mounting panels to treated-wood frame

[RShackleford]

I'm really thinking, after all this talk of warping of the wood, that I want to replace the 2x8 purlins with IronRidge XR1000 rails. It will add about $500 to my cost. I hope I can convince the building inspector to allow it. The permit has the engineer's drawings, and that validates the 6x6 posts and the 2x12 beams. Then I can show him one of the auto-generated designs from IronRidge's online tool, which allows you to support a 4x4 array of panels with 4 pairs of 14ft XR1000s, and the rails supported in only two places. The two together kinda verifies what I want to do; except not really, because my PEs design is an integrated whole. In particular, there's the issue of diagonal bracing; but I think I can manage that. The braces from the posts to the beams can be the same. I can keep a 2x8 at each side, but attached to the face of the 2x12s, to allow those N-S diagonal braces. What's then missing is the one diagonal piece running along the underside of the purlins (shown in the leftmost drawings).

[RShackleford]

It looks like there are diagonal braces in the design which may have given the engineer enough confidence that all he needed to account for was uplift and side loading.

That was my impression.

That is perhaps why he specified that particular model of post anchor. Simpson does make post anchor's that counteract bending forces better than the one spec'd in those drawings. Some embed in the concrete of the piers but may have required more concrete.

I actually spec'd the post anchors. I made sketches just kinda using my intuition to size parts, and he went over them and said "yeah, that all works". But, he added tons of diagonal bracing.

[Ampster]

It looks like there are diagonal braces in the design which may have given the engineer enough confidence that all he needed to account for was uplift and side loading. That is perhaps why he specified that particular model of post anchor. Simpson does make post anchor's that counteract bending forces better than the one spec'd in those drawings. Some embed in the concrete of the piers but may have required more concrete.

[J.P.M.]

No, he really didn't share any detailed calcs w/ me. I'm not a ME, but I assume you mean sideways "torques" at the bottom of the piers ? There are a LOT of stiffening diagonal braces.
I'll attach his drawings (w/ identifying info removed) ...

Thank you for the sketches.

I was referring to the bending force and the resulting stresses in the post at the point where the post meets the concrete that's caused by the horizontal component of a design wind vector or other external loading.

Think of a broomstick in a vertical orientation embedded in concrete. Consider the concrete fixed. The question is: If you apply a, say, 100 lbf horizontal load to the broomstick at its farthest point from the embedment (that is, the top of the broomstick), will the broomstick fail ?

That simple situation is similar to array support posts but more complicated in that the reactions along the posts, particularly at intermediate supports/diagonals, needs to be checked along with other things.

Not a knock, but looking at the drawings and not having the calcs makes it impossible to check a design. Just curiosity on my part.

Looking at the drawings, I guess I'd also be curious about the treatment of the drilling/bolt holes in the wood, particularly with respect to occasional/alternating/wind induced vibrational loading and the possible resulting fatigue stresses and their consequences. In the end, NOMB, but that's one of the several reasons why I wouldn't be using wood as array support material in the first place.

[RShackleford]

Uplift is necessary but not necessarily complete information for design calcs.
Did the engineer mention or identify a value for the induced moment in the 6 X 6 posts at the point where they meet the piers ? Just curious.

No, he really didn't share any detailed calcs w/ me. I'm not a ME, but I assume you mean sideways "torques" at the bottom of the piers ? There are a LOT of stiffening diagonal braces.
I'll attach his drawings (w/ identifying info removed) ...

[J.P.M.]

Not really, to support the beams, which are 20ft-long twin 2x12s. I just gave PE sketches of what seemed reasonable to me, and I used the 6x6s. Cost savings of using 4x4 would be minimal (three 8ft pieces, one each for high end, cut in half for low end) and that sure feels flimsy to me

PE didn't share his computations, but I believe he said uplift was somewhere around 1000lb per pier. Piers are spec'd as 24"-diameter by 30" high (20" required to be below grade), which comes out to a little less than 8 cu-ft each. In fact, I can't imagine anything close to the design winds occurring; the biggest wind event we've had here in over 50 years, a hurricane which was producing 50-70 mph gusts when it arrived here ~100 miles after landfall, did not produce winds anywhere close to that strength at ground level. What it did do is to knock down a lot of trees; so my wind risk is not that the solar array is too flimsy or poorly-anchored, but rather that it's smashed by falling trees. Better check if I need an insurance rider !

Uplift is necessary but not necessarily complete information for design calcs.
Did the engineer mention or identify a value for the induced moment in the 6 X 6 posts at the point where they meet the piers ? Just curious.

[RShackleford]

Wow. 6x6 posts are pretty big. What was the uplift wind design criteria to come up with those posts?

Not really, to support the beams, which are 20ft-long twin 2x12s. I just gave PE sketches of what seemed reasonable to me, and I used the 6x6s. Cost savings of using 4x4 would be minimal (three 8ft pieces, one each for high end, cut in half for low end) and that sure feels flimsy to me

PE didn't share his computations, but I believe he said uplift was somewhere around 1000lb per pier. Piers are spec'd as 24"-diameter by 30" high (20" required to be below grade), which comes out to a little less than 8 cu-ft each. In fact, I can't imagine anything close to the design winds occurring; the biggest wind event we've had here in over 50 years, a hurricane which was producing 50-70 mph gusts when it arrived here ~100 miles after landfall, did not produce winds anywhere close to that strength at ground level. What it did do is to knock down a lot of trees; so my wind risk is not that the solar array is too flimsy or poorly-anchored, but rather that it's smashed by falling trees. Better check if I need an insurance rider !

