Pre-Galvanized vs Hot Dipped Galvanized vs Dura-Green

[Stringpark]

Hello,

Thought I'd take a survey. I'm planning out a ground mount racking system for 18 panels in portrait (6 columns and 3 rows, due to space constraints) and am currently looking at Cooper B-Line strut. Pre-Galvanized and Dura-Green are roughly the same price from my supplier, coming in at around $800 for the channel pieces alone. Hot Dipped Galvanized is more than double the price, coming in at around $1700 for the channel alone. I'd like for the racking system to last roughly as long as the panels themselves. I live in a rural, mountainous area, and away from salt water. Are folks having good success with Pre-Galvanized or Dura-Green strut in this environment? There's not much info out there on the internet directly comparing Dura-Green to Pre-Galvanized, but one document I found from Unistrut indicated that Pre-Galvanized was a step up from their similar "Perma-Green III" finish when it comes to corrosive environments.

Any input is appreciated. Thanks!

[bcroe]

Treated wood is very cheap but limited life. I use 6061 aluminum with 18-8 stainless
hardware, pretty much immune to corrosion. If you get snow, read my thread
Snow Tolerant PV Mounting Bruce Roe

[J.P.M.]

Treated wood is very cheap but limited life. I use 6061 aluminum with 18-8 stainless
hardware, pretty much immune to corrosion. If you get snow, read my thread
Snow Tolerant PV Mounting Bruce Roe

+1. Just be careful for galling on the St. Stl. threads. I always specified aluminum-bronze nuts for st. stl. pressure vessel bolting. Never seize is another way to go. But be assured, st. stl nuts and bolts used together will gall up.

Galvanic corrosion from Al - St. Stl. contact is of some concern but those materials are close enough together on a galvanic series chart not worry about unless a lot of un/improperly grounded equipment is present.

[Stringpark]

Thanks so much for the responses so far! I have seen others on the forum mention aluminum as well and I do have some questions about it, though I'm afraid to steer the thread too far away from the topic of strut finishes as I suspect I'll probably still end up using strut. I say that for a couple reasons:

1.) A quick Google search didn't appear to turn up any aluminum suppliers in my area (though I could definitely look harder).
2.) I've already put in a lot of leg work on a strut design, researching and understanding the loading properties of strut, figuring out assembly, and working up a complete design that I'm comfortable with at this point.

For the basic strut design, I was leaning heavily on the IronRidge design tool, which uses either 2" or 3" sch 40 pipe for the rail supports. Conveniently, single strut appears to have similar beam loading properties as 2" sch 40, and double strut similar to 3" sch 40, so their tool was quite helpful in figuring out a sound design. For aluminum though, I wouldn't know where to begin. It does interest me however, as others have said it can be inexpensive, and the corrosion immunity is a big plus. So, at the risk of going off topic, here are couple questions about aluminum:

1.) Would you estimate that the price for DIY aluminum racking is comparable to the price I mentioned for Pre-Galvanized strut above (for the array configuration I mentioned)?
2.) Do you have any pics or designs guides I could reference to get started with designing an aluminum rack?

Thanks again!

[bcroe]

First decide if your construct will allow easy seasonal tilt. A design that is difficult to change
in the end just will not be done. Do you want it to be useful after snow? Consumer supplies
do not very well adapt to tilt change.

Aluminum price has increased a lot in the last decade. Bruce Roe

[J.P.M.]

Thanks so much for the responses so far! I have seen others on the forum mention aluminum as well and I do have some questions about it, though I'm afraid to steer the thread too far away from the topic of strut finishes as I suspect I'll probably still end up using strut. I say that for a couple reasons:

1.) A quick Google search didn't appear to turn up any aluminum suppliers in my area (though I could definitely look harder).
2.) I've already put in a lot of leg work on a strut design, researching and understanding the loading properties of strut, figuring out assembly, and working up a complete design that I'm comfortable with at this point.

