Firewalls for Wood Construction

What is a firewall?

A firewall is a term that is used in the construction industry to describe a fire-resistive-rated wall or fire-stop system, which is an element in a building that separates adjacent spaces to prevent the spread of fire and smoke within a building or between separate buildings. A firewall is actually one of three different types of walls that can be used to prevent the spread of fire and smoke.

 Types of fire-resistive-rated walls: 

 The three types of fire-resistive-rated walls are firewalls, fire barriers and fire partitions. They are listed in order from the most stringent requirements to the least. A firewall is a fire-resistive-rated wall having protected openings, 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. A fire barrier is a fire-resistive-rated wall assembly of materials designed to restrict the spread of fire which continuity is maintained.  A fire partition is a vertical assembly of materials designed to restrict the spread of fire in which openings are protected.  Each type has varying requirements and the table below displays some of the differences between them.

fire-resistive-rated-wallsWhat are some of the typical uses of each type of fire-resistive wall? 

As the requirements for each type of wall vary, so do the uses. Typical uses of each are as follows:

  • Firewalls – party walls, exterior walls, interior bearing walls
  • Fire barriers – shaft enclosures, exit passageways, atriums, occupancy separations
  • Fire partitions – corridor walls, tenant space walls, sleeping units within the same building

How do you determine whether your wood building design needs a firewall?

The 2012 International Building Code (the IBC, or “the Code” in what follows), which is adopted by most building departments in the United States, is the resource we are using in this discussion. (As a side note, it’s possible your city or county has supplemental requirements, and it is best to contact your local building department for this information up front.)

To determine your fire-resistive wall requirements, review these chapters in the 2012 IBC:

  • Chapter 3, Identify Occupancy Group – typically Section 310 (“Residential Group”) for wood construction
  • Chapter 5, Select Construction Type – Section 504, Table 503
  • Chapter 6, Determine Fire-Resistive Rating Requirements – Table 601, typically Type III wood-constructed buildings require a two-hour fire separation for the exterior bearing walls

What are typical fire-resistive wall designs? 

 Information for one-hour, two-hour designs, etc. can be found in tables 721.1(2) and 721.1(3) of the Code provide information to obtain designs that meet the rating requirements (in hours) for your building, including the walls and floor/roof systems. The GA-600 is another reference that the Code allows if the design is not proprietary.

How do I know whether the structural attachments I specify for the wall and roof assemblies meet the Code requirement?

Once the wall or floor/roof assembly design is selected, the Designer must ensure that the components of the wall do not reduce the fire rating. The Code requires that products which pierce the membrane of the assemblies at a hollow location undergo a fire test to ensure they meet the requirements of the design. ASTM E814 and ASTM E119 are the standards governing the fire tests for materials and components of the fire-resistive wall. There are several criteria that the component in the assembly must meet: a flame-through criterion, a change-in-temperature criterion and a hose-stream test.

Simpson Strong-Tie has created the DHU hanger for use with typical two-hour fire-resistive walls for wood construction.The DHU hanger has passed the ASTM E814 testing and can be used on a fire-resistive wall of 2×4 or 2×6 constructions and up to two 5/8″ layers of gypsum board. The DHU and DHUTF have both an F (Fire) and a T (Temperature) rating.

dhutf-dhu-hangersThe DHU/DHUTF hanger has two options, a face-mount version (DHU) and a top-flange version (DHUTF).  The hanger doesn’t require any cuts or openings in the drywall, which ensures reliable performance; no special inspection is required.  To install the hanger, gypsum board must first be installed in a double or single layer, at least as deep as the hanger.  For installation, apply a two-layer strip of Type X drywall along the top of the wall, making the base layer a wider strip (bottom edge is 12″ or more below the face layer, depending on jurisdiction).  Then install ¼” x 3½” Simpson Strong-Tie Strong-Drive® SDS screws through the hanger and into top plates of the wall.  Since the hanger is more eccentric than typical, the top plates of the wall must be restrained from rotation. The SSP clip can be used for restraint, but the design may not require it if there is a sufficient amount of resistance already in place, such as sheathing, a bearing wall above, or a party wall as determined by the designer.  See the photos and installation illustration below for guidance or visit our website for further information.

