IBC Seismic question (again)

[JAE]

This is just a throwback to an earlier post (see thread507-101687) where the question came up about the IBC code and its reference in Chapter 35 to the AISC Seismic Provisions.

The AISC initially put out its Seismic Provisions in 1997 and then added a Supplement No. 1 in 1999 and then a Supplement No. 2.

No. 2 really altered the provisions.

But the IBC 2000 reference AISC Seismic Provisions with Supplement No. 1 only (see errata for the IBC on the ICC website).

So the question came up - as an engineer - do we keep to the technical aspect of the code and just use Supplement No. 1? Or do we "realize" that No. 2 is the result of better understandings and a consensus among engineers and it should be followed?

Technically, if a governing agency adopts the IBC 2000, then only Suppl No. 1 applies. What do you think? This just came up again and we are wrestling with it.

 

[FalsePrecision]

Since the Code references other entities such as AISC, AITC, ASCE, etc., I use those entities as the "bottom line". This may not be entirely legally correct, but the IBC has many "holes" in it for many reasons that are another subject entirely.
My 2 cents- use the Supplement #2.

 

[Taro]

Be careful about using Supplement 2 with 2000 IBC. They are not entirely compatible. For example, Supplement 2 removes most of the ductile detailing requirements for Ordinary Concentric Braced Frames. Because of this, OCBF's are now limited to short buildings in high seismic categories. So unless you have a very good understanding of the changes, you should stick to Supplement 1. And if you do decide to use Supplement 2, make sure the building official OKs it.

 

[JAE]

Taro - take a look at MrStohler's comments on the thread link in my first post....down near the middle he seems to make the case that using Supplement No. 2 is better because it is "more up to date" and based on better knowledge. Your point about being careful using incompatible things is well taken.

Once the IBC 2003 takes hold (or the 2006) and is more pervasively adopted, I guess this problem goes away.

 

[FalsePrecision]

I hope this isn't muddying the waters, but my philosophy:

Option #1 - follow the IBC code edition blindly, no matter what.

Option #2 - follow the "root source" - i.e. the AISC.

If I know for a fact that Option #2 will result in a safer, more up-to-date, or generally more sensible design philosophy,
I take that route. In my experience, Plans checkers aren't very aware of the IBC "contradictions", and my opinion is to provide a more "technically correct" analysis.
If the Plans checker DOES have a different opinion, then its time to negotiate at that point.

 

[haynewp]

What I have decided is this: I think what you use should be as conservative as required by the Building Department (Code and supplements). If what the Official requires is less stringent than the latest version of Code or supplement (not adopted by the department) then your engineering judgment to protect the public may require you to use that newer research data. It's a real pain to have to look at it both ways, but I just try to envision myself in court and what questions I might be asked.

 

[uda]

I am looking for a program (in CD) came with IBC 2000 for Seismic Parameters. If any body has that CD please let me know if you are not using. I appreciate that. I will provide my address later.
Thanks,
uda

 

[vmirat]

We have always maintained the philosophy that the more stringent criteria applies. However, the client should be made aware of the situation if it costs more to construct than using the current code criteria. Of course, the client will always want the cheapest option, but you have addressed the issue for the record. If the client does go with the cheaper solution, then I would get a signed statement from them proving that they were informed of the situation.

Code compliance is determined by the jurisdiction in control, not by what we feel is better. For example, the UBC used to allow guardrails to have ladder effect but the National Building Code (BOCA) did not. Does that mean we should have used BOCA for some things and UBC for others? On the other hand, there is no code restriction againt exceeding the code - only economics. If we start deciding what should be used, then where do we stop? Should we be responsible for research under way that might affect the project (i.e. let's stop the design and see what the results are)? What about non-US criteria? What if we used engineering criteria that had not yet been adopted by Regional Building and something was in error?

I just think you can drive yourself crazy trying to decide what is the right thing to do versus meeting code.

 

[FalsePrecision]

vmirat-
Just a difference in opinion. If you blindly follow the Code, as you advocate, what do you do for a wind analysis under the IBC 2000? The wind requirements in that Code are an unwieldy mess. The IBC 2003 partially fixed the problem. The IBC 200 Code wind requirements are derived and "dumbed-down" from ASCE-7. It makes more sense to read and understand ASCE than to wonder about the vague intents of IBC 2000. Using ASCE-7 is perfectly legal.

 

[ChuckerD]

The question that I would ask myself in making this judgment is whether using Supplement #1 would result in providing a design that was deficient in some aspect.
Certainly Supplement #2 offers more "up-to-date" information and thus different design requirements.

Perhaps a fringe comparison, but in Charleston SC they still used SBC 1997 while the rest of the state had adopted IBC 2000 as the governing building code. Obviously quite a change in seismic methodologies had occurred between these two codes (and probably a five-fold increase in design base shear going to IBC). Are the SBC 1997 seismic design parameters going to give me a deficient design? I don't personally have a definitive way to answer that with any certainty. Would I choose to use the IBC 2000 seismic criteria because it more "up-to-date"? It would take a very persuasive argument for me to do so.

 

[vmirat]

ChuckerD,
We have the same situation here. The state adopted the 2000IBC but the county stayed with the 1997 UBC because the 2000IBC had so many corrections. I go back to my previous post where I said that it may be up to the client to decide whether you use the current code or a more up-to-date code which may have more stringent (read "more money") requirements. But then you open yourself up to the insanity of decided which parts of which code costs less to use.

 

[ChuckerD]

vmirat,
I totally agree with you. It is 1000 times easier to justify using the references specifically defined in the governing building code accepted by the reviewing official.

Trying to justify a piece-meal approach using various design specs is going to be much more difficult than proving that older design methodologies create a life safety issue and should not be used.

Until the alarms sound that all buildings designed under a previous specification are DEFICIENT and must immediately be upgraded to meet the "state-of-the-art" provisions and requirements, I think we should not feel the least bit uncomfortable using an older spec.

 

[JAE]

In terms of DEFICIENT (per ChuckerD), there seem to be two conflicting ways to look at it:

1. The updated codes, simply by implication, assume that the earlier codes were "not good enough" (i.e deficient?) and therefore a better code requirement is necessary.

yet....

2. The governing agencies rarely take the position that building owners must upgrade their buildings to meet these new provisions unless 20% or so of the building's value is being renovated.

 

[ChuckerD]

So would we consider ASD steel design to be deficient since the only updated specs published since 1986 address LRFD steel design?

I think sometimes we can get trapped in a "newer-is-better" mindset. Are the new generation of slow-pitch softball bats better because they hit the ball twice as hard as older models? Not if you're the pitcher. Since you can't hear tone of voice on a text document - that last statement is anecdotal but very truthful (I'm a pitcher).

 

[ChuckerD]

Please forgive my misdating of the 1989 Edition of ASD.