Aluminum column base plates 2

[structuresguy]

Hi

I need to design an aluminum structure with 14" square tube columns bearing on concrete piers. I am not too familiar with the design of aluminum and am looking for references on the design of base plates. The aluminum design manual has nothing for base plates that I can find. I am proceeding on the basis of similiar behaviour to steel base plates. Plus I will add vertical stiffeners to help things out a bit. I need a rigid connection at the base.

Also, what would you use for anchor rods? I need 8-1" diameter rods using A36 steel. I guess I could call for the A36 rods and hardware to all be hot dipped. Would I be better using stainless, if so, anyone know the allowable tension stress for 316 stainless?

Thanks

 

[ishvaaag]

Stainless will be better, sure.

http://www.askzn.co.za/tech/tech_grade_316.htm

http://www.imp.mtu.edu/patents/castcon/castcon.316.html

http://www.assda.asn.au/316.html

UBC-94 has table 20-I-B for allowable shear stress in fillet welds for aluminum alloys.

 

[kenvlach]

Fresh concrete is very corrosive toward aluminum due to its high pH of ~12. As concrete weathers, its surface pH gradually drops, but I cannot say that it will ever be safe for aluminum.
In order to proceed w/o altering your plans a great deal, I suggest that you let the concrete fully cure, with ample washing, then apply a penetrating sealant.

 

[structuresguy]

Thanks for the replies. The info on the stainless is much appreciated.

Kenvlach: Ya, concrete and aluminum dont go too well together, thats for sure. I was planning on coating the aluminum with an epoxy or bitumous coating at any of the contact surfaces between the concrete and the aluminum. Or maybe even applying a thin sheet of plastic (Teflon or PVC or something like that) under the base plate to separate it from the grout bed. It has to be something that will separate the bottom of the base plate from the grout when the grout is poured under the plate.

 

[openam]

I'm looking at making a base plate out of aluminum also. Have you found any information on it yet? I'm thinking about designing it in RISA Base, but putting in the Aluminum strengths, i.e. E = 10,100 ksi and Fy = 12*0.8 = 9.6 ksi for 6061-T6 thicker than 1/2" and within 1" of a weld. Does anyone think that this would be a problem? Or do you have any other suggestions?

 

[boo1]

Consider Delrin as a spacer. Soft spacers my elastally yield.

 

[3dboy]

well, concrete and aluminum is only your first corrosion problem, galvanic action between your steel bolts and aluminum is your second problem, just make sure you account for all of this stuff, especially since you are trying to get a rigid base.

 

[Ron]

I have designed many aluminum baseplates. Use same process as steel, except remember that the allowable stress for welded areas is much lower than surrounding aluminum. I use gusset plates up the side of the column to negate bending in the base plates and to compensate for lost stress at welds. Edge distances for bolts are covered in the aluminum manual. Use stainless steel anchor bolts, typically 316 stainless threaded rod. Yield varies but around 40ksi is typical. Isolate baseplate from concrete with bituminous paint.