Thanks for posting that ....
I think this clarifies what happened in the process but I think the road to coming to an agreement on how to handle this is just beginning. The exemption should have referenced Eq. (30-7) for zone 4 structures and was not printed that way. Subsequently, based on research, it is recommended that the base shear in designing for drift for tall structures should have a minimum value to account for "fling" effects.
Now, I think the questions are:
Q1: Is the base shear from EQ. (30-7) the shear that should be used when designing for drift even when the building framing is designed for the same base shear?
Q2: Should a new equation be introduced that produces a middle ground?
Q3: Should the "ACTUAL" period obtained from Method B or computer analysis be used without the 1.3 Ta cap when designing for drift? This could potential create a design more stringent than that used to design beams & columns with the 1.3 Ta cap.
Q4: What happens in the 2000 IBC and 2003 IBC?
Q5: What should we be doing now in design, follow the printed version or use the erratta?
Feel free to add to the list.
On an outside note, I only recently began working for a firm that designs a lot of tall buildings and probably never would have cared about this issue unless it affected me directly. Perhaps this is what Dennis is seeing regarding his concerns with the impacts the code has on residential.
Call me stupid, but I think that if the provisions in the code caused grief to a committee member's day to day work, it is more likely to be discussed and remedied. Maybe this is why Mr. Hooper pushed the issue with ICBO. I don't know the breakdown of all the varoius committee, but could the lack of response on the points Dennis has made over the course of two-three years be attributed to the lack of committee members practicing on residential structures? On the otherhand, maybe everyone's just too busy to tackle these issues.
Gerard Madden, P.E.
Middlebrook + Louie, Structural Engineers
71 Stevenson Street, Suite 2100
San Francisco, CA 94105