I took a couple of structural engineering/design classes in college, so I should really know how to calculate how much weight a steel beam (or pipe, in this case) can hold. But, I’m having a hell of a time remembering, and all the online resources I’ve found either want to calculate for me (using variables I vaguely remember), or get straight into calculus (thank you, wikipedia). So, I turn to my fellow dopers.
The situation is this - I am supposed to be roasting a suckling pig on Tuesday. Since I am poor, and also into DIY, I’m planning on just buying a steel pipe to use as a spit, trussing the pig to it, and rotating it either by motor (if I get inspired) or by hand (if not). I’m actually not too worried about my pig breaking my spit (it’s only 35 pounds), but I’d kind of like to remember how to go about doing the calculation. All I really remember is that I need the moment of inertia of my cross section, the moment force (pounds times lever arm?), and the fact that I have pinned joints at either end of my beam. Can someone point me to a good, fairly basic resource to jog my memory? Mostly, I need something where the equations come with glossaries for terms, because I just don’t recall. Thanks!
You’ll need to account for the distribution of weight when you calculate the peak value of M. A pig isn’t a point mass.
Timoshenko and Gere can tell you how. It’s always amused me how Timoshenko has been dead since 1970, but they keep his name on new editions of the book anyway.
Don’t forget to check the strength of whatever it is supporting the pipe.
You can’t really roast a pig on a straight spit. Once the pig has cooked a bit, the spit will turn & the pig won’t. And by then it’s real hard to fix. The usual outcome is epic fail.
You’ll want some kind of fork attached (welded?) to the spit which puts some off-center prongs into the pig’s body. Or weld a 6-8" by however long paddle lengthwise along the spit which the pig can straddle & then you tie the feet together beyond the paddle edge.
As pointed out above, pigs aren’t point masses. And you can always drill a hole through the pipe and put a transverse rod through it, to which you can tie your pig. Then it’ll rotate with the beam even after it’s partly cooked. Engineering!
This sounds like some sort of traditional engineering saying.
One grizzled old engineer to another at the pub:“Ayup. As I was saying to young Wilberforce over there, a pig isn’t a point mass, and we’ll all have to keep at it if our overheads aren’t to make us go under… but you know how kids are. He’ll have to learn the hard way, just like his dad did.”
Sweet, thanks guys! Doing a force diagram or two really helped me remember what the hell I was doing. I’ve got my maximum moment and shear - any pointers on finding allowable values? Engwiki is proving difficult to navigate, especially when I can’t really remember my terms.
As for piggage - the method I had recommended, and was planning on trying, was drilling a few holes in the spit, and trussing the pig’s spine to it with wire. And, yeah, no galvanization, just plain steel for me.
Euler-Bernoulli Beam Equation has some nifty beam calculators where you input your boundary and loading conditions, and material properties, and it spits out all the necessary stuff (max stress, graphs, etc.).
