How do I calculate stress in an eccentrically loaded tension rod.

[R3345]

I'm designing a eccentrically loaded lifting linkage and I'm wondering the best way to calculate stress as there is a combination of tension and bending.

The linkage is made up of 1m long 25mm diameter round bar with a rod end on one end. The other end has a plate welded along the side of the rod. The plate has a hole for loading, so the loading is say 50mm from the axis of the rod.

I've played around with working out the lateral component of the force and then applying beam bending calculations but I need to know how to consider tensile and bending stress combined.

Apologies for the crude doodle

 

[msquared48]

It's just P/A +- M/S where the eccentric distance for the moment is the distance between the centerline of the pull points and the centerline of the small section of the rod at the juncture with the larger plate.

Mike McCann
MMC Engineering

 

[azcats]

P/A + M/S will give you the stress - but doesn't really deal with allowable loads or interaction equations.

I'd reference the AISC spec for those but I doubt they're applicable to a lifting rig and the required safety factors.

 

[R3345]

Great. thanks

 

[msquared48]

Contrarily, I believe that the formulas are applicable here as the moment is directly, as in linearily, related to the tension force, mot a random and variable applied moment as with a concrete column interaction equation.

Mike McCann
MMC Engineering

 

[azcats]

I guess my question is whether or not AISC allowable loading and interaction equations are applicable in a rigging or lifting environment. I thought there was another or additional standards to be met when designing rigging.

 

[dhengr]

RonanT:
That is just not a good detail for a tension link (lifting link), however you calc. the stresses and actually detail and manufacture the link. You will get very high stresses at the point where the eccentric plate terminates on the rod. And, that’s a bitchy detail to make well, without any stress raisers, etc. Your red force arrows should be on the same line, and they don’t appear to be. And, the rod should be on that line too, with pin eyes at each end. You should work very hard to make that happen, or the tension member should not be a rod. You better open a few Engineering Mechanics and Strength of Materials text books or you shouldn’t be doing these kinds of problems. The load could fall on someone’s head and hurt them. This engineering thing is serious business if you want to do it right.

 

[JLNJ]

As dh said, the rod will try to straighten out to align itself with the line of force.

The moment is uniform, so there is no good place to reduce the section to just a rod. Be careful with this one!

 

[R3345]

Thanks for the advice.
I should say this is not lifting/ hoist equipment in the traditional sense.
The linkage passes through a limited space so the eccentricity and size reduction is necessary. The example I gave is very much a simplification though, (the rod is actually part of a telescopic assembly which allows the rod to move toward the line of force.)
I just wanted some advice on preferred combined stress calculations for eccentric loading of tension members. I have been referring to text books also.

I am not working on a brand new design so much as modifying an existing one. The company has always employed a high safety factor based on physical testing with this type of design.