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January 30, 2019 Gregory A. Kopp, PhD, PEng
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<br />The instrumentation was adequate for the measurements that were made, and is consistent with
<br />current state-of-the-art practice. Pressure measurements were made with a system capable of
<br />resolving fluctuating and peak pressures. Several hundred pressure taps were used, and the tap
<br />layout is appropriate. Thus, this condition is met.
<br />Neither ASCE 7-05, ASCE 7-10, or ASCE 49-12 discuss data processing methodologies, but SEAOC-
<br />PV2-2017 and ASCE 7-16 indicate that these also need to be examined and discussed. This is done below.
<br />
<br />ASCE 7-16 section 31.6 and SEAOC-PV2-2017
<br />The assumptions made in the model construction of the array are clearly articulated and represent state-
<br />of-the-art practice with respect to panel thickness, gaps between panels, tap layout, etc. Detailed drawings
<br />of the actual PanelClaw product are not included so these cannot be compared in detail within the CPP
<br />report. However, basic geometric parameters of aerodynamic importance are provided in Appendix A.
<br />The array size meets the requirements for the number of panels to establish a set of loads for large
<br />arrays. As the CPP report notes, for projects with smaller or irregular array sizes, judgement must be used
<br />in design decisions.
<br />The roof zones are examined by using a range of array placements on the roof surface. CPP defines a
<br />more refined set of roof zones than SEAOC and ASCE have. I believe these are reasonable since the
<br />testing was done for a single racking system with a large test matrix of array placements and setbacks, and
<br />is based on a thorough understanding of the array and building aerodynamics. The differences with ASCE
<br />7-16 and SEAOC result from the fact that these standards must account for a range of arrays, including tilt
<br />angles in a relatively simplified manner. The effects of the tilt angle and deflectors will also alter the size
<br />and shape of the roof zones, as I have found in my own studies. In any case, the CPP report is an
<br />appropriate use of the well thought out experimental database, and the justification given is clear.
<br />The CPP report is consistent with the wind load methodology presented in ASCE 7-16 and SEAOC-
<br />PV2-2017. It presents peak, area-averaged pressures as a function of tributary area, where the area is
<br />normalized by the building wall size in a similar manner as SEAOC. These are depicted in Figures 3-4, 3-
<br />5, 3-6, 3-7, and 3-8. The wind load coefficients in these figures appear to be reasonable, noting that the
<br />coefficients are more than 50% lower than those in ASCE 7-16 and SEAOC-PV2-2017. This appears to be
<br />due to the shape of this particular system, which should be effective at pressure equalization, but may be
<br />sensitive to increased loads if there is some lifting of some panels. While the choice of tributary area is
<br />discussed based on lifting of the array and load sharing, caution should be exercised in the choice of
<br />tributary area since lifting of the array may lead to increased wind loads.
<br />The design formulation for the wind loads also includes a parapet factor that is based on that in ASCE
<br />7-16. This was actually based on data obtained by CPP, which have been published in the Journal of Wind
<br />Engineering and Industrial Aerodynamics.
<br />A factor for the edge of the array is also included, which, once again, is based on the comprehensive
<br />test matrix for this project. The conditions for the use of the data are reasonable, and based on the authors’
<br />knowledge and understanding of the array aerodynamics.
<br />Aside for the parameters highlighted above, the additional constraints required by ASCE 7-16 and
<br />SEAOC-PV2-2012 are met.
<br />I would add that, since the range of buildings that the array will be used on is large, CPP have provided
<br />guidance in Section 4 (and elsewhere in the report) for the handling particular design issues (e.g., the
<br />presence of other roof-top equipment) without the back-up of experimental data. This guidance is provided
<br />based on their expert opinion, and I have no objections to it as I agree with the underlying assumptions.
<br />However, as new data are published on some of these topics, I would recommend that Panel Claw re-visit
<br />these issues every couple of years.
<br />
<br />CONCLUSIONS AND RECOMMENDATIONS
<br />I have no significant general criticisms of the report; it is clear, comprehensive, reporting on a study
<br />that uses state-of-the-art knowledge of wind loads on roof-mounted solar arrays. Thus, it is my opinion,
<br />based on the discussion above, that the CPP report meets the requirements of ASCE 7-16 Chapter
<br />31, ASCE 7-10 Chapter 31, ASCE 7-05 Section 6.6.2, ASCE 49-12, and SEAOC-PV2-2017.
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