] [Thread] [Date] [Author]

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

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
> 
> From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
> To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
> Subject: RE: '97 UBC section 2316 comparision to 2000 IBC section 2306
> 
> Chapter 16 of 2000 IBC does not say you can automatically take a 1.33
> allowable stress increase for all materials when wind or seismic forces are
> used, with the exception of foundation soil pressure per 1616.1.
> 
> IBC 1605.3.2 says to see the material's chapter for determing wind/seismic
> allowable stress increases for "alternate basic load combinations".  IBC
> 1605.3.1.1 says that for the "basic load combinations" you can't use
> wind/seismic increases except for wood load duration factors.  The idea for
> wood in the IBC is to use the load duration factors out of NDS 2.3.2.3.
> 
> ACI 530-99 Section 2.1.1.2.3.2 says when you can use an 1.33 increase for
> masonry and AISC ASD 89 Section A5.2 says when you can use an 1.33 increase
> for steel.
> 
> 
> 
> -----Original Message-----
> From: utej(--nospam--at)attbi.com [mailto:utej(--nospam--at)attbi.com]
> Sent: Thursday, April 11, 2002 10:14 AM
> To: seaint(--nospam--at)seaint.org
> Subject: '97 UBC section 2316 comparision to 2000 IBC section 2306
> 
> 
> The 2000 IBC section 2306 references the NDS for ASD 
> design and I do not see any deletions or substitutions 
> similar to the '97 UBC section 2316.2.  The '97 UBC 
> section 2316.2 states that "When using Section 
> 1612.3.2....the 1/3 increase shall not be used 
> concurrently with the Load Duration Factor, CD."  It 
> also revises the NDS table 2.3.2 to use CD=1.33 for 
> earthquake and 1.6 for wind for members and nailed and 
> bolted connections exhibiting Mode III or IV behavior.
> 
> I could not find these deletions and substitutions in 
> the 2000 IBC and in IBC section 1605.3.1.1 it states 
> that "Increases in allowable stresses specified in the 
> appropriate materials section of this code....shall not 
> be used with the load combinations of 1605.3.1 except 
> that a duration of load increase shall be permitted in 
> accordance with Chapter 23."  There is no statements 
> that I could find in the 2000 IBC that stated that one 
> shall not take the 1.33 or 1.6 load duration factor 
> increase in conjunction with the 1.33 stress increase 
> such as the '97 UBC section 2316.2 states.
> 
> Are these purposeful omissions and modification of 
> design requirements in the 2000 IBC compared to the 1997 
> UBC or perhaps an oversight? 

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