Reprinted From "Guide to the Use of the Wind Load Provisions of ASCE 7-02"
By Kishor C. Mehta and James M. Delahay
The questions and answers below came directly from Chapter 4 of "Guide to the Use of the Wind Load Provisions of ASCE 7-02" by Kishor C. Mehta and James M. Delahay. Additional FAQ's appear in the guide. It is available through the ASCE publications department for $49.00 nonmember / $36.75 member. The guide will not be released until December, but you can reserve your copy now by calling ASCE at 1-800-548-2723.
1. Is it possible to obtain larger scale maps of basic wind speeds (see Figures 6-1, 6-1a, 6-1b, and 6-1c) so that the locations of the wind speed contours can be determined with greater accuracy?
No. The wind speed contours in the hurricane-prone region of the United States are based on hurricane wind speeds from Monte Carlo simulations and on estimates of the rate at which hurricane wind speeds attenuate to 90 mph following landfall. Because the wind speed contours of these figures represent a consensus of the ASCE 7 Task Committee on Wind Loads, increasing the map scale would do nothing to improve their accuracy.
2. IBC Figure 1609 gives the 3-s wind speed at the project location. However, according to the Notes, Figure 1609 is for Exposure C. If the project location is Exposure B, what is the proper wind speed to use?
The basic wind speed in IBC Figure 1609 or ASCE 7-02 is defined as a 3-s gust wind speed at 33 ft above ground for Exposure Category C, which is the standard measurement. The velocity pressure exposure coefficient, Kz, adjusts the wind speed for exposure and height above ground. However, for simplicity the coefficient is applied in the pressure equation, thus adjusting pressure rather than wind speed. Use of Kz adjusts the pressures from Exposure C to Exposure B.
3. If the design wind loads are to be determined for a building that is located in a special wind region (shaded areas) in Figures 6-1, 6-1a, 6-1b, and 6-1c, what basic wind speed should be used?
The purpose of the special wind regions in these figures is to alert the designer to the fact that there are regions in which wind speed anomalies are known to exist. Wind speeds in these regions may be substantially higher than the speeds indicated on the map, and the use of regional climatic data and consultations with a wind engineer or meteorologist are advised.
4. In the design of the main wind force-resisting systems (MWFRS), the provisions of Figure 6-6 apply to enclosed or partially enclosed buildings of all heights. The provisions of Figure 6-10 apply to enclosed or partially enclosed buildings with mean roof height less than or equal to 60 ft. Does this mean that either figure may be used for the design of a low-rise MWFRS?
Figure 6-6 may be used for buildings of all heights, whereas Figure 6-10 applies only to low-rise buildings. Section 6.2 defines low-rise buildings to comply with mean roof height h £ 60 ft and h not to exceed least horizontal dimensions. Pressure coefficients for low-rise buildings given in Figure 6-10 represent "pseudo" loading conditions enveloping internal structural reactions of total uplift, total horizontal shear, bending moment, etc. (see Section C6.5.11). Thus, they are not real wind-induced loads. These loads work adequately for buildings of the shapes shown in Figure 6-10, but they become questionable when extrapolated to other shapes.
5. Do I consider a tilt-up wall system to be components and cladding (C&C) or MWFRS or both?
Both. Depending on the direction of the wind, a tilt-up wall system must resist either MWFRS forces or C&C forces. In the C&C scenario, the elements receive the wind pressure directly and transfer the forces to the MWFRS in the other direction. When a tilt-up wall acts as a shear wall, it is resisting forces of MWFRS. Because the wind is not expected to blow from both directions at the same time, the MWFRS forces and C&C forces are analyzed independently from each other in two different load cases. This is also true of masonry and reinforced-concrete walls.
6. Section 22.214.171.124 provides for a minimum wind pressure of 10 lb/ft2 multiplied by the area of the building or structure projected onto a vertical plane normal to the assumed wind direction of MWFRS. Does this provision apply to low-rise buildings?
It should. There was some confusion in ASCE 7-98 provisions for low-rise buildings where it was difficult to interpret application of loads on building frames using the two cases of loads at each corner. Figure 6-10 in ASCE 7-02 clarifies with illustrative sketches the application of loads on low-rise buildings, and only one table of pressure coefficients is provided. In addition, Note 6 is added to account for minimum total horizontal shear, although this provision does not guarantee minimum 10 psf on the projected area of the building.