For the basic strut design, I was leaning heavily on the IronRidge design tool, which uses either 2" or 3" sch 40 pipe for the rail supports. Conveniently, single strut appears to have similar beam loading properties as 2" sch 40, and double strut similar to 3" sch 40, so their tool was quite helpful in figuring out a sound design. For aluminum though, I wouldn't know where to begin. It does interest me however, as others have said it can be inexpensive, and the corrosion immunity is a big plus. So, at the risk of going off topic, here are couple questions about aluminum:

1.) Would you estimate that the price for DIY aluminum racking is comparable to the price I mentioned for Pre-Galvanized strut above (for the array configuration I mentioned)?
2.) Do you have any pics or designs guides I could reference to get started with designing an aluminum rack?

Thanks again!

Nothing wrong w/galvanized. Just make sure you cover all the bases - things like isolating the galv. from any concrete and other things usually unknown or not mentioned. to or by DIYers.

Regardless of the material selected, what has the leg work of strut design entailed ?
How about the design of rest of the structure ?
What have you done and what methods have you used to calculate loadings on the structure ?
What hoops do you need to jump through for permitting, both for municipality and power company ?

A bit off topic (so far), but what about things like array elec. design, wiring, grounding, other electrical issues, panel fixation, etc. ? Just wonderin'.

[peakbagger]

I have a couple of arrays made with superstrut. I live in the mountains well away from the coast. I did no galvanic isolation on the first array which is 17years old so its aluminum PV frames bolted directly to superstrut in multiple places.This is an array that is hung off a wall that has cedar siding. I had installed standoffs between the strut and the wall but in one area the cedar is touching the strut and there is some corrosion at the contact point. I dont see any corrosion where the panels bolt directly to the strut but there is a horizontal strut at the base of the panels that forms a lip that the bottom of the panels rest on that has some galvanizing loss. It also acts as a small gutter and is the most exposed to the weather.

My pole mount array is about 14 years old. It started out with different panels and was upgraded substantially at one point which required partial disassembly so I got partial autopsy after the first panels were replaced after about 5 years. These were used when I got them and came from different sources. Some came off highway traffic signs and had some corrosion when I got them. I didnt see a lot of point corrosion between the frames and the strut from my installation but it did look like the panels that came with prior corrosion had more at the attachment points. The array was assembled with mostly stainless steel fasteners using nylock type nuts. I did have one or two galled connections but given that the panels were 60 watt of panels, I had lots of connections. On occasion during the original build, I used hardware store zinc plated and galvanized fasteners and they were far more impacted by corrosion. Some came apart hard and I threw them all away while I reused the SS fasteners. When I swapped to the newer larger Evergreen panels I was far more careful to use Neverseeze on all SS connections and cut way back on any new galvanized fasteners. I also bought some thin HDPE plastic sheets from Grainger and made up some washers with a slot cut out of them. When I assembled the panel frame to the strut I would get the hardware started and then slip a washer between the strut and the panel frame then tighten things up. This galvanically isolates the strut from the frame. I see little or no deterioration anywhere on this frame.

I would definitely recommend going with SS fasteners with a dab of neverseeze on all threads. I used the paste, I have seen a stick version that is less messy but havent used it. One thing about SS fasteners is the typical hardware store charges a big premium. I order boxes of them from a discount marine supplier (Hamilton Marine in Maine). I can usually buy a box of 100 for what a hardware store charges 5 or 10 individual pieces. Its the way to go except when you need long bolts and would have little use for the extras.

My third small roof array is 5 years old with factory racking and roof mounts. I rigged and installed everything myself (on all three arrays) and must admit its nice to have little plastic baggies of exactly the right hardware in my tool pouch and a factory engineered system. It is also a lot lighter weight and quicker to install.

An important thing to realize with using unistrut is its not an engineered system as far as code is concerned. My first two arrays were done relatively early in the solar game. I am in a rural area with no inspection and I didnt worry about having an engineered mounting system but did follow the rest of the electrical code in place at time. Both arrays were some of the earlier grid tied arrays in my region. The newest array was done completely to code including an engineered racking system. I do have a PE stamp and in pinch I could sign off on my own racking systme but nice not to need to. Unless you have the skills or know someone who does to ensure code compliance on the racking design I strongly suggest buying a commercial system

[J.P.M.]

