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..

Midrise of Steel

Guest blogger Jeff Ellis, engineering manager

Guest blogger Jeff Ellis, engineering manager

The number of midrise structures constructed using light-frame cold-formed steel (CFS) certainly seems to be increasing each year. As with any material, there are benefits and challenges, especially in areas of moderate to high seismic risk. This post will discuss these as well as potential solutions.

Light-frame CFS midrise construction often uses ledger floor framing primarily to facilitate the load transfer detailing at the floor, tension anchorage (tie-downs or hold-downs) and compression chord studs or posts designed to resist the amplified seismic overturning loads. CFS framing is typically thin and singly symmetric.

Various CFS Construction Floor Framing Methods

Various CFS Construction Floor Framing Methods

 

 

 

 

 

 

Amplified Seismic Load

The AISI Lateral Design standard (AISI S213-07/S1-09) Section C5.1.2 requires that the nominal strength of uplift (tension) anchorage and the compression chord studs for shear walls resist the lesser of (1) the amplified seismic load or (2) the maximum load the system can deliver when the response modification coefficient, R, greater than 3. The amplified seismic load is defined as the load determined using the ASCE 7 seismic load combinations with the overstrength factor, Wo, which may be taken as 2.5 for CFS framed shear wall systems with flexible diaphragms.

Typically, the maximum the system can deliver to the uplift anchorage or chord studs is taken as the forces determined using the nominal shear strength of the shear wall assembly tabulated in the seismic shear wall table in S213 multiplied by 1.3. The S213 commentary accounts for the tabulated loads being based on Sequential Phased Displacement (SPD) rather than CUREE cyclic protocol and the degraded backbone curve. See the Structure magazine article that discusses the design of CFS framed lateral force-resisting systems.

Continuous Rod Tie-Down Systems

Light-framed CFS over three stories often use continuous rod tie-down systems rather than cold-formed steel hold-downs to resist shear wall overturning forces as they offer increased load capacity. Neglecting the dead load contribution, the amplified seismic load requirement for CFS shear walls using an R greater than 3 results in an 80% increase in the load used to size the continuous rod tie-down system compared to design level loads. For shear walls using an R greater than 3, it is important to note on the design drawings whether the uplift loads shown are ASD, LRFD, amplified ASD or amplified LRFD so the appropriate tie-down system may be designed.

Continuous Rod Tie-Down System Resisting Shear Wall Overturning Forces

Continuous Rod Tie-Down System Resisting Shear Wall Overturning Forces

Continuous rod tie-down systems are designed not only for strength, but also checked to ensure they do not deflect too much to cause the top of shear wall drift to exceed the code limit or to exceed the 0.20” vertical story deflection limit required by some jurisdictions and ICC-ES AC316. Take-up devices are used in CFS framed structures to take-up construction and settlement gaps that may occur.  AISI S200 Section C3.4.4 states that a gap of up to 1/8” might occur between the end of wall framing and the track. The vertical elongation of the continuous rod tie-down system includes rod elongation (PL/AE) and the take-up device deflection due to the seating increment and the deflection under load.

In addition, coordination is important in using continuous rod tie-down systems in CFS structures because the walls are often prefabricated offsite. An example is the consideration of the appropriate detail for the steel bearing plate installed at the floor sheathing in the story above to resist the uplift (tension) force from the story below.

One possible detail is to install the bearing plate in the bottom CFS track under all the CFS chord studs, but it’s important to ensure the bottom track flanges are deep enough to screw them to the stud flanges as the bearing plate can have a thickness of 1 ½” or more and typical tracks use 1 ¼” flanges. It is also important to ensure that the bearing plate width fits in the track. Another possible detail is to install the bearing plate under the CFS track under all the CFS chord studs.  However, then it must be cut into the floor sheathing and may cause the bottom track to be raised at the bearing plate. For this detail, the floor shear transfer must be detailed through the ledger into the CFS framing.

