For lumber diaphragms, allowable shear values have been shown in the IBC tables with 12" mail spacing for supporting members and just the outside spacing would change to get more shear. Now that the IBC refrences the NDS Wind & Seismic Tables it seems that the supporting members are required to be spaced at 6" o.c. instead of the traditional 12" o.c. Does anyone know background on why this would have changed or if I'm reading something wrong? Maybe the NDS has just always been this way but I have not noticed because I have not needed to use those tables until the 2012 IBC adopted them.
It also seems the allowable loads are significantly greater in the NDS tables than what the IBC gave. For example, IBC table for basic unblocked sheathing with 10d nails and 19/32" sheathing, 2" nominal edge, and case one pattern gives 285 plf. The same set up in the NDS (Wind & Seismic provisions) gives 800/2 = 400 plf. Has anyone else come across this. Is this higher load correct and just a result of the increased supporting member (interior field) spacing? Any comments greatly appreciated.
It also seems the allowable loads are significantly greater in the NDS tables than what the IBC gave. For example, IBC table for basic unblocked sheathing with 10d nails and 19/32" sheathing, 2" nominal edge, and case one pattern gives 285 plf. The same set up in the NDS (Wind & Seismic provisions) gives 800/2 = 400 plf. Has anyone else come across this. Is this higher load correct and just a result of the increased supporting member (interior field) spacing? Any comments greatly appreciated.
