that would be Radius X 5/8 (or radius X .625) the tube manufacturer should have specs on the tube radius, or you can get some radius gauges or sketch it.

I may have seen somewhere where the radius of the tube should be 2 times the wall thickness but don't qoute me on that!

hope that helps!

I think you would have to measure the radius of the actual tubing you are welding.  HSS shapes are produced to ASTM A500.  This spec specifies "the radius of each outside corner shall not exceed three times the nominal wall thickness specified" (Sec. 10.6).  But wait!  Table 3 of ASTM A500 specifies outside dimension tolerances by specifying the "Large flat dimension tolerance", plus and minus as:  0.020" for flat dimensions of 2 1/2" or less, 0.025" for flat dimensions of 2 1/2" to 3 1/2", 0.030" for flat dimensions of 3 1/2" to 5 1/2" and 0.01 x large flat dimension over 5 1/2".

So there can be some variation in ourtside radius right from the mill and I think you would have to just measure whatcha got.

Bob G.

Maybe I can simplify, maybe not?

The outside radius on a 1/2 wall steel tube will be 3/4 inch.  You cn measure this with a radius gauge.

D1.1 does state weld size 5/8*R so that gives you a minimum weld size of .46 or 5/16 for a prequalified weld.

If the weld size is 1/4 inch as you say, remember weld size for a groove weld is specified within parentheses, and the weld size has been specified by an engineer this means the weld is not prequalified and must be qualified by test.

The other option is to ask the engineer, who probably doesn't know anything about weldment design, to increase the weld size on the print.  Of course by doing this you increase the cost of that weld exponetially.

I am assuming you understand that weld size for this type of weld is depth of penetration and not the width of the weld face.

Hope this helps.

If you were to fill a flare bevel groove flush to the top and cut a cross section out of it, you would see that the weld did not penetrate clear to the bottom of the joint (where the radius tangent meets the plate at the root).  So you cannot say the weld size is the distance from the top surface to the tangent point - it is a lesser dimension. 
The 5/8r means that if you fill the joint flush, the size, or throat, can be assumed to be equal to whatever the corner radius is multiplied by 5 and divided by 8 (same as .625 times the radius).

This does not mean you have to fill every joint flush, or that you have to provide that particular weld size.  It simply means that if you fill the joint flush, that's how you determine the size you have.

If a smaller weld size is required -less than flush- it would be a good idea to weld a mock-up joint and cross section it so you can determine the actual throat. 

And be careful,  the ratio for SMAW, FCAW-S, and SAW is 5/16r - not 5/8r.