Fire Wall Double Foundation? 1

[jrfroe]

I'm working on an 1800 square foot single story CMU addition to an existing building. Since the existing building (built in the 1950's) is over the allowable height per IBC Table 503 (2009 IBC) for the construction type, we need a double fire wall that can't be tied to the existing building (per IBC 706.2). The existing building wall will provide the fire rating for one side of the wall, and the new building wall will provide the fire rating for the new building. This isn't a problem for walls above the foundation, but the city plan examiner is insisting that the separation has to extend through the foundation. The existing building is supported on steel piles with pile caps that extend approximately 6' into the space to be occupied by the new building. The new addition will be supported on micropiles, and is adjacent to the existing building on the south and west sides. The problem I'm having is that the southwest corner of the new building is over an existing pile cap and I don't think it is reasonable to not support the corner of the new building off of the existing pile cap, as I would need to design a fairly significant cantilevered grade beam that extends over the cap, and I'd be concerned with CMU wall cracking due to deflection of the cantilever. The existing pile cap supports a 12 story column, and I'm not adding enough load to overstress the existing foundation.

Does anyone know if there is something I'm missing in IBC that would prohibit having a common foundation between two sides of a double fire wall?

 

[DaveAtkins]

If you can prove the existing wall has adequate structural stability to remain standing as a cantilever wall for 4 hours (or whatever fire rating is required), then you don't even need a second wall.

But to answer your question, I have never heard of requiring separate foundation walls for adjacent fire walls. I don't think foundations fail during a fire.

DaveAtkins

 

[hokie66]

It sounds like the "city plan examiner" needs an injection of a bit of common sense.

 

[concretemasonry]

Possibly a transfusion instead of an injection.

Dick

 

[chicopee]

For a 4hr rated fire wall, wouldn't you also need a parapet to extend ( I think 3') above the roof?

 

[KootK]

I usually use combined foundations as well. Logically, however, there is one problem with the common foundation. If the firewall adjacent the new building is dragged down by it in a fire event, it's conceivable that the combined foundation might be so damaged in the process as to also bring down the other fire wall at the same time. A typical case might lead to one designing the foundation to withstand the overstrength yield moment of a block wall applied to the supporting foundation. If your foundation has been designed to preclude such a possibility, I'd express that to your reviewer. It may put their concern to rest.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jike]

Normally, one foundation will suffice for the double firewall. I could possibly see requiring two foundations if one might be a retaining wall where the collapse one fire wall could pull over the common foundation wall, but other that that one foundation should be adequate.

 

[DETstru]

I've done these many, many, many, times and I've never used two footings. No issues with AHJ's.

The IBC defines a "Firewall" as (Chapter 7):

A fire-resistance-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof, with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.

It states "from" the foundation and "through" the roof. My interpretation (and all the architects I've worked with) is that it means from the top of the foundation. You can easily argue this since they specifically say "through" the roof. If they wanted the separation through the foundation, they would've said so just like they did for the roof.

 

[steellion]

I agree that independent foundations are unnecessary - and also KootK has a good point to check the foundation still works if one of the walls is lost.