I'm fabricating a steel frame for a tray for my 4wd. The side rails are 65 x 30 x 2.5mm RHS, and the intermediate cross members are 40 x 40 x 2.5mm SHS. The 40 x 40 cross members are being butted into the 65mm side of the side rails, 15mm below the top to give a coaming above the deck.

tray crossmember to rail joint.jpg

I have a question for the experienced fabricators please; is it best practice to weld all the way around the joint, or just two opposite sides? When I built my tandem trailer 30 years ago, I was advised to weld only 2 sides of the joint where the drawbar went under the front frame member, to avoid 'break-out' resulting from a HAZ all around the joint (that was one length of RHS laid across another, not a butt joint like this). I'm wondering if it would be wise to apply this principle to these joints? This tray will see some hard work on outback roads and I want it to last.
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