How to calculate solar rail feet on a commercial solar system, running parallel to the rib

Tuesday, January 25, 2022
Training
by
Veli Markovic

Solar rail on commercial roofs, how many feet?

In past presentations we have looked at solar panel rail framing from the perspective of parallel to the rib and perpendicular, examining how we calculate the amount of rail and minimize wastage.

In this presentation we look at putting together a simple spreadsheet that calculates the number of feet required for a rail run that is perpendicular to the rail.

Rail feet placing is determined by engineering based on wind loads, roof height and other criteria and this determines max spacing but when running rail perpendicular to the rib the final spacing is also determined by the space between the ribs, centre to centre.

The Assumptions

Like all scenarios involving commercial solar calculations we have to make some assumptions that include the following:

  • Spacing between the ribs centre to centre is 233.3 mm
  • Due to thermal expansion max unbroken length of rail is 30 metres
  • Calculations concerning actual rail length are already known
  • Max spacing for feet determined by the engineer in this case is 1000 mm for all examples

Rail Length

In this example we are dealing with a total rail length of 11,410 mm

Rail cantilever and correct feet overall spacing

As well as the max spacing the engineer has said the cantilever can’t exceed 300 mm either end so:

- We subtract 600 mm in total from the rail length of 11,410

Rail cantilever and correct feet overall spacing

As well as the max spacing the engineer has said the cantilever can’t exceed 300 mm either end so:

  • As we have max feet spacing between the feet of 1000 mm
  • We just divide 10,810 / 1000 = 10.81
  • Have to round up to 11 and then add 1
  • Total of 12 feet x 2 ( top and bottom rail) = 24 feet

But hang on, the spacings between ribs are 233.3 mm so the max spacing we can achieve is 933.2 mm ( 4 x 233.3)

Rail cantilever with correct increments

So the question is,  does this alter anything at all with this 11 x panel configuration?

  • We just divide 10,810 / 933.2 = 11.58
  • Have to round up to 12 and then add 1
  • Total of 13 feet x 2 ( top and bottom rail) = 26 feet

So even with a really short rail run we are talking 2 x extra feet

Imagine if there were 20 rows with the same configuration!

Using the 1000 mm max figure we would be 40 feet out.

Feet aren’t that expensive BUT waiting around for stock to be delivered that was not originally accounted for means, potentially, guys on site not having anything planned to do and the trickle down effect that this implies.


And the cantilever changes!

We have been given a max cantilever of 300 mm each side but what happens when we use the correct rib spacing?

Let’s look at our 11 x panel example:

  • We use the 933.2 mm increment
  • There are 12 spaces
  • Total is 12 x 933.2 mm = 11,198.4 mm
  • Remember our total rail length is 11,410 so cantilever on each side is
  • 11,410 - 11,198.4 = 211.6 mm and divided by 2 this means 105.8 mm!

Laying out the feet

With this example we know feet are being placed every 933.2 mm and this means:

Feet placed every 4 x ribs so this is easy from an install preparation point of view but is this all?

What about a bigger rail run?

What about a bigger rail run? 

Say 29 x panels @ 29,860 mm:

  • We just divide (29,860 -600) / 1000 = 29.26
  • Have to round up to 30 and then add 1
  • Total of 31 feet x 2 ( top and bottom rail) = 62 feet

But using the corrected figure of 933.2 we end up with 66 feet.

Again if we had a 20 row system this means an additional 80 feet!

Cantilever: different on both ends

Now, as we are restricted to the distances between the ribs, achieving an equal cantilever on both sides rarely occurs as one extra foot is required to maintain the max engineered feet spacing.

For example, our 29 panel scenario results in 33 feet for the top rail which has a total length of 29.860 mm. 

Our feet spacing is at 933.2 mm .

Let’s look at the sums. 

We have 33 feet but this means 32 spaces so 32 x 933.2 = 29,862.4 mm!!. 

This is 2.4 mm greater than the entire length!

Ok let’s try 31 x 933.2 = 28,928.2 mm which is 913.8 mm for the cantilever so we divide by two and this results in 465.9 mm each end. 

Too much!!

Cantilever: what’s going on?

The engineer has stipulated a max of 300 mm and with the 31 space, 32 foot calculation we have exceeded this and that is why we need the extra foot.

In the above image we have a commercial roof. The white vertical line syndicate the max span of 933.2 mm.

So we can see that the last foot on the right hand side is not on the white line which signifies the 933.2 increment.

Our total rail length is:

  • 29,860 mm
  • Distance between ribs 233.3 mm
  • Max distance between feet is 933.2 mm
  • 31 x 933.2 = 28,929.2 mm
  • Difference between 29,860 - 28,929.2 = 930.8 mm

So we have to:

  • Add another foot 3 x rib length spacings 
  • So from foot 32 
  • Add 699.9
  • So now distance between first foot and last is 
  • 28,929.2 + 699.2 = 29,629.1

Now difference between full rail length and feet spacing is 230.9 mm

And if you make the cantilever even both sides this equates to 115.45 mm per side

So we can see that the last foot on the right hand side is not on the white line which signifies the 933.2 increment.

Conclusion

  • Assess the site and determine feet perpendicular or parallel to rib
  • If perpendicular see max feet spacing and how that relates to rib spacing
  • Make sure your feet spacing do not exceed max engineering stipulations
  • Prepare site drawings so all involved understand the design and what they need to do

If you’d like to see what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:

https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial

https://www.greenwoodsolutions.com.au/news

https://www.greenwoodsolutions.com.au/commercial/customer-stories


About the author

Veli Markovic

CEC Designer
Veli has nearly two decades of experience in the renewable industry. He is passionate about providing people with valuable education and is highly regarded throughout the industry as an educator and operator.
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