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Title: '97 UBC section 2316 comparision to 2000 IBC section 2306

Here's my response to this posted in Oct. 2001. The archives show the full discussion.
**********
The 1/3 stress increase is totally unrelated to the load duration factor for
wood. The 1/3 stress increase is a load combination adjustment factor that
accounts for the reduced probability that two or more loads, other than dead
loads, acting concurrently will each attain its maximum at the same time. such
adjustments are applicable to all materials, but it unfortunately got
incorporated on the resistance side of the equation rather than the loads side.
it has been moved from the resistance side of the equation in ASCE 7 (and IBC)
to the load side of the equation in the form of a 0.75 adjustment (inverse of
1.33). so, you are still permitted the adjustment, but it's now taken on the
loads in accordance with IBC 1605.3.1.1.

The ASCE 7 Commentary section C2.4.3 and NDS Commentary section 2.3.2.3 has more
detailed discussion of this for those interested.

HTH

Buddy Showalter, P.E.
AF&PA/AWC

From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: '97 UBC section 2316 comparision to 2000 IBC section 2306

Chapter 16 of 2000 IBC does not say you can automatically take a 1.33
allowable stress increase for all materials when wind or seismic forces are
used, with the exception of foundation soil pressure per 1616.1.

IBC 1605.3.2 says to see the material's chapter for determing wind/seismic
allowable stress increases for "alternate basic load combinations".  IBC
1605.3.1.1 says that for the "basic load combinations" you can't use
wind/seismic increases except for wood load duration factors.  The idea for
wood in the IBC is to use the load duration factors out of NDS 2.3.2.3.

ACI 530-99 Section 2.1.1.2.3.2 says when you can use an 1.33 increase for
masonry and AISC ASD 89 Section A5.2 says when you can use an 1.33 increase
for steel.

-----Original Message-----
From: utej(--nospam--at)attbi.com [mailto:utej(--nospam--at)attbi.com]
Sent: Thursday, April 11, 2002 10:14 AM
To: seaint(--nospam--at)seaint.org
Subject: '97 UBC section 2316 comparision to 2000 IBC section 2306

The 2000 IBC section 2306 references the NDS for ASD
design and I do not see any deletions or substitutions
similar to the '97 UBC section 2316.2.  The '97 UBC
section 2316.2 states that "When using Section
1612.3.2....the 1/3 increase shall not be used
concurrently with the Load Duration Factor, CD."  It
also revises the NDS table 2.3.2 to use CD=1.33 for
earthquake and 1.6 for wind for members and nailed and
bolted connections exhibiting Mode III or IV behavior.

I could not find these deletions and substitutions in
the 2000 IBC and in IBC section 1605.3.1.1 it states
that "Increases in allowable stresses specified in the
appropriate materials section of this code....shall not
be used with the load combinations of 1605.3.1 except
that a duration of load increase shall be permitted in
accordance with Chapter 23."  There is no statements
that I could find in the 2000 IBC that stated that one
shall not take the 1.33 or 1.6 load duration factor
increase in conjunction with the 1.33 stress increase
such as the '97 UBC section 2316.2 states.

Are these purposeful omissions and modification of
design requirements in the 2000 IBC compared to the 1997
UBC or perhaps an oversight?