Designing Overhangs on Gable Ends

It seems that each major hurricane tends to teach those of us in the construction industry some lesson. With Hurricane Andrew, the lessons were the importance of protection from windborne debris, and the importance of proper construction of gable ends.

There are two main areas where gable ends can fail. One is a failure of the hinge at the connection between the top plate of the wall and the gable end framing, if the gable end is not balloon-framed with continuous studs. This is now addressed in the International Residential Code. Since 2009, Section R602.3 has required that “Studs shall be continuous from support at the sole plate to a support at the top plate to resist loads perpendicular to the wall. The support shall be a foundation or floor, ceiling or roof diaphragm or shall be designed in accordance with accepted engineering practice.”

For existing construction, the International Existing Building Code specifies a method for retrofitting gable ends in Appendix C.  For new construction, Simpson Strong-Tie shows a couple of solutions for bracing top plates of gable ends in our High Wind–Resistant Construction Application Guide on Page 48.

Figure 1, Gable Wall Bracing Methods

Figure 1, Gable Wall Bracing Methods

The other common wind-related failure at gable ends is uplift of the roof decking at the overhang. This can be from two causes: inadequate nailing of the sheathing to supporting framing, or inadequate connections of the framing at the rake edge that supports the roof. As far as this author can tell, this area of light construction is not covered in the International Residential Code for wood framing, but it is covered for cold-formed steel framing, where Section R804.3.2.1.2 contains requirements for “Rake overhangs.” The two methods shown are the cantilever outlooker (Option 1) and the ladder outlooker (Option 2).

Figure 2, IRC Gable Overhand Details

Figure 2, IRC Gable Overhand Details

Figure 3, Gable End Wind Damage

Figure 3, Gable End Wind Damage

In the photo above, it appears that the cantilevered outlooker method was used, and that there was a failure of the outlooker connections at the gable end and the first full truss. If you look closely, the end nails from the full-height truss that were in the end of the outlookers can be seen in a couple of places.

If a truss roof is used with this method, the gable truss is manufactured 3½” shorter than the others. Then a 2×4 outlooker is placed over the dropped gable, and butted into the side of the adjacent full-height truss. Then the barge or fly rafter is attached to the end of the cantilevered outlooker. At the overhang, wind can cause uplift on both the bottom and top surface. The uplift at the end of the outlooker imparts an uplift force at the gable truss, which must be resisted by a tension connection such as a hurricane tie, and a downward force at the connection to the full-height truss.

Figure 4, Cantilevered Outlooker Method

Figure 4, Cantilevered Outlooker Method

The other method commonly used to support the sheathing and the barge rafter is the ladder method. With this technique, lookout blocks are used to connect the barge or fly rafter back to the gable framing. One way this can be constructed is as a full ladder, with parallel fly rafter and ledger with block framing in between. Either this assembly can be constructed on the ground and then raised and fastened in place, or it can be built in place at the overhang. Or there are also examples where a ledger is not used, and the block framing is just connected directly to the top chord of the gable truss or gable rafter. This method is less wind-resistant, and in literature is limited to a 12″ overhang.

Figure 5, Ladder Outlooker Block Method

Figure 5, Ladder Outlooker Block Method

If the gable overhang is to resist wind loads properly, it must either be designed, or constructed in accordance with some pre-engineered prescriptive detail. Figure 4 shown above was originally published in a Simpson Strong-Tie Technical Bulletin, the High Wind Framing Connection Guide. But this Guide is no longer published. As shown earlier in Figure 2, there are some prescriptive details in the IRC for cold-formed steel construction. These are limited to an overhang length of 12″ and apply for up to 139 miles-per-hour ultimate wind speed. For wood-framed construction, comparable details are contained in the American Wood Council Wood Frame Construction Manual. For the cantilevered outlooker method, connection design loads are published for various wind speeds. Cantilevered outlookers are permitted to extend out up to 24 inches, while the ladder outlookers are only permitted to extend out 12 inches. See below for excerpted figures and tables from the Wood Frame Construction Manual, courtesy of the American Wood Council.

Figure 6, WFCM Gable Overhang Design (courtesy, American Wood Council, Leesburg, VA)

Figure 6, WFCM Gable Overhang Design (courtesy, American Wood Council, Leesburg, VA)

In addition to the framing design, the connection of the roof decking at this location is critical. If you’re building to traditional construction methods, with 6″ nail spacing at panel edges and 12″ nail spacing at interior supports, the close nail spacing ends up at the nonstructural outer member, while the nailing at the actual roof edge over the gable is only 12″ on center. As shown in the details above, newer documents do indicate the importance of spacing the nails over the gable end at the closest spacing, both because these are subject to the highest withdrawal loads and because this is the edge of the diaphragm for transfer of lateral loads.

The Journal of Light Construction has a discussion of the unbraced gable end overhang on one of their Forums.

The Florida Division of Emergency Management provides some information on wind resistance of gable overhangs and some possible means of retrofitting them here.

Have you seen or designed with different methods for framing gable overhangs?

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.

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.

Timber Tower Research Project

In 2009, Simpson Strong-Tie participated in the NEESWood Capstone Test, which was the final experiment in a multi-year study to test and evaluate the seismic performance of various wood-framed buildings. The Capstone Test was a six-story apartment building constructed and tested at the E-Defense test facility, located in Miki, Japan. More information about the Capstone Test is available here.

NEESWood Capstone Test. Photo credit: Simpson Strong-Tie.

NEESWood Capstone Test. Image credit: Simpson Strong-Tie.

I only mention the NEESWood testing because I thought six-stories was pretty tall for wood-framed construction, since U.S. building codes limit us to four or five stories in wood. I recently came across a research project by Skidmore, Owings & Merrill LLP (SOM) for something just a tad taller than that. Looking to minimize the carbon footprint by using timber as the main structural material, SOM published a report for the design a 42-story, 405-ft. tall building. The solution utilizes mass timber for the main structural elements with reinforced concrete at highly stressed areas. The project used the Dewitt-Chestnut Apartments, a 42-story reinforced concrete structure built in 1965, as the benchmark building.

SOM's

SOM Timber Tower. Image credit: Skidmore, Owings, Merrill website.

Abstract for the Timber Tower Research Project along with links to the full report and sketches are on SOM’s website.

So, what do you think of a 42-story wood-framed building? Let us know by posting a Comment.

– Paul

Join Me for a WoodWorks Webinar on Acceptance Criteria and Testing Methods for Connectors

I have the privilege of presenting a free WoodWorks webinar, “Testing and Product Evaluation of Products for Wood-framed Construction” on Thursday, October 25 at 10 am PST. Here is the webinar description:

Products that are not covered by the code are used in many if not all buildings. While the code permits a single engineer to review and submit to a building official and a single building official to review and approve a product not covered by the code, many feel a more robust process is needed to ensure that these products meet the code intent. Also, many code or evaluation reports are used not just for one project, but for multitudes of projects in numerous locations. Test setups can affect the performance and load rating of products.

Several private entities have been created over the years to assist the industry by developing public and transparent processes to develop test requirements, load rating requirements, design and detailing requirements, and ongoing quality compliance as well as product evaluation methodologies. This webinar will discuss various acceptance criteria and testing methods used for products used in wood construction, such as ASTM D7147 and ICC-ES AC155 to further advance the knowledge of these test methods and processes for those in the wood construction industry.

This will be a great opportunity for those of us who work in wood frame construction to discuss code and test requirements for connectors and other products for wood. I hope you’ll consider joining us! To register for the webinar, click here.

– Paul