MWFRS wind pressures are calculated using the ‘envelope procedure’ contained in Chapter 28 of ASCE 7-10. The velocity pressure exposure coefficient for a building located in Exposure B with a 7.6-m (25-ft) MRH is 0.70 per ASCE 7-10 Table 28.3-1, and a factor of 0.6 adjusts the pressures associated with a 700-year mean return period wind to allowable stress design. The velocity pressure calculates to 1.12 kPa (23.4 psf).
ASCE 7-10 Figure 28.4-1 shows the external pressure coefficients for interior and end zones for two cases—winds generally perpendicular to the ridge and winds generally parallel to the ridge. Wind perpendicular to the ridge produces the highest external wall pressure coefficients. Reactions at the top of the bearing wall are determined by summing overturning moments about the top of the leeward wall for both load cases and determining the controlling reaction to use in the design. Horizontal projections are used in the analysis. The out-of-plane MWFRS pressure on the wall at interior zones is calculated as 0.83 kPa (17.3 psf).
Load Combinations 1, 2, 3, and 4 model gravity only loads (dead load, live load, and/or snow load). Load Combinations 5, 6a, and 7 include MWFRS loads. Load Combination 6a controls for the load combinations that include wind loads. The bearing walls must resist distributed loads from the attic floor and roof and out-of-plane MWFRS loads proportional to the width of their tributary areas. Using NDS column, beam, and combined bending and axial load provisions, the interaction value calculated per NDS Equation 3.9-3 is 0.46.
ASCE 7-10 provisions for calculating C&C loads are used assuming a minimum effective wind area of L2/3. By observation, negative external pressure coefficients are greater than positive external pressure coefficients. Thus, negative external pressures and positive internal pressures (windward) create the greatest C&C pressures. A C&C pressure of −1.22 kPa (−25.5 psf) is calculated for this example. Applying the C&C pressure as a bending load on the studs calculates a bending stress to bending strength ratio of 0.76—even larger than the combined bending and axial interaction calculated with MWFRS loads.
A deflection check using C&C loads reveals an L/Δ of 273. Assuming either a flexible finish or gypsum-type finish, code deflection limits are typically L/180 and L/240, respectively. While 2x6s could be designed to work for this example from a strength standpoint, the minimum deflection limit for this application of L/180 would be exceeded with 2×6 studs.
Conclusion
Major structural elements should be designed for MWFRS loads, and secondary cladding elements should be designed for C&C loads. Components and assemblies receiving loads both directly and as part of the MWFRS should be checked for MWFRS and C&C loads independently. In cases where components and assemblies must be designed for lateral wind loads, the controlling design case
will often be wind acting alone. However, each load combination should be considered thoroughly before being dismissed.
Now more than ever, designing buildings to withstand potentially devastating high wind forces is one of the greatest challenges construction professionals face. Given wood’s resilient properties in these situations, communities are less likely to sustain damage and should have a faster time getting back up on their feet.
John “Buddy” Showalter, PE, joined the American Wood Council (AWC) in 1992, and serves as vice president of technology transfer. His responsibilities include oversight of publications, website, helpdesk, education and other technical media. Showalter is also a member of the editorial boards for Wood Design Focus (Forest Products Society) and STRUCTURE (National Council of Structural Engineers Associations [NCSEA]/American Society of Civil Engineers/Structural Engineering Institute [ASCE/SEI], and Council of American Structural Engineers [CASE]) magazines. Before joining AWC, Showalter was technical director of the Truss Plate Institute. He can be reached at bshowalter@awc.org.
Bradford Douglas, PE, joined AWC in 1986, and serves as vice president of engineering. He directs a program aimed at developing state-of-the-art engineering data, technology, and standards on structural wood products, systems, and assemblies for use by design professionals and building officials. Douglas is on several standards development committees of organizations including ASTM, American Society of Civil Engineers (ASCE), and the U.S. Federal Emergency Management Agency’s Building Seismic Safety Council (BSSC). Since 1987, he has also served on U.S. model building code committees for the Southern Building Code Congress International (SBCCI), the International Council of Building Officials (ICBO), and more recently the International Code Council (ICC) dealing with proper design of wood buildings to resist high wind and seismic loads. He can be reached at (202) 463-2766.
David K. Low, PE, is president of DK Low & Associates, a consulting firm in Charlottesville, Virginia. He has practiced electrical and structural engineering for over 35 years. Since 1998, Low has focused on the effects of natural hazards on the built environment with an emphasis on high wind and flood effects. He is a member of numerous forensic engineering teams that assess building performance after floods, tornadoes, and hurricanes and is a principal or contributing author of several publications for improved building performance and resilience. Low is also a member of code and standard development committees for the National Fire Protection Association (NFPA) and the American Society of Civil Engineers (ASCE). He can be reached at (434) 295-6364.