8. When can I use the one-third stress increase specified in some material standards?
When using the loads or load combinations specified in ASCE 7-02, no increase in allowable stress is permitted except when the increase is justified by the rate of duration of load (such as duration factors used in wood design). Instead, load combination #6 from Section 2.4.1 of ASCE 7-02 was added for the case when wind load and another transient load are combined. This load combination applies a 0.75 factor to the transient loads ONLY (not to the dead load). The 0.75 factor applied to the transient loads accounts for the fact that it is extremely unlikely that two maximum events will happen at the same time.
9. Why can the wind directionality factor (Kd) only be used with the load combinations specified in Sections 2.3 and 2.4 of ASCE 7-02?
In the strength design load combinations provided in previous editions of ASCE 7 (ASCE 7-95 and earlier), the 1.3 factor for wind included a "wind directionality factor" of 0.85. In ASCE 7-98, the loading combinations used 1.6 instead of 1.3 (approximately equals 1.6 x 0.85), and the directionality factor is included in the equation for velocity pressure. Separating the directionality factor from the load combinations allows the designer to use specific directionality factors for each structure and allows the factor to be revised more readily when new research becomes available.
10. What exposure category should I use for the MWFRS if the terrain around my site is Exposure B, but there is a large parking lot directly next to one of the elevations?
Section 6.5.6 of ASCE 7-02 provides general definitions of Exposures B, C, and D; however, the designer must refer to the Commentary for a detailed explanation for each exposure. The exposure depends on the size of the parking lot, its size relative to the building, and the number and type of obstructions in the area. Section C6.5.6 of the Commentary includes a formula (Eq. C6-2) that will help the designer determine if the terrain roughness is sufficient to be categorized as Exposure B. Note that, for Exposure B, the fetch distance is 2,630 ft or 10 times the structure's height, whichever is greater. Also note that the Commentary provides suggestions for determining the "upwind fetch surface area."
For clearings such as parking lots, wide roads, road intersections, underdeveloped lots, and tree clearings, the Commentary provides a rational procedure and an example to interpolate between Exposure B and C; the designer is encouraged to use this procedure.
11. What pressure coefficients should be used to reflect contributions for the underside (bottom) of the roof overhangs and balconies?
Sections 126.96.36.199.1 and 188.8.131.52.2 specify pressure coefficients to be used for roof overhangs to determine loads for MWFRS and C&C, respectively. No specific guidance is given for balconies, but use of the loading criteria for roof overhangs should be adequate.
12. If the mean roof height, h, is greater than 60 ft with a roof geometry that is other than flat roof, what pressure coefficients are to be used for roof C&C design loads?
Section 184.108.40.206.3 permits use of pressure coefficients of Figures 6-11 through 6-16 provided the mean roof height h < 90 ft, the height-to-width ratio is 1 or less, and Eq. 6-22 is used.
Note 6 of Figure 6-17 permits use of coefficients of Figure 6-11 when the roof angle 0 > 10°.
14. Under what conditions is it necessary to consider speed-up due to topographic effects when calculating wind loads?
Section 6.5.7 of the Standard requires the calculation of the topographic factor, Kzt, for buildings and other structures sited on the upper half of isolated hills or escarpments located in Exposures B, C, or D where the upwind terrain is free of such topographic features for a distance of at least 100 h or 2 mi, whichever is smaller, as measured from the crest of the topographic feature. Kzt need not be calculated when the height, H, is less than 15 ft in Exposures D and C, or less than 60 ft in Exposure B. In addition, Kzt need not be calculated when H and Lh is less than 0.2. h and Lh are defined in Figure 6-4. The value of Kzt is never less than 1.0.
15. What constitutes an open building? If a process plant has a three-story frame with no walls but with a lot of equipment inside the framing, is this an open building?
An open building is a structure in which each wall is at least 80% open (see Section 6.2). Yes, this three-story frame would be classified as an open building, or as "other" structure. In calculating the wind force, F, appropriate values of Cf and Af would have to be assigned to the frame and to the equipment inside.
