Question: I have a mitered tube steel truss connection (round sections) with a small angle between the chord and the branch, 16 degrees. The chord is a 10 inch tube and the branch is a 6 inch tube. AISC Design Guide 24 limits you to a 30 degree minimum branch angle. Is that one of the limitations of the underlying research, or is there a different reason for the limitation? Since the capacity has the sinϴ of the branch angle in the denominator, I would guess that using a 30 degree angle in lieu of the 16 degree angle should provide a conservative result, is that correct? I meet all of the other limitations of AISC 360-10 Table K2.1A. The angle I’m using is a limitation of the structure’s geometry. It’s a submerged structure, so I’d like to use a mitered joint rather than a knife plate that would leave the interior of the tube open to the water.

Secondly, AWS D1.1 Figure 3.5 (AISC 13th Edition Manual Table 8-2) limits you to a 30 degree minimum heel angle (ϒ). I understand that I can’t have a pre-qualified joint in the small portion of the connection that is less than 30 degrees, but I’m planning on neglecting that portion of the weld for capacity and only using the weld there to seal the connection. Is there anything else to watch out for in the weld at small acute angles such as this?

Answer: The limitation requiring acute branch angles greater than 30 degrees, in AISC Design Guide No. 24 and AISC 360 (which is taken from International Institute of Welding recommendations), is primarily due to the problems of producing good welds in the heel positions at very low angles. Smaller angles may be acceptable, although not pre-qualified, if you can confirm with the fabricator that proper structural quality welds can be achieved. The difficulty is greatest for square/rectangular HSS connections, but tends to be less of a problem with round-to-round HSS connections – which you have – because that small acute angle is only at one point (the chord crown position). At other positions around the round branch member this local intersection angle “opens up”. I would agree that a fully-sealed welded joint is preferable to a knife plate connection, in your situation. Thus, in conclusion, you need to liaise with the fabricator to obtain confirmation that a specific effective throat can be obtained in the heel, while welding all around.