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IBC 2000 (and 2003) load combination question

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The equations given in 1605.3.2 came from the UBC (why?, I don't know - it
couldn't be political could it?).
Regarding your question about consideration of the "dead load only" case,
see the last sentence of Sec. 1605.1 (IBC 2000): "Effects from one or more
transient loads not acting shall be investigated." Live loads are considered
to be transient loads, so that means that Formula 16-13, for example,
requires you to consider D+L+Lr and D+L and D+Lr and D. The Code doesn't
explicitly list all of those permutations, but they are required based on
that statement in 1605.1.
Allen Adams, S.E.
RAM International

-----Original Message-----
From: Clifford Schwinger [mailto:clifford234(--nospam--at)]
Sent: Sunday, April 04, 2004 3:58 PM
To: seaint(--nospam--at)
Subject: IBC 2000 (and 2003) load combination question

Section 1605 in IBC 2000 specifies load combinations.

Section 1605.2.1 lists the strength design load
Section 1605.3.1 lists allowable stress design load
Section 1605.3.2 lists "alternate" (allowable stress)
basic load combinations.

The load combinations in 1605.2.1 and 1605.3.1 are
pretty much in agreement with each other (accounting
for the differences between LRFD design and ASD
design.) I can't figure out what planet the 1605.3.2
load combinations came from. There seems to be no
consideration for the "dead load only" condition or
for the "dead load + wind load" condition in this
group of equations.  To tell the truth I never even
noticed the 1605.3.2 equations before. Whenever we do
ASD design we use the 1605.3.1 equations.  What is the
logic behind the 1605.3.2 equations and why have two
groupings of ASD load combination equations? Where did
they come from? Why not just always use the 1605.3.1
equations when you're doing ASD design? Why is there
no "dead load only" or "D+W" equation with the
1605.3.2 equations? What am I missing?

(These load combinations (and confusion) remain
basically the same in IBC 2003.)


Cliff Schwinger

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