Mono Scissors Trusses

When designing mono scissors trusses, be careful with the bottom chord condition at the upper bearing.

While it is technically possible to have the bottom chord extend over the upper bearing, that type of design causes all the weight of the truss to come down on the front edge of the wall. This could be a problem for lighter-gauge cold-formed steel walls.

Plus, you must be careful to ensure that the height difference between lower and upper bearings is correct. In the Aegis Design program, the vertical dimension between bearing heights is given and the end of the truss, but you are really concerned about the inside face of the upper bearing.

A better design option would be to increase the bottom chord slope slightly, so the chord will clear the upper wall by a few inches. Then, you can extend the end vertical down to the top of the upper bearing.

This is a more stable design. You have full contact with the bearing surface, plus you have exact control over the vertical distance between bearing points. In some cases where the bottom chord does not have sheathing directly applied, you can raise the bottom chord slope by a large amount and save money by have shorter webs.