Continuous Rod Tie-Down System Steel Bearing Plate Coordination Issues

Continuous Rod Tie-Down System Steel Bearing Plate Coordination Issues

Concrete Tension Anchorage

The concrete tension anchorage is designed according to ACI 318 Appendix D using the continuous steel rod material and size in accordance with S213 to have the nominal strength to resist the lesser of the amplified seismic force or the maximum load the system can deliver. ACI 318-11 Section D.3.3.4.3 offers four force limits for design of concrete tension anchorage design in Seismic Design Category C through F:

(1)   The concrete nominal tension anchorage strength shall be greater than 1.2 times the ductile steel rod nominal tension anchorage strength

(2) The anchorage design strength shall be greater than the maximum tension force that can be delivered by a yielding attachment;

(3) The anchorage design strength shall be greater than the maximum tension force that can be delivered by a non-yielding attachment; and

(4) The anchorage design strength shall be greater than the amplified seismic force.

Typically either option (1) or (4) is used where (1) would lead to less concrete required than (4) if the bolt is efficiently sized while (4) would be required for such conditions as a vertical irregularity.  See the concrete anchorage and podium anchorage SE Blog posts for more details.

ACI 318-11 Section D.3.3.4.3 Anchorage Design Options

ACI 318-11 Section D.3.3.4.3 Anchorage Design Options

CFS Wall Stud Bracing

CFS studs are typically thin and singly symmetric and thus require bracing. AISI S211 (Wall Stud Design Standard) permits two types of bracing design that cannot be combined; sheathing based or steel based. There are limits on the stud axial strength when using sheathing braced design. It’s important to identify on the drawings that the sheathing braces the studs and another load combination must be used for the stud design.

2012 IBC Section 2211.4 requires stud bracing to be designed using either AISI S100 (North American Specification) or S211 (Wall Stud Design Standard). S100-07 Section D3.3 required nominal brace strength is to be 1% of the stud’s nominal compressive axial strength, but S100-12 Section D3.3 changes this to the required brace strength is to be 1% of the stud’s required compressive axial strength (demand load). In addition, D3.3 requires a certain stiffness for each brace. AISI S211 required brace strength is to be 2% of each stud’s required compressive axial strength for axially loaded studs and, for combined bending and axial loads, be designed for the combined brace force per S100 Section D3.2.2 and 2% of the stud’s required compressive axial strength.

There are two primary types of steel stud bracing systems: bridging and strap bracing. U-channel bridging extends through the stud punchouts and is attached to the stud with a clip, of which there are various solutions such as this post on Wall Stud Bridging.  Bridging bracing requires coordination with the building elements in the stud bay. It installs on one side of the wall, and does not bump out the wall sheathing. It also requires periodic anchorage to distribute the cumulative bracing loads to the structure for axially loaded studs often using strongback studs and does not require periodic anchorage for laterally loaded studs since the system is in equilibrium as the torsion in the stud is resisted by the U-channel bending.

Flat strap bracing is installed on either side of the wall and at locations other than the stud punchout. It bumps out the sheathing and requires periodic anchorage to distribute the cumulative bracing loads to the structure for axially and laterally loaded studs.

Beam

Strap and Block Bracing

Strap and Block Stud Bracing Anchored Periodically to Structure Using Strongbacks

Beam2

Bridging and Clip Bracing Anchored Periodically to Structure Using Strongbacks

Bridging and Clip Bracing Anchored Periodically to Structure Using Strongbacks

Bridging and Clip Bracing Anchored Periodically to Structure Using Diagonal Strap Bracing

Bridging and Clip Bracing Anchored Periodically to Structure Using Diagonal Strap Bracing

Light-frame cold-formed steel construction has been used successfully for many projects, but there are challenges  that must be addressed to ensure code compliance and desired performance. Some beneficial resources for designing CFS structures are the SEAOC 2012 IBC Structural/Seismic Design Manual Volumes 1 and 2 and the Cold-Formed Steel Engineers Institute’s (CFSEI) website where you can find technical notes and design guides.

What have been some of your observations or challenges in designing cold-formed steel midrise structures?