Dave Merrick indicated that there is an exception to this for one and two
story wood frame residences. I assumed he was quoting the code, but he may
have been paraphrasing.
Can you confirm whether or not the exception as he noted appears or is his
comments reflective of a commentary of this section of the code.
From: Cain, William [mailto:bcain(--nospam--at)ebmud.com]
Sent: Friday, May 12, 2000 3:50 PM
Subject: RE: 2000 IBC and Cantilever Column issue
The 2000 IBC reads:
1618.104.22.168 Combination Framing Factor. The response modification
coefficient, R, in the direction under consideration at any story shall not
exceed the lowest response modification coefficient, R, for the
seismic-force-resisting system in the same direction considered above that
story, excluding penthouses. The system overstrength factor, W0 , in the
direction under consideration at any story shall not be less than the
largest value of this factor for the seismic-force-resisting system in the
same direction considered above that story. In structures assigned to
Seismic Design Category D, E or F, if a system with a response modification
coefficient, R, with a value less than 5 is used as part of the
seismic-force-resisting system in any direction of the structure, the lowest
such value shall be used for the entire structure." BTW, R for an Inverted
Pendulum System, Cantilevered column system = 2-1/2 per Table 1617.6."
A structure is designated a Seismic Category D where Sds is greater than or
equal to 0.50g or Sd1 is greater than or equal to 0.20g. Higher spectral
accelerations can raise the designation to Category E and essential
structures can raise the designation to Seismic Design Category F.
Sds is basically 2/3 of the mapped maximum considered earthquake short
period spectral acceleration on a fault adjusted for soil/rock conditions
and Sd1 is the similar value at 1-second period.
I'm guessing you won't like the above. ;<)
Bill Cain, S.E.
From: Dennis S. Wish [SMTP:dennis.wish(--nospam--at)gte.net]
Sent: Friday, May 12, 2000 12:23 PM
To: SEAINT Listservice
Subject: 2000 IBC and Cantilever Column issue
Does anyone know if the 2000 IBC has provided rhetoric that supports
SEAOC's revised position on embedded cantilever columns in wood framed
structures with wood diaphragms? In other words, is it clearly stated that
the lower "R" is recommended in the plane of the braced frame or
cantilevered column only and that the rest of the structure need not be
penalized by reducing the overall R value in the direction of force?
I know that this is best left to the engineers judgment, but I
received a call from a company that was concerned as to how this opinion of
the professional community was incorporated into the next code provisions.
Can anyone comment?
Dennis S. Wish, PE
Structural Engineering Consultant
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