typical-installation-over-2-layers-drywall

 

 

Fire Protection Considerations with Five-Story Wood-Frame Buildings: Part 2

Bruce Lindsey is the South Atlantic Regional Director for WoodWorks The Wood Products Council, which provides free project assistance as well as education and resources related to the design of nonresidential and multi-family wood buildings. Based in Charlotte, NC, Bruce’s multi-faceted career with the industry spans 20 years and includes architectural design, structural design and roles within the engineered wood products industry related to marketing, product management, distribution, consulting and sales.

Last week’s post reviewed some of the common questions WoodWorks receives from engineers designing five-story, Type III wood-frame buildings—including those related to fire retardant-treated building elements, and fire-rated floor and wall assemblies. This week, we extend that conversation to another common issue—details and fire rating of floor-to-wall intersections.

The fire rating of an exterior wall assembly in Type III construction causes a detailing issue where the floor intersects the exterior wall assembly. There are no testing criteria established by the code for system intersections of any material, so detailing must rely on code interpretation. The two points of interpretation focus on continuity of the two-hour wall fire rating and the FRT requirement.

Section 705.6 of the 2012 IBC 1 requires that an exterior wall have “sufficient structural stability such that it will remain in place for the duration of time indicated by the required fire resistance rating.” The ‘interruption’ of the floor in the plane of the exterior wall may be seen by authorities as affecting the structural stability. It is not clear how designers are to comply with this language; for that reason, the language has been removed in the 2015 IBC.

The implication of FRT continuity is derived from the primary requirement that Type III buildings have noncombustible exterior walls. FRT wood is permitted in these walls per IBC Sections 602.3 and 602.4. Since the noncombustibility or acceptable FRT alternative is intended to reduce fire exposure to other buildings, some code officials require FRT material in the plane of exterior walls through the floor intersection. The degree to which a building official believes that the rim joist, floor joist and/or sheathing present a risk of fire spread will determine the degree of FRT material required through the floor-wall intersection.

The manner in which this floor-to-wall connection can be detailed first depends on the type of framing being used—traditional platform framing or semi/modified balloon framing. Platform framing relies on the fact that the floor system bears directly onto the wall below. Semi-balloon framing relies on hangers to support the floor framing.

Typical platform-framed floor-to-wall intersections have been accepted by many jurisdictions without any special detailing according to the rationale that the area of intersection represents “floor framing” and not “wall framing.” In these intersections, the “floor” is not required to be FRT and its fire resistance is limited to one hour. This is similar to the floor conditions found in Type V construction; where such conditions obtain, it’s also logical to extend the same detailing allowances at this intersection to Type III buildings.

While local code interpretation varies widely, a variety of detailing concepts have arisen across the country as possible solutions to this issue.

In one solution, a solid sawn, glulam or engineered rim board is used to create continuity of the two-hour rating through the plane of the wall by using the charring capability of the rim board calculated using Chapter 16 of the NDS. Variations of this detail include a built-up rim board. In some solutions, the member closest on the outside of the wall may also be FRT to provide some degree of FRT continuity. If continuity of FRT through the floor for the entire width of the wall is also required, the entire thickened rim board and possibly the first sheet of floor sheathing may need to be FRT. In some scenarios without heavy FRT requirements, a hanger is not needed if the rim board width that can accommodate the charring is narrower than the width of the wall and the joist can bear on the top plate itself.

Another option is to use a continuous 2x block to achieve one hour of fire resistance, again calculated using Chapter 16 of the NDS. The second hour of resistance is provided by the horizontally applied drywall on the underside of the floor. While the two layers of drywall may not be in the plane of the wall, they still provide two hours of fire endurance. This detail may or may not require that the block and the floor sheathing be FRT, depending on the FRT continuity interpretation. Variations of this detail include an option where the blocking is moved inside the plane of the wall between the joists. Some jurisdictions object, citing concerns about fires starting in the floor cavity. There are other measures, such as fire blocking or cavity sprinklers, provided to minimize spread of fire in these situations. The same question could be asked about fires starting within a wall cavity.

