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Using Engineered Wood in 2-hour Fire Resistance Wall Construction
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With the recent increase in multifamily construction, we are seeing more instances where 2-hour rated walls are required. This leads to questions concerning how to properly frame the intersection of the floor and wall assembly. 2-hour fire rated walls are required in some buildings based on Table 601 and 602 of the International Building Code. The expectation is that fire protection extends through the rim location of the floor assembly. Some typical locations you will see a 2-hour fire rated wall requirements include mechanical shafts, elevator cores, stair ways, and exterior load bearing walls.

The traditional approach of platform framing (using a rim board and floor framing on top of a stud wall) is still the preferred method of construction both for economics and speed. A traditional approach to protect the rim board is to install the same quantity of gypsum protection used on the walls below to protect the rim above.  This method creates complications for the project due to the need of the gypsum applicator to install during the framing stage in additional to completion of framing. This involves scheduling multiple trips to the job site and extra fees.  Another method is to balloon frame the wall with custom stud heights, apply gypsum and ledger the floor system to the wall. There are also new hangers available to be support floor joists through two layers of gypsum in lieu of a ledger, but this does not solve the problem of having the gypsum installer providing multiple job site visits.

The growing trend at these continuously supported intersections is to install a large section of wood for fire endurance in lieu of a smaller section with multiple layers of gypsum. The rationale behind this is that if you have a large enough section of wood, after two hours of burning, there is still sufficient section remaining to have capacity to transfer loads defined by the designer of record.

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5 ¼ Parallam® PSL may be a suitable material at continuously supported locations to meet the 2-hour fire rated wall requirement based on the following code provision.

Type III and V Construction: The IBC (section 722) allows calculated fire resistance of exposed wood members from Chapter 16 of the ANSI/AF&PA National Design Specification for Wood Construction (NDS). Calculating fire resistance per the NDS is valid for Parallam PSL as stated in section 4.3.1 in our ESR 1387 code report. Contained within the NDS is a formula to determine the effective char rate based on time exposure and nominal char rate. Per testing listed in our TJ-1500 specifiers guide (Fire Rated Assemblies and Sprinkler Systems), Parallam PSL has a nominal char rate of 1.5″/hour.  When looking at a 5-1/4″ section of Parallam PSL, the effective layer of char is 3.2″ for a 2 hour time period, assuming only one face exposed to the fire, leaving the remaining section to be checked for load adequacy.

It is important to note that designers must always verify compliance of this approach with the authority having jurisdiction, especially for applications that require fire retardant treatment in the code. This is because Parallam PSL cannot be treated with fire retardant.

What are the benefits of using Parallam PSL in this application?

There are other unique benefits to using a solid piece of Parallam PSL over other traditional materials and methods described above.  Most already know the reasons not to use dimension lumber, but they’re worth reviewing. First,dimension lumber does not come in depths that are compatible with TJI® Joists and Weyerhaeuser structural composite lumber. Even if it did, the material may be “green” or at best kiln dried down to 15% moisture content (MC), resulting in significant shrinkage compared to other engineered lumber floor components. This can lead to structural and aesthetic problems.

This same concern exists with glued laminated timbers. They may be manufactured and published with the same depths as engineered wood products, however, Glulam height tolerances for beams allow up to minus 3/16″. Besides the variability in manufacturing, one must also consider that the lam stock used to make Glulams are permitted to be up to 16% MC at time of manufacture, resulting in typical finished product MC of 12-14%.  These two items can lead to potential serviceability and structural issues.  Parallam PSL and TJI joists are manufactured under tight quality control targets, allowing the products to be compatible from a MC and product depth standpoint.  The MC of Parallam and TJI joists is approximately 4-6%, resulting in a dimensionally stable floor system.

In review, there are alternate methods to achieve 2-hour fire rating at the floor and wall intersection that are code recognized utilizing readily available materials. Review of the structural and fire requirements should be coordinated through the Engineer and Architect on the project to ensure approval by the authority having jurisdiction. For more information on Parallam PSL visit our website.

About Jim Anderson

Jim Anderson, P.E. is an engineer for Weyerhaeuser supporting Trus Joist Engineered Wood Products for the past 15 years. His last 10 years have been focused on support for the Western US markets, including reviewing large multi-family structures.