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RE: IBC 2000 (and 2003) load combination question

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The load combination lists in the IBC have always seemed a bit bizarre to
me. 

One additional difference between 1605.3.1 and 1605.3.2 to note is that the
second set of equations permits the old "1/3 allowable stress increase" with
wind or seismic, whereas the first set of equations only permits a 0.75
reduction factor when including "two or more transient loads". 

Based on the wording in 1605.1, I don't see why the code lists Formulas 16-1
and 16-7. Dead load alone rarely governs design, and it still must be
"considered" without transient loads. 

What baffles me the most is that between 1605.2.1, 1605.3.1, and 1605.3.2,
there are combinations in some lists that are not represented in other
lists, but which seem appropriate for consideration. Commentary on why these
combinations vary so much would be helpful to the users. 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


> -----Original Message-----
> From: Allen Adams [mailto:aadams(--nospam--at)ramint.com] 
> Sent: Monday, April 05, 2004 4:27 PM
> To: seaint(--nospam--at)seaint.org
> Subject: IBC 2000 (and 2003) load combination question
> 
> 
> 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)yahoo.com]
> Sent: Sunday, April 04, 2004 3:58 PM
> To: seaint(--nospam--at)seaint.org
> 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
> combinations
> Section 1605.3.1 lists allowable stress design load 
> combinations 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.)
> 
> TIA,
> 
> Cliff Schwinger

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