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RE: IBC 2000 load combinations

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According to my notes from a wind design seminar, the 1.3 factor in the
Alternate Basic Load Combinations for allowable stress combinations in IBC
2000 "accounts for whether the code includes the wind directionality
factor". The 1.3 factor is used when ASCE 7 is used for wind loads, which
includes the directionality factor; the factor reduces to 1.0 for wind from
other codes, which generally do not include the directionality factor. (What
doesn't make sense to me is that 1.3 is not the inverse of 0.85? Why don't
they just say to use kd = 1.0 for such conditions?) 

According to the commentary to ASCE 7, the 1995 code used a wind load factor
of 1.3, with the directionality factor "hidden" in the load factor. The 1998
code uses a wind load factor of 1.6 but includes the directionality factor
(typically 0.85) in the wind load development. Thus: 1.3/0.85 = 1.53 rounded
up to 1.6 (erroneously noted to be rounded to 1.5 in the commentary). 

If you do use ASCE 7-98 for development of wind loads and use the Alternate
Basic Load Combinations, then the 1.3 factor for wind does partially offset
the 1/3 allowable stress increase. 

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


> -----Original Message-----
> From: Dwain Hendershot [mailto:dhendershot(--nospam--at)lawsonelser.com] 
> Sent: Tuesday, November 11, 2003 10:55 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: IBC 2000 load combinations
> 
> 
> 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)engin.umich.edu]
> Sent: Tuesday, November 11, 2003 12:50 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: IBC 2000 load combinations
> 
> 
> 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...
> 
> "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 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?
> > >
> > > Thanks in advance.
> > >
> > > Regards,
> > >
> > > Dwain Hendershot
> 
> 
> 
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