American Forest & Paper AssociationJuly 2, 1996 TO:AF&PA/AWC Design Professional Members RE:Considerations in Wind Design of Wood Structures The following paper entitled Considerations in Wind Design of Wood Structures is provided for your information. As a design professional, you are undoubtedly aware of the various wind load provisions adopted by many local, state and model codes. This article is intended to clarify the application of the wind load provisions incorporated in ASCE's Minimum Design Loads for Buildings and Other Structures, ASCE 7-88 (which are carried into ASCE 7-93 unchanged). Specific discussion on stud design is warranted as follows:
This information is tabulated to point out that not only can loads be significantly higher than are often assumed, they also can vary with the assumption about how to use MWFRS and C&C loads.
If you have any comments or questions regarding this information feel free to contact the engineering department at AF&PA. The e-mail address is INTERNET: awcinfo@afandpa.org. Enclosure Considerations In Wind Design Of Wood StructuresBradford K. Douglas, P.E. Proper design of wood structures to resist high wind loads requires the correct use of wind load provisions and member design properties. A thorough understanding of the interaction between wind loads and material properties is important in the design process. Wind load provisions vary in the local, state and model building codes currently used in the United States. Most of these provisions are based on wind engineering research conducted over the last 50 years. Proposals to change current code provisions are the result of interpretations of new state-of-the-art wind engineering research. The wind load provisions of the national load standard ASCE 7-88 Minimum Loads for Design of Buildings (formerly ANSI A58.1) include general wind load provisions which, in turn, are used as the basis for wind load requirements in most U.S. building codes. For the purposes of this article, references to wind loads have been limited to the provisions found in ASCE 7-88. Wind Load ProvisionsDesign wind load provisions in ASCE 7-88 are based on wind speed data collected during severe wind events in the United States. The windspeed contours provided in ASCE 7-88 are presented in terms of fastest-mile windspeeds (FMW). A fastest-mile windspeed is based on the time required for one mile of wind to pass a stationary point. The FMW data in ASCE 7-88 have been statistically adjusted to a 50-year mean recurrence interval with an annual probability of occurrence of 2 percent. The data has also been adjusted to a reference height of 33 feet and Exposure Category C, which assumes a flat, open terrain with scattered obstructions. Provisions in ASCE 7-88 adjust the FMW value derived from the windspeed map to pressures associated with shorter, peak gusts. The wind load provisions of ASCE 7-88 also provide adjustments for variations from the reference conditions such as increased windspeeds during hurricane events, different exposure conditions, different elevations, and localized peak gusts associated with localized funneling and turbulence. The adjusted peak gust pressures are the pressures used for designing buildings and their components. ASCE 7-88 contains separate provisions for the design of major structural elements using "Main Wind-Force Resisting System" (MWFRS) loads and secondary structural elements using "Component & Cladding" (C&C) loads. In building design, MWFRS loads are not true loads, but "pseudo-loads" developed to induce critical stresses in the main structural elements of the structure. These loads were developed by enveloping major structural actions induced on a building from various wind directions and for various building geometries, roof heights and roof slopes. Due to the interactions which occur from loads on different surfaces, MWFRS loads are considered time-dependent loads. C&C loads have been developed to represent load increases resulting from localized peak gusts occurring over small areas as a result of localized funneling and turbulence. Localized load increases can approach 300% at corners and ridges for certain configurations and require special considerations when designing for these loads. In wood structures, wind damage surveys have indicated that these localized loads can cause failures of connections in small areas which can affect the overall Main Wind-Force Resisting System. Since C&C loads attempt to address a "worst-case" loading on a particular element during the wind event, these loads are considered time-independent and are not intended for use when considering the interaction of loads from multiple surfaces. When designing a structural wood member, a decision must be made whether a member is a MWFRS element, a C&C element, or a hybrid of both systems. ASCE 7-88 defines the MWFRS as an assemblage of major structural elements which provide support to secondary members and cladding. Components and cladding are defined as structural elements which are either directly loaded by the wind or receive wind loads originating at relatively close locations, and which transfer these loads to the MWFRS. However, some elements such as light-weight roof trusses, load-bearing studs, and structural sheathing have been identified in both systems since they resist time-dependent interaction loads and time-independent localized loads. One suggested interpretation is to design these elements for the interactions using the MWFRS loads they would receive as part of the MWFRS and, separately, design these elements for the single C&C load they would receive if they were only a C&C element. In many cases this would require at least two checks; however, differences in the load cases and estimated stresses make it both necessary and beneficial to separately consider both cases. Moreover, under certain common conditions, elements can be pre-engineered for C&C loads eliminating the need to check this case for each element. Load CombinationsSeveral building codes, including the National Building Code and the Uniform Building Code, allow an increase of 33 percent on allowable stresses for all materials when designing for wind or seismic loads. Many explanations have been forwarded by engineers and building officials in an attempt to rationalize this increase including claims that the increase is an overstress factor to adjust from elastic design to plastic design or an additional load combination factor to adjust for a lower probability of occurrence; however, a consensus has not been reached. Similarly, an equivalent reduction is permitted on loads under certain conditions for wind and seismic design in ASCE 7-88. In ASCE 7-88 a structure is designed for 100% of the total load when one transient load is acting alone, 75% of the total load when two transient loads are induced on the structure simultaneously, and 66% of the total load when three transient loads are considered simultaneously. The justification for these reductions in ASCE 7-88 is more