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RE: IBC 2000 load combinations

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I agree with this interpretation, but it still seems we're losing the 1/3
increase in the alternate load combinations.  Essentially the 1/3 increase
is being "washed out" when applying the 1.3 factor to W if all you are
looking at is a wind force only, say on tension-only x-bracing.  Whereas in
UBC you applied a similar wind load with a 1.0 in front of W and still got
the stress increase.

Any comments?

Thanks again,

Dwain Hendershot

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)]
Sent: Tuesday, November 11, 2003 12:50 PM
To: seaint(--nospam--at)
Subject: RE: IBC 2000 load combinations


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 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 appears...

"regular" load cases:

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

equivalent "alternate" load case:

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 it would give me warm
fuzzies if someone verfied that what I believe makes sense.


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)]
> 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
> > come up with is that maybe the IBC is referencing ASCE7-95.  The wind
> > factors appear to have been increased in ASCE7-98 according to its
> > commentary section C2.3.2.  Is this a correct interpretation?
> >
> > Thanks in advance.
> >
> > Regards,
> >
> > Dwain Hendershot

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