Ground mount for solar panels in pressure-treated wood 3D model
Thanks for the input. I will be mounting the panels with rails and clamps that are either attached to the rafters with through carriage bolts or alternatively wood screws 7x120mm so that the panels are fully supported on the backside. I will be using a hybrid inverter and 16kwh battery from sigenergy. I have found a contractor who charges reasonably for the materials and is OK with me doing a lot of the installation myself. I am an electrician but don't have my own authorization, and to take advantage of the green technology tax deduction, I need to go through a firm. I'm paying 1060kr per panel for solitek glass/glass in all-black 370w per panel. Then I prepare with scaffolding, rails, dig for power supply, install the substation, etc.
It's getting close to the time to call off material for the structure, but I am a bit stuck in the decision-making process about which fastening to use for the rails. The rails are aluminum, which places some demands on the fastening to avoid the risk of galvanic corrosion that could eventually weaken the aluminum. Although my project is not near the coast, so there's no significant amount of salt (electrolyte) here, everything needs to be long-lasting, so stainless steel it is.
I have considered using through-fastening with threaded rod and lock nuts, but it feels like an unnecessarily complicated fastening that would require drilling 10mm holes through the entire 45x170 beam, which is why I've started looking closer at screws.
The rails in question are designed for fastenings around M10. However, I've had a hard time finding wood screws with a 10mm shaft that aren't of the type French bolt, which feels rather outdated. What I can find that is fully threaded, stainless, and with a shoe head is SPAX 8x100mm https://www.spax.com/sv-sv/p/hi-for...s-4cut-rostfritt-stal-a2-diameter-o-8-mm.html
These can be obtained at a reasonable price at the hardware store.
Would they be sufficient as fastening for the panels considering wind load, etc.? The rails will be fixed with a c/c measure corresponding to the roof trusses, which will be right at the seams of the panels that measure 1074mm in width.
As usual, grateful for any input.
PS.
If I understand the datasheet correctly on the screw here.
Specified pull-out parameter 12N/mm2;
Does it mean that one can calculate on the core of the screw (5mm) 5mmxPI=15.7mm circumference x the length of the thread (80mm)=1256mm2
1256mm2x pull-out parameter= 15072N
Can it be correct that a screw can withstand 15kN? If that is the case, this should work more than well provided that C24-rated wood can withstand everything.
I have considered using through-fastening with threaded rod and lock nuts, but it feels like an unnecessarily complicated fastening that would require drilling 10mm holes through the entire 45x170 beam, which is why I've started looking closer at screws.
The rails in question are designed for fastenings around M10. However, I've had a hard time finding wood screws with a 10mm shaft that aren't of the type French bolt, which feels rather outdated. What I can find that is fully threaded, stainless, and with a shoe head is SPAX 8x100mm https://www.spax.com/sv-sv/p/hi-for...s-4cut-rostfritt-stal-a2-diameter-o-8-mm.html
These can be obtained at a reasonable price at the hardware store.
Would they be sufficient as fastening for the panels considering wind load, etc.? The rails will be fixed with a c/c measure corresponding to the roof trusses, which will be right at the seams of the panels that measure 1074mm in width.
As usual, grateful for any input.
PS.
If I understand the datasheet correctly on the screw here.
Specified pull-out parameter 12N/mm2;
Does it mean that one can calculate on the core of the screw (5mm) 5mmxPI=15.7mm circumference x the length of the thread (80mm)=1256mm2
1256mm2x pull-out parameter= 15072N
Can it be correct that a screw can withstand 15kN? If that is the case, this should work more than well provided that C24-rated wood can withstand everything.
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Thanks for the quick input.K knegolf said:
Stainless steel carriage bolts going through a 170mm timber are quite hard to find, not to mention expensive. What is the difference between that anchor screw and the SPAX screw that I linked to?
I found that table on essve and a similar screw to the one I found, which listed the permissible load in different weight types, including C24-classified timber. An 8mm screw with 80mm thread like the one I'm looking at from SPAX would manage 225KG in axial load. The reported figures are supposed to contain safety factors according to Eurocode.
If I count on my panels on average being attached with 2 screws per panel, I have a fastening that can handle 450kg for a panel with an area of about 1.7m2. I don’t know how this can be translated into wind loads, so I'm gratefully accepting help in this regard.
Does it have to be stainless, galvanized should suffice?K knegolf said:
If it needs to withstand such loads per 2 screws, then through bolts are required. I doubt the wood can withstand that much with regular wood screws, I believe these will give over the years with the movements...
The reason for stainless is that aluminum can be eaten away by zinc over time. This occurs mainly if there is some form of electrolyte in the picture, i.e., impurities in water that hit the fastening, such as saltwater, but also other alkalis or acids that are naturally around us. The combination of aluminum/galvanized is not recommended outdoors.Rejäl said:
I was playing around with a calculator to calculate wind load per square meter that I'm not really competent to use, but I get that I must reach a wind speed of 45m/s to exceed 450kg per panel. The values I've used are as follows
Height: 1.5m
Zone: A
Terrain type: 2
Reference wind speed: 45m/s
Turbulence factor: 1
Topographic factor: 1
Density: 1.25
Safety factor: 0.91
Load factor: 1.5
The calculation gives me a wind load of 2625N/m2, which lands at 4.46kN on a panel of 1.7m2.
As I said, this is far above my knowledge level, but I assume the calculations assume it's a vertical plane hit directly by the wind. In my case, the panels are at a 20-degree angle.
Now, this calculation is more of a fun thing to play with that I've probably misinterpreted, but if I'm not completely off track, the 8x100mm screw should be fully sufficient. But as you mentioned, it should perform over time.
I would use hot-dip galvanized carriage bolts for that, as they will definitely hold. Then you can monitor if the aluminum feels more theoretical than likely over the years... Your wooden framework won't last forever either... Regular wood screws will likely jiggle loose with the wind forces since you will have so few...K knegolf said:
I think you are overanalyzing this.
You won't need such heavy screws.
I have set up 2x5 panels at 45 degrees on 2x3 ground screws (Jula's ground anchors/screws) on a support beam of 120x45 and truss of 95x45 cc 1200.
On these, I attached the rails with something like a lag screw through the rail.
Pictures are available in my profile.
This has withstood storms for several years without problems.
So the construction you are discussing doesn't need particularly heavy attachments, cover the gables with some scrap wood and it's safe.
You won't need such heavy screws.
I have set up 2x5 panels at 45 degrees on 2x3 ground screws (Jula's ground anchors/screws) on a support beam of 120x45 and truss of 95x45 cc 1200.
On these, I attached the rails with something like a lag screw through the rail.
Pictures are available in my profile.
This has withstood storms for several years without problems.
So the construction you are discussing doesn't need particularly heavy attachments, cover the gables with some scrap wood and it's safe.