Structural Aluminum Tube in Oxidation Ditch

[profplan]

I am designing a pair of vertical aluminum round tubes (pipe columns) 4 ft apart, 23 ft high, and braced (laced) at quarter points using smaller aluminum tubes welded to the vertical. They support an aluminum walkway platform and a mechanical mixer for aeration of wastewater in an oxidation ditch. 8" diameter verticals and 3" diameter bracing satisfy the stresses, 6061-T6. The mechanical mixer weighs less than 1000 lbs.

My questions relate to using the appropriate materials for the application.


1. Should I use "Round Tube" or "Pipe" sections as listed in the Aluminum Association Manual for this structural application.

2. How do I specify the welds for the 3" to 8" tubes. I am familiar with welding of structural steel, is this the same, for aluminum to aluminum. I'll need to weld aluminum base and cap plates also.

3. I have been told by the mixer manufacturer that vibration during operation (20 rpm) is negligible. However, do you think fatigue should be a design criterion?

4. Is aluminum good for this application, or is stainless steel preferable? The design stresses in aluminum are low, less than 10 ksi for non-slender elements.

Thanks.

 

[Ron]

profplan....your environment is aggressive for aluminum. Stainless steel might be a bit better for this application, considering aluminum doesn't do particularly well in wastewater environments. Better than carbon steel, but still susceptible to aggressive corrosion.

As for the welding, keep in mind that your allowable stresses will go down significantly in the weld area. Even though your stresses are less than 10ksi, in a weld area you could exceed the allowables.

 

[profplan]

Ron,
Thanks. I thought aluminum was comparable to stainless steel. If I use stainless steel, is welding of members the same as structural steel?

 

[Ron]

Yes. Depending on the alloy, you can use processes similar to carbon steel and there's no reduction in allowables for welding.

 

[ash060]

Also, the filler material for the welding has to be stainless as well

 

[profplan]

Ron/ash060,

Thanks for your valuable posts. The project calls for 316L stainless steel, ASTM 312/269 for pipe/tube, and ASTM A240/276 for plates/shapes. Where do I find uniform section properties and design manual/guides for stainless steel? I checked the SSINA website, but it seems you have to contact the individual manufacturers for design information.