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# RE: IBC 2000 load combinations

• To: seaint(--nospam--at)seaint.org
• Subject: RE: IBC 2000 load combinations
• From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
• Date: Tue, 11 Nov 2003 12:49:43 -0500 (EST)

```Dwain:

I believe you have that a little backwards.  I might be wrong, but I
believe that the alternate load combinations ALLOW the 1/3 increase for
when you then are essentially increasing the wind load by the 1.3 factor.
In otherwords, it seems that you can use the increased allowable stresses
(increased by 33% on one side of the equation) when compared with the load
combinations that increase the wind load (increased by about 30% on the
other side of equation...at least the wind load is).  If you use the
"regular" (i.e. NOT the alternate) load combinations, then the IBC appears
to be doing away with the 1/3 increase with _NO_ 1.3 factor in front of
the wind.

Or if this helps...it appears...

D + (W or 0.7E) + L + (Lsubr or S or R) <= allowable stress (eq 16-10)

D + L + 1.3W + S/2 <= 4/3 * allowable stress (eq 16-15) [at least for
those materials that permit the 1/3 stress increase]

Again, I could be wrong in my interpretation...so it would give me warm
fuzzies if someone verfied that what I believe makes sense.

HTH,

Scott
Ypsilanti, MI

On Tue, 11 Nov 2003, Dwain Hendershot wrote:

> So it seems that IBC is essentially doing away with the 1/3 increase by
> adding the 1.3 in front of the W, correct?
>
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Tuesday, November 11, 2003 9:34 AM
> To: Seaint@Seaint. Org
> Subject: Re: IBC 2000 load combinations
>
>
> Dwain:
>
> The 2000 IBC references the ASCE 7-98.  The referenced standards used in
> the 2000 IBC are listed in chapter 35.
>
> The reason for the 1.3 appear to be due to the fact that the alternate
> allowable stress load combinations in section 1605.3.2 allow the use of
> the good ol' 1/3 allowable stress increase that has traditionally been
> used in most materials.  The "main" ASD load cases (section 1605.3.1) does
> not seem to allow the use of the stress increase.
>
> Just my take on it from a brief look.
>
> HTH,
>
> Scott
> Ypsilanti, MI
>
>
> On Tue, 11 Nov 2003, Dwain Hendershot wrote:
>
> > Section 1605.3.2 Alternate basic load combinations indicates that ASCE7
> wind
> > loads must be increased by a factor of 1.3 but you get applicable
> increases
> > in allowable stresses.  Section 1605.3.1 does not increase the same wind
> > load but does allow an increase to wood shear walls for duration per
> chapter
> > 23.  It seems to me that using the alternate load combinations penalizes
> you
> > 30% when checking allowables for shear walls.  The only explanation I can
> > come up with is that maybe the IBC is referencing ASCE7-95.  The wind load
> > factors appear to have been increased in ASCE7-98 according to its
> > commentary section C2.3.2.  Is this a correct interpretation?
> >
> >
> > Regards,
> >
> > Dwain Hendershot
> >
> >
> >
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