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..

Wood-framed Deck Design Resources for Engineers

This week’s blog was written by David Finkenbinder, P.E., who is a regional engineer working out of the Simpson Strong-Tie Ohio branch which services 24 states through the Northeast, Midwest, and Mid-Atlantic. He graduated from Penn State with a B.S. in Agricultural and Biological Engineering in 2004 and earned his M.S. in Civil Engineering with a focus on Structural Engineering from Virginia Tech in 2007. His master’s thesis investigated the splitting strength of bolted connections in solid-sawn lumber and structural composite lumber. Since joining Simpson Strong-Tie in 2007, David has shown a passion for deck safety and has served on committees developing prescriptive information and building code provisions for decks. Here is David’s post.

“Decks cause more injuries and loss of life than any other part of the home structure. Except for hurricanes and tornadoes, more injuries may be connected to deck failures than all other wood building components and loading cases combined.”

This quote, taken from Washington State University’s magazine article Making Decks Safer, underscores the critical importance of proper deck design, construction, and maintenance. An engineer who is encountering their first deck may be surprised that the deck design resources available are not as plentiful as he/she might have expected. The following resources can be helpful start:

For decks built to the IRC, the book Deck Construction Based on the 2009 International Residential Code provides a review of applicable code provisions and related commentary. The book gives background on important durability considerations such as flashing at points where the deck connects to an adjacent structure. The book also briefly discusses variations with IBC provisions, which can be significant for examples such as minimum guard height and live loads.

The American Wood Council (AWC) has several tools available in addition to using the NDS for wood member and connection design. Calculators for evaluating simple span joists and single fastener connections are available in both web-based and mobile app format. Technical Report 12, which was the topic of our May blog post, provides the ability to design connections with a gap between members, or with members having a hollow cross section. AWC’s DCA6 – Prescriptive Residential Wood Deck Construction Guide presents information for common deck details and a commentary covering important considerations for alternate designs. While the guide is helpful, please note that it is limited in scope to single level residential decks and does not address wind or seismic design.

Researchers at Virginia Tech and Washington State University conducted laboratory testing and published information to help in several common topics needing attention. An article in the May 2008 issue of Structure Magazine featured test performance of ledger-to-band joist connections using bolts or lag screws – this information has since been adopted into the IRC.

For lateral design there has been some uncertainty regarding lateral loads that can be generated by occupants, and if the magnitude of such is significant in comparison with wind and seismic forces calculated from ASCE 7. Tests were conducted of occupants performing several types of movement on a deck floor configuration. Separate articles summarizing results for each load type were published in the Summer 2013 issue of Wood Design Focus, along with a fourth article on the lateral performance of IRC ledger attachments (online copies of the articles courtesy of Professional Deck Builder magazine: Wind Loads; Seismic Loads; Occupant Loads).

Our January 2013 post, Corrosion: The Issues, Code Requirements, Research, and Solutions, touches on the corrosion considerations that are significant for most projects as well.

Have you found any other resources that have been helpful in your designs? Let us know by posting a comment.

Wood Design Education Opportunities

Designing wood structures properly requires a broad knowledge base of a variety of materials and how they go together.  However, it can often be difficult to find educational opportunities for designers to learn about wood design or keep up with new technologies on wood construction.

Fortunately, there are some unique chances this summer to increase your knowledge about wood as a construction material.

There is a short course titled Advanced Design Topics in Wood Construction Engineering, being held May 21 and 22 at Virginia Tech University in Blacksburg, VA.  It is intended for designers, inspection professionals and builders that want to expand their general knowledge of wood as a building material and their knowledge of building design beyond the introductory level.  The agenda includes sessions on Decay Processes, Design for Durability, and Insects that Attack Wood; Wood Shrinkage Issues in Construction; Lumber Grading Methods and Design Values; Design of Built-Up Beams and Columns; Glulam Beam Design; Evaluating Structural Capacity of Fire-Exposed Timber Beams and Columns; Multiple-Bolt Wood Connection Design; Basics of Diaphragm and Shear Wall Design; Post-Frame Building Design and Diaphragm/ Shear Wall Tests; Creep of Solid-Sawn Joists, I-Joists, and Floor Trusses; Design Considerations for Preventing Flat Roof Failures from Gravity Loads or Sustained Live Loads; Wood Truss Design Responsibilities; Wood Truss Repair Design Techniques; Permanent Truss Bracing Design Basics; and Lateral Design of Decks.

You can find more information about the Virginia Tech Short Course here. Web registration ended May 14, 2014; you can register by calling the Conference Registrar  (540) 231–5182 up to the first day of the course.

If you feel like travelling, the World Conference on Timber Engineering (WCTE) will be held in Quebec City on August 10-14.  WCTE is an international biannual event focusing on timber engineering, engineered wood products and design of timber structures.   The conference theme is “Renaissance in Timber Construction.” Information on the conference can be found here.

But you don’t have to necessarily travel far to get quality training on wood design.

WoodWorks is a cooperative venture of major North American wood associations, research organizations and government agencies that aim to encourage and assist architects, engineers and others in the use of wood in non-residential and multi-family buildings.  WoodWorks deliver knowledge to designers in three main ways:  webinars, short 2-3 hour seminars and Wood Solutions Fairs.  Upcoming webinars include Mixed Use Podium Design, Changes to Wood Design Standards and Healthy Buildings.  Seminars scheduled for June focus on Cross Laminated Timber in California, Pennsylvania, Texas, and Washington.  Finally, Wood Solutions Fairs are excellent all-day events where attendees can choose from more than 15 classes in six sessions throughout the day.  The Fairs also include exhibits to allow for networking with building product manufacturers.  Upcoming Wood Solutions Fairs are May 22 in Chicago, August 27 in Washington, DC, October 23 in Portland, Oregon, and November 12 in Arlington, Texas.  Here is a full schedule of WoodWorks events.

If you just can’t get out of the office, or you don’t like to travel, there are still ways to keep up with the wood industry.  Several groups offer webinars or self-study classes on various subjects.

WoodWorks, mentioned above, is a good resource. The American Wood Council (AWC) is the voice of North American traditional and engineered wood products, representing more than 75% of the industry.  AWC’s engineers, technologists, scientists, and building code experts develop state-of-the-art engineering data, technology, and standards on structural wood products for use by design professionals, building officials, and wood products manufacturers to assure the safe and efficient design and use of wood structural components. AWC also provides technical, legal, and economic information about wood design, green building and manufacturing environmental regulations advocating for balanced government policies that sustain the wood products industry.  AWC has begun offering regular webinars on various subjects with complimentary registration.  Upcoming webinars include the AWC Prescriptive Residential Wood Deck Construction Guide on May 22, AWC Web-based Calculators and Other Resources on June 24, and Prescriptive and Engineering Design per the 2012 WFCM will be offered some time in the fall.  Also, AWC has a comprehensive library of e-courses on their website as well as a helpdesk via email, info@awc.org.

In addition, the International Code Council offers a variety of online training classes as part of their ICC Campus Online.  Most have a nominal fee, but several are available free of charge.  They have a Catalog of Classes on their website.

And finally, don’t forget about resources available from Simpson Strong-Tie. These resources range from full and half-day workshops offered at various locations throughout the country to online courses you can take from the comfort of your own office.  Many of these courses come with CEU credits and some also offer AIA credits.  And if you would like a personal visit, such as a lunch-and-learn, contact your local sales rep, or one of our regional offices and ask to speak with the training manager.

Do you know of any other good events coming up?  Keep the conversation going.