Calculated Fire Resistant - Swap out Timber with Equivalent Sistered 2X?

[Ryan Schultz]

Let's say you use the following to determine that a 3"x5.5" timber will be enough for a certain fire X rating.

https://www.awc.org/pdf/codes-standards/publications/tr/AWC-TR10-1510.pdf

Is it possible to swap out the 3"x5.5" timber with (2) sistered 2x6's?

 

[ADAguy]

Heavy timber is just that, monolithic members (they burn "slower than sistered members), not sistered, sistering only works for structural support increases,

 

[e hilton]

AD ... i would agree with your assessment of the slower burn time, but I’m wondering if the is any code that confirms it.

 

[steveray]

Could you use calculated fire resistance in IBC 722 or wherever it is?

 

[ADAguy]

Yes, there are test charts that allow for alternate combinations in lieu of ....

 

[Ryan Schultz]

It looks like my AHJ might allow it.

 

[Builder Bob]

NO - when a fire occurs, the metal heats up and chars the member that the nail or fastener in tid into - which loses strength and pulls out. Also humidity and temperature fluctuations will slowly pull the nails out or as the lumber dries out, the nails may pull out - Have you never seen a nail pop, missing nails in a deck, or nails slightly pulled out.

If they are using thru bolts, nuts, and washers to "sister" the members, we may talk but being based on mechanical devices that rely on skin friction to hold them in place - Not only no, but Heck no.