IBC 2000 - Seismic-Force-Resisting System

[TypeIV]

Has anyone used option #7 for a Seismic-Force-Resisting System (Structural Steel Sytems Not Specificaly Detailed for Seismic Resistance)? I am designing a single story office building and I don't particularly want to mess with seismic detailing.

 

[JAE]

This item (7) is usually used in very low seismic regions. What area is your project in?

 

[TypeIV]

The project is in Oklahoma City with Ss=0.342g and S1=0.085. Soil Site Class = C, which results in Seismic Design Category = B.

 

[JAE]

Well, you'd have to weigh the relative costs of the detailing in seismic, using higher R values, verses the sledgehammer approach with an R=3 (lots higher seismic forces)..

Simply compare the R=3 seismic forces with your wind loads to see if there is a significant difference. If the R=3 loads are comparable to wind - that would probably be the way to go.

 

[MrStohler]

Go with R=3!

Using under IBC Chapter 22, using R=3 allows you to by-pass the AISC Seismic Provisions. Using any greater value for R would mandate that you conform to the AISC Seismic Provisions even though your project is SDC = B.

We have done the comparison for braced frames using R=3 and no seismic detailing or R=5 for OCBF's (if allowed) and R=6 for SCBF.

The member weights to drop with R=5 or R=6 IF seismic forces controled their design. BUT you would be amazed at the increase in connection weight and field work for R=5 or R=6. This is primarily due to the fact that one MUST design for the full tension capacity of the brace (see thread487-100957), unless you do a non-linear pushover analysis.

The only other item to consider is the effect of the increased seismic forces on the foundation system. This might not be an issue in low seismic areas where wind controls design of the lateral force resisting system.

 

[JAE]

MrStohler - I read through the referenced thread and agree that for special moment frames the more complex pushover or inelastic analyses are probably required to determine the "max. force delivered by the system". However, for more simple structures, such as an ordinary braced frame, with a metal deck diaphragm, usually the max. force delivered to the system is either the overturning force on the footings or the failure of the metal deck in shear.

These make for a more simple determination of the max. force delivered vs. a moment frame.

Also, the decision to go with R=3 or a higher R isn't always just a slam dunk decision - it really needs to be compared to the wind forces that otherwise might control. In a 90 mph vs. a 120 mph wind zone, the R=3 vs. R=higher could be affected.

Thanks for the thread reference.

 

[MrStohler]

JAE - Your suggestions are what I initially thought as well until I asked AISC. True the foundations might control the force that can be delivered to the system if a spread footing is used. This does not apply to other foundation systems.

The more complicated analysis not only also applies to moment frames but braced frames as well. Consider the AISC definitions of OCBF and SCBF both require the structure to withstand some inelastic deformations, in a brace this means full yield.

Please refer to the following:

AISC Seismic Provisions 2002 Sections 13 and 14 (including Commentary)
“Design of Special Concentrically Braced Frames”; Structural Steel Education Council Steel Tips May 2004
“Seismic Behavior and Design of Special Concentrically Braced Frames”; AISC Journal 2001 3rd Qtr.
“Seismic Design and Steel Connection Detailing”; 2003 NASCC Proceedings
“A Commentary on Seismic Provisions for Special Concentrically Braced Frames and Special Truss Moment Frames”; 2003 AISC Proceedings


Also TypeIV is in OKC, wind speed = 90 , this matches the work our firm did to compare R=3 vs. R=5 and R=6. It was no contest, EVERY aspect (connection material quantity, weld volume, bolt count, etc.) of the connection cost increased when a higher R value was used.

 

[JAE]

But isn't a roof diaphragm also a part of the "system"? I would think so. Even if the inelastic brace force is much higher, the LEAST amount of load delivered into the brace would be the deck tearing away. It is the weak link in the chain of the load path.

The metal deck does not fail inelastically (as far as I understand it) and thus it would apply...not the inelastic brace force or a force from a higher order analysis that would not consider the deck.

What do you think?

 

[MrStohler]

JAE- The concept of using "the maximum force, indicated by analysis, that can be transferred to the brace by the system" only applies to SCBF, R=6 (OCBF must be designed for yield of the brace).

