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Tirza Austin
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posted 06-03-2022 01:02 PM

The question relates to the ASCE 716, Section 29, regarding the lift under PV modules. We are doing work designing PV solar rooftop attachments. We were specifically directed by the engineering department of the AHJ to use the area of a SINGLE module, instead of the array area for a contiguous module array (no gaps beyond the clamp width) to develop the GCp, GCrn and the Gamma a equalization factor . The question is, to determine the pressure under the modules, should we be using the area of the SINGLE module, or of the contiguous array in each of the nomographs for GCp, GCrn and the Gamma a equalization factor?

**** This question was submitted to ASCE staff and posted here so it can be answered and viewed to share knowledge with the entire ASCE community.

Sean Homem
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posted 07-06-2022 11:54 PM
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I recommend using ASCE 7-22, which is now consistent with the 2017 edition of SEAOC-PV2. And I suggest reading all of Chapter 29 of ASCE 7-2. 2

The prescriptive pressure coefficients are plotted against the NORMALIZED effective wind area. This is not the same as the effective wind area. The normalized EWA is determined by the formula in ASCE 7 and reflects the building dimensions. The effective wind area (EWA) of the PV module reflects the load sharing of the array. If each PV panel is mechanically anchored, then an EWA of one panel is appropriate.

The air pressure equalization factor (ϒa) shown in the lower graph is only used when the PV panels are parallel to, and 10 in. or less from the flat part of the roof. The minimum space between panels in both directions must be 1/4 in. Often the clamps between the panels define that space, which usually is at least ¼ in. In that case, external pressure coefficients for the roof cover may be used, and reduced by multiplying by ϒa.