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RE: IBC 2000

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James:
Your interpretation of UBC 97 is correct; the one-third stress increase is
only applied to combinations that include wind and seismic loads.

However, the concept of a one-third stress increase has fallen out of
favor, and the newer codes and standards no longer allow it. An excellent
article on this can be found in the Fourth Quarter 1977 'Engineering
Journal' published by AISC, entitled "The Mysterious 1/3 Stress Increase"
by D. S. Ellifritt. That article can be downloaded off of the AISC website.
I highly recommend it in order to understand the reasons for the change in
philosophy.

IBC 2000 refers to "transient" loads, which essentially are any
non-permanent loads. Charlie Carter's definition yesterday was excellent:
"Transient loads are those that have a significantly different arbitrary
point in time value versus their maximum lifetime value." For example, one
minute a conference room is empty, and the next it is full of people - the
Live Load is transient. On the other hand, the soil pressures on a basement
wall are (or may be) pretty constant for the life of the structure - you
may not want to consider it a transient load. The British and Eurocodes are
more clear on the meaning of this than the IBC, but it is essentially as we
have explained. When two or more transient loads act simultaneously, the
code (IBC) allows you to multiply those loads by 0.75 to account for the
reduced probability that each of those loads would be at their peak values
simultaneously. Note that, unlike the one-third stress increase, this 0.75
factor can not be applied to the Dead Load term. The 0.75 factor permitted
by the IBC is not philosophically the same as the old one-third stress
increase.

So again, my explanation yesterday was not applicable to the UBC, which is
governed by a different philosophy on load combinations. My explanation was
applicable to the IBC.
Allen Adams, S.E.
RAM International



-----Original Message-----
From: "Lutz, James" <JLUTZ(--nospam--at)earthtech.com>
To: seaint(--nospam--at)seaint.org
Subject: RE: IBC 2000

I've never heard this interpretation before. I thought only wind and seismic
were transient loads. See for example 1997 UBC 2209, where allowable stress
increases are only permitted for wind and seismic load combinations. I think
I will stick with this interpretation.

-----Original Message-----
From: Allen Adams [mailto:aadams(--nospam--at)ramint.com]
Sent: Thursday, October 03, 2002 8:23 AM
To: seaint(--nospam--at)seaint.org
Subject: FW: IBC 2000


"Transient" loads are any non-permanent loads (Wind, Seismic, Live, Roof,
but not Dead Load). This means, for example, that the Load Combination
given by Formula 16-9 of Section 1605.3.1 becomes:

   D + 0.75L + 0.75(Lr or S or R)

but if there is no Roof Load it is

   D + L

or if there is no Live Load it is

   D + (Lr or S or R).

You have to check all three combinations.

The 0.75 factor can only be used if there are multiple transient loads
simultaneously applied. It is to account for the lower probability that the
peak values of those loads will occur simultaneously.
Allen Adams, S.E.
RAM International



 -----Original Message-----
From: 	Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent:	Wednesday, October 02, 2002 9:47 AM
To:	'SEAINT'
Subject:	IBC 2000

Section 1605.3.1.1 Load reduction allows a reduction of the combined effect
of two or more transient loads. ASCE 7-98 omits the word "transient".

Does "transient" mean short term load such as Wind and Earthquake only or
does is it meant to include various forms of Live Load also?

I cannot find any definition of "transient" in the code.


Jim K.


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