1901 N Fairview St & Bldg A, B, C, F & 1919 N Fairview St Bldg E - Plan

Home Browse Search

January 30, 2019 Gregory A. Kopp, PhD, PEng page 2 of 4 ASCE 7-16 Section 31.2, ASCE 49-12 (as well as ASCE 7-10, Section 31.2 & ASCE 7-05, Section 6.6.2) ASCE 7-16 section 31.1 and 31.2 (ASCE 7-10 and ASCE 7-05, section 6.6) note that “wind tunnel testing shall be permitted in lieu of” the prescriptive procedures in the other chapters, but that “tests for the determination of mean and fluctuating forces and pressures shall meet all of the following conditions,” which are provided in ASCE 49-12. There are several conditions that need to be met, which are indicated in the italicized text below: 1. The natural boundary layer has been modeled to account for the variation of wind speed with height. The CPP wind tunnel experiments make use of a boundary layer wind tunnel with a relatively long test section (shown in the Figure 2-1). The set-up for this experiment included a barrier and spires at the inlet to the test section, and roughness elements placed on the ground. The array models were placed on a turntable at the downstream end of the test section. Photographs in Appendix B provide evidence of the test set-up, which appears to be conventional for such wind loading experiments at large scales where only the lower portion of the atmospheric boundary layer is simulated. The mean velocity as a function of height is provided in Appendix C. The authors state that the wind tunnel simulation, which was conducted at a scale of 1:40, represents a suburban terrain, i.e., terrain category B is ASCE 7-16. Thus, this condition is met. 2. The relevant macro- (integral) length and micro-length scales of the longitudinal component of atmospheric turbulence are modeled to approximately the same scale as that used to model the building or structure. This is typically the most difficult requirement for wind tunnel studies of small structures to meet because of the relatively large model scales, such as 1:40 in the current study, which are usually needed to resolve geometric details. The turbulence intensities depicted in Appendix C are lower than typical for suburban terrain (category B). The spectra, along with the supporting text, indicate that the intent is to model the scales of turbulence corresponding to 3 sec gusts and smaller. This is an approach that has been broadly used for examining buffeting wind loads on bridges and is appropriate for solar arrays mounted on the roofs of low-rise buildings. Nevertheless, the integral scale is within a factor of 2 of the target, which is the acceptable range. 3. The modeled building or other structure and surrounding structures and topography are geometrically similar to their full-scale counterparts. The interpretation of ASCE 49-12 regarding the wind tunnel models for a roof-mounted solar array is that the geometry of the array must be matched and placed on an appropriately sized building. A wide range of array sizes, building sizes, and array placements on the roof were made. Tests were conducted with parapets, with the array placed in many different locations on the roof. The array was always placed on a single, isolated, rectangular plan-shaped building with sharp edges. This usually provides worst case loads and is consistent with how the wind load provisions for low-rise buildings in ASCE 7 were developed. In fact, this is a comprehensive data set with sufficient information such the roof and array zones can be obtained from the data. Thus, this condition is met. 4. The projected area of the modeled building or other structure and surroundings is less than 8 percent of the test section cross-sectional area unless correction is made for blockage. The blockage in the wind tunnel tests was not reported, but it is stated that it is less than 5%. Thus, this condition is met. 5. The longitudinal pressure gradient in the wind tunnel test section is accounted for. This criteria is discussed in the report and is met. 6. Reynolds number effects on pressures and forces are minimized. Equation (2-6) in ASCE 49-12 indicates that the Reynolds Number should be larger than 11,000, while the ASCE Manual of Practice #67 indicate that it should be above 10,000 (on p.15 of the 1999 version). The values of the Reynolds Number for this study are well above this threshold. Thus, this condition is met. 7. Response characteristics of the wind tunnel instrumentation are consistent with the required measurements. PCC2.2.4 1901 N Fairview St & Bldg A, B, C, F & 1919 N Fairview St Bldg E 4/11/2024