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The Mysterious 1/3 Stress Increase REREREDUX (was RE: 2000 IBC v 1999 UBC code question)

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I don't think you can make this statement definitively. Duane Ellifritt's
now (in)famous article in the AISC Steel Journal in 1977, titled "The
Mysterious 1/3 Stress Increase" gives an account of Ellifritt's dead-end
journey to get to the bottom of that quirky phenomenon. One thing he figured
out is that "no one knows" why the 1/3 increase was ever put there in the
first place, and that it is a great example of how engineers often just
accept what they're taught by their "betters" without question.

This is from the "Steel Interchange" column in "Modern Steel Construction"
magazine, December 1999.


(David MacGregor
September 1999)

ASCE 7-95 section 2.4.3, part (b) states that the effect of two or more
transient loads may be reduced provided that the allowable stress is not
also increased. AISC's ASD Manual, 9th Ed., section A5.2 allows a 1/3 stress
increase provided that the loads are not "calculated on the basis of
reduction factors applied to design loads in combinations," and gives ANSI
A58.1, which was updated as ASCE 7, as an example. My questions are:

Is it acceptable to use the load combinations specified in ASCE 7, but not
to reduce them and use a 1/3 stress increase when designing steel members?
May the 1/3 stress increase be used when designing for a Dead + Wind

Duane S. Ellifritt
November 1999

In my opinion, the answers to your questions regarding ASCE-7 and the 1/3
stress increase are:

Having said that though, I feel the need to qualify my answer and also tell
you that not every member of the AISC Specification Committee agrees with me
on this.

At first glance, ASCE-7 seems to prohibit a 1/3 increase in stress if the
loads have already been reduced because of loads acting in combination. In
fact, the wording is: (b) the allowable stress shall not be increased to
account for these combinations (underlining mine).

But what if the 1/3 stress increase is not to account for loads in
combination? Then is it permitted? It has long been my position that the
stress increase (which has been allowed for at least 100 years) was never to
account for simultaneous action of two or more loads, but to ameliorate the
effects of wind which was always applied as a static force.

On the other side, one can argue that modern wind forces are developed
taking into account the gusty and localized nature of wind, so there is no
need for a correction factor. I can understand this logic, too. However,
most damage in wind storms is to glass, certain wall, roof and wall panels,
and the forces on these small-tributary area items have been dramatically
increased over the last two decades, reflecting the real behavior of wind.
You might refer to my article "The Mysterious 1/3 Stress Increase," in the
2nd Quarter 1977 AISC Engineering Journal.


-----Original Message-----
From: awcinfo(--nospam--at) [mailto:awcinfo(--nospam--at)]
Sent: Tuesday, October 30, 2001 1:21 PM
To: seaint(--nospam--at)
Subject: 2000 IBC v 1999 UBC code question

...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.

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