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Cantrilever Diaphragm Limitations 1

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msquared48

msquared48

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In older IBC and UBC codes, there used to be restrictions on the size of a wood horizontal diaphragm that could be cantilevered from a structure. Very applicable to deck or roof canopy construction where it is attached to a much larger structure with shear walls.

I cannot find any such limitation in the 2009 IBC, area or otherwise, other than the allusion in Figures 2308.12.6(3) and (4) where the 6 foot dimension is given. Other than that Sections 2308.1.1 and 2308.4 seem to say if it is engineered, it is OK.

Any other insight?

Mike McCann
MMC Engineering
 
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Mike,
I think that it is found in the Special Design Provisions for Wind & Seismic section of the NDS (published by the American Wood Council). The limitation for cantilevered diaphragms, which must be rigid for open front buildings, shall be permitted to cantilever past the outermost supporting shearwall a distance, Lc, of not more than 25' or two thirds of the diaphragm width W, whichever is smaller. It is shown in Figure 4.2.5.2 of my version of the NDS.

I hope that this helps.
 
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Yes. That was also in the 2006 IBC, but could not find it in the 2009.

I just woindered if the thinking has changed here, or am I missing something?

Mike McCann
MMC Engineering
 
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In the 2009 IBC, Section 2306.2 Wood Diaphragms states that diaphragms shall be designed to the AF&PA SPDWS, which is the Special Design Provisions for Wood Construction. My guess is that the IBC took a lot of the wood stuff out of their book so we have to buy the reference materials.

But, if you go to WindSeismic.pdf you can view a read only version of the SDPWS for free.
 
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Mike,
Thanks for the correction. I never could type worth a damn.
 
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