Shear Lag

[Mike Mike]

Hi all, what is the minimum weld length for tension members? We are designing a ton of welded knife plate connections for HSS10x10x1/4 tension and compression braces. The LRFD tensile capacity is 371k. The LRFD axial load is +-100k, so we only need (4) 6" long 3/16" fillet welds, and we only need a U factor of around 0.27. But to comply with case 6 in the table below, we have to specify 10" minimum. Why does the table state l >= H? Were shorter welds not tested? Would you be comfortable pushing this recommendation? It seems we have mixed opinions in my office.

 

[JAE]

The provision that weld lengths should be at least as long as the gap between them (perpendicular to the line of force) has been in the code for a long time.

A commentary from the 9th Edition for its section B3 states:
"...tests (Kulak, Fisher, and Struik, 1987) have shown that flat plates, or bars axially loaded in tension and connected only by longitudinal fillet welds, may fail prematurely by shear lag at their corners if the welds are separated by too great a distance. Therefore, the values of U are specified unless the member is designed on the basis of effective net area as discussed below."