Firewall as shear wall

[sc123]

can a firewall be used as a shear wall for lateral force resistance? i have a high roof area in a building which has a flexible roof diaphragm. i have masonry walls on all 4 sides but one of my walls is firewall separating the high roof area from an adjacent low roof area. can i connect my roof diaphragms on the high and low sides of the firewall to the firewall for lateral resistance or is connecting to a firewall not permitted?

 

[JAE]

Connecting to a firewall is permitted on one side IMHO as a firewall, by definition, must be provided whether one side or the other is burned down. To attach to a wall for shear resistance implies a very strong connection which will not release if the braced adjacent structure fails.

In this case, you really need two separate walls, each serving as a shearwall independently...most shearwalls brace the structures in the direction of the wall length, but rely on the structure to brace it perpendicular to the wall length.

 

[RARWOOD]

JAE

I found this thread very interesting but have several questions. What does IMHO stand for?

Also you talk about connecting to the firewall on one side, indicating that a firewall must be provided whether one side or the other is burned down. Does this mean that if you attach structural elements to the fire wall you are required to have two seperate walls?

I am asking this question because it is hard for me to imagine a case where you would have no structural attachment to the fire wall. For example assume that you have a three story apartment building with a masonry fire wall between sections with no structural attachment to the wall.
this would mean that the wall would be unsupported from the foundation to the top of the wall.

 

[JAE]

RARSWC,

IMHO = In my humble opinion

Some building codes require firewalls to divide up buildings into individual sections, each protected from the other sections from fire. This is an attempt by the codes to manage/limit the spread of fires in buildings to manageable areas.

When they require firewalls, they require that the wall be independent of the fire damage that may occur on one side or the other. In other words, the codes require that the wall remain standing if one side or the other collapses.

This requires that the connections from structure to wall to structure be able to release or break off should the structure on one side collapse. Very difficult to achieve although numerous engineers have attempted it.

The other option is to provide two distinct walls, each capable of providing the necessary fire resistance and each attached to their adjacent structure on one side, but not the other - so if one structure collapses, the other wall remains standing.

This is how I understand it - hope it is clear.

 

[jike]

There are several ways to design firewalls to remain standing for the hourly rating.

1) Free standing cantilevered wall (not connected to either structure)

2)Double fire wall (one attached to each structure

3) One wall with breakaway connections to release one structure, if it tries to collapse. the other structure must stablize the firewall. (Difficult to achieve)

4) Tied Firewall where the structure on either side is rigidly tied to the firewall. The structure on each side is capable of resisting the collapsing force of the other structure.

5) One firewall tied to with a rated (same as wall) floor or roof on one side.

 

[apetr26542]

I've struggeled with this also, I know that using a firewall to bear on is not permitted, but I am tying into one for building lateral stability

 

[JAE]

Nice summary jike!

 

[jike]

I know of no requirement that does not permit you to bear on a firewall or to use it for a shearwall. It is more a issue of how can you rational the collapse of the structural frame without imposing a significant pulling force on the firewall that may cause it to be pulled over.

 

[RARWOOD]

JAE

Thank you for your reply!


The others have added interesting comments also.

I will need to ponder on the topic to see what I can add based on my wood background.

Some codes in the past required fire cuts on wood beams supported in pockets of masonry walls. A fire cut was a triangular cut back from the face of the wall to the back of the beam bearing. I think the logic was that if the beams support burned away that as the beam dropped that it would rotate out of the masonry pocket with out pulling the wall down. I doubt that a beam pocket would be allowed in a fire wall.

Also in the past the BOCA code required the use of self releasing wall plate boxes or approved hangers for wood beams entering masonry walls. At one time I worked for a lumber trade association and help present educational seminars to Building Officials using the BOCA. I inquired several times to see if any of the Building Officials could explain what a self releasing wall plate box was I never found any one who could answer the above question. So if any one out there knows what a self releasing wall plate box is, I would appreciate being educated.

 

[jike]

You can have a pocket in a firewall, but the wall behind it needs to meet the firerating of the wall, generally by grouting cores solid.

Another consideration is to have an adequate distance between the steel framing and the firewall. Before steel collapses, it will expand and sufficient distance needs to be provided so that the expanding steel does not damage the firewall. See FM of NFPA for more info.

 

[RARWOOD]

In my experience I have found that a lot of people bear on the fire walls and use the individual floors to brace the wall. Whether this will meet code of not I don't know.

One of my thoughts on the subject is to tie in the wood floor members and sheathing by attaching a leger beam to the wall at each floor or roof level.

The attachment of the wood ledger may be some what of a problem. I would assume you can't bolt through the fire wall, as I would think the bolts would transfer heat through the wall. There are a number of products out there that should allow a ledger to be attached from one side.

The other problem is that the value of bolts perpendicular to the grain in wood is fairly low. However depending on the load a ledger is a good solution. I would probably use a 3 1/8 wide glulam. If the loads are higher a steel angle could be attached to the wall to support the wood ledger.

My next step would be to use face mounted hagers to support the beams framing into the ledger. A good hanger for this application would be a U.S.P. GHF style hanger. GHF hangers use 1/4" x 3" NA25 nails.

My logic here is that I believe if the beams wood support away from the wall collapses that the wood beams would either roll out of the GHF hangers or would pry out the hanger nails. I think this would occur without pulling down the wall.

Since I have never actually used this approach in practice, the above idea only represents my thoughs on how I would approach the problem.

Look forward to hearing other opinions.

 

[kscottr]

Just rember that a Fire Wall must have its own footing and be self supporting. Typically where a fire wall occurs within a building,most of the time you have two rated walls that support the structure on either side. Different building codes may have different requirements.
Scott, R.A.
Virginia