ASME BTH-1: Pin Flexure

[thenley]

I'm evaluating a number of existing lift beams located at a dam using ASME BTH-1 for below-the-hook lifting devices. In checking my calculations, my coworker told me I should be checking the flexure of some of these lifting pins that are inserted into the gantry crane lifting hook eyes.

I had only been checking the shear in these pins because a) the pins have a very tight tolerance (less than 0.01") in comparison to the hook eye diameter, and b) the hook thickness is approximately the same as the span between the lifting plates of the lift beams. I mean, I've never checked flexure in a double-shear bolted connection, why would I do that for a pinned connection? I did not think flexure would be an issue or even need to be considered, but my coworker disagrees and his calcs, which treat the pin as a simply supported span with a point load at the midpoint (M=PL/4), show that the flexural capacity in these pins is insufficient for lifting their intended loads (which is ridiculous, since many of these beams have been in service for 50+ years already without any issues).

At any rate, it's not a problem to include pin flexure in the calculations, but is it necessary to check the flexure in these pins? If so, the allowable bending stress would be based on Eq. 3-25, yes? The client is not going to like hearing that they'll need to replace all of their lifting pins, so I want to make sure we're not giving them inaccurate information.

 

[dhengr]

Thenley:
I’m not sure I understand what “ gantry crane lifting hook eyes” are, could you provide a sketch of this lifting arrangement around the pins in question, with some dimensions and reasonably good proportions? When you have a tight fit btwn. the pin (pin o.d.) and the holes in the lifting plates (the i.d., the plate hole dia’s.), I think that’s your .01"; and when the thickness of your “lifting hook eye” pl. is a reasonably tight fit btwn. the two lifting pls. on the lifting beams, you do not normally check pin bending. Your thinking is correct that the primary mode of failure of the pin will be in shear, and your comparison to a bolted connection and our normal shear calc. check is reasonable, although somewhat limited because the faying surfaces in the bolted connection are in intimate contact with each other. As the gap btwn. the pls. grows, relative to the pl. thicknesses and the dia. of the pin, at some point you do start to have a combined stress problem where the pin is primarily in shear, but you are starting to induce some bending stresses too. Imagine what the pin bearing looks like on the two pls. at each of the gaps, it will be at its max. near the gap and may go to zero (or reverse direction) someplace in the thickness of the pls. A bending lever arm would be from the 1/3rd points of these bearing triangles, plus the gap. If the pin fit is tight, there would likely be some reverse curvature in the pin (moment fixity), so PL/4 is too high a moment. A smaller pin in larger pl. holes could be more of a bending problem, and have bearing contact neared the edge of the pls. Maybe that’s a shackle with a large throat in comparison to the lifting lug thickness, with a pin hole much larger than the shackle bolt. That’s why you put doubler plates on each side of the lifting lug, with a specific hole dia., to match a specific shackle size. Always read the commentary in the codes, where it exists, it often gives some insight into the thought process and reasoning. Look up Hertz bearing stresses and dig out a few Theory of Elasticity, Advanced Strength of Materials or Machine Design texts and look into the design of pins, lifting lugs, bushings, etc. It becomes a pretty complex problem, pretty quickly, if you don’t make some conservative simplifying assumptions, which the ASME BTH-1 does.

Once you’ve done all these fancy calcs., it might be just as important to inspect the equipment and set up an inspection program/routine before using the equip. You should be looking for any cracks, nicks/dings in the edges of pls. with high tensile stresses, wear, permanent deformation, bent pins, serious deformations in pin holes, etc. etc. These are all potential stress raisers, particularly in tension regions, and can lead to problems far quicker than a few percent error in your design assumptions .

 

[thenley]

Thanks dhengr! Don't really have time to sketch this out and attach it, but I think you got the gist of what I was trying to say. And I finally talked some sense into my coworker, he agreed that flexure would not control for these lifting pins (and certainly not to the extend of PL/4); he didn't seem to understand that the pin/hook and hook/lift plate clearances were minimal.

Inspecting and load testing these lift beams is indeed on the agenda. We're running the calcs to ensure these various beams, frames, and devices meet current safety standards (i.e. ASME BTH-1) and then they will all be inspected and load tested later this year. This particular client has done a terrible job of keeping records of their lifting devices (of which they have over 50!), so we're doing our due diligence and then there will be a periodic testing and deep inspection schedule established from that point.

 

[boo1]

post your calcs

 

[seaaggie]

How do we get around the ASME BTH-1 statement...
"Shear forces and bending moments in the pin shall be computed based on the geometry of the connection."

 

[boo1]

ASME BTH-1 3-3.3.6 Pin Design.

 

[dhengr]

Seaaggie:
Come on Seaaggie, get on the stick and use a little engineering judgement and imagination, if you want to pretend to be a Structural Engineer. No one is suggesting ‘getting around anything,’ I’ve suggested facing the problem head on, with some engineering experience and judgement. Did you bother to read the whole thread, my post in particular? You might learn something by reading the entire tread slowly enough, and giving it a little thought in that process, so that you got something out of it. Many times, it seems a waste of time to try to write a meaningful and educational post, because today people are so busy, and in such a hurry for THE answer, that they don’t have time to read, and think, and learn anything. And, for goodness sake, they certainly shouldn’t have to do a little text book or ref. book reading and literature research on their own, to learn and understand a problem. If the code doesn’t spell it out to the letter and there isn’t a worked example in a design guide, into which they can plug their numbers, they are lost. I think I speak for many of the older engineers here on E-Tips, when I say we are unhappy with this cookbook approach to engineering; and to the codes being written in this ‘cookbook and exact recipe,’ fashion, ever more complex to cover every conceivable condition, so that nobody has to think a little about how things actually work. We aren’t doing any real engineering any longer, we are just plugin and grindin, either into code formulas or into software, with little idea of what we are really doing.

 

[seaaggie]

dhengr,
Dude, If you don't know how to answer the question, just say so. Could have saved a lot of writing.

 

[boo1]

seaaggie, I am sure dhengr could you your work for you if he wanted. But as a mentor he was encouraging you to do it first. Do the math and post up your work and we can help you.