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1.7 allowable increase gone? 2003 IBC
- To: <seaint(--nospam--at)seaint.org>
- Subject: 1.7 allowable increase gone? 2003 IBC
- From: "Chad Druffel" <CDruffel(--nospam--at)AHBL.com>
- Date: Fri, 12 Sep 2003 18:35:36 -0700
IBC Section 1618.104.22.168
indicates that an allowable stress increase of 1.7 can be used with the
amplified seismic forces, which is not a change for me...great.
Except, this only applies when you are using the simplified analysis procedure,
which is only allowed to be used for relatively small structures (and I'm
assuming that the simplified analysis is overly conservative anyways, if this
code follows how the 97 UBC worked).
When using the
equivalent lateral force procedure, you are directed to ASCE 7-02, which in
turn only permits the use of a 1.2 allowable increase for the amplified
loads! Is this correct? I thought that it must be a typo so
I searched for an errata to the 7-02, but nothing is out there. This is a
whopping 42% increase in force. Does anyone have input? Looks like
I'm almost being forced to switch to strength design.
A conflict is that the
seismic provisions for steel structures (AISC 341-02) specifically indicate that
the 1.7 increase may be used.
This brings up an
additional point. What has happened to the 1/3 stress increase for steel
design when used with non-amplified seismic or wind loads? The 9th
Addition AISC had it in there, but it was then removed by Supplement #1
(December 17, 2001), and instead put the responsibility on the applicable
building code. IBC 2003 does not reference a 1.33 allowable increase for
steel design as far as I have seen (they in turn put the responsibility back on
AISC), except in the case of the amplified seismic load.
Chad P. Druffel,