Steel Roof Cantilevers

[jjeng2]

I have a small(approx. 50'x 50') addition to an existing retail building. The addition is bounded on three sides by existing walls and there are a lot of corners and angles. In trying to come up with a preliminary scheme and col layout, I figured it would be better to minimize disturbance to existing footings and spend a little more on roof steel as opposed to all sorts of underpinning and footing work. My column layout results in two corners having about 7' cantilevers on each side(perpendicular to each other). I also offset some columns on one side so the new roof cantilevers about 5' to an expansion joint where it meets the old roof. My gut feeling is this is not great practice but may be the best alternative. Tying into the existing columns is not possible in several spots and will require reinforcing the columns in others. I wanted some opinions on doing this and also on analyzing it. I think I can analyze the two spandrel beams that cantilever into each other as a multi span beam(as if the beams were paralel) with a hinge at the corner.

 

[DaveAtkins]

I have no problem with your conceptual layout, but I don't agree with your last statement. Either do a 3D analysis, or check the members by hand, one at at time, to ensure stability. Of course, member stresses and deflections must also be acceptable.

And don't forget to do a lateral analysis.

DaveAtkins