Building Separation at Firewall for Multistory Wood Framed Buildings

[NCSU02]

On a multistory wood framed project the architect is looking to use UL U925 with AAC block to achieve the fire rating. This detailing does not have a gap between the wood framed walls and the AAC block.

Per ASCE 7-10 section 12.12.3 "All portions of the structure shall be designed and constructed to act as an integral unit in resisting seismic forces unless separated structurally by a distance sufficient to avoid damaging contact as set forth in this section."

Since we do not extend the floor diaphragm across the firewall we are creating 2 buildings on the same property adjacent to one another. The requirements above appear to directly contradict each other.

Based on the anticipated shearwall deflections we would want to account for 4" of differential movement between the 2 buildings (ASCE 7-10 equation 12.12-2).

How are other engineers detailing this condition?

 

[JAE]

There needs to simply be a gap large enough to allow both buildings to move separately and not impact during an event. ASCE 7 has methods of calculating that gap (square roof of sum of squares for instance - see the drift and deformation section of ASCE 7).

Usually for demising wall situations one wall can be the "fire" wall and the other framed typically with the gap between them, closed off at the exterior walls and roof with an expansion joint type slip connection of some kind. At corridors there are floor slip plate systems available. I don't have any details to share (sorry).

 

[NCSU02]

The square root of the sum of squares is the ASCE7-10 equation 12.12-2 I referenced above. That was the initial thought of what we would need, but the UL detailing specifically notes that the AAC block is tied off to the walls on both sides and it also shows no air gap.

Since fire walls are a common condition and they typically act as building joints (unless you are in seismic design category D) it seems odd that the UL detailing would create a condition that does not work for the building joint requirements. There are a lot of UL details that have the fire wall tied off to the structure on both sides. Do other engineers require an alternate firewall detail that has 2 separate fire rated walls with an air gap between?

 

[JAE]

I'm not sure if Design U925 is intended for spanning over building joints like what you have here.
This appears to be an attempt to create a product that serves as a fire-separation wall to meet the demands of the IBC to fully separate the structure into parts (not interconnected) and also meet the requirement that if one side collapses due to fire the other side will not.

In your case you not only have a fire separation but also a movement joint due to building size, shrinkage, wind and seismic forces. Two different things.

You always have to be careful not to shoe-horn the U925 into your situation where you'd have to alter it - thus voiding the U925 rating.
I realize I'm not solving your issue here very welld (sorry!). If you try to use U925 but separate it from one of the sides, you then have a situation where your attached side might collapse and leave the non-attached side exposed to more flames.

Seems like you'd have to use two U925's to allow for a 4" movement joint. Or use some other fire protection assembly.

 

[JLNJ]

You ask a good question. I'm not familiar with U925 specifically, but time and again I have seen movement joints which don't get the attention they deserve.

Maybe you can put something sacrificial in there which would just crush in an event and still meet the intent of the code "firewall". I'm guessing most architects detail it with no gap and let the chips fall where they may.

We often specify and place building joints which account for longitudinal movement, but what about the transverse direction? I've received a lot of blank stares from architects when I tell them that the relative transverse movement is greater than the longitudinal movement. They just want to specify an "expansion joint" which simply squishes longitudinally and then call it good.