[SunEagle]

I'm liking the idea of replacing my 2x8 purlins with IronRidge XR1000 rails. I'd stick with my PE-approved design for the piers, 6x6 posts, and 2-ply 2x12 beams running E-W. A bit more expensive, but should mitigate wood-warpage concerns and make panel installation easier with the UFOs.

Wow. 6x6 posts are pretty big. What was the uplift wind design criteria to come up with those posts?

[RShackleford]

I'm liking the idea of replacing my 2x8 purlins with IronRidge XR1000 rails. I'd stick with my PE-approved design for the piers, 6x6 posts, and 2-ply 2x12 beams running E-W. A bit more expensive, but should mitigate wood-warpage concerns and make panel installation easier with the UFOs.

[bcroe]

Not being a PE, and already having an array up, I decided to get 4 dozen extra panels (not in
use)up quickly to increase output at day extremes and under clouds. Hopefully the experiment
would last at least 5 years, its at 7 now and looking decent. All the wood hardware is hot dipped
galvanized, all the aluminum is 18-8 SS. Once anything is in the wood a while, it will likely
break off before being removed. Almost all holes were done on a drill press for accuracy.

The buried pieces went in, then a laser was used so the holes for the horizontal cross pieces
would all be in line. Aluminum rails were attached to the A frame by long SS bolts. You can
just see a big linoleum square between the aluminum and wood.

So panels are BOLTED to aluminum, no clips. The aluminum runs ground the panel frames,
connected to a common ground and 10 ground rods. Bruce Roe

NSlow.jpgNSsEnd.jpgNSAjig.jpg

[RShackleford]

most of the movement of the wood is going to happen as it initially dries out. So you will want to let your lumber sit, stacked with space around each board for at least a few days.

I thought of doing the build in two stages. Let the 6x6s and 2x12s dry out as you say, then install the posts and the 2x12 beams. Then let that assembly and the loose 2x8s dry for a couple weeks or months, then paint everything, then install the 2x8s onto the beams.

304 vs 316: 304 is perfectly suitable, and makes up the vast majority of SS parts used in solar. 316 is usually only used in Hawaii or other beach front locations.

Ok, solved that at least.

I would caution against using standard washers as clamps. They likely do not have the strength required. The IronRidge UFO has been extensively designed tested to withstand the required uplift forces. Washers have not. I would either use the mounting holes, or find a clamp that is designed for solar panels.

Mounting holes seems like a bad idea, as they'd be less forgiving or movement of the wood (clamps allow some movement of the wood relative to the panel, in one dimension at least, holes allow none. They do make extra-thick fender washers, 1/8" thick. Maybe best would be to use a clamp like https://www.renvu.com/Solar/SolarMou...p-Dark-302030M or https://www.solarflexion.com/product-p/c225imc-24.htm and toss the bolt and replace it with my lag screw.

And don't try to ground using washers. Again, solar panel grounding devices are extensively designed and tested for this purpose. You don't want to compromise on safety.

Argh ok, guess I have to use those Tyco grounding lugs. They are specified to use 10-12 awg wire. I thought such grounding required 6awg though.

[J.P.M.]

No doubt. I choose wood for cost. Hope it doesn't end up being peenywise-poundfoolish, but I'm committed to it now. Hopefully I can make it work with some of the advice being given here.

I would too if first cost and no concern for tradeoffs about future maint. or any concerns about how such things as effects on property resale value had no value to me.

[RShackleford]

When considering material for support for flat solar panels, based on its properties, metal is probably a better overall choice than wood for such duty.

No doubt. I choose wood for cost. Hope it doesn't end up being peenywise-poundfoolish, but I'm committed to it now. Hopefully I can make it work with some of the advice being given here.

[RShackleford]

I built a patio cover two years ago using treated 2 x 4s.I didn't paint them and they are doing fine. They span 9 feet and are on 24" centers. There are two rows of panels so the structure is actually stiffened by the 4 Iron Ridge rails bolted to them.

So the IronRidge rails run perpendicular to the 2x4s ? I suppose I could do that, but my wood frame is constrained to what the PE gave me. So I'd have to clamp the panels on the short sides (10" zone from corners). Or are the rails parallel to the 2x4s and just bolted to the top edge of them ? Anyhow, it adds considerable expense, especially if I can't buy them locally and avoid freight shipping.

I selected the 2 x 4s for small tight knots to minimize warpage. The treated wood I see today is much less susceptable to warpage than the utility grade mudsill that used to be sold. The incisor marks and the pressure treating process tends to dry it out more evenly in my opinion.

This is MCA treated and #1 grade, so I am optimistic.

I bought inexpensive SS lag bolts and embedded them according to the Iron Ridge engineering. I used 1 1/8 SS fender washers and 1/4" bolts which fit in the top slot of the rails.

Now I'm confused. If you used IronRidge rails, why didn't you use their UFO clamp bolts ? Or did you use the lag bolts to attach the rails to the 2x4s ? Would you point me to the IronRidge page where they talk about SS lag bolts being used ? I thought their system was all metal down to the concrete piers.

[emartin00]

most of the movement of the wood is going to happen as it initially dries out. So you will want to let your lumber sit, stacked with space around each board for at least a few days.

304 vs 316: 304 is perfectly suitable, and makes up the vast majority of SS parts used in solar. 316 is usually only used in Hawaii or other beach front locations.

I would caution against using standard washers as clamps. They likely do not have the strength required. The IronRidge UFO has been extensively designed tested to withstand the required uplift forces. Washers have not. I would either use the mounting holes, or find a clamp that is designed for solar panels.

And don't try to ground using washers. Again, solar panel grounding devices are extensively designed and tested for this purpose. You don't want to compromise on safety.