I have a couple of arrays made with superstrut. I live in the mountains well away from the coast. I did no galvanic isolation on the first array which is 17years old so its aluminum PV frames bolted directly to superstrut in multiple places.This is an array that is hung off a wall that has cedar siding. I had installed standoffs between the strut and the wall but in one area the cedar is touching the strut and there is some corrosion at the contact point. I dont see any corrosion where the panels bolt directly to the strut but there is a horizontal strut at the base of the panels that forms a lip that the bottom of the panels rest on that has some galvanizing loss. It also acts as a small gutter and is the most exposed to the weather.

My pole mount array is about 14 years old. It started out with different panels and was upgraded substantially at one point which required partial disassembly so I got partial autopsy after the first panels were replaced after about 5 years. These were used when I got them and came from different sources. Some came off highway traffic signs and had some corrosion when I got them. I didnt see a lot of point corrosion between the frames and the strut from my installation but it did look like the panels that came with prior corrosion had more at the attachment points. The array was assembled with mostly stainless steel fasteners using nylock type nuts. I did have one or two galled connections but given that the panels were 60 watt of panels, I had lots of connections. On occasion during the original build, I used hardware store zinc plated and galvanized fasteners and they were far more impacted by corrosion. Some came apart hard and I threw them all away while I reused the SS fasteners. When I swapped to the newer larger Evergreen panels I was far more careful to use Neverseeze on all SS connections and cut way back on any new galvanized fasteners. I also bought some thin HDPE plastic sheets from Grainger and made up some washers with a slot cut out of them. When I assembled the panel frame to the strut I would get the hardware started and then slip a washer between the strut and the panel frame then tighten things up. This galvanically isolates the strut from the frame. I see little or no deterioration anywhere on this frame.

I would definitely recommend going with SS fasteners with a dab of neverseeze on all threads. I used the paste, I have seen a stick version that is less messy but havent used it. One thing about SS fasteners is the typical hardware store charges a big premium. I order boxes of them from a discount marine supplier (Hamilton Marine in Maine). I can usually buy a box of 100 for what a hardware store charges 5 or 10 individual pieces. Its the way to go except when you need long bolts and would have little use for the extras.

My third small roof array is 5 years old with factory racking and roof mounts. I rigged and installed everything myself (on all three arrays) and must admit its nice to have little plastic baggies of exactly the right hardware in my tool pouch and a factory engineered system. It is also a lot lighter weight and quicker to install.

An important thing to realize with using unistrut is its not an engineered system as far as code is concerned. My first two arrays were done relatively early in the solar game. I am in a rural area with no inspection and I didnt worry about having an engineered mounting system but did follow the rest of the electrical code in place at time. Both arrays were some of the earlier grid tied arrays in my region. The newest array was done completely to code including an engineered racking system. I do have a PE stamp and in pinch I could sign off on my own racking systme but nice not to need to. Unless you have the skills or know someone who does to ensure code compliance on the racking design I strongly suggest buying a commercial system

IMO, good information professionally presented. Thank you.

One of several reasons galvanized bolting can sometimes corrode faster is if/after it's un/refastened and reused and retorqued, especially if the bolt or nut, or both, has ever gone into local yielding from over/improper torqueing or tightening at some point.

I've used antigalling compound for stuff around the house 1X/awhile, but far less so for stuff I designed for others. When I did, I made sure it was moly disulfide. Beware of stuff that's not. Read the label.

For some pressure vessel and some structural stuff, some codes make you jump through testing hoops that are a PITA or disallow thread lubes or some types altogether. Some food processing applications don't allow it. Long, boring story and mostly not applicable to what we're talkin here, but if interested, for such times, for pressure vessels/piping, stainless bolting and Al-Bronze nuts work well but cost a lot.

For structural and also for pressure vessel design, sometimes, depending on the application, if galvanized or other surface treatment such as painting is not used for some reason, 400 series stainless can be used with 300 series stainless and the galling problem can be mostly avoided IF the joint is bolted SLOWLY. I'd still use moly disulfide in that case for added insurance. My experience is that the actual tightening method and professional workmanship is what controls the joint integrity in that case.

[Stringpark]

Thank you all for the responses and your interest in helping me get this sorted out! Oddly, I'm afraid I'm having some trouble posting. I wrote a response to the various questions asked and it looks fine when previewing, but when posting, only the first few sentences show up. Don't know if it's holding me to character count (being a new member) or what.

