Ground Mount Brace Opinions ?

[Syberdog]

I've got my T6061 ground-rack frame pretty much complete... I'm almost ready to pour the concrete for my ballast blocks and then install the rails.



I will be using IronRidge XR1000 rails E-W, spliced together for a total of 33' in a 2Px10 configuration. My 5" T-posts are 8'-9" OC and the front of the 3" Angle front supports are 5' from the post. The 30-degree, 3"x1/4"x10' angled table extends about 3'-11" upward from the T-post. The Ballast blocks are 24"x96"x18" with the main post set another 18" in conc. I'm in a ASCE 7-05 90mph wind zone with 0 snow. Prevailing winds are from the SSW but storms come in from the East and North.

I have 5' lengths of 2"x1/4" angle I originally thought of using as a brace between the front and main posts but now I have an alternate idea, as I am considering whether the top cantilever would bend in a high wind vs supporting the frame from distorting.
I suppose both would be a safe option but adding another brace may be somewhat down the road.
My question is...

As is, would it be better to brace the upper table to the main post OR brace the front and main posts as shown in these examples ?



OR

In the case of supporting the top cantilever, is there a formula to calculate the optimum angle? I have 5' of brace but can cut it if necessary.

Thanks for your help
(ignore the rusty steel bolts...SS hardware is on order- the local bigbox is 4x the cost)

sd

[kb58]

Between the two, the second does more good. The first redundantly reinforces two points that are already rigidly located.

If anything, having a diagonal run from the lower left corner in the first picture to the center of the long span would be even better as far as preventing wind deflection.

BTW, how is the aluminum being protected from corrosion?

[NEOH]

After you pour the concrete your two "posts" will be stiffest at the bottom.
So, what is accomplished by joining the two stiffest locations together? ( as shown in your photo #2 )
You have essentially the same triangulation before and after.

Consider this ...
http://image.made-in-china.com/43f34...unt-System.jpg

Cut you large triangle in half and create two smaller sets of triangulation = less flex

[Syberdog]

Thanks for that idea, however you may notice that in the pic the vertical rear post is at the back-topmost point of the table so splitting the triangle does help in that case. If I have any excess flex, it probably will be in my E-W span (I hope not- those XR1000 rails are supposed to span 12' and are used in 9' spans with more weight)

I came up with the horizontal brace while reviewing many ground rack manuals. It seems that in most cases if a brace is added, it is from the main post to the front OR if it is a single post design, the brace angles up, directly to the front table beam- however these racks almost always use piers in the ground so they would have potential flex & tilting that the ballast doesn't. The rear vertical support makes more sense to me but it is at a steep angle, so I'm unsure if it will add the necessary stiffness. It appears it would at least help some, but again I'm unsure how to determine the best angle for it.

Without structural calcs or a 'rule of thumb' positioning...I could position the angle at the end of the table beam or 1/4, 1/3, 1/2 the cantilever distance and set the brace on the post wherever it falls. In addition I wonder if an additional hole will weaken the support post any.
The rear fin of the T is 1/4" x 3" and I plan on using 5/16" bolts for the 2" brace. The main post uses 1/2" and the front connection is 3/8".

[J.P.M.]

Does the design need professional review/stamping ? Have the wind loads been calc'd ?

After the load/wind/seismic calcs, but before review/stamping, I'd respectfully suggest you consider NEOH's suggestion/reference as a modification. Cantilevers have their place, but if not needed, a design is probably better off without them.

Aluminum will protect itself from most atmospheric corrosion by the aluminum oxide layer that forms on the surface. Same to a lesser degree of protection for galvanic corrosion. Steel fastener (bolting) materials of C - steel are closer to Al than other materials on a potential chart, but rust. St. Stl. to Al is farther apart. than C. Stl. - Al, but the bolting won't rust. Galvanized bolting offers some protection. d

[bcroe]

The 6061 aluminum handles the elements quite well. After 4 years of northern IL weather, mine
looks like new. However, it does not want to be put directly in concrete. It needs a protective
coating for that. 18-8 SS hardware is compatible and hold up if not over torqued. I use a second
nut to be sure nothing will rattle loose. Bruce Roe

[Syberdog]

Thanks for that info JPM and bcroe...
I'm in a pretty lenient county- I have a "no inspection required" permit and a waiver letter for the Elec. Co. When I did the additions to my house a few years ago all they checked was the setback when I was done. However I don't want my panels blowing away either. No, I have not done all the wind calcs (some) but I have compared much of my design with engineering letters and charts for similar and larger systems.

