Aluminum Base Plate Detail 1

[KootK]

Disclaimer: I am using this post as a means to expedite / circumvent my own research. I do realize that Google is my friend. I'm a little pressed for time and resources unfortunately.

So... I've got some 4 m tall aluminum public art that needs to be connected to a concrete grade beam. And I'm not an aluminum expert by any means. I'd appreciate any advice here regarding the design of this connection. Things that I'm concerned about:

1) Fatigue in the pole / base plate connection.
2) Vibration issues similar to those encountered with light standards.
3) Aluminum / concrete corrosion. What needs isolating from what here and how would that best be accomplished?
4) For something of this scale, I don't need to debond/prestress the anchor bolts for fatigue resistance do I?
5) Anything that I should know about RHS section availability?
6) Anything I should know about aluminum welding?
7) Any issues with tight radius bending of aluminum pipe (wall thickness)?

That's a long list, I know. Feel free to answer any item that tickles your fancy without addressing the rest. And, if you can think of anything that should be added to my list, don't hesitate to table it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[manstrom]

Aluminum welding significantly reduces the strength of the material. Like 2/3 less than the original capacity.

There will be galvanic action between galvanized steel and aluminum. Use stainless steel and a bond breaker.

I would not worry about prestressing the bolts.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[MotorCity]

The weak link in the chain here appears to be the pole/baseplate/anchor rod connection. The easy way out (for you as the engineer) would be to coat the aluminum and cast the pole into the foundation. That way, you only need to check the pole for bending and design the classical pole foundation. The downside of course is that the owner needs to foot the bill for a longer pole.

Might not be an option, just throwing it out there.

 

[KootK]

Use stainless steel and a bond breaker.

Thanks Manstrom. Bond breaker is at the U/S of base plate and is meant to isolate the base plate from the grout pad?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

I would seriously consider a grounded, sacrificial zinc anode attached to the aluminum pole. However, like a boat, this would be a maintenance item.

Mike McCann, PE, SE (WA)

 

[KootK]

@MotorCity. That is something that I should have added to my list. The architect would love to have an embedded detail if it were possible. I'm concerned that I'll never be able to solve the concrete-aluminum corrosion issue to my satisfaction however. Remember, it's public art. Any corrosion of the aluminum will earn me a "fail". Also, I'd like to have a way to field adjust the angle of the posts in the field.

How would you recommend coating the aluminum?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MotorCity]

My suggestion does not help with being able to adjust the angle of the posts, but.... we embed aluminum in concrete on occasion (handrail posts, process pipes through walls, etc). Basically all you need is a bond breaker to prevent direct contact between the aluminum and concrete. We typically specify that the aluminum be coated w/ an asphalt mastic that extends an inch or two beyond the face of the concrete. Works great.

 

[PittEng88]

It's been awhile since I worked with aluminum, but are a few tips:

1.) Anodized aluminum is typically the way to go when it comes to corrosion protection, just be aware that anodizing can have a negative effect on the aluminum's fatigue resistance.

2.) For the aluminum/concrete interaction, I would suggest using an epoxy coating on the baseplate. Also, I would specify stainless steel anchor bolts for the connection.

3.) Welding aluminum can greatly reduce the allowable stresses of the member, so try to use these sparingly. I understand that you are probably going to have to use welds to attach the baseplate, just be aware of this reduction.

4.) The Aluminum Design Manual does address fatigue design. As compared to steel, aluminum has about one quarter to one half the fatigue strength. Also, temperature plays a big role in aluminum's fatigue strength.

Hopefully this helps. If you need any clarification let me know.

 

[rb1957]

could you have a stub tube cast into the foundation, steel is more compatable with concrete ? then I'd fish the Al tube over the stub and a couple of through bolts to join them together. being aware of the corrosion issues, youcan take steps to mitigate it ... coatings, paints, sacrifical nodes, ... you could place a sacrifical Al tube between the post and the pole and replace every couple of years.

btw, it's not "T6061" as written in the sketch, but 6061T6 or T4.

Welding is never good with Al (6061 may be one of the better weldable alloys, but it still sucks), and at the point of maximum stress. I'd rather a separate base fitting, something to bolt to the concrete foundation and then bolt onto the pole.

another day in paradise, or is paradise one day closer ?

 

[manstrom]

Another recommendation. Leave a round box out in the pile cap - 8" dia x 12" or so. Set the tube after the fact and fill around it with grout.

Saves a lot of expensive hardware and headache.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[dhengr]

KootK:
Your sketch attachment above has reduced the scroll (left, right ,left) to a large (pretty wide) single page. So, that’s a real improvement, with a little adjustment to start with, I can see the whole page without having to scroll laterally. Thanks much for your effort, keep fiddling, but the problem really seems to be a system problem actually, not your problem. This has only been a problem for the last few months, and seems to involve how the forum admin. allows you to post sketches in the body of a thread now. It is just easier for me to scroll up-n-down with my mouse wheel than it is to scroll laterally, when the page explodes laterally. Your sketch with the grade bm./pile torsion thread is a little too wide, I have to scroll to read the last four or five words of most lines. You know what was different about the two different sketches, as you experiment. The problem does seem to be a function of the pixel width of the sketch that you post, or some such. I don’t know how to post a sketch like you are doing, but it would seem that they could limit the width of the sketches allowed, and that might solve the problem. Am I the only one having this problem?

