Impractical high dead load required to counter wind uplift remains Achilles heel of ballasted rooftop solar installations.
Apply wind load from any national code and calculate ballast, for example using this spreadsheet . It is very likely that ballast will be unacceptably heavy.
So far, designers have had to rely on pressure coefficients applicable to buildings. Fortunately, based on the work of SEAOC Solar Photovoltaic Systems Committee backed by extensive wind tunnel tests, ASCE 7-16 now includes provisions for determining design wind loads on rooftop solar panels. Section 29.4.3 is specific to solar panels on buildings with roof slopes less than 7º and limitations on the panel length, tilt, height above the roof etc.
There are several other factors that modify the design wind loads based on the presence of parapets, panel length and distance from edge of the roof. Resulting pressures are lower for sheltered interior panels as compared to the ones near edges.
The new code provisions are quite elaborate and we are not getting into their intricacies here. Perhaps the most relevant aspect for ballast design is the graph below which relates nominal net pressure coefficient (GCrn)norm to normalized wind area An.
As the tributary normalized wind area increases, nominal net pressure coefficient reduces similar to components and claddings provisions. The challenge is to arrive at correct tributary area which depends on the stiffness of interconnection between adjacent mounting structures.
Even with reduced pressure coefficients, ballast requirement may still be high specially near the edges of an array. One option is to reduce ballast and allow some portion of the array to uplift. Effective wind area then depends on the vertical deflection that the array is allowed to undergo. Smaller deflection limits typically require consideration of smaller effective wind areas.
Author- Mr. Rudra Nevatia, Consulting EngineerAuthor- Mr. Rudra Nevatia, Consulting Engineer
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