cantilever lengths max - need back-up

[scherry]

OK, we got rid of the thirteen foot conical cantilever over the garage with the slate roof that took the two steel beams weighing 3800 pounds apiece (I may be exaggerating slightly, but not much) due to expense. I have managed to find criteria for trusses stating that the maximum cantilever using a strut is .25L, L being the span. And I know about the rule of thumb of twice the back span...but I cannot seem to get through to this guy... is there anything else anyone knows of that gives a more solid...reason than common sense not to use a cantilever with say, oh .44L? I mean, I CAN make it work, I suppose, uplift at the opposite end, bracing of the compression chord, check the stresses check the deflections and make it stout, but it is SO stupid. Any help anyone can give me would be appreciated.

 

[scherry]

sorry guys, did not mean to be unclear, the cantilever I referred to above as ".44L" is incorrectly noted - it is actually .71L, with L being the backspan length.

 

[AJUK]

I always work on a rule of thumb of 1/3L max cantilever to 2/3 min backspan.

Without seeing the actual detail it is difficult to judge but if you have reservations over your design, go with your gut feeling. If it looks right it usually is right but if it looks wrong that's because it is wrong!

 

[SRO]

scherry:
I assume by "this guy" you mean your client? If you feel uncomfortable using a cantilever that isn't 1/3 - 2/3 just tell him it's "not economically feasible". Clients understand $$$, not stresses, moments, & uplift reactions.

 

[scherry]

actually, "this guy" is "arch the bobitect". Unfortunately, he is a friend of the client, and he is designing his dream structure without much regard, in my opinion, for the comfort or wallet of his friend. One "design" he offered me as a solution for the late unlamented 13 foot cantilever was a 6" steel beam with a back span of nine feet to the 13 foot cantilever... according to his calculations it only deflected a half an inch at the tip...except that he forgot to convert from feet to inches and didn't quite understand the deflection formulas. He suggested that he needed a refresher course over a beer sometime...I am far away though, and he could not see my face when he said that. I will try the cost issue...since it is true. thanks guys. I appreciate the help. And the chance to pound the keys if not my fists...

 

[StephenEd]

I'm guessing he assumed total fixity when he "computed" the deflection. Model in the actual beam with its corresponding rotation at the support and the deflection may go up considerably.

While I agree that clients don't understand shears and moments, they sure can see deflection. Most lay people are surprised when they find out the deflections we deal with and allow in a structure. Show him the 1 1/2" deflection and see what his reaction is.