For example, I reviewed 9 ballast systems and the average weight per panel is 620# where mine is 638# (total weight is similar too). Ballast is in the ground to help negate friction and over tipping issues. bearing capacity should be ok.

Rail span is a tricky one to compare but a 5x4 rack can have a 9' clear span and sticks a lot higher into the wind. Many of the span charts suggest I should be good up to 120 mph. The AE Rayport uses galv sheet-metal in a 8'-9: span so I'm taking the chance the XR rails will hold two panels high over that distance. I'm keeping my splices close to the supports. If they flex too much I will have to come up with a support fix. Since everyone typically uses stainless bolts I figured that would be the way to go.

We do have a lot of calcium in our water -I can't control the concrete truck so I'll research it... do you have any suggestions for coating the aluminum before pouring?

I found this..."The aluminum industry suggests protecting the conduit with bituminous or asphalt-base paint or other inert or insulating coatings."
So I have a bucket of roofing adhesive asphalt - think that would sufficiently work to coat it?

Not much I can do now about the portion of T-section already in conc.
thanks
al

[J.P.M.]

The 6061 aluminum handles the elements quite well. After 4 years of northern IL weather, mine
looks like new. However, it does not want to be put directly in concrete. It needs a protective
coating for that. 18-8 SS hardware is compatible and hold up if not over torqued. I use a second
nut to be sure nothing will rattle loose. Bruce Roe

Bruce: Good catch. I forgot about the hydrogen reaction, and coating with a petrochemical like roof cement, etc. or such like below embedment. My bad and apologies to the OP.


On the bolting: Plain old C. stl. is closer to Al than st. stl. with respect to galvanic potential, but then there's the rust problem. To avoid galling the St.Stl threads, use 400 series bolting or use thread lube if 300 series (18-8). Use a torque wrench and don't overtighten. Same for locking nuts.

[kb58]

Coming from an automotive and aircraft background, we never leave aluminum bare. While it'll last longer in an arid environment, it'll still corrode, but as long as you're fine with it, okay.

[Syberdog]

Thanks for reminding me JPM, I planed on using anti-seize (learned a long time ago the hard way). I don't know if it will do any good but in addition to the flat & lock washers, I have a external-star washer for the head side, to help with the bonding issues. I thought the Torque recommendations found in the various OEM install manuals tended to look pretty low for 3/8" & 1/2" (20 & 30 Ft.Lbs) but if that's all it takes..... thread locker or another nut might be a good idea.

So kb58 are you suggesting paint or ??? the exposed alum.?
hmmm... flake purple or green spiderweb....

[J.P.M.]

Coming from an automotive and aircraft background, we never leave aluminum bare. While it'll last longer in an arid environment, it'll still corrode, but as long as you're fine with it, okay.

The way I leaned it, the Aluminum Oxide coating that forms on the surface of new (shiny) aluminum from contact with the air is pretty corrosion resistant and so prevents most subsequent chemical attack (as differentiated from galvanic action), including further oxidation from contact w/the atmosphere.

[sdold]

The way I leaned it, the Aluminum Oxide coating that forms on the surface of new (shiny) aluminum from contact with the air is pretty corrosion resistant and so prevents most subsequent chemical attack (as differentiated from galvanic action), including further oxidation from contact w/the atmosphere.

Pure aluminum is very corrosion resistant, but the stronger structural alloys corrode more easily.

[kb58]

Agreed, but unless the (very likely 6061) aluminum the OP is using has a coating of pure aluminum on it, it will corrode. There are various approaches, anodize, paint, and my favorite, Alodine.

[kb58]

As mentioned, galling is a very real problem with stainless hardware. When building the car, I've test fit parts together, spinning a stainless nut on by hand to a stainless bolt. You can feel galling start to happen, as the nut will slowly start getting harder to turn. Right then you have to stop and if you're real lucky, you Might get it off. Most of the time though the two will stick so well together that you'll shear the fastener. Remember, that's with a hand-tightened nut.

If you have a lot of stainless hw to assemble I highly recommend nickel-based anti-seize.

[bcroe]

I don't have a problem with stripping bolts, but I keep forgetting that most haven't had 7 decades of
experience to get their wrench arm accurately calibrated. I have doubts about lock washers. What
alloy are they made of, is it compatible with exposure and the other metals? If the materials in the
stack up held together by a bolt can move at all under stress or shrink slightly, the lock washer will
loose its tension. Putting a second nut against the first will never come loose, even if its rattling
around in the hole. This can be a problem with electrical connections involving insulation in the
stackup.

Extra nuts also protect any exposed threads: I'm the only guy I know that can reuse
muffler clamps several times. Bruce Roe