Not Recommended

While you may get away with this, a back of an envelope calculation says otherwise. I would not put a 4000 lb vehicle on 15" lengths of 3/4" SCH 40 black iron pipe.

The load on each pipe will be approximate 1/4 of the total weight or 1000 lbs.

If you, conservatively, consider the pipe to be a simple beam with a point load in the middle, the bending moment on it will be 3750 in-lbs.

The moment of inertia of 3/4" SCH 40 pipe is 0.037 in^4.

The bending stress on the pipe is then 53209 psi.

The yield stress for black iron is around 35000 psi! (Please verify this.)

Now, if you consider that the pipe is not really a simple beam and then do a more sophisticated analysis by considering the entire system, then I surmise that the stress on the pipe would be closer to 1/2 of 53209. Still, that doesn't leave much design margin.

Lastly, keep in mind that the above calculation is for a static load. However, this ramp will have to handle a dynamic load which will be greater than a static load both vertically and longitudinally. Note that statically, the longitudinal load is very small. Dynamically, it is much larger. So beware.

In summary, I think that in the end of the day, after a lot of work, you will have a structure that is both heavy and weak.

Originally posted by

**Bob61**View PostThe load on each pipe will be approximate 1/4 of the total weight or 1000 lbs.

If you, conservatively, consider the pipe to be a simple beam with a point load in the middle, the bending moment on it will be 3750 in-lbs.

The moment of inertia of 3/4" SCH 40 pipe is 0.037 in^4.

The bending stress on the pipe is then 53209 psi.

The yield stress for black iron is around 35000 psi! (Please verify this.)

Now, if you consider that the pipe is not really a simple beam and then do a more sophisticated analysis by considering the entire system, then I surmise that the stress on the pipe would be closer to 1/2 of 53209. Still, that doesn't leave much design margin.

Lastly, keep in mind that the above calculation is for a static load. However, this ramp will have to handle a dynamic load which will be greater than a static load both vertically and longitudinally. Note that statically, the longitudinal load is very small. Dynamically, it is much larger. So beware.

In summary, I think that in the end of the day, after a lot of work, you will have a structure that is both heavy and weak.

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