Machine Design 6-Min Solutions - Prob 30

Problem 30 of the Machine Design 6-Min Solutions book is the deflection analysis of a contrived welded structure. In the problem, there's a stubby 18 inch long beam that has two longer beams welded along its length at 6 and 12 inches respectively (sort of a tuning fork looking thing). Each of the longer beams are 48 inches long. Everything is square steel stock.

The question is to predict the deflection of the second long beam at its end. Everything is deflecting under its own weight. The deflection of the stubby beam is neglected, but its rotation is not.

In the solution, the author accounts only for the twist of the stubby piece due to the moments imparted by the two long beams. She does not account for the deflection of the long section under investigation as a cantilever under its own weight.

I agree with her calculation of the assembly's twist, but I think she should have also added in the sag of the long section...

Has anyone else done this problem? Any comments/insights to offer?

Thanks,

Rocco

Rocco,

I remembered that problem from when I did it last fall, because I thought the same thing, along with thinking it was unrealistic to neglect the deflection of the 18" piece. This sort of work is actually similar to what I do for a job, so I was extra ticked that they didn't handle it as I would have. If they wanted to make it simpler they could have asked for the angle of twist, instead of the deflection on the beam. I think most of the quetions in the 6 min solutions are reasonable, but this one was the exception. I don't remember how far not using those threw you off, was it enough to push you into one of the other answears, or would you still have picked the right one? That may have been why they left it with just the simple solution.

Just my take,

John