Just took a brief poll in the office to talk about the 5psf horizontal loading required by the code for wood stud design on walls per [IBC 1607.13]. Interesting...
The commentray for the IBC 2003 talks about the loading being related to pressurization, or nominal impact loads. The transient nature of these loads seems to indicate the use of the Cd=1.6 factor.
The poll reveals that most of us are using a Cd = 1.6 for our combinations involving wind and seismic as applies to the Fb bending criteria. For the D+L load case, we are NOT applying the 5psf or 1.6 factor.
Steve points out that the 1997 UBC [1611.5] is actually a LOT more clear about the 5psf: It specifically mentions that the 5psf need not be applied simultaneously with the wind or seismic loads, and further gives deflection criteria...
Steve and I speculate that the 5spf may be used in lieu of actual horizontal seimic loads on wall (unless there is a heavy "special case" phenomenon) , and wind should be applied as per standard wind calcs. The question is whether to apply the 5psf as a live load to the D+L load case. When you "factor" (excuse the pun here) in the 97 UBC, it seems that we should consider the 5psf in this case as well (what Cd to use though?)
So to summarize:
The commentray for the IBC 2003 talks about the loading being related to pressurization, or nominal impact loads. The transient nature of these loads seems to indicate the use of the Cd=1.6 factor.
The poll reveals that most of us are using a Cd = 1.6 for our combinations involving wind and seismic as applies to the Fb bending criteria. For the D+L load case, we are NOT applying the 5psf or 1.6 factor.
Steve points out that the 1997 UBC [1611.5] is actually a LOT more clear about the 5psf: It specifically mentions that the 5psf need not be applied simultaneously with the wind or seismic loads, and further gives deflection criteria...
Steve and I speculate that the 5spf may be used in lieu of actual horizontal seimic loads on wall (unless there is a heavy "special case" phenomenon) , and wind should be applied as per standard wind calcs. The question is whether to apply the 5psf as a live load to the D+L load case. When you "factor" (excuse the pun here) in the 97 UBC, it seems that we should consider the 5psf in this case as well (what Cd to use though?)
So to summarize:
- With the Seismic combination - we are generally using 5spf in lieu of hor seismic loads, with Cd = 1.6 (for flexure and axial)
- With wind combinations - the wind does not generally apply with the wind loads to a 2x4 wall (the main point of concern)
- With dead and live combos - we have NOT been applying the 5psf - but should. On this note, we're floating out the idea that we may apply a Cd =1.6 for flexural component of the design only. Cd for other aspects will remain at 1.0.
5 comments:
In response to summary item#3, the method of using 2 different Cd factors for a single load combination simplifies the calculation to 1 run and is great for a quick calculation. In my understanding, we are allowed to multiple both the Fc and Fb by 1.6 when wind (bending) is combined with dead (axial). However, applying this higher duration factor would require that more combinations be checked with lower duration factors. Eventually I would like to make a spreadsheet with allowable axial loads based for typical heights and common wind pressures for our area. If I follow that ambition, I will post it on the server and blog it's location.
...to substantiate the use of a single CD factor applied to both Fb and Fc for each load case, see "Problem 13: Beam Column Check" in the "Example: Structural Wood Design Solved Problems, 2005 Ed." accompanying the 2005 NDS. If you work backwards from the calculated F'c (p.112), it shows that CD=1.6 (p.106) was used when only D+S is applied axially to check the biaxial capacity of the column. The calculation lacks a second design check where the column should be checked for axial capacity alone (CD=1.15 or 1.00) when the wind is removed and (first order) short duration bending is not present.
Per NDS 2.3.2.2 The load duration factor, CD, for the shortest duration load in a combination of loads shall apply for that load combination.
So the choice of CD is based on load source alone and not configuration of load applied to the member.
"Composite-Repetitive" Factors:
Table 3.1.1.1 of the NDS 2008 Edition (formerly found in IBC Tbl. 2306.2.1) provides increased repetitive factors that may be used in lieu of the Cr factors for studs walls lined with interior GWB and exterior sheathing. It specifies that the out-of-plane moment-inducing force be wind. Does anyone agree or disagree that these "composite-repetitive" factors can apply to the 5 psf "live"/impact (transient) load for interior shearwalls.
"P-Delta Effect":
Also, what are your thoughts on including a second-order P-delta effect in stud design due to the eccentricity caused by bending from wind. My thinking is to include a preset eccentricity based on the allowable code deflection. Thoughts?
I have generally understood the 5 psf to be a conservative uniform load that represents a fairly large mixture of possible shorter and longer-term loads. The examples I've seen are people leaning on the wall and moment induced by post-installed cantilevered items like shelving and lights that they don't bother telling us about. As a result, I generally do not use the 1.6 factor, since shelving can induce a moderate amount of long-term loading.
As for P-Delta, for typical studs and posts I generally use an eccentricity of .5" for all loads that "should" be concentric, to account for unbalanced loading conditions. In cases with significantly loaded posts and columns (example: Single beam between two large storefront windows), I do include P-delta effects due to wind.
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