] [Thread] [Date] [Author]

[utej(--nospam--at)attbi.com]

I also noticed in notes, that ICBO handed out at the 
ICBO 381i seminar on the 2000 IBC, statements that 
said "'97 UBC section 1605.3.1 - Basic load combinations 
(ASCE 7)" and that the "ASCE 7-95 load combinations 
intially adopted in 1997 UBC revised in IBC to be more 
consistent with ASCE 7-98."

In looking at an earthquake load and the demand over the 
capacity for the UBC sections 1612.31.1 and 1612.3.2 to 
the IBC sections 1605.3.1 and 1605.3.2 I came up with 
the following comparisions:

1/1.4 = 0.70

                  1997 UBC               2000 IBC

Basic Comb. [0.75*0.70/CD=1.33]=0.39  [0.75/CD=1.6]
=0.47   
 
Alter. Comb. [0.70/CD=1.33]=0.53      [0.70/CD=1.6]=0.44

So it appears as if you get a slightly larger factor of 
safety for the Basic Load Combinations of the 2000 IBC 
compared to the '97 UBC (as the ICBO notes stated they 
have been corrected in the 2000 IBC) and a slightly 
smaller factor of safety for the Alternate Basic Load 
Combinations of the 2000 IBC compared to the '97 UBC by 
using the Wood Load Duration factor (CD) of 1.6 for wind 
or seismic as allowed in the IBC.



> There are no restrictions on the 1.6 load duration factor in the 2000 IBC.
> 
> Sorry I didn't understand your original question.
> 
> Buddy Showalter, P.E.
> AF&PA/AWC
> 
> ***********
> 
> From: utej(--nospam--at)attbi.com
> To: <seaint(--nospam--at)seaint.org>
> Subject: Re: '97 UBC section 2316 comparision to 2000 IBC section 2306
> 
> Thank you for your response.  I do realize that the 1/3 
> stress increase is unrelated to the load duration 
> factor, CD, for wood.  
> 
> My original question was trying to help determine if the 
> 2000 IBC allows a designer to use the 1.6 load duration 
> increase for wood for seismic loads where the 1997 UBC 
> did not allow a 1.6 load duration increase for seismic 
> loads for fasteners in wood if mode III or IV behavior 
> was not exhibited nor does the 97 UBC allow for the 1.6 
> increase for seismic for mechanical fastenings ('97 UBC 
> section 2316.2).  In addition, the '97 UBC stated that 
> the 1.6 wood load duration factor for wind was allowed 
> only when members or nailed or bolted members exhibited 
> Mode III or IV behavior.  It appears as if the IBC lets 
> a designer use the 1.6 wood load duration factor for 
> wind or seismic with no restrictions.  Is this correct?
> 
> Also, '97 UBC section 2316.2 states that when using the 
> 1.6 wood load duration factor increase for either 
> earthquake or wind, it should not be combined with the 
> 1/3 stress increase in section 1612.3 (this applies to 
> both the basic and alternate load combinations).  The 
> IBC appears not to have limitations on the use of the 
> 1.6 wood load duration factor as the 1997 UBC.
> > Here's my response to this posted in Oct. 2001. The archives show the full
> > discussion.
> > **********
> > The 1/3 stress increase is totally unrelated to the load duration factor
> for
> > wood. The 1/3 stress increase is a load combination adjustment factor that
> > accounts for the reduced probability that two or more loads, other than
> dead
> > loads, acting concurrently will each attain its maximum at the same time.
> > such
> > adjustments are applicable to all materials, but it unfortunately got
> > incorporated on the resistance side of the equation rather than the loads
> > side.
> > it has been moved from the resistance side of the equation in ASCE 7 (and
> > IBC)
> > to the load side of the equation in the form of a 0.75 adjustment (inverse
> > of
> > 1.33). so, you are still permitted the adjustment, but it's now taken on
> the
> > loads in accordance with IBC 1605.3.1.1.
> > 
> > The ASCE 7 Commentary section C2.4.3 and NDS Commentary section 2.3.2.3
> has
> > more
> > detailed discussion of this for those interested.
> > 
> > HTH
> > 
> > Buddy Showalter, P.E.
> > AF&PA/AWC

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