How do I calculate steel sizes?

[bigspike]

Does anyone know where to find simple tables or tools for figgerin out what size & thickness steel is needed for typical backyard projects? Like trailers, cranes, and stuff.

I am a woodworker / contractor by trade and have fiddled around for years building the occasional trailer, bracket and hitch carrier. I have been pretty good at building strong and durable projects by lookin at the metal and guessin. But that is really not a good process. For all I know I am buying steel that is way heavier than I need or running the risk of failure.

I know nothing is better than testing before use and for the hitch carrier, after I built it and installed it on the car, I jumped up and down on it and it didn't bend so I knew it would support less than I weigh. It also didn't weigh so much that the springs on the car kept if off the ground. LOL

I don't have an engineering degree but I can use a computer. If there is an online calculator or something?

What I need immediately is to design some lifting arms for my trailer, sort of like a boom truck would have. I need to know what size square steel tubing would be required.

For example, if I attach two arms to the rear of a trailer with drop legs to support the load to the ground. The arms are 60" apart and attach at the top to a crossbar that is 60". Attach a pulley to the center of the cross bar, run a cable from a winch located on the tongue 10' back and through the pulley down to a sling attached to a log that weighs as much as 2 tons. If the arms are against stops about 24" above and 24" back from the pivot. With the arms at a 45 degree angle and 10' long. see picture.

As the log is lifted it will be dragged toward the trailer. When the hook reaches the pulley, the arm boom will be pulled towards the front of the trailer dragging the log onto the trailer. When the log is as far forward as desired, the arms will have tilted as far forward as the front stops allow and the winch is reversed to lower the log onto the trailer. A pair of ratchet straps will hold back the arms from slamming forward as they pass vertical.

So then with 8' sticking past the stop, how do I figure out how to select a cross section shape and how thick the steel needs to be? I think that if the log weighs 4000#, the level fulcrum pressure would be 1/5 to 4/5ths or 20,000# ??? at the pivot. But at a 45 degree angle isn't it less? Or is the amount insignificant?

Assuming I drill a hole thru square tube and use a solid round pin. What size pin and what additional bracing at this point? And the cross bar would have the 4000# plus the force of pulling the log forward on it.

I know...liability and all. I am not asking anyone to assume any liability. Just point me to some tables or something so I can mess up the calculations on my own. For wood framing there are all sorts of span tables for determining what size joist for a cantilever. Surely there is something for steel.

[peter.blais]

Unfortunately, it's not really simple enough to slap a calculator on it.

If you wanted to know what stress level is acceptable for a straight beam, without any bends, with no holes in it... Well, that's simple enough and could easily be made into an online calculator. Then it'd just be a matter of figuring out if that stress level is below the acceptable stress levels- either by failure from yield, fatigue, or unacceptable deflection.

BUT- as soon as you throw in some more complicated geometry, for example a hole, bend, or weld... It quickly becomes too complicated to be easily solved by a casual observer.

You will either need to do significant research, or- more practically- find something somebody else already engineered that does the same thing, and copy / modify that design, making it "robust".

Sorry there's no easy way out-


PS: Just looked at your example... Just go look at a small crane or something that will lift loads at an angle similar to that. The worst case scenario will probably be with the boom all the way extended. Remember that if you set up the cable like that, the cable is also pulling down on that boom, so the weight lifted is effectively multiplied by some number, from probably 130-170%, depending on the geometry.

Remember that your trailer needs to be strong enough to absorb the entire weight of the load and then quite a bit, all in one spot (your mounting spot for the boom).

Keeping the boom from rotating will take some serious force / torque.

Pete

(Mechanical Engineer)

[Geof]

You could take my "bush engineer" approach. Make a guess about how heavy the boom arms need to be; in this case I would start with 3" square with a 3/16" wall. Build everything and test it with a load at least three times the planned working load; actually I would start at the working load and move up from there.

If I reached the threefold load without failure I would figure I am safe. But if it failed before this I would bump up the wall thickness.

Actually I think the weak spot in your design will prove to be the pin going through the bottom of the boom. It will be necessary to weld on cheek plates so the bearing area on the pin is several times larger than would be the case just using the wall of the tube.

[bigspike]

I didn't think it would be easy and I didn't expect to find "Structural Engineering for Dummies" on Amazon. I do have Machinery's Handbook, but the calculations are as yet beyond my understanding. I do understand that weld quality has a substantial influence on the load carrying ability as well as holes and such.

I have already carried twice the required load on the trailer, with the "log" loaded by a forklift. What I am attempting to build is a loading arm to load without the forklift.

I assumed that I would need cheek plates at the pins. The previous picture did not show the arm from the back of the trailer so I have added one.

I am considering adding a pair of angled bars from the corners in and offset 6" from the cross bar at the pulley. The drop legs at the back corners are to reduce the uplift pressure on the hitch and truck.

The whole point of posting this question is to avoid guessgineering. An afternoon of light reading in the Machinery's Handbook provides a general equation. But what is the allowable deflection before collapsing, the modulus of elasticity, moment of inertia, section of modulus and stress in extreme fiber of regular mild steel as available at the local metal supply?

I refer to a log as a simple way to refer to the actual load which is a 36-42" wide x 36-42" high bundle of assorted lumber offcuts from 1/8 x 3/4" to 2x2" and ranging from 10-18 feet long. Which precludes the possibility of dragging onto the trailer up a ramp.

[TomB]

A couple of years ago I needed some analysis quickly so I downloaded and used a freeware SW program called LISA. The freeware version is a bit limited in that it only allows 500 or so nodes but that worked for my problem, albiet in two steps. In the first step I used a coarse model to establish boundry conditons for a critical area and then I did a finer grain but smaller problem to investigate the effect of a sharp corner. I latter did the same problem by analysis and got about the same answer.

That approach might be the fastest way to get a SW package that was useful for a problem like bigspike posed. But it might be a big step to correctly use a modeling program without some theoretical background. I was probably making implicit use of my ancient mechanical engineering degree as I read the instructions and set up the two models.

Tom