CMU Firewall

[SpiderM]

I am designing a cmu block firewall. It is intended to have aluminum "fall away" clips on each side of the wall. My question is-- Can I consider these clips as a brace point for the wall, our do I need to design the wall as a free standing cantilevered wall (two and three stories)?

Thanks

 

[BAretired]

You cannot consider the wall braced by "fall away" clips on each side.

Moreover, the concept of using fall away clips on each side of a two or three story firewall is wrong. Much better to tie the wall to both sides, then, if you have a fire on one side, the wall is still braced by ties on the other side.

BA

 

[OHIOMatt]

Another option to consider would be two separate walls. This way the wall and footing can be designed as if the wall is supported and will help you achieve what you are trying to do.

One other concern is aluminum clips. What temperature is required to have them 'break' away? I have always used ties made of zinc with a melt point of 792 degrees F.

 

[concretemasonry]

A fire wall rating is defined environmentally as being able to resist the heat and passage of flames and gasses for a prescribed period of time.

If the wall lateral support fails to provide support, it becomes a structural system where it must be designed as a cantilever or reliable support must be provided to provide lateral support for life and safety.

Commonly, bond beams and bolts to the wall at the intermediate levels are used. This can provide lateral support in both directions in case one side is destroyed. This assumes the flooring system is capable of providing the stability as BA suggests.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JLNJ]

I have always wonder about connecting the wall to both sides. You don't want the falling side to pull the other side down, so what force are you to plan on resisting? FM has some forces generated by certain wall systems on a common wall. They can be significant.

I like the two walls, connecting each to its own side. At least I understand how it works.


As far as the AL clips go, how is one to know that the clips will heat up before the wall tries to fall and pull everything down? To me the logic in the clips seems a bit lacking.

 

[BAretired]

There is no question that a double firewall is the best solution. The difficulty is in getting the owner to spend the additional money when he knows that the code permits a single firewall.

BA

 

[SpiderM]

Sorry for the slow reply. I was out of the office for a few days.

I should offer some more information. The each side of the cmu wall is free standing. Neither side would be dependent on the cmu wall for ANY support. I am trying to avoid constructing an 8 meter cantilevered wall. I want allow for one side to burn to the ground, while the other is still standing. Furthermore, I don't want the wall to fall on anyone during the fire fight or the cleanup afterward. Perhaps I'm hoping to have my cake and eat it too.

(The ironic thing here is that both buildings are constructed of cold formed steel. The structure itself is noncombustible. Please don't ask, I'll have to write an epistle to explain how we got here. Suffice it to say, governmental bent and suspenders and duct tape and super glue and etc., etc. etc.)

 

[BAretired]

Commentary C of User's Guide - NBC 2005 "Structural Integrity of Firewalls" considers four design approaches...Double Firewall, Cantilever Firewall, Tied Firewall and Weak-link connections.

The double firewall and cantilever firewall are easy to understand but expensive to construct.

For a tied firewall, the structure on each side must be capable of resisting the lateral forces from the fire side. The commentary provides some guidance in determining the horizontal forces from a collapsing structure.

For weak-link connections, the commentary states "If a weak link is provided on each side of the firewall, the link on the fire side will break away while the link on the non-fire side will not."

I don't know how to detail the weak links in order to be confident of that result. If the weak link on the non-fire side breaks away first, the firewall will collapse toward the fire, defeating the entire purpose. For that reason, I have never used the weak link approach.

BA

 

[SpiderM]

Thanks BA. That will definitely guide me in the right direction. Thanks a million.

 

[BAretired]

You're welcome.

BA