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Design ballast for solar PV racking systems mounted on flat roofs

Task:

Racking systems mounted on flat roofs are mostly secured by ballast to withstand uplift and sliding forces due to wind actions. In this context, flat roofs are defined as roofs having a slope of less than ± 7° (cf. ASCE/SEI 7-10) so that with view to the wind flow over them it can be assumed that the flow separates on the roof edge or parapet.

Typical for the flow over flat roofs is the forming of edge vortices due to cornering flow. These forms of flow also called "delta wing vortices" present high local rotational speeds and create correspondingly high suction effects on the roof, especially in the corner and edge zones.

 

Solution:

Wind tunnel studies conducted in the I.F.I. wind tunnel laboratory have shown that many parameters affect the wind loads on racking systems mounted on flat roofs. Parameters such as tilt angles of modules and deflectors, venting gaps, row spacing, parapets, PV array setback from the roof edges play a major role in the wind load calculation.

Typically, design pressure coefficients are given separately for different array and roof zones. Furthermore, the necessary ballast for the securement of ballasted roof mount solar systems depends on the stiffness of connecting members. If wind forces on highly loaded zones of arrays can be largely redistributed by the interconnected substructure, the benefits of load sharing are applicable. The building itself also needs to be stiff which may be generally assumed for low-rise buildings not higher than 60 ft.

 

Advantage:

The above brief summary of parameters dictating the wind loads on roof-mounted solar PV arrays shows that simplified assumptions have to be made regarding wind tunnel studies from which design wind loads for a wide range of building and array dimensions are to be determined.

According to SEAOC-PV2-2012 wind tunnel reports on solar roof mounted structures need to be peer-reviewed by an independent expert. In this way I.F.I. wind tunnel reports have been approved several times. I.F.I. Institute for Industrial Aerodynamics is also a City of LA Department of Building and Safety (LADBS) approved laboratory for wind tunnel testing of buildings and structures, Testing Agency License Number TA 24830.