Fall away firewall connection?

[ml13]

Does anyone have a recommendation on a "fall-away" connection at a CMU firewall (extending above the roof) and plywood roof sheathing?

My local building official said he thought that the "masonary" people would might have a proprietary connection available. Anyone heard of anything similar (I have a call into to my local chapter now).

Thanks.

 

[Stress02]

I'd be intrested in what they say.

 

[KootK]

Damn fine question. A coworker of mine encountered the same problem on a recent project. Here's what we did:

The plywood is nailed to a double 2x4 sill plate. That sill plate is bolted to the horizontal leg of a steel angle. The vertical leg of that same angle is oriented upwards and then expansion bolted to the CMU wall. However, instead of a regular hole for the bolt, we provided a slot in the vertical leg that runs from the underside of the bolt all the way to the toe. Vertical support for the angle is provided by a roof truss positioned snuggly below the horizontal leg of the angle.

The idea behind this weirdness is that, once the truss burns away, the angle will fall away from the expansion bolt via the slot provided. However, in the mean time, you can use the CMU wall for shear. It's pretty tough to describe the detail textually; if you provide me an email address, I'd be happy to email you a copy of our detail.

Don't get the wrong idea here though. I don't think, even for a minute, that this detail would actually work the way that the code thinks it should. I suspect that A) The truss below the angle wouldn't be burnt away when you'd need it to be and B) The nut on expansion anchor bolt would be tight enough to prevent the angle from just falling away.

It satisfied the building offical however. That, and we just couldn't come up with anything better. If somebody knows of a better way, I'd love to hear about it.

 

[jheidt2543]

I do believe that this topic has been discussed in another thread not so long ago.

I was looking for a "fall away connection" for steel joist
bearing on a masonry fire wall, such that in the event of a fire and the joist failed, they would not pull over the masonry fire wall. I called the SJI, Vulcraft, and the Masonry Society all of whom said they do not have a recommend a connnection that satifies this condition.

The NCMA Tek Note 5-8A "Details for Concrete Masonry Fire Walls" has some useful informaton. You can go to

http://www.ncma.org

and follow their links.

I can't believe that this is not covered somewhere. I would think that UL would have tested some details for this. It must be that the perceived liability of suggesting a detail is holding this back. It seems to me that this would be a great research project for a grad student.

I eventually came up with my own detail and submitted it to the building department for review and approval. They told me they don't have a standard "approved" detail either, they look at each submission separately.

Talk about reinventing the wheel each time!

 

[KootK]

jheidt,

Is there any chance that you'd be willing to describe the detail that you came up with? I'm very interested in this particular issue.

thanks.

 

[jheidt2543]

Sorry for this long post, but I thought I would give the entire thought process behind this detail. Friendly, for the good of the order, comments and critiques are welcomed, even encouraged!

Background: The project required a fire separation wall between an existing steel framed building and a new steel framed addition. However, due to poor soil conditions which would have required undercutting for the new column footings and required underpinning of the existing adjacent footings, we opted for a load bearing masonry fire wall to support new steel roof joist. The wall would be 33’ tall of 12” thick reinforced block.

Problem: Provide an acceptable “breakaway” joist connection so that joist failure in the event of a fire would not pull the wall down and yet provide lateral stability to the masonry wall and wind uplift resistance for the roof joist.

Proposed solution #1 was suggested to me by the Technical Services Dept. of the Heckman Co., to use anchors made of Zinc 710, a metal that melts at 792 degrees F. The anchors would be one of their standard designs, but special ordered in the Zinc 710 metal. I decided against this for my situation because of the possibility that the joist could fail at mid span in a fire before the temperature reached the melting point at the wall anchor.

Proposed solution #2 was suggested to me by the Technical Services Dept. of the Steel Joist Institute, although they did not provide a sketch or “officially approve” it. The detail consisted of a standard joist plate with a “Z” strip of sheet metal welded to the plate along each side of the joist shoe. The joist was to be slipped loose into the “hold down” clip made of the Z strip. This provided uplift, but I also needed lateral stability, so I rejected this too.

Proposed solution #3, which is the one I selected, consists of a standard joist bearing plate with a 2” long section of HSS 5”x 3”x 1/4” with a ¼” thick plate end cap,the HSS welded on its side to the joist bearing plate. The joist can be slid loose into the open end. The far end of the joist is welded to a supporting beam. The upper portion of the HSS shoe provides uplift restraint for the joist (clearance is about ¼”); the end cap provides lateral stability for the masonry wall in one direction and the joist shoe can slide out of the connection should a fire cause collapse of the joist (only the end 2” of the joist is covered). Depending on how paranoid one is, grease or a Teflon pad could be set on the joist plate to provide a better sliding surface, but I didn’t call for that. The project is out for bid now, so I’ll have to see what the fabricators and erectors think of this. I did get preliminary approval from the building inspection department, but final approval is pending.

I had a different detail for lateral support of the masonry wall in the opposite direction because the adjacent existing building was independently framed.