We have an aluminum window sill with a .1" width X .3125 depth. The outside edge of the groove has a .025 continuous barb. We are installing a .050 aluminum formed sill into the groove. The edge of the sill that goes into the groove has a lanced barb every 6", 050" deep and 3/16" inside the edge. The overall thickness of the sill at the barb is .1, the same as the groove that it is fitting into. We get a tight friction fit into the groove and a mechanical lock as the lanced barb on the sill catches the continuous barb inside the groove. The Architect wants to know in pounds the pull out holding strength of this design. IS there formula to estimate that strength? Thanks. Curt

There is no simple formula for this, as there are too many variables (material specifications, surface finish, dimensions of the barb, tolerances of the parts, etc). I suggest that you call the manufacturer of the sill to see what they specify for minimum pull out force for this system. Or you could run a few experiments to see what you get for the pull out force, but the problem is that over time the results may change. If the architect has a minimum value that he's looking for, and your experiments show that you exceed it by a factor of 2 or more, I would think you're probably OK.