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Wind Loads on Sign Structures Using ASCE Standard 7-98

New wind load provisions are explained

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Three areas of the United States have different sign-engineering standards. However, the entire country will adopt the International Building Code (IBC). Consequently, we asked James A. Gerwig, P.E. and S.E., to explain the new standards. Jim is the chief structural engineer of the Las Vegas division of Young Electric Sign Co. (YESCO), which uses the new standard on a limited basis. He has a B.S.E.S. in civil engineering from the University of Alaska and a master’s in the same field from Cornell University. He was a Lecturer in Structural Engineering at the University of Nevada, Las Vegas from 1981 to 1983. Open your copy of the ASCE Standard 7-98 (330 pages, available at www.asce.org or amazon.com) to see how it impacts sign design, engineering, estimating, manufacturing and installation departments. In June’s column, Jim will apply new standards to a few examples. The recently introduced International Building Code (IBC) uses the American Society of Civil Engineer’s standard ASCE 7-98 for computing wind loads on buildings and other structures, including signs. The wind load provisions in ASCE 7-98 are considerably more comprehensive, and complex, than those used in the Uniform Building Code (UBC), one of the Codes being superseded by the IBC. The goal of this article is to assist the reader in reviewing some of the wind load provisions of ASCE 7-98 that apply to the primary sign structure and offer some insight as to how these forces are computed and applied. When using the provisions of ASCE 7-98 for computing wind loads on sign structures, the basic approach is to first compute the design wind pressure (qz) at height "z", (Eq 6-13, page 30) and then multiplying the computed pressure by factors for the shape of the sign, i.e. force coefficient (Cf), a gust response factor (G or Gf) and the projected area (Af), Eq. 6-20, page 33. The design wind pressure is computed by calculating the wind stagnation pressure (0.00256 V2) and then modifying it using factors for height (Kz), topography (Kzt), wind directionality (Kd) and an importance factor (I). In addition the design wind pressure may be modified for altitude if sufficient climatic data is available. One major difference between the ASCE 7-98 and the 1997 Uniform Building Code (UBC) and most "older" specifications is that the time reference for the basic wind speed is different. The UBC used a "fastest mile" wind speed; ASCE 7-98 uses a 3-second gust speed. The 3-second gust speed is what weather commentators commonly report. Fig. C6-1, page 268, presents a graph that shows the relationship between average gust speed and hourly mean speed (V3600). A 3-second gust speed of 90 miles per hour (MPH) is approximately equal to a fastest mile speed of 75 MPH. ASCE 7-98 also uses a one hour mean wind speed in some computations. Both wind speed and time duration must be specified to properly define a design wind speed. ASCE 7-98 does not use vertical wind "zones" to represent changes in wind speed with height; instead, a value for Kz is calculated for each height above grade being considered. Kz is a function of the exposure of the site under consideration. Exposure classifications include "A" for large city centers with numerous tall buildings, "B" for urban, suburban, or wooded areas, "C" for open terrain with scattered obstructions, and "D" for flat unobstructed areas exposed to wind flowing over open water (excluding shorelines in hurricane prone regions). Detailed requirements for each exposure are provided in ASCE 7-98 and the commentary. The factor Kzt is used to modify the wind speed when certain topographic factors are present. For example, an escarpment is a cliff or steep slope separating two levels or gently sloping areas; when wind is blowing across the lower area towards the escarpment there is a speeding up for the wind near the top of the escarpment, i.e. a constant volume of air is forced through a reduced cross sectional area. At some point the speed up effect attenuates with height above the escarpment top and with distance back from the escarpment edge. Three topographic features are presented: 2D escarpment, 2D ridge, and 3D axisymmetrical hill, along with criteria as to when the wind speed up effect has to be considered (Table 6-2, page 39). When a value of Kzt greater than unity is required, it may not always be conservative to use only the top, or centroid, of sign elevation when computing wind loads on sign panels; Kz increases with height, but Kzt increases, and then decreases, with height. Since the wind loads are a function of the product of Kz and Kzt intermediate elevations may have greater values than either the top or bottom elevations. For both solid and open signs the wind directionality factor "Kd" is 0.85 (Table 6-6, page 61). Kd may only be applied when used in conjunction with load combinations specified in ASCE 7-98 sections 2.3 and 2.4 (page 5). Section 2.3.2 provides combinations and load factors for strength design (LRFD) of specifically authorized materials (steel and wood not concrete). The load factor for the wind component in a number of combinations has been changed to 1.6 from 1.3, used in previous standards. Some of the combinations have also been modified and/or expanded. The load combinations for allowable stress design (ASD) are contained in section 2.4.1 and provisions for ASD load reductions are in section 2.4.3. In using these provisions the loads are reduced; as a result, the one-third stress increase commonly permitted in ASD is NOT allowed. Of particular importance to sign designers is the statement "…the combined effects due to the two or more loads multiplied by 0.75 plus the effects due to dead loads shall not be less than the effects from the load combination of the dead load plus the load producing the largest effect". For most sign structures the controlling load combination will be dead plus wind (D+W) without any reductions or stress increases permitted. The values for the wind load importance factor "I" are listed in Table 6-1 on page 55. The definitions for the category classification that the importance factor is based on are presented in Table 1-1 on page 4. Most sign structures would be category II. One of the factors that will have the greatest impact on the design of sign structures is the gust response factor (G or Gf). This factor reflects how the structure responds to the wind gust. For wind load analysis ASCE 7-98 classifies structures as either "rigid" or "flexible or dynamically sensitive." A rigid structure is defined as having a fundamental frequency greater than or equal to 1 Hz, i.e. a period of 1 second or less. If the structure is classified as rigid either a value for G of 0.85 may be used or a more complete analysis may be made to determine G (Eq. 6-2, page 29). The more complete analysis includes factors for structure height, width, depth and exposure parameters. The computation of Gf (required for flexible or dynamically sensitive structures) is considerably more complex than computing G (used for rigid structures). Computing Gf includes factors for structure height, width, depth, fundamental frequency, structural damping and exposure parameters. It has been my experience that very few signs under 50 feet in height would be classified as flexible or dynamically sensitive. Sign structures over 75 feet high, however, usually are classified as flexible and require that Gf be computed. In the definition of "flexible buildings and other structures" previous versions of the ASCE 7 standard included "buildings and other structures that have a height exceeding four times the least horizontal dimension;" this restriction is not in current standard. The wind load provisions in ASCE 7-98 apply to the majority of sign structures, but there are limitations. For example topographic channeling effects and vortex shedding are beyond the scope of the standard. A discussion of the limitations of the analytical procedures can be found in the commentary (C6.5.2, page 245). For sign structures force coefficients are presented in Tables 6-11 and 6-12, pages 66 and 67. Signs are classified as either "at ground level" or "above ground level". A monument type sign would be typical of an at ground level sign, while an elevated billboard would be classified as above ground level. For an at ground level sign the height to width ratio is used to determine the force coefficient. For an above ground level sign the panel ratio, larger dimension/ smaller dimension, is used. When evaluating what force coefficient should be used, the overall airflow around the structure should be considered and not just arbitrarily defined width to height ratios. Note 4 of Table 6-11 states: 4. To allow for both normal and oblique wind directions, two cases shall be considered: a. resultant force acts normal to the face of the sign on a vertical line passing through the geometric center, and equal to 0.2 times the average width of the sign.
The first requirement indicates a uniform wind pressure applied normal to the sign face. The second requirement indicates that the total wind force must be applied with an eccentricity equal to 0.2 times the width of the sign (or panel) under consideration. Note that this requires that the total wind force, i.e. total area times pressure must be used, and not the application of the design wind pressure to partial areas of the sign. This requirement can have a profound effect on the design of sign structures.

