames and you'll see what I mean.

McEngr

I think your guy is somewhat correct, becuase Part III is intended as an overlay of Part I. When you convert allowable strength to nominal strength per Part III, you are essentially switching into LRFD.

 

[McEngr]

I think your guy is somewhat correct, becuase Part III is intended as an overlay of Part I. When you convert allowable strength to nominal strength per Part III, you are essentially switching into LRFD.

This is the most recent debate that we've had at work regarding load combinations.

Currently, it is acceptible to design with ASD, but the phi-factors need to be incorporated in the allowable stress increase. My boss interprets it this way:

The lowest phi outside of bearing on concrete (which is 0.65), is phi=0.75. Therefore, if one is to use the ELF (not the Simplified), then it is permissible to use a 0.75*1.7=1.275 allowable stress increase.

I think that the guy interviewing is correct when it comes to parts not listed in part III, but nearly all low-rise LFRS's are included in part III, which causes me to wonder... Is he talking about dual LFRS's when you have to account for the lowest R-value and check for 30%+100% orthogonal effects???

Either way, I think that it's highly debatable until AISC 13th edition is adopted, but by then, I'm sure all hell will break lose in the SE community out here on the west coast. )

 

[kevo_55]

McEngr,

The AISC 341-05 reads like like the 02 and 99 versions. (But MUCH better for ordinary concentricly braced frames)

Honestly though, I believe that you can use ASD for low or for high rise structures. The required detailing requirements is given by IBC and the detailing changes between parts I and III is just a scaling factor. So, I'm not sure why LRFD is required for high rise.

If you can tell, I'm not a big fan of LRFD. :winko:

 

[engchck4]

"fooled SK Ghosh"

ok- whatever...

 

[Mike]

Just to keep thisdiscussion going ..cause I liked it :2cents:

What do you guys think of Special Moment Frames (SMF) in SDC D and the various requirements of FEMA 350 and 353? I have a project which is giving me real had time. Now, can you guys help me out? For example, by merely stating "refer to FEMA this and that on the plan" can I get out of this...or do I have to be specific and write spec's from the FEMA? This is my first SMF project.

Thanks.

 

[kevo_55]

Mike,

I doubt that you have to go as far as look into FEMA for this type of stuff. Think of FEMA has the theory behind how those "R" factors and the like are chosen. Most likely you only need to dig into AISC's seismic provisions.

The first thing that I must ask is what code are you designing to? 2000 IBC? 2003 IBC? 2001 CBC? 2006 IBC?

 

[McEngr]

Mike,
I doubt that you have to go as far as look into FEMA for this type of stuff. Think of FEMA has the theory behind how those "R" factors and the like are chosen. Most likely you only need to dig into AISC's seismic provisions.

The first thing that I must ask is what code are you designing to? 2000 IBC? 2003 IBC? 2001 CBC? 2006 IBC?

Yeah,

I tend to agree with Kevo on this. FWIW. The biggest thing to watch out for on special moment frames is the "dog-bone" on your beams.

McEngr