As solar tariffs continue to fall and competition heats up, cost cutting on mounting structures is an obvious first choice because their under performance will not be noticed - until a major climatic event occurs.
In a previous article, buckling of section, member and assemblage as a primary design criteria for solar pv mounting structures was mentioned en passant. This article expands on the theme by answering some of the related frequently asked questions.
Can solar panels provide lateral torsional buckling restraint to purlins ?
Codes allow bracing by covering material
- By rigid diaphragm action capable of transferring forces to the supporting structure
- By arranging equally loaded pairs of purlins facing each other
Option 1 is not applicable due to discontinuity between adjacent solar panels. Option 2 would firstly, result in flange of one of the opposing pairs of purlins to point in up-slope direction which will accumulate dirt and water and secondly, subject the panel to axial forces which may cause micro-cracks in cells. In fact, as neither flange is effectively stabilized by panels, purlin supports or intermediate bracings, if any, need to be designed for torsional moments or equivalent forces PL1 and PL2 shown below:
What is the appropriate effective lengths of columns in supporting frames ?
Tendency of designers is to use alignment charts for unbraced frames as shown below:
This method assumes that all columns in a storey buckle simultaneously. A different approach e.g. by Le Messurier which accounts for difference in column heights and loads is more appropriate here.
Should I use pinned or fixed base condition for columns ?
A pinned base has more maneuverability during erection as compared to a fixed base. Moreover, simultaneous action of uplift and bending moment as in case of a fixed base poses challenges for foundation design.
How do I ensure stability in the longitudinal direction ?
Bracings in end bays and connecting their top ends to a line of purlins designed for additional axial compression should suffice.
The alternative method that you suggest for effective lengths of columns seems to be iterative as column loads are not known a priori. Isn't there a direct method ?
Yes and very appropriately it is called as direct analysis method ! Download MASTAN 2 software and give it a shot.
I have come up with an innovative profile for columns / rafters / purlins but the effective width calculations are a big nuisance. Can you suggest a way out ?
Direct Strength Method is exactly what you need. CUFSM is a free but very sophisticated software for DSM.
Is there any other type instability that is possible ?
Yes, vibration due to vortex shedding followed by galloping can be disastrous. This type of instability is observed in flexible components such as cantilevers. A wind tunnel test is highly recommended for flexible mounting structures.
Author- Mr. Rudra Nevatia, Consulting Engineer
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