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Tracking PV Array Foundation Design Help

martinwinlow

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Feb 6, 2021
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Hi All,

I have a 10 module (each pretty much 1m wide x 2m high in portrait orientation) dual tracking array to install - making the array 5m wide x 4m high. I'm after some help with the foundation design. I am planning to use a reinforced concrete base attached to the bedrock which is only 500mm below soil surface using cement grout-fixed steel rebar (tho stainless rebar is an option as it's pretty wet here and I'm only 3km from the sea).

I had in mind a flat-topped pyramid shaped base 120mm square at the bottom, stopping about 600mm above soil level and the portion below ground having vertical sides (ie a flat-topped pyramid sitting on a box).

I'd like to come up with a suitable design of the reinforcing and concrete to bedrock interface ie how much rebar I'd need and of what diameter and what size/depth of rebar penetration into the bedrock. I'd rather not use resin for the rock fixing to reduce the cost. I have access to a portable/dismantle-able 20T hydraulic press (with pressure/force gauge) and thought I could do a couple of trial pins and then test their pull strength (up to 20T anyway).

From an online calculator I gather the deployed array at approximately 45 degree inclination in a 40mph wind (the most I'd be likely to use the array in before it automatically levels itself to reduce wind load) is 2800N. The main support column is 2m high at the point where the array pivots on the column giving a 'lever' length of 2.6m at bed rock level. Am I on the right track thinking that if the concrete to bedrock pins are 600mm out from the centre of the support column's axis that the loading at the pins would be 2800N x 2.6m/0.6m = 12000N?

If I then shared that load by, say, 4 x 12mm diameter steel pins along each side of the base (12 in all) would I be right in thinking that the shear strength of the pins (at 30KN for 12mm diamiter) will be way more than adequate? So, it's then just a case of ensuring the pins are deep enough and that the cement/sand grout is strong enough to withstand the pull-out forces... As I say, I'll do a couple and test them to 20T and see how it goes... I guess could test them all...

Does this all sound reasonable or am I way off? Does anyone have any professionally designed examples they would be willing to share here (or by PM)?

Regards, MW
 
using cement grout-fixed steel rebar
I’d use that BUT ALSO use very very fat and deep anchor bolts in multiple locations so that the leverage angle of the array does not defeat the rebar
how much rebar I'd need and of what diameter and what size/depth of rebar penetration into the bedrock
that depends on the makeup of the ledge- how frangible/fracturable is it? Rebar is cheap metal, and 1/2” rebar is not that strong. Plus multiple angles should be used to avoid extreme shear or tension loads. It use 5/8” or 3/4” plus deep bolt anchors.
 
main support column is 2m high at the point where the array pivots on the column giving a 'lever' length of 2.6m at bed rock level
It’s more than that- leverage angle is from the top of the load.
do a couple and test them to 20T and see how it goes...
12mm diamiter)
I would use fatter bolts, it’s been over ten years since I’ve stumbled my way through any calcs for uplift and shear. But I’d want 3/4” or 19mm alloy 10.8 if you can get threaded rod in 10.8
Don’t forget that cyclic, dynamic loads will fail fasteners well within design tolerance with any movement. Ever seen failed crane pivot plate retainers? When out of tolerance, undertorqued, or overtorqued they fail by stretching aka “yielding.”
anyone have any professionally designed examples they would be willing to share here (or by PM)
Nobody with credentials that I’ve ever met would do that without a site visit. The stuff they would tell you freely you already looked up.
 
Thanks for your replies. I dug the hole 2 days ago only to find what I anticipated being bedrock was just a layer of heavy glacial rocks with light, gravelly clay beneath. It always tickles me to think this stuff hasn't seen the lght of day for 12 thousand years or so...

So, back to the drawing board! I think I'll try a base of concrete 600mm (2') thick but augur some 150mm (6") re-barred fingers off the bottom about 1m (3') long and at about 45 degrees, perhaps 8 in all and cast it all in one go. I'm trying to minimise the amount of concrete I use as it is quite an awkward site, we have no access to cranes or concrete pumps or, FTM, ready-mixed concrete but we do have a good sized mixer and all the free sand and ballast you could ever want - one of the benefits of living on an island. Nevertheless, it's going to be a ~4m3 by the time I have an 2' diameter concrete 'column' poking out of the ground 2' or so...
 
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