Light-framed 3-hour interior load-bearing firewall with large eccentric loads.

[Prestressed Guy]

I know, the title is a non-sequitur.

I have an architect who has specified a 3-hour wall in a light-framed fire station (essential structure). On one side of the wall there is a 2-story living area with 13’ joists and roof trusses bearing on the wall. On the other side of the wall I have 59’ roof trusses. The reaction on each truss is 560# and 2320#.

The architect has called out the wall a single 6” steel stud wall with 3 layers of 5/8” GWB on each side. He has shown the trusses bearing on a ledger beam that is through bolted to the wall but this will defiantly not work.

I am not seeing any way to proceed with this concept. It would need to meet the requirements for a tied fire wall and I cannot see that happening.

Has anyone ever done anything like this? Oh, and just for fun, it is an SDS-D structure.

 

[BAretired]

Does such a wall meet your code requirement for a three hour firewall?

What is an SDS-D structure?

BA

 

[manstrom]

2 narrow walls with CH studs in the middle? Or CMU?

Not sure how you will get a 3 hour rating unless you have gyp running under your ledger .

 

[Prestressed Guy]

Thanks for the replies.

BAretired, SDS-D seismic design category D. importance factor for seismic = 1.5, for wind, 1.25. in other words, the shear walls have high lateral loads with lots of nails.

The wall assembly is 6" CFMF 20ga minimum with resilient channel and 3 layers of type-X GWB on one side and 3 layers of type-X GWB on the other side. this assembly meets the 3-hour rating but I cannot see any way to support the structural loads and also make it stable with a fire induced collapse on either side.

manstrom. Whehehehat are CH studs?

The architect has shown the 3 layers of type-X GWB under the ledger but then there is no way to support the ledger loads. The reaction of each truss on the 59' bay is 2320lbs. This is not possible over 1.875" of gypsum.

 

[Archie264]

In as much as possible I try to concentrate on just the structural elements and leave other important design considerations -- such as fire-rating -- to others. That's out of both desire and necessity as it's all I can do to keep up with structural issues and I know almost nothing about the requirements of other disciplines. But you mention designing for stability in the event of a fire-induced collapse on one side. Is this a code requirement I need to consider? If so could you provide me a reference?

(That's not a facetious question, by the way; if it's a requirement I'd like to know about it. Standing by to look dumb(er).)

 

[manstrom]

CH studs are metal studs shaped like a C and H put together. You can find a 3 hour rating in here.

http://www.usg.com/content/dam/USG_...g-shaft-wall-systems-catalog-en-SA926.pdf.pdf

I would recommend 2 separate walls with a CH stud fire wall between them. You should no try to install a ledger spanning over 3 layers of gyp. It just won't work. The separate walls can meet the requirement for one side of the building to fall away and the other to remain (I am unclear on the specific architectural req't. It is either fire "wall" or fire "separation" that must do this. The architect should know)

Archie, I'm finding myself wrapped up in fire rating issues more and more lately. Fire ratings affect structural more than any other specialty. It is important that we have a good grasp on requirements. While the architect *should* know this stuff, it's us who ends up having to fix it.

 

[Prestressed Guy]

Archie. IBC-12 section 202 defines Fire Wall as “A fire-resistance-rated wall having protected opening, 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 is the architects responsibility to specify the fire rated assembly and therefore is outside of my scope of work. once he specifies the assembly I need to make sure that it meets the structural requirements of this assembly. As a case in point, the UL assembly that he has specified calls for minimum 20 ga studs but just to carry the design load if it was concentric would require 14 ga 6" studs. in addition to that I need to find a way to get the bearing load into the studs without violating the requirements of the assembly such as (3) layers of GWB continuous.

 

[Prestressed Guy]

Manstrom, that HC section is new to me. very interesting. I am at a toss up between using two load bearing fire rated walls (one for each side) or calling for a column and beam line that has 3 hour protection.

One thing that I found is that if you use the double wall system, then each of the walls only needs to have a 2 hour rating to get the 3 hour assembly. The problem with the double wall system is that the 59' bay is a single story with 18' plate height and it will need to stand with an 18' unbraced length. That will require some pretty good studs.

CMU may be looking better all the time.

 

[XR250]

I have done this in the past and they constructed the firewall, sheathed it and then built a narrow additional bearing wall on each side. Total pain in the ass and adds thickness.