Questions Answered: Making Wood Connections Work for Two-Hour Fire Walls

Issue #10232 - November 2018 | Page #90

By Paul McEntee, S.E.

In this article, Paul McEntee follows up on our July 25 webinar, Making Wood Connections Work for Two-Hour Fire Walls, by answering some of the interesting questions raised by attendees.

We presented an hour-long webinar recently about using fire wall hangers in Type III wood-frame buildings, how they are tested and what the code requirements are for these buildings. In case you weren’t able to join our discussion, you can watch the on-demand webinar and earn PDH and CEU credits online.

As with our previous webinars, we ended with a Q&A session for the attendees. But clearly, we needed more time to chat. Tom Evans, our project manager for hangers, and I answered as many as we could in the time allowed, but we were quickly overwhelmed. Following the webinar, we sat down and answered all of them. Here are some of the commonly asked questions and their answers, but if you’d like to see the full list, it is online.

Fire Wall Hanger Code Definitions and Requirements

Isn’t there a difference between two-hour firewalls and two-hour fire-rated walls?

A two-hour fire-rated wall includes all wall types with a two-hour rating. A two-hour fire wall is more specific and has the requirement of maintaining its independence and integrity regardless of whether the adjacent floors or roof fails.

I would like a clarification — how would the provisions of the 2015 IBC Section 706.2 Structural Stability be compliant with this solution for a “Fire Wall”? I see how this would work with a Fire Partition and a Fire Barrier.

The typical application for use in Type III construction would be a “Fire Barrier” because a wall needs floors on both sides for structural stability. A “Fire Wall” application may be at an interior area separation wall where there is framing on both sides of the wall. So if one side burns away, the floor framing on the other side is there to provide stability. In a lot of multi-family projects, the walls are double 2x4 walls with an airspace, primarily for sound. That also helps meet the fire requirements, since the framing and wall on one side could burn away leaving the other wall still intact. However, I suppose technically that’s not a fire wall but two independent fire barriers.

Could you explain the char rate again and how it is used for a fire rating determination?

NDS® Chapter 16 and AWC Tech Note 10 have nominal char rate definitions and equations to calculate effective char rate and required char depth for up to two-hour resistance. So you calculate the thickness of wood required to achieve the desired rating and provide at least that thickness to ensure the fire will not char through the wood in that time. If you are designing heavy timber, you oversize the beam/post by the required char depth. However, there are more details to it in the AWC Tech Note that need to be considered. Remember, this is a tool to rate char of a “standard” fire, thus a comparison tool to compare to other similarly rated systems. Actual fires and conditions will, of course, vary in intensity from the standard.

It is important to note that the F and T ratings and Intertek listing for the Simpson Strong-Tie DG series is based on an actual ASTM E814 two-hour fire test — i.e., not based on calculations alone.

Is it acceptable to use non-FRT material in an exterior bearing wall of Type III? Shouldn’t the material be noncombustible?

The IBC reference is below. Noncombustible material is required, but fire-retardant-treated is also permitted in the section. So non-FRT would not be allowed for the wood framing in Type III.

602.3 Type III. Type III construction is that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of any material permitted by this code. Fire-retardant-treated wood framing complying with Section 2303.2 shall be permitted within exterior wall assemblies of a two-hour rating or less.

Fire Wall Hanger Installation

On an interior wall with hangers on both sides, is the plywood subfloor (diaphragm) permitted to be continuous over the wall, or is it discontinued by the wall sheathing?

That is up to the local jurisdiction. In most cases, the decking is continuous to the outside face of the wall plates. The type of detailing that is being discussed is for a fire barrier, not a code-defined fire wall.

Does the hanger need to resist fire in these assemblies? Is intumescent paint required?

For the two-hour rating of the wall, the hanger does not need to have fire resistance. If you’re talking about the floor-ceiling assembly, the hanger would have to be part of the rating. However, to our knowledge, floor-ceiling fire tests are typically done without hangers. In many one-hour rated assemblies, the 5/8” gypsum ceiling provides much of the rating. Intumescent paint is not required on Simpson Strong-Tie hangers unless specifically called out on the plans.

I am concerned about the slot in the drywall around the hanger. Doesn’t this affect the rating?

The notch that is cut in the drywall around the hanger does not reduce the two-hour rating for the wall. In fact, that is the entire purpose of running ASTM test E814. The drywall notches were part of the fire test and are fully described in our design, which is found in our literature and in the Intertek Directory. Please feel free to contact Simpson if you have questions about that.

Fire Wall Hanger Design/Load Rating/Testing

What’s the typical reduction factor for face-mounted hangers’ capacity when they are attached over wood structural panel sheathing or drywall?

We have allowable loads for DG and DGH hangers installed over wood structural panel sheathing available on the product pages. Reductions are (roughly) 5–10%, but specific values are available in an engineering letter. The hanger is not intended to be installed over drywall, but can be installed over the sheathing.

Face-mounted hangers installed through drywall could see a significant reduction. This is especially true with nails, as they have limited bending strength to span through the gypsum board (drywall).

For lighter loads, the DU/DHU hangers are designed to be installed over two layers of drywall. For higher loads, we have tested several face-mount hangers that use Strong-Drive® SDS Heavy-Duty Connector screws installed through two layers of drywall into solid headers, and they perform well. Load information will be published soon. A face-mount hanger mounted on a wood structural panel over a solid beam has no reduction, provided that the sheathing is well attached to the beam. See engineering letter L-C-FCMNTSHTG18 for more information.

The more a hanger stands off a wall the more moment from the loads get into the framing along with shear. How do you deal with that?

The vertical offset causes a moment in the hanger. The resisting moment is supplied by a couple made from two horizontal forces: (1) nails in shear at the top flange; and (2) bearing of the hanger against the wall which occurs against the face of the wall top plates.

The footnotes for the load tables specify a reduced load capacity for joists spaced less than 16“ on center. Is the load governed by the capacity of the top plate?

Correct. There is a certain amount of torsion induced into the top plate. We tested for 16″ o.c. joist spacing. Presumably the reduction would be linearly proportional to a reduced spacing. At this time we do not take advantage of increased spacings.

Miscellaneous Questions

What makes a fire-rated hanger different than any other hanger?

Just to clarify, the hanger itself is not a fire-rated hanger. The hanger has been evaluated in its use in a rated two-hour wall assembly. However, what makes the DG series different from most other joist hangers is the short cantilever section through the drywall notch, which allows it NOT to require notching into the wall cavity.

Can this series of hangers be used with a single layer of drywall?

These hangers can be used with single-layer systems, because the drywall is not required to achieve the allowable loads. However, Simpson Strong-Tie does not claim the F and T ratings for a one-hour system using one layer of gypsum drywall. For a single layer, we have much more economical hanger solutions in the technical bulletin T-C-TFWALL16. However, the standard hangers installed over drywall require the drywall to be installed first.

Are these the only hangers or methods that Simpson Strong-Tie provides for two-hour fire-rated approved walls?

No. We also have the DU/DHU/DHUTF series hanger which installs over the drywall using Strong-Drive® SDS Heavy-Duty Connector screws into the wall top plates. The information can be found in our Wood Construction Connectors catalog (C-C-2017) on pages 196–197 or on our website.