For application of the wind load across the sign face the only "simple" continuous geometric distribution, that I know of, that would result in an eccentricity of 0.2 is the compliment of a parabola. Application of such a load distribution could become extremely tedious.

As a result, I suggest that, for this loading condition, the wind pressure be multiplied by 1.67 and applied to 60% of the panel width, i.e. from the left edge of the panel to 60% of the panel width, and as a second load case from 40% of the panel width to the right edge of the panel. These conditions produce the full wind force on the structure with eccentricities of 0.2 to the left and right of the panel center. The eccentric loading conditions may result in significantly higher forces in some sign structure members.

For a symmetric billboard type structure with supports at the ends and a simply supported sign face the distribution for the first case would be 50% of the total load to each support. The eccentric loading condition would result in a distribution of 70% to the support on the side of the wind load and 30% to the other support. If the supports were located one quarter of the width in from each end the eccentric loading would result in a distribution of 90% and 10%. A center-mounted billboard with a single tubular column would not be as adversely effected by the eccentric loading because of the high torsional capacity of tubular members, the horizontal torsion tube and verticals supporting the sign face would have to be designed for significantly higher forces. When the vertical supports are coupled together with horizontal components that transfer shears, moments and torsion between the vertical supports the analysis is more complex and usually requires that a three-dimensional computer analysis program be used; fortunately this approach can, usually, reduce the increase in support design forces associated with the eccentric wind load case. It is very important that connection computations and detailing be consistent with the assumptions made in the computer model.

The wind load provisions in ASCE 7-98 are considerably more comprehensive, and complex, than the provisions in many of the Codes being replaced by the IBC. On the positive side the provisions allow for the computation of wind loads for a wide range of structures without reverting to "blanket" or overly conservative "worst-case" coefficients. The downside is that, in my opinion, some of the methodology in conjunction with the symbols and notation used can be confusing, resulting in a loss of "feel" for the loads being computed.
 

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