A third option is a slight variation of the second. Instead of using blocking to achieve the one hour of fire resistance, one layer of drywall can extend up behind certain proprietary top flange joist hangers (for SST example, click here). This provides one hour of fire resistance in the plane of the wall, and the second hour is provided by the drywall on the underside of the floor. Some contractors find this detail difficult to accommodate because of construction sequencing — the drywall crew typically does not arrive on site until after rough framing is complete. A variation seen in some areas is using a top-chord-bearing truss, which eliminates the hanger hardware and minimizes the non-treated penetration in the plane of the exterior wall. Addressing full FRT continuity may be more difficult with this variation depending on the truss manufacturer.

A fourth option requires relatively new concepts using connector solutions that allow two layers of gypsum to be applied behind the floor joist connection to the wall (for SST example, click here). Hardware solutions can be a useful option to have available when an Authority Having Jurisdiction is particularly wary of maintaining the integrity of a rated wall assembly, but Designers should consider both the labor and the cost of these details to determine the best fit for the project.

In addition to regional nuances and differing (and evolving) code interpretations, there isn’t one solution that fits all applications. Designers should determine the local availability of FRT products, review manufacturer product specifications and discuss the proposed solution with their jurisdiction.

Available Support

If you’re designing a mid-rise wood building and have questions—e.g., about fire and life safety, lateral and vertical loads, how to address shrinkage, etc.—I encourage you to contact your local WoodWorks regional director. The WoodWorks website (woodworks.org) also offers a wide range of technical information on mid-rise structures and we welcome inquiries to the project assistance help desk (help@woodworks.org).

1Information is based on the 2012 International Building Code unless otherwise indicated.

Fire Protection Considerations with Five-Story Wood-Frame Buildings Part 1

Bruce Lindsey[1]Bruce Lindsey is the South Atlantic Regional Director for WoodWorks The Wood Products Council, which provides free project assistance as well as education and resources related to the design of nonresidential and multi-family wood buildings. Based in Charlotte, NC, Bruce’s multi-faceted career with the industry spans 20 years and includes architectural design, structural design and roles within the wood products industry related to marketing, product management, distribution, consulting and sales.

As a regional director for WoodWorks, my job is to provide technical assistance related to the design of nonresidential and multi-family wood buildings. I’ve been with the program since it launched in 2007 and, although we support a full range of building types, I’ve seen a steady increase in the number of design professionals looking for information and support related to mid-rise wood structures in particular.

Reasons for this are summed up in a recent Wood Solution Paper by my colleague, Lisa Podesto, PE, Maximizing Value with Mid-Rise Construction, in which she points out that wood-frame construction is a cost-effective choice because it allows high-density use (five stories for many residential occupancy groups, six for office) at relatively low cost, while providing other benefits such as construction speed, structural performance, design versatility, sustainability, and a light carbon footprint.1

In particular, WoodWorks gets a lot of calls from engineers designing five-story, Type III wood-frame buildings, since the structural challenges are considerably different than they are for buildings up to four stories. We provide technical support (at no cost), from conceptual design through construction of a project, helping to work through issues such as the following:

Fire Retardant-Treated Building Elements

Type III buildings are required to have fire retardant-treated (FRT) exterior walls, and designers often struggle with how to specify FRT. While preservative-treated products are typically applied under a set of prescriptive requirements according to the American Wood Protection Association (AWPA) U1 standard, FRT wood is defined in IBC Section 2303.22 and differs from preservative-treated specification because treatments include proprietary formulations and application processes that instead meet a performance standard. Each of the treatment formulations has it’s own recommendations with regard to corrosion resistance of fasteners and strength reduction factors for wood members and connections. Full recommendations can be found in individual evaluation reports from FRT suppliers. Engineers might consider using the worst-case reduction factors for design to allow contractors the flexibility to source FRT from different suppliers.