16. When is a gable truss in a house part of the MWFRS? Should it also be designed as a C&C? What about individual members of a truss?
Roof trusses are considered to be components since they receive load directly from the cladding. However, since a gable truss receives wind loads from more than one surface, which is part of the definition for MWFRS, an argument can be made that the total load on the truss is more accurately defined by the MWFRS loads. A common approach is to design the members and internal connections of the gable truss for C&C loads, while using the MWFRS loads for the anchorage and reactions. When designing shear walls or cross-bracing, roof loads can be considered an MWFRS.
In the case where the tributary area on any member exceeds 700 ft2, Section 220.127.116.11.3 permits it to be considered a MWFRS. Even when considered a MWFRS under this provision, the top chord members of a gable truss would have to follow rules of C&C if they receive load directly from the roof sheathing.
17. Flat roof trusses are 30 ft long and are spaced on 4-ft centers. What effective wind area should be used to determine the design pressures for the trusses?
Roof trusses are classified as C&C since they receive wind load directly from the cladding (roof sheathing). In this case, the effective wind area is the span length multiplied by an effective width that need not be less than one-third the span length or (30)(30/3) = 300 ft2. This is the area on which the selection of GCp should be based. Note, however, that the resulting wind pressure acts on the tributary area of each truss, which is (30)(4) = 120 ft2.
18. Roof trusses have a clear span of 70 ft and are spaced 8 ft on center. What effective wind area should be used to determine the design pressures for the trusses?
Following the approach of question #17, above, the effective wind area is (70)(70/3) = 1,633 ft2. The tributary area of the truss is (70)(8) = 560 ft2, which is less than the 700-ft2 area required by Section 18.104.22.168.3 to qualify for design of the truss using the rules for MWFRS. The truss is to be designed using the rules for C&C, and the wind pressure corresponding to an effective wind area of 1,633 ft2 is to be applied to the tributary area of 560 ft2.
19. Metal decking consisting of panels 20 ft long and 2 ft wide is supported on purlins spaced 5 ft apart. Will the effective wind area be 40 ft2 for the determination of pressure coefficients?
Although the length of a decking panel is 20 ft, the basic span is 5 ft. According to the definition of effective wind area, this area is the span length multiplied by an effective width that need not be less than one-third the span length. This gives a minimum effective wind area of (5)(5/3) = 8.3 ft2. However, the actual width of a panel is 2 ft, making the effective wind area equal to the tributary area of a single panel, or (5)(2) = 10 ft2. Therefore, GCp would be determined on the basis of 10 ft2 of effective wind area, and the corresponding wind load would be applied to a tributary area of 10 ft2. Note that GCp is constant for effective wind areas less than 10 ft2.
20. A masonry wall is 12 ft in height and 80 ft long. It is supported at the top and at the bottom. What effective wind area should be used in determining the design pressure for the wall?
For a given application, the magnitude of the pressure coefficient, GCp, increases with decreasing effective wind area. Therefore, a very conservative approach would be to consider an effective wind area with a span of 12 ft and a width of 1 ft, and design the wall element as C&C. However, the definition of effective wind area states that this area is the span length multiplied by an effective width that need not be less than one-third the span length. Accordingly, the effective wind area would be (12)(12/3) = 48 ft2.
21. If a monoslope roof over an open building is virtually flat, what force coefficients from Figure 6-18 should be used?
A requirement for the use of Figure 6-18 is that the wind shall be assumed to deviate plus or minus 10 from the horizontal. Accordingly, the values of Cf corresponding to a roof angle of 10 should be used. The wind forces may be directed either inward or outward, and both cases should be checked.
22. A trussed tower of 10 x 10-ft2 cross section consists of structural angles forming basic tower panels 10 ft high. The solid area of the face of one tower panel projected on a plane of that face is 22 ft2. What force coefficient, Cf , should be used to calculate the wind force? What would the force coefficient be for the same tower fabricated of rounded members and having the same projected solid area? What area should be used to obtain the wind force per foot of tower height acting (1) normal to a tower face, and (2) along a tower diagonal?
