Solar Tracker I Beam Reinforcement 1

[floodrod]

Hi all,

So I messed up and need advice. I'm building a solar tracker on a pole mount. Pretty large, 20' x 12' on a 11' high mast.

So I went to the scrapyard to find a pipe and ended up bringing home an I-Beam. It's 16' length.. I poured 3 1/4 yards of concrete (13,000 lbs) and mounted the pole 5' deep so now it's sticking up 11 ft.

I placed the array frame on the mount and now the I beam is not firm enough. Torsion and opposite web action will definitely buckle this beam when weight + wind are applied..

So I cut 8 angle irons and welded lateral bracing evenly spaced. This helped substantially, but I do not think it is sturdy enough yet.

My next idea is to get Sonotubes and 15 bags of concrete and encase the whole beam.

Will a 12" diameter concrete column with an I-beam encased be strong enough to hold about 750 pounds of panels and framing?

Thanks!

 

[BAretired]

I think you need something a little sturdier than that.

What size is the steel column? It must be pretty small if it fits inside a 12" diameter column. What is the orientation of the 20' x 12' tracker...is it horizontal or sloping? Is the weight concentric with the column? Will there be snow load on it? How will wind affect it?

If you use a concrete column, do you intend to provide reinforcement in addition to the steel column? If so, do you intend to drill and epoxy the vertical bars into the concrete base?

Provide a sketch or photograph of what you have built so far. I'm not clear on how the angles were placed or how they were connected to the concrete base.

BA

 

[floodrod]

Thanks for the reply.

The tracker will constantly change orientation following the sun both North and South and East and West. The main weight will be mostly Facing South. The array will almost never be horizontal, or facing north unless a storm is coming. then I may make it horizontal to limit wind resistance. Snow load should be minimal, as it will have a pretty big downward angle in winter when the sun is low. It will most never be concentrically over the beam. Wight will definitely be mainly on southern side, and east/west when tracking.

I certainly can drill in 5/8" rebar and epoxy them in if needed. The beam is about 5.5" X 8.25". so a 12" tube should give almost 2" of concrete on each side.

The lateral braces I welded are like this-

http://www.lusas.com/products/images/3span_bracing_results_cropped.gif

One flat of the L is welded to the web, and the other flat is welded to the inner wall of the outer H pieces. I installed 4 on each side, and plan on installing 1 larger angle at the bottom and anchoring it into the concrete with wedge anchors or epoxy rods.

I appreciate the advice

 

[floodrod]

Edit this line: "One flat of the L is welded to the web, and the other flat is welded to the inner wall of the outer H pieces"

I meant- One flat of the L is welded to the web, and the top and bottom sides of the other flat are welded to the inner H frame.

 

[BAretired]

A 20' x 12' solid sign carries a lot of wind pressure. Your tracker is likely not solid, i.e. there are probably openings for the wind to blow through. To make an intelligent assessment of the supporting column, you need to be able to calculate the axial load and bending moment.

If the weight is P and its eccentricity is e, the gravity load moment is P.e; if the wind force is W and it is applied at a height of h above the base, the wind moment is W.h. Taken together, the maximum moment is P.e + W.h. Wind load W is a variable which depends on the wind velocity in your area as well as the size, orientation and openness of the tracker.

If the tracker is unsymmetrical, wind will also cause torsional moments on the column which need to be considered in your design.

We know P = 750#. We don't know e.
We know h = 11'. We don't know W.

There may be other features about your installation that should be determined before sizing the column, but this should give you a rough idea of how to proceed.


BA

 

[jayrod12]

How deep did you drill your concrete plug. You say you embedded the steel 5 ft, is that the total depth of concrete as well. My bet is you're going to have some issues with the plug over time, especially considering it will be under a permanent state of overturning.

 

[floodrod]

Hi Jayrod,

The foundation is 52" X 50" by 5' deep. Surrounded by undisturbed earth on the 3 main support sides- With 7/8" rebar cage. It took about 3.25 yards of 300 psi concrete to fill. Yes the beam goes to the bottom of the foundation.

I tried to follow the 1/3 buried 2/3 exposed logic. I followed advice from a person who made a 14' x 14' array on a 6x6 wood post that withstood hurricane force winds. He used 2 yards of concrete in a smaller hole.

I am planning on drilling and epoxying 6 pieces of #5 rebar into the foundation that will extend up to to the top of the beam and cross rebars going through holes in the beam connecting to the vertical rebars. Then encasing the whole structure with 4000 psi concrete column. I am trying to decide how large diameter to make the column.

Available sonotubes are 12" 14" 16" diameter

Thanks!

 

[canwesteng]

Might as well make it real beefy. Either that or hire someone to do engineering on it. You should get some circular ties for the vertical rebar though, ditch the cross(?) rebars.

 

[BAretired]

A 20' x 12' rectangle has an area of 240 SF. If it is solid like a sign, it would attract a wind pressure of about 30 psf under heavy wind when the tracker is in the vertical position. That is a force of 7200# which produces a moment of 79,200'# at the top of the concrete base.

The I-Beam you installed has dimensions of 5.5" x 8.25" which sounds like a W8x20. It has about half that capacity in strong axis bending and much less in weak axis bending.

You would be well advised to engage a structural engineer to review your project and advise how to proceed from this point on.

BA

 

[floodrod]

Beefy beefy beefy... I can do beefy..

The max worst angle the array will be is 70 degrees. And the weak axis is between a house and a mountain side, so pretty good wind shielding.

I can go up to 16" column encasing the I beam with 6 or 8 extra rebars epoxied into the concrete base. I'll post a job for an engineer to evaluate but not sure if it will go through..

Any resources about concrete column strength that a non-engineer can understand?

Thanks

 

[BAretired]

The best advice I can offer you is to get help from a qualified engineer, not just for the column design but for the tracker structure as well. A 12' x 20' rectangular structure is going to be subject to significant wind moments when supported only at the midpoint. You need professional help, so please get it locally.

BA

 

[canwesteng]

On the flip side, the consequence of failure is low. Although I wouldn't want to give any design advice to a non engineer on the forum. Concrete columns are pretty complicated

 

[drawoh]

(
On the flip side, the consequence of failure is low...

That depends on where it lands does it not?

--
JHG

 

[floodrod]

I don't want failure, I am OK with overkill.

I can attach FOUR 2" steel sch-40 pipes coming down from top of the beam tied into the foundation 2 feet out from the beams base. Making lots of triangles..

Then when I encase the beam, will also encase a all four pipe at the bottom and top and fill the pipes with concrete.

http://files.engineering.com/getfile.aspx?folder=6ce7d80f-9f29-4cab-aedc-950fbc145fa0&file=pipes.jpg

 

[canwesteng]

What would it cost to get a non-stamped sketch from a structural engineer for this? Might be the way to go

 

[floodrod]

Ok i am speaking to a local engineer about getting this designed and stamped.

 

[floodrod]

Spoke to a licensed engineer today, and he did some quick numbers over the phone. I mentioned the 30 psf figure and he said I only need to calculate somewhere around 13-16 psf. My local codes call for 90 MPH wind load rating, but the location is shielded heavily. Basically, he said it would come out to about 40,000 to 50,000'# at the top of the pole.

He is going to give me an estimate for the stamped drawings.

Also speaking to another engineer who is licensed in other states. Seeing if he will evaluate the tracker frame along with the pole..

Guess we will see...