straight-forward, allowing load reductions based on the reduced probability of occurrence of multiple maximum transient loads occurring simultaneously. Allowable Design StressesOnce the induced loads on a wood member or connection have been determined, that element can be designed. Structural wood members and connections should be designed using the appropriate provisions of the local building code. For the design of solid-sawn wood members and general connections, the codes normally reference or include provisions from the National Design Specification® for Wood Construction (NDS®). Included in NDS design provisions are various adjustments to design values. Among these adjustments is the duration of load (CD) factor. Wood strength properties have been observed to exhibit increased capacities under shorter durations of maximum load. This phenomenon has been analyzed extensively in the U.S. and in countries around the world. To account for this phenomenon in design, the U.S. Forest Service, Forest Products Laboratory in Madison, Wisconsin developed a duration of load curve, commonly called the "Madison Curve", which relates the maximum load-carrying capacity to a given load duration. Most wood member design properties and connection capacities in the NDS are based on 10-minute test values reduced for the effects of growth characteristics, stress concentrations, safety and duration of load. The duration of load adjustment reduces a 10-minute value to a 10-year value by dividing the 10-minute value by a factor of 1.6 based on the "Madison Curve". During a severe wind event, maximum peak wind gusts on a structural member or connection have been approximated to have a cumulative duration of approximately 1-10 seconds. Worst case estimates by wind load experts have indicated that over the life of a structure the cumulative duration of these maximum loads would be less than 1 minute. While this estimate of duration would justify an increase in excess of 1.6, the 1991 NDS provisions specify an increase of 1.6 for wind loading which returns the design values for the wood members or connections to 10-minute design values. While a duration of load increase is allowed for most design properties and connections, there are a few important exceptions. For lumber, a duration of load increase is not permitted for compression perpendicular-to-grain (Fc^), and Modulus of Elasticity (E) design values. These properties are based on deformation and stiffness limits, which are not directly affected by the duration of load phenomenon. For panel product systems, published design capacities for shear walls and diaphragms are expressed in terms of the test duration and need only be adjusted for long-term loading. In addition, information on proprietary products and systems should be reviewed to determine if CD adjustments of design capacities are permitted for those products. Design ExampleA 36'x60' one-story wood-frame building is to be built on a site located in a 100 mph fastest-mile wind zone and on terrain representative of Exposure C. The walls will be constructed using 8-foot studs spaced 16 inches on center. The roof will be constructed using trusses spanning 36 feet spaced 24 inches on center and having 2 foot eave overhangs. The mean roof height will be approximately 13 feet and the roof angle will be approximately 15 degrees. The base velocity pressure can be calculated using the following equation: qc = 0.00256Kc(IV)2 Using the calculated base velocity pressure, MWFRS design loads can be determined using the following equation: pMWFRS = qcGhCp-qc(GCpi) For C&C design, the "effective" load area of the component must be determined to determine the external pressure coefficients. For rectangular load areas, ASCE 7-88 allows the area to be calculated as, A=L2/3. For this example, the C&C design loads for studs can be calculated using the following equation and inputs: pC&C = qc(GCp)-qc(GCpi) Using the equations and values given above, loads for design of the exterior load-bearing studs can be derived. Tabulated below are design loads for the MWFRS and C&C load cases:
After determining the design wind loads on the structure, building components and assemblies can be designed. All pertinent load combinations should be considered. In ASCE 7-88 the following load combinations should be considered for allowable stress design:
Under most design conditions, many of these load combinations can be dismissed. For the design of load-bearing studs in the example case, it is assumed that the building will be located in an area that receives little or no snow, that rain can not pond on the roof, and that roof live loads will not be present during a high-wind event. In addition, the studs only support the roof and ceiling loads, therefore, a special case for floor live loads need not be considered. Given these assumptions, only load combinations 1-3 need to be considered. For load combinations 1-3, live and dead loads in the structure must be determined. Tabulated below are the assumed roof and ceiling live and dead loads.
The duration of load adjustment and induced loads exerted on the studs for each load case and combination are tabulated below.
The final step in design of the studs is to choose a member which has sufficient design capacity to resist the induced loads tabulated above. For this example Hem-Fir #2 - 2x4 was chosen. The following tabulated base design values were taken from the NDS Supplement: Fb = 850 psi Applying the appropriate adjustments and checking each load combination as follows:
The other cases considered under load combination 3, dead plus wind, can be calculated in a similar manner. Tabulated below are the load/resistance ratios for each load combination and load case.
In the above example, stud design is limited by the C&C load case. This is not uncommon and in most cases can be considered the controlling limit in wind design of loadbearing and non-loadbearing exterior studs. However, until sufficient boundary conditions are placed on this simplification, both MWFRS and C&C load cases should be considered. SummaryDetermination of wind loads and material resistances must be considered together. Adjustments of reference wind conditions to extreme-value peak gusts require designers to make similar adjustments to design properties to ensure equivalent and economic designs. Major structural elements should be designed for MWFRS loads and secondary cladding elements should be designed for C&C loads. Components and assemblies which receive 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 often will be wind acting alone. However, each load combination should be considered thoroughly before being dismissed. As the wind load provisions in ASCE 7-88 and the model building codes continue to change, the wood industry must keep abreast of these changes. Efforts must be made to improve engineering knowledge and procedures to ensure adequate design of structures in high wind areas. REFERENCES
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