How does your office treat firewalls?

[samdamon]

Several recent projects in my office have featured stand-alone firewalls. The need for these fire walls usually rises when an addition is being built next to an existing structure. (We use the BOCA code on most of our jobs). The walls are required to have a specified minimum fire rating, and also must be independant of the structure on both sides of the wall. The idea is that the structure on one side of the wall can fall (due to fire etc), and the firewall will remain standing, providing protection for the other area of structure behind it.

Due to the "stand-alone" requirement, the firewalls typically have to be designed as vertical cantilevers from the foundation level. On a multi-story building, this gets to be an interesting engineering problem, and an expense for the owner.

The wall has to be checked for both seismic and wind loads. But, what loads to use?

For wind, do we treat it as an exterior wall, albeit a temporary one? Since the firewall is separated from the interior of the structure there will be no build-up of internal pressure on the interior wall face, hence it seems that only a portion of the code-mandated wind loads for components and cladding should be used (just the external component).

For seismic loads, if the fire wall is treated as a cantilever (Which it really is) the lateral force can get up to the 50% range of the walls' weight. Whew. This happens even though we have only moderate seismicity here in NJ. Some engineers I have talked to do not treat the wall as a cantilever, but as an "architecural element". The seismic loads reduce to the 20% to 30% range then.

What is the practice in your office? How do you guys approach firewalls? Any comments would be greatly appreciated.

 

[ishvaaag]

In Spain we don't have such requirement, but the structural elements need be of prescribed resistance to normalized fire. So the wall can be part of the structure. Other than for 2 or 3 plants tall, I see your requirement quite aberrant, for it is so structurally, and one may gain the required resistance to fire by other means, particularly making use of the extant resistance to fire of the whole building properly detailed.

 

[Taro]

As you noted, cantilever walls become impractical for multistory buildings. Luckily, there are several other options besides cantilever walls. Here are a couple...

1. Use double walls with one wall anchored to each portion of the building. If one side collapses, there is still a wall standing with the other side.

2. Use a single wall that is tied to both sides. The wall and the structure on each side must be strong enough to resist the forces causes by one side collapsing.

A good reference for fire walls is NFPA 221: Standard for Fire Walls and Fire Barrier Walls, 2000 Edition.

 

[samdamon]

Taro-

Using a double wall would only work if the new and existing structures have the same geometry (in plan) where they meet. That is, they are both the same size where they meet. This was not the case in the two examples I mentioned above. Unfortunately! In both projects, the code required the firewall to extend beyond (in plan) than the actual interface area of the addition and new building.

Using a single wall tied to both structures would not work for the same reason. However, if the geometry of the two buildings allowed it, there would still be the problem of what loads to use, which was really the question I was hoping for comment on. There is no guidance in BOCA for loads caused by the fire-induced collapse of one side of a structure, but there is for wind and seismic forces. But its not clear what forces to use. I'm not sure how one could reasonably predict loads caused by a partial fire-induced collapse, any way.

Will take a look at the NFPA ref though. Thank you.

 

[LPPE]

I've heard something about 'fire' bolts that you can tie your floor systems to the wall, but they will break away when there is a fire. Try Hilti.

 

[mgardina]

We stoped designing cantilever firewalls like you are noting. We have gone to (2) gypsum board shaft type walls or (1) gypsum board wall with fusable connections that burn away on the fire side. Try UL U336.

Otherwise we have desiged the wall as non-structural element for seismic load since it will be typically be betweend two buildings or 5 psf blast load. I believe Factory Mutual has some recommondations.

 

[JAE]

We are doing what mgardina states - use two walls each tied to their respective sides. For an existing building/new building situation, you can extend the NEW fire wall out beyond the edge, and meet code - both walls do not have to extend as long as the single extension meets the required rating.

Cantilevering the wall is not really a viable option for the cost issue that you've now realized...especially in multi-story buildings.

Interior loads - I would check for component seismic forces against 5 psf interior partition load.

 

[samdamon]

Thank you gents for your comments. Much appreciated.