As unreasonable as it sounds for SDC B in OKC, all of the literature available, including contacts to AISC's Solution Center, is very specific that the phrase which provides the exception in SCBF's requires a very complicated non-linear analysis. It all follows out of the definition of SCBF per Seismic Prov Section 13.1 (see the reference reading from my prevous response)

Our firm has been down this road in another part of the USA that has adopted IBC 2k. Unless SDC is D or worse use R=3. AISC even says so when one attends their seminar "Practical Steel Design".

 

[JAE]

MrStohler,

You refer to the AISC Seismic 2002. This and Supplement No. 2 is not referenced in IBC 2000.

For the IBC 2000, only the 1997 AISC Seismic provisions and the Supplement No. 1 applies. So your statement "the maximum force, indicated by analysis, that can be transferred to the brace by the system" only applies to SCBF" is true for only the 1997 AISC Seismic with Supplement No. 2 applied. Supplement No. 2 may not have been adopted by the specific city or governing authority as most all editions of the IBC 2000 did not include supplement no. 2.

So in some cases, technically, an engineer is faced with the quandry of which parts or editions of the AISC Seismic spec to use - the legally adopted code (which may not include supplement no. 2) or the "latest" specifications which would include supplement no. 2.

This is sort of a nit-picking point in a way, but legally and technically, the provisions you refer to may not apply. So TypeIV would have to check to see what's correct for the jurisidiction of his/her project.

I will check out the AISC website and see if I can find some of your references on this per Supplement No. 2. If Supplement No. 2 applies, then what you describe makes perfect sense.

 

[haynewp]

I don't understand how it allows you to limit the connection to the max force that can be transferred by the system and not the brace yield force. If the brace doesn't yield, then the frame cannot perform inelastically and the R=(higher than 3) doesn't seem justified.

I would think that the max force the system has to deliver would have to be at least equal to the force:

1) Enough to yield the brace
2) Force from R=3

whichever is less.....I am just thinking out loud here

 

[JAE]

haynewp - I have alwyays been a bit befuddled by the "max. force delivered" concept - It seems like it was trying to accomplish this:

[red] The whole building system is made up of a complete load path which results from acceleration of building elements, through the walls, diaphragms, braces, etc. and eventually comes out at the foundation.

Each of these elements along the path have design requirements that are dictated by the code. The braces have especially detailed requirements as they tend to be key to the stability of the structure. So they have special provisions that attempt to allow stable yielding of the brace and thus attempt to absorb the energy without falling down.

The brace, however, may not ever "see" the yield force because it may be that an element on the load path PRIOR to the brace MAY MEET THE CODE PROVISIONS, yet fail prior to developing the yield in the brace members.....thus, you don't have to design the brace member higher than that limiting element's force.[/red]

What this does is allow a weak link somewhere prior to the brace that may or may not be ductile - that always bothered me.

What MrStohler is pointing out, it seems to me, is that the AISC and NEHRP finally caught this issue and responded to it in the latest specs.

 

[haynewp]

I haven't been using the AISC 2002 provisions, I just downloaded it.

Yes, I see the "max force delivered" was removed for OCBF's and in supplement#2 to the 97. I am normally req'd to use IBC2000 so I guess I would fall into the "quandry" as you describe.

But by going with the latest provisions for OCBF, I think I would be going above the 97 requirements w/supplement #1. Therefore I am justified in choosing to do so even if the latest provisions are not referenced by the IBC2000.

 

[MrStohler]

JAE - I disagree with your assertion that Suppliment No. of the 1997 AISC Seismic Provisions does not apply. Altough Supp #2 is not referenced in the IBC 2k, it is clearly a part of the 1997 AISC SP even though it was published in Nov. 2000 (AISC even publishes the 97 SP with Supp #2 incorporated).

Related Question: would you assert that one could take a 1/3 stress increase for ASD even though ASD Supp #1 removed this provision? IBC 2k does not reference this Supp. either.

My point is that it is that we should not be "code lawyers", we should use sound engineering judgement as to considering the effects of code revisions not referenced by the building code. Just because a provision is not specifically excluded or include does not mean that we should neglect it. That is why I reference the 2002 SP, although this code is not referenced by IBC 2k, it can provide insight into the intent of the code authors.