Most of the quick thoughts on your detail have already been expressed, but maybe in slightly different words. There is no way that your base pl. to pipe or pipe to pipe at the knee are nice details to do quickly or simply. The welding really screws up (I mean down) the allowable stresses on the 6061T6 tempered alum. at the two critical locations on the structure. You’ve bought tempered aluminum with a fairly good allowable stress, and the welding reduces it back to soft aluminum. It can be re-tempered (heat treated) after welding, if you detailed it correctly, and if you have the time, money and quantity to make this practical. We applied a bitumastic product (can’t name it off the top of my head) to the aluminum surface at any mating surfaces with steel or conc. I believe SS A.B’s. should be o.k. But, all in all, your detail shows almost everything wrong and difficult about working with and fabricating aluminum as a structural material. Ron has done more aluminum design than I have, if he comes around. I’d try to sell doing this in steel and then pay for galvanizing, (chrome plating?), fancy epoxy paint system, color to match the art; and all the aluminum problems go away. Can the sharp knee be replaced with a gently curved/formed shape?

 

[BUGGAR]

Some pretty, water jet cut gussets between the baseplate and post will lower stresses on welds and the resultant fatigue stresses as well. Maybe something to compliment the acorn nuts. Bring out the landscape Architect in you.

 

[Ron]

KootK....I've done hundreds...yes hundreds of embedded column designs in aluminum. Some have been in place for almost 30 years without issue.

Isolate the aluminum post from concrete by dipping in clear urethane coating. This can give you about 15 to 20 mils of coating. Make sure you seal the post so that water doesn't get inside

 

[dhengr]

BUGGAR:
The gussets likely add more in the way of stress raisers and irregularities in the stress flow paths, etc., as relates to any fatigue issues, so they are not likely an improvement on that account. Furthermore, 12, 16, 18" above the top of base pl. where the gussets terminate on the pipe, you still have basically the same max. bending and torsional moments and axial loads in combination; all on a weaker pipe section because the welding has eliminated the temper in a sizeable portion of the pipe cross section. Move a few inches above this location, and then the pipe section is back at full strength. The cleanest welding detail is just a good girth weld btwn. the pipe and the base pl., but you still have to deal with the weaker pipe section near (just above) the weld, so the pipe size must be increased to deal with this lower allowable stress.

 

[SAIL3]

dhengr...one could add a ring @ the top of the gusset pls but not weld it to the pipe...this would eliminate the total circumferential weakening of the pipe...still left with vertical strips of weakened areas of the pipe @ the gussets but is not as damaging as a complete circumferential weakening of the pipe...not sure, right now, how one would analyze a pipe with ,say, 4 vertical weak areas...

 

[BUGGAR]

dhengr

Indeed, a large enough pipe will overcome any weakness induced by welding and the fatigue potential. But rethink the use of gussets where there are size restrictions. Blodgett has a good write up on how to design gussets, albeit for steel; the principles apply for aluminum as well.

 

[MikeHalloran]

Manstrom has a great idea there; just sink the pole ends in a bucket of epoxy grout.


Mike Halloran
Pembroke Pines, FL, USA

 

[KootK]

Thank you everyone for the valuable tips. I've incorporated several of them into the details below. I'd love to get some more feedback on this arrangement if possible. Here's a summary of my intention:

1) The base plate will be sacrificial. It's sole purpose will be to provide field adjustment to the frames before the pocket is epoxy grouted. After the epoxy grout is installed, moment resistance will be provided by compression reactions against the sides of the frame, pole barn style. The base plate welding will weaken the aluminum frames locally but I'll assume that to be of no consequence with regard to the final load resisting mechanism.

2) Primary corrosion protection will be provided by a clear urethane coating.

Some follow up questions that I have:

1) There will have to be a hole in the embedded portion of the frame for electrical conduit. That will have to be watertight. Any ideas for that? Alternately, could the pole be dipped in urethane prior to welding on the base plate and then repainted afterwards in order to get coverage on the inside of the pole?

2) The consensus seems to be that stainless steel anchor bolts in contact with the aluminum frame isn't a big deal. Are we in agreement there? No isolation required between the aluminum and the anchor bolts?

3) Is there concern that a bimetallic corrosion cell will form between the aluminum and the rebar in the grade beam? I'll be using galvanized rebar in the grade beams. I've read some things that seem to suggest that bimetallic corrosion between Aluminum and galvanized rebar is less of an issue than is bimetallic corrosion between aluminum and regular rebar.

4) MS^2's idea about using a sacrificial anode appeals to me. The only options for the anode are magnesium and zinc, neither of which is very far from Aluminum on galvanic potential charts. Given that, is it still reasonable to expect reliable performance from such a system? Could such a system be considered a silver bullet in the sense that it would be highly reliable and nothing else would be required? As long as there is electrical connection between the anode and cathode, is there any reason that the anode couldn't be stored in an accessible hatch somewhere away from the aluminum frames?

5) Preliminary feedback from suppliers indicates that the "knuckle" may be a problem to fabricate. The architect would likely allow an increase in the radius of curvature but probably not enough to resolve the issue completely. Would the scheme shown below be workable? Or would a solid section be too difficult to bend? How ugly would the finished welds look?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.