I also appreciate the additional explanation provided by SEAOC
It is clear that consideration of near-field effects drove the
development of 1997 UBC Equation 30-7, 2000 IBC Equation 16-38, and
2000 NEHRP Provisions (Third Ballot Composite) Equation 126.96.36.199-4.
I would suggest that the various code writers generally had the
following definitions of "near field" in mind during code work.
1997 UBC: Sites with Nv greater than 1.0
2000 IBC and 2000 NEHRP: Seismic Design Category E or F (S1>0.75g)
In the 2000 NEHRP Provisions the "near-field" equation applies to
structures in Seismic Design Categories E and F; this indicates
consistency between intent and application. It is not clear to me why
the application of the "near-field" equation is not strictly tied to
the "near-field" condition in the UBC and IBC. In the 1997 UBC, the
"near-field" equation applies not only to "near-field" conditions but
also to vast areas of Seismic Zone 4 that are not "near-field." In
the 2000 IBC, the "near-field" equation applies to structures in
Seismic Design Categories E and F AND (as an afterthought and
departure from the NEHRP Provisions) to structures where S1 is greater
than or equal to 0.6g.
As indicated in the SEAOC Seismology Committee Reply, although there
are concerns about tall structures subjected to near-field pulses, the
equations in question were not crafted with that issue in mind. Again
Let's decide what we've learned; carefully consider, craft, and
coordinate modern provisions; and thus improve the quality, coherence,
and useability of our codes.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
From: Gerard Madden [mailto:GMadden(--nospam--at)mplusl.com]
Sent: Thursday, February 08, 2001 2:24 PM
Subject: Re: SEAOC Seismology Committee Reply
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"
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
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