Type of Aluminum for....... 2

[KeepingUSnum1]

I'm having a bit of trouble deciding on a type of aluminum to have outriggers built out of. I little bit of background on my objective first:

Objective:

Design an outrigger system that can stabilize while supporting all of the weight of a 4,200lb trailer system, plus 2,000lbs of fuel with a 60' mast in up to 90mph winds. The trailer is the M1102 LTT and the 90mph winds are what I was told to design the maximum to.


I have done my design and calculations and will need outriggers of just over 14' long to achieve a safety factor of about 1.5. Due to the length of the trailer though each outrigger will need to be about 7' long, folded in about center and then they will be supported again once extended.

As a newly graduated mechanical engineer I am not familiar with what type of aluminum to use.

I was thinking of using 6061-T6, but I've read that after 6061-T6 gets welded it needs to be reheat treated to achieve the T6 strength again. As do all aluminums after welding I assume? Will that be a problem with heat treating 7' long sections?

Is this a good choice of aluminum for this? Anyone have suggestions for alternatives?


Thank you for your time reading this. I know it was a lot.

Alan
USN

 

[Ron]

Yes, 6061-T6 is a good choice. It is a readily available structural alloy that performs well. As for the welding, generally the reduction in allowable stress is figured into the design. For instance, the allowable stress is reduced in any member within 1 inch of the weld. That stress reduction is major, but I've designed over 600 welded aluminum structures and it generally works out fairly well.

You can then design the locations and configuration of the weldments with this in mind. It is usually more critical in bending than other stress areas. Also, for tubular structures, if the weld does not exceed a certain percentage of the area, then a reduction is not necessary.

All of this info is contained in the Aluminum Association's Aluminum Design Manual, either ADM 2000 or ADM 2005.

Good luck.

 

[KeepingUSnum1]

Ron,

A 1" HAZ may be a problem using rectangular tubing varying from 1"x2" to 2"x3" both with a 1/8" wall thickness.

My outriggers are currently just a rectangle frame made up of the 2"x3" and then a "zig-zag" of the 1"x2" for more support. I was planning on having each of the "zig-zag" tubes welded to the next as well as to the frame. All of the tubes will be oriented so that the long dimension is vertical. That 1" HAZ seems like it would weaken the whole system an awful lot.

Maybe it is my inexperience with this. Do you have any pointers? Maybe, have the 6061-T6 riveted instead of welded?

Alan

 

[Ron]

If I understand your configuration, the "zig-zag" tubes will be welded like truss diagonals to the inside face of the two tubes. Since they are welded to only one side of the tubes, you do not have to take the reduction for the larger tubes, but it would be taken for the zig-zag tubes.

This exclusion is contained in ADM 2000, Section 7.1 of the ASD section.

The partial welded section are treated as noted in 7.1.2 "Members with Part of the Cross Section Weld-Affected"

 

[KeepingUSnum1]

Ron,

Yes, you understood me correctly.

I will be putting in for the ADM 2005 tomorrow. Hopefully it will help provide some more confidence in my design.

Thank you for your help!

Alan

 

[Ron]

Alan,
Good luck. Post back if there's anything we can help with.

Ron