 

[JAE]

MrStohler,
Your disagreement about Supp No. 2 - I understand where you are coming from. We engineers must protect the safety and welfare of the public, codes are minimum standards, and updated specs or codes, whether adopted by a city/state or not should be seriously considered in our design because many times they reflect new knowledge and research.

That said, we as engineers are still somewhat "going out on a limb" when we decide, on our own, to use a different code or standard than what has been specifically adopted by a governing jurisdiction.

For example, in California, the UBC some years ago adopted certain seismic code articles that were questioned by many reasoned engineers. Various cities and counties, responding to these concerns, decided to NOT adopt the code, or revised the code provisions specifically to reflect those concerns. Now, as an engineer, do you use the "updated/new/revised/current" code provisions, or do you respect the "law" that governs the specifically adopted code?

As an engineer, I place on all my plans the specific codes that the design was based. I do this to communicate to the permitting authority what the design is based on so that they can, in good faith, provide the permit to build. Now if I choose to use a more recent code provision and don't show that on my plans, I am essentially practicing a level of fraud. If I do show them on my plans, I am telling the permitting authority that I'm not meeting their specific code requirements....its just a quandry that we face. Just because something is new, doesn't mean its right or more conservative....

Just some thoughts on it.

Your related question about the 1/3 stress increase - ??? don't quite follow why you directed that at me (if you even did)

 

[MrStohler]

Re: the 1/3 stress increase.

IBC 2k 1605.3.2 permits an increase in the allowable stress, if the materials section (Ch 22) or when the referenced standard permits an increase. The AISC-ASD code did permit this until Sup #1 was issued (after the issue of IBC 2k). I contend that one should not use the stress increase any longer since the 1989 ASD manual has been amended to remove this provision, but using the reasoning cited above for excluding AISC SP Supp #2 would allow the stress increase.

My point is that we should recognize that not all codes are coordinated and we need to review the changes that are made in codes issued after the "governing" code. If research were to show that some provision of the current code is lacking we should endeavor to make the appropriate changes in our design even if not specifically required by "governing" code.

 

[haynewp]

Shouldn't the Building Official have input on whether you have to design by a more stringent supplement or revision than the adopted Code?

If I design by the mandated Code, have I done wrong by not hunting down the most recent revisions and supplements to all materials I use for design? Even if none of this research is referenced within the Code that is required per the Official?

 

[MrStohler]

I think TypeIV dropped out of this discussion several posts ago, but to respond to haynewp, who asks:

If I design by the mandated Code, have I done wrong by not hunting down the most recent revisions and supplements to all materials I use for design? Even if none of this research is referenced within the Code that is required per the Official?

Answer: It depends; From a legal viewpoint, one needs to answer whether a supplement can be considered part of the original document referenced by the governing code even if the supplement is issued after the code. My point of view is yes. I have not consulted with attorneys about this. But given that the courts will generally view engineers as experts we should be expected to be current in our field of practice.

Any Building Official out there care to comment? Please, your input is welcome.

Warning wild change of topic coming:

Engineers frequently compare our work to that of other well educated professionals (doctors, lawyers, etc). These professions require the practitioner to be current in their knowledge. If we are to have a valid comparison we must also maintain a similar standard. I am not suggesting that we should have complete knowledge of all research work published in every journal, but most of us tend to focus our work in specific areas. It is not difficult to be aware of new publications in these fields and have a grasp of their meaning. There are several free publications that help engineers keep current as well as professional societies. The internet has vast quantities of free information (be sure to check out the 2005 version of AISC, the 2nd draft version is now out)

 

[haynewp]

I still believe that if you decide to use a supplement that is not referenced in the original "Code" ("Code"=Code plus any State or City revisions), the new design approach has to be at least as conservative as what you would have gotten through using the stipulated "Code" alone. Wouldn't you agree with that?

I think being current with research is great but you still have to design to achieve the minimum as required by the jurisdiction.

 

[MrStohler]

Your Q: "...the new design approach has to be at least as conservative as what you would have gotten through using the stipulated "Code" alone. Wouldn't you agree with that?"

My Ans: Yes, my original response to TypeIV's question did just that.

Point of debate: Whether a supplement can be considered part of the original document referenced by the governing code even if the supplement is issued after the code is issued.

My contention: Yes