The gross area of one panel face is (10)(10) = 100 ft2, and the solidity ratio is 22/100 = 0.22. For a tower of square cross section, the force coefficient from Figure 6-22 is as follows:
Cf = (4)(0.22)2 - (5.9)(0.22) + 4.0 = 2.90
For rounded members, the force coefficient may be reduced by the factor
(0.51)(0.22)2 + 0.57 = 0.59
Thus, the force coefficient for the same tower constructed of rounded members with the same projected area would be
Cf = (0.59)(2.90) = 1.71
The area, Af, used to calculate the wind force per foot of tower height is 22/10 = 2.2 ft2 for all wind directions.
23. In calculating the wind forces acting on a trussed tower of square cross section (see Figure 6-22), should the force coefficient, Cf , be applied to both the front and the back (windward and leeward) faces of the tower?
No. The calculated wind forces are the total forces acting on the tower. The force coefficients given in Figure 6-22 include the contributions of both front and back faces of the tower, as well as the shielding effect of the front face on the back face.
24. If the pressure or force coefficients for various roof shapes (e.g., a canopy) are not given in ASCE 7-02, how can the appropriate wind forces be determined for these shapes?
With the exception of pressure or force coefficients for certain shapes, parameters such as V, I, Kz, Kzt, and G are given in ASCE 7-02. It is possible to use pressure or force coefficients from the published literature provided these coefficients are used with care. Mean pressure or force coefficients from other sources can be used to determine wind loads for MWFRS. However, it should be recognized that these coefficients might have been obtained in wind tunnels that have smooth, uniform flows as opposed to more proper turbulent boundary-layer flows. Pressure coefficients for components and cladding obtained from the literature should be adjusted to the 3-s gust speed, which is the basic wind speed adopted by ASCE 7-02.
25. Section 6.2 of the Standard provides definitions of glazing, impact resistant; impact-resistant covering; and wind-borne debris regions. To be impact resistant, the Standard specifies that the glazing of the building envelope must be shown by an approved test method to withstand the impact of wind-borne missiles likely to be generated during design winds. Where does one find information on appropriate test methods?
Section 6.7 of the Standard refers to two ASTM standards. These standards give test method and performance criteria of glazing, doors, and shutters when impacted by wind-borne debris.
26. The Standard does not provide for across-wind excitation caused by vortex shedding. How can one determine when vortex shedding might become a problem?
Vortex shedding is almost always present with bluff-shaped cylindrical bodies. Vortex shedding can become a problem when the frequency of shedding is close to or equal to the frequency of the first or second transverse of the structure. The intensity of excitation increases with aspect ratio (height-to-width or length-to-breadth) and decreases with increasing structural damping. Structures with low damping and with aspect ratios of 8 or more may be prone to damaging vortex excitation. If across-wind or torsional excitation appears to be a possibility, expert advice should be obtained.
27. If high winds are accompanied by rain, will the presence of raindrops increase the mean density of the air to the point where the wind loads are affected?
No. Although raindrops will increase the mean density of the air, the increase is small and may be neglected. For example, if the average rate of rainfall is 5 in./h, the average density of raindrops will increase the mean air density by less than 1%.
29. What wind loads do I use during construction?
ASCE 7 does not address wind loads during construction. Construction loads are specifically addressed in the standard SEI/ASCE 37-01, Design Loads on Structures during Construction.
30. Can the pressure/force coefficients given in ASCE 7-02 be used with the provisions of ASCE 7-88, 7-93, 7-95, or 7-98?
Yes, in a limited way. ASCE 7-88 (and 7-93) used the fastest-mile wind speed as the basic wind speed. With the adoption of the 3-s gust speed starting with ASCE 7-95, the values of certain parameters used in the determination of wind forces have been changed accordingly. The provisions of ASCE 7-88 and 7-02 should not be interchanged. Coefficients in ASCE 7-95, 7-98, and 7-02 are consistent; they are related to 3-s gust speed.
31. Is it possible to determine the wind loads for the design of interior walls?
The Standard does not address the wind loads to be used in the design of interior walls or partitions. A conservative approach would be to apply the internal pressure coefficients GCpi = ±0.18 for enclosed buildings and GCpi = ±0.55 for partially enclosed buildings. Post-disaster surveys have revealed the failure of interior walls when the building envelope has been breached.