[Mike90250]

Hello,

Thought I'd take a survey. I'm planning out a ground mount racking system for 18 panels in portrait (6 columns and 3 rows, due to space constraints) and am currently looking at Cooper B-Line strut. Pre-Galvanized and Dura-Green are roughly the same price from my supplier, coming in at around $800 for the channel pieces alone. Hot Dipped Galvanized is more than double the price, coming in at around $1700 for the channel alone. I'd like for the racking system to last roughly as long as the panels themselves. I live in a rural, mountainous area, and away from salt water. Are folks having good success with Pre-Galvanized or Dura-Green strut in this environment? There's not much info out there on the internet directly comparing Dura-Green to Pre-Galvanized, but one document I found from Unistrut indicated that Pre-Galvanized was a step up from their similar "Perma-Green III" finish when it comes to corrosive environments.

Any input is appreciated. Thanks!

To get your posts to complete, you need to use a plain Jane text editor like notepad and generic fonts. The apostrophe issue is causing a lot of post's truncation. Plain windows fonts are fine, cell phones and "upgraded" browser fonts or word processor embedded stuff triggers the anti-spam truncation.

[Stringpark]

**Thanks Mike90250! It did seem to stop at the apostrophe, so I'll try it again with apostrophes removed.**

Thanks for the additional responses! I've not done this before, so your experienced advice is valuable to me. Let me try to answer a few questions.

First decide if your construct will allow easy seasonal tilt. A design that is difficult to change
in the end just will not be done. Do you want it to be useful after snow?

Im planning on a 30 degree fixed tilt. We do get snow here, but not so much that it would be a great bother to tend to it with broom. I do like the idea in your thread about spacing the rows 6 inches apart, so I may incorporate that.

Regardless of the material selected, what has the leg work of strut design entailed ?
How about the design of rest of the structure ?
What have you done and what methods have you used to calculate loadings on the structure ?
What hoops do you need to jump through for permitting, both for municipality and power company ?

A bit off topic (so far), but what about things like array elec. design, wiring, grounding, other electrical issues, panel fixation, etc. ? Just wonderin.

Honestly, probably not a lot of work for someone experienced, but for a first-timer like me, it represents a considerable amount of planning. The main part was just trying to understand beam loading and how strut compares to the schedule 40 used by the IronRidge tool. Being that their tool is for landscape only, I had to understand how to adapt what the tool was giving me for my portrait configuration. As far as calculation goes, I was relying solely on the output of the tool, as dont know how to do this otherwise (hence my apprehension to switch to aluminum, where I dont have a tool to guide me). The end result was a design based largely on this picture https://goo.gl/images/uJ2rmJ and tweaked with the span figures from the IronRidge tool.
As far as permitting goes, the county doesnt seem to be overly particular here. I spoke with the inspector and he just said to use a lot of concrete, which I intend to do.
For the other stuff, Im much more comfortable with the electrical part and most of that work has already been completed. Im pretty much down to just installing the panels themselves. For panel fixation, if I go with strut, Ill probably use the Snapnrack mid-clamps, as it looks like they will fit within the strut channel. For end clamps, I intend to use square strut washers underneath the panel to clamp the lip of the frame to the strut.

peakbagger, thank you for your comments. My main question is about longevity of the different finishes of strut (hot dipped vs pre-galvanized, etc.). I think that the gold superstrut is basically a pre-galvanized finish, so your comments are very helpful! At this point, do you foresee your 17 year old strut lasting another 20 years?

Thanks again everyone!

[peakbagger]

In industrial use in a far nastier environment then my house, I have seen a lot of 30 to 40 year old strut in electrical rooms. It self heals but if you cut it or grind it you definitely need to touch it up with a cold galvanizing product. Even with a touch up, the cold galvanizing spray is usually the first thing to fail. As for my piece that is getting impacted by cedar siding, at some point I need to touch it up.

[J.P.M.]

**Thanks Mike90250! It did seem to stop at the apostrophe, so I'll try it again with apostrophes removed.**

Thanks for the additional responses! I've not done this before, so your experienced advice is valuable to me. Let me try to answer a few questions.