Fire-Rated Wall Assemblies

While all Type III construction requires two-hour fire-rated exterior walls, it can be challenging to find tested assemblies that meet this criterion. When looking for these assemblies—and indeed all assemblies—it is helpful to keep a few things in mind:

  • Structural panels may add to fire resistance – Many assemblies may not show wood structural panels in the approved assembly, but exterior walls usually require wood sheathing for lateral resistance of the building, sometimes on both sides of the wall. The addition of wood structural panels to assemblies should not diminish the fire rating, as acknowledged in the General Notes section of the Gypsum Association Fire Resistance Design Manual, which allows their addition. The second rule in Ten Rules of Fire Endurance Rating by Tibor Harmathy, presented in the American Wood Council publication, CAM for Calculating and Demonstrating Assembly Fire Endurance, says, “The fire endurance does not decrease with the addition of further layers.” Another resource that may assist designers is the ICC-ES Evaluation Report ESR-2586, Performance Standards and Qualification Policy for Structural-use Panels, which states, “Structural-use panels may be installed between the fire protection and the wood studs on either the interior or exterior side of fire-resistance-rated wood frame wall and partition assemblies described in the applicable code, provided the length of fasteners is adjusted for the added thickness of the panel.”
  • FRT studs may be used – For Type III construction, FRT wood is also a requirement in exterior wood wall assemblies, in addition to the two-hour rating. Some two-hour-rated assemblies may not specifically state that FRT studs may be used, but the UL Guide Information clarifies that FRT may be used in place of non-treated wood in any assembly.

Fire-Rated Floor Assemblies

Both Type IIIA and Type VA construction require one-hour-rated floor assemblies. Even when using Type B, generally considered unprotected construction, with a residential occupancy, floors between dwelling units still need protection per IBC Section 711.3.

  • Floors less than 10 inches deep – As with wall assemblies, finding fire-rated floor assemblies that meet the design parameters can be challenging. In mid-rise applications, it is common for designers to go to great lengths to minimize the floor depth in order to maximize the plate height at every level and still stay beneath the overall height limit of the structure. However, there are few available UL assemblies with a minimum joist depth of less than 10-inch nominal. Designers can use either IBC Section 721 with the Deemed to Comply tables, or Section 722 on calculated fire resistance to address this issue.
  • Using structural composite lumber in floors – While a similar lack of published options is true of assemblies with structural composite lumber (such as laminated veneer lumber, laminated strand lumber or parallel strand lumber), the argument for using these products in fire-rated assemblies lies in their ICC-ES reports. The section under Calculated Fire Resistance states that the fire resistance of an exposed wood member—solid sawn, structural glued laminated timber (glulam) and structural composite lumber—can be calculated using Chapter 16 of the National Design Specification® (NDS®) for Wood Construction, which implies that the fire resistance is equal to that of solid sawn members. The structural adhesives used can withstand temperatures beyond that of wood.
  • Heavy timber corridor decking – Some designers use a heavy timber decking over corridors allowing taller plate heights and/or unencumbered area for utilities to run above a drop ceiling. This accomplishes a one-hour resistance by using char calculations for exposed wood elements as outlined in Chapter 16 of the NDS stipulated as an alternate method in IBC 722.1.

If you’re designing a mid-rise wood building and have questions—e.g., about fire and life safety, lateral and vertical loads, how to address shrinkage, etc.—I encourage you to contact your local WoodWorks regional director. The WoodWorks website (woodworks.org) also offers a wide range of technical information on mid-rise structures, and we welcome inquiries to the project assistance help desk (help@woodworks.org).

1For more information, visit www.woodworks.org/why-wood

2Information is based on the 2012 International Building Code unless otherwise indicated.

… To be continued in next week’s blog with information on details and fire rating of floor-to-wall intersections..