ASCE 7-10 12.2.3.1 - Townhomes

[JNEnginr]

Hey, I know this has been discussed before for larger-scale structures, but does anyone have any thoughts on using vertical combinations of R Values on town-homes? I have a light framed single family dwelling, 3 stories tall, in SDC D. Long and narrow and simple. I want to use Gyp (R=2) on my 3rd floor as my lateral force resisting material, and OSB sheathing on my 2nd and 1st floors. According to 12.2.3.1, i have to use R=2 throughout, which kills my design. I could sheath the 3rd floor, and then i'd have R=6.5 throughout, but before i do that, i wanted to get everyone's opinion on the "exception" note, and see if a 3 story town-home would meet that exception, and if not, exactly why. Just too much mass?

Thanks!

 

[mike20793]

Can you not use the IRC instead of IBC? It may help you out to stay in there instead of going to the IBC and ASCE 7. I have never used the exception and would be hesitant to since the gyp board really doesn't give much of an inelastic behavior before failure. Also, what is the demising wall assembly for the townhomes? Do you have a shaft liner and discontinuous floor sheathing? I'm not sure townhomes quite fits the definition of detached one and two family dwellings since they can often be interpreted as multifamily, depending on fire rating requirements.

Edit: After digging into the definition of detached homes, it seems that townhomes would not apply because of the property line and permitting requirements for the townhomes as a whole rather than individual, detached units.