Im planning on a 30 degree fixed tilt. We do get snow here, but not so much that it would be a great bother to tend to it with broom. I do like the idea in your thread about spacing the rows 6 inches apart, so I may incorporate that.



Honestly, probably not a lot of work for someone experienced, but for a first-timer like me, it represents a considerable amount of planning. The main part was just trying to understand beam loading and how strut compares to the schedule 40 used by the IronRidge tool. Being that their tool is for landscape only, I had to understand how to adapt what the tool was giving me for my portrait configuration. As far as calculation goes, I was relying solely on the output of the tool, as dont know how to do this otherwise (hence my apprehension to switch to aluminum, where I dont have a tool to guide me). The end result was a design based largely on this picture https://goo.gl/images/uJ2rmJ and tweaked with the span figures from the IronRidge tool.
As far as permitting goes, the county doesnt seem to be overly particular here. I spoke with the inspector and he just said to use a lot of concrete, which I intend to do.
For the other stuff, Im much more comfortable with the electrical part and most of that work has already been completed. Im pretty much down to just installing the panels themselves. For panel fixation, if I go with strut, Ill probably use the Snapnrack mid-clamps, as it looks like they will fit within the strut channel. For end clamps, I intend to use square strut washers underneath the panel to clamp the lip of the frame to the strut.

peakbagger, thank you for your comments. My main question is about longevity of the different finishes of strut (hot dipped vs pre-galvanized, etc.). I think that the gold superstrut is basically a pre-galvanized finish, so your comments are very helpful! At this point, do you foresee your 17 year old strut lasting another 20 years?

Thanks again everyone!

Although it may seem a useless question at this point since from what you write it looks like code compliance will not be an issue, but I'll ask again in a different: What external loads did you use to determine the suitability of the strength of the structure and how did you arrive at the magnitude of those loads ?

[Stringpark]

Although it may seem a useless question at this point since from what you write it looks like code compliance will not be an issue, but I'll ask again in a different: What external loads did you use to determine the suitability of the strength of the structure and how did you arrive at the magnitude of those loads ?

I don't think I know enough to answer the question intelligently, but I'll just describe basically what I did to come up with a design that I am comfortable with. I'm sure it's not perfect, but I think it should be a solid structure. So what I did was to look up the beam loading of 1-5/8" single and double strut and found it to be similar to the beam loading of 2" and 3" schedule 40 pipe, respectively. Armed with this information, I went to the IronRidge design tool and loaded it up with my array specs (incidentally, since the tool only supports landscape, I had to tell it 4 columns of 5 rows in order to achieve a similar total area to my configuration). The output of the tool gave me figures for North-South pier spacing, pier depth, and concrete volume that should be right for my approximate array size and tilt. I was also able to toggle back and forth between 2" and 3" pipe to understand the max span of each under various loading (by adjusting the number of rows), which in turn helped me understand an acceptable span for the strut. When finished, I also added pier to pier diagonal E-W supports and N-S supports to match the picture I referenced earlier. I figured this should make up for any shortcomings in my adaptation of the IronRidge tool. I hope this answers the question at least somewhat. Thx!

[peakbagger]

If in doubt, "formula P" applies for most structural problems. Formula P is simply if in doubt add more pounds of steel . Its close cousin is "formula H", the taller a cross section can be the more load it can take although the key thing is that the strength is only when its directly in line with the height. Lateral bracing definitely has to be in place or bad things happen. Its obviously not that simple as someone has to be aware of the potential loads before they can try to account for them. Static loading is usually easy but dynamic loads like wind uplift are a lot tougher. I am not a structural engineer so I can get the static loads covered but dynamic loads are a whole other story.

I remember running my proposed configuration with high snow loads and a high wind zone. The various manufacturer's calculation programs would come up with a solution for close spacing to the roof but whenever I tried to open up the gap the programs would generally say they didn't have any systems that would fit. I expect there was a parameter set in the software that made gross assumptions about what was a complex calculation. No doubt if I had a structural engineer with expertise he could of come up with a fix but I just accepted a stock solution and took the hit during hot days in the summer.