Rail Thermal Expansion

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jaggedben said:
Solar racking rail. This is the PV forum.

I doubt anyone has coallated and published such a chart.
Click to expand...
We have a job with 80ft runs of Snap-n-Rack and the manufacturer says they recommend no longer than 56 feet. I would assume such a calculation would be subject to the temperature differential for a given area. Here in the Northeast it can be -10 degrees and as high as 100 degrees ambient with higher temperatures on a rooftop. This is an important consideration so the racking and supports don't loosen.
 
I found this chart for sheet roofing materials...


The chart shows that aluminum would expand about 1.229 inches at 80ft using the 100 degree column.

Next I need to determine what is an acceptable expansion length before a thermal break is needed. SnapNrack said about 56ft. According to that chart, thermal expansion is about .768in at 50ft using the 100 degree column.

3/4 of an inch seems to be too much?

100-120 degree F delta should be about 30-40ft max rail length then a thermal break....for all aluminum rail.
 
shortcircuit2 said:
I found this chart for sheet roofing materials...


The chart shows that aluminum would expand about 1.229 inches at 80ft using the 100 degree column.

Next I need to determine what is an acceptable expansion length before a thermal break is needed. SnapNrack said about 56ft. According to that chart, thermal expansion is about .768in at 50ft using the 100 degree column.

3/4 of an inch seems to be too much?

100-120 degree F delta should be about 30-40ft max rail length then a thermal break....for all aluminum rail.
Click to expand...
The ASHRAE manual says that for the Boston area, the winter design temperature for 99.6% is 7.7°F and the summer design temperature is 90.8°F, also at 99.6%. In any given year, the chance that the extreme range is between these temperatures is 99.6%. So it looks like you could get away with using a delta of 85°F instead of 100°F.
 
Old rule of thumb for us was ~30' or so, about three rails (Pro Solar rail at the time).
Interestingly steel is just about half as expansive as aluminum - and PVC is over double that of aluminum (or over 4 x steel).
(PVC is VERY problematic on a roof, but that is a different topic.)
 
Also, wouldn't we want to compare the expansion rate of the building itself with the aluminum rail?
I have always wondered about this.
Does the whole building expand when the sun shines? Albeit at differing rates?
 
Opinions vary on whether you gap the rail and the PV panels (use end clamps for the panels on both sides)
....or just leave a rail gap underneath a PV panel (panels continuous over gapped rails).
Just thought I'd complicate the issue.
You're welcome.
 
Zee said:
Opinions vary on whether you gap the rail and the PV panels (use end clamps for the panels on both sides)
....or just leave a rail gap underneath a PV panel (panels continuous over gapped rails).
Just thought I'd complicate the issue.
You're welcome.
Click to expand...

I've never seen a manufacturer put the second "option" in writing. It would defeat the purpose of an expansion joint, and it would concentrate the thermal movement beneath an individual module, thus risking damage to that particular module. You are much better off if you coordinate the joint to occur at the module gap, and leave space for both end clamps and bonding jumpers to be installed on the top of the rail. You also need to exclusively fasten only one rail to the splice joint, otherwise you also defeat the purpose.

It's like using expansion joints to meet 300.5(J), and then strapping the stub-up below the fitting. Or like casting the stub-up directly in concrete without a sleeve or box-out to allow vertical movement. It defeats the entire purpose of the expansion joint, if the stub-up beneath it is restricted from moving.
 
gadfly56 said:
The ASHRAE manual says that for the Boston area, the winter design temperature for 99.6% is 7.7°F and the summer design temperature is 90.8°F, also at 99.6%. In any given year, the chance that the extreme range is between these temperatures is 99.6%. So it looks like you could get away with using a delta of 85°F instead of 100°F.
Click to expand...
I have seen the ASHRAE "design" temperature charts. But the in the real world on the rooftop in the direct sun, it gets much hotter under that array. Is it reasonable to go by the ASHRAE chart?
 
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