Failure to plan for wind uplift on rooftop equipment can lead to significant roof damage or even catastrophic failure. High winds can impact solar installations as well as pipelines, especially when loaded on extensions.
The KnuckleHead System can be loose laid, but where wind uplift is a factor, it is recommended that KnuckleHeads be mechanically fastened and fully bonded using adhesive. Mechanically fastened KnuckleHeads require all-purpose screws to attach the Universal Base [P/N 2001] to the decking. TABLE 1 summarizes important pullout information on a common all-purpose fastener when used on a variety of decks.
TABLE 1: All Purpose Fastener Specifications
Bonding with Adhesive
Fully adhered KnuckleHeads require the use of GREEN LINK Adhesive/Sealant. The tensile and shear strength resulting from the use of GREEN LINK Adhesive/Sealant is summarized in TABLE 2. It is important to note that with a fully adhered solar KnuckleHead application where no mechanical fastener is used, the uplift value is limited by the tensile strength of the weakest component in a roofing assembly such as the interface between the insulation and the insulation facer.
Table 2: GREEN LINK Adhesive/Sealant Properties
A hybrid system uses both mechanical fasteners and GREEN LINK Adhesive/Sealant. In this case, the maximum allowable uplift force is based solely on the pullout strength of the fastener as shown in TABLE 3.
TABLE 3: Suggested KnuckleHead Support and Uplift Resistance Values (per unit)
Wind Dynamics Calculations
Wind loads can be calculated, and computed values can then be physically applied onto prototype assembles to validate the relative strength and robustness of rooftop support products. Below is a force calculation of a KnuckleHead extension subjected to 75 mph wind conditions, followed by recommendations for maximum allowable height of extensions.
Wind is a flow of gasses that behaves according to the principles of fluid dynamics. Conservation of fluid momentum explains the dynamic pressure created by the flow of gas. Three-dimensional fluid flow problems are extremely complex; ninety percent of the computational effort is spent on the last ten percent of accuracy.
Calculating Drag Force
Figure 1: Drag Coefficients
Drag force (Fd) quantifies the effect that a dynamic fluid pressure, in this case wind, has on an object. There are several variables that determine the magnitude of forces created.
Density (ρ) of gas is an important variable and is directly proportionate to drag force. In short, denser air creates more drag force than dry air.
The velocity (v) exponentially increases drag force: thus, for example, if velocity doubles drag force quadruples.
The frontal area (A) of the object exposed is directly proportionate to the drag force; thus, doubling the wind exposed frontal area will double the drag force.
Drag coefficient (CD) relates to the shape and roughness of the object and its effect on wind flow patterns. Wind flow might be laminar over a smooth surface, but turbulent over an irregular surface. Froude number, Mach Number and Reynolds Number relate to the flow conditions such as the compressibility of air and its flow characteristics as turbulent versus laminar quality. Flat plate at 90 degrees to the flow is assigned a drag coefficient of 0.05. Here, solar panels, wires and cables are assigned values between 1 and 1.3. Other geometrical objects are assigned values according to Figure 1 (drag coefficients are found empirically using wind tunnel testing). Thus, the formula for determining drag force is as follows:
Wind Loading on a Solar KnuckleHead
The wind loading on the Solar KnuckleHead can be approximated by considering the wind exposed area and splitting it into two distinct sections (Figure 2, Back view). Section 1 is the 6.73” inches of tube of 3” diameter. Section two is the head approximated as a rectangle with 3.76 height x 5.7” width. Drag coefficient of 1.15 is assigned to Section 1 while 1.5 to Section 2. Combined effect can be calculated adding moments created by the drag forces on the assembly as shown in the Side View (Fig. 2). The drag force from Section 1 (2.2 lb-force) acting 3.4” above the base applies 0.617 pounds of torque to the Universal KnuckleHead base. The Solar Head (Section 2) creates 3 pounds of drag at 33 m/s (75 mph): this force applies an additional 2.15 foot pounds of torque to the base.
Figure 2: Wind profile of Solar KnuckleHead and Riser
Figure 3: PV rack body diagram
Figure 4: PV rack and module diagram
The following table summarizes the combined effect of the wind load on the Solar KnuckleHead and the different height risers at 75 mph.