Lateral Stability of PPC Beams
Lateral Stability of PPC Beams
(OP)
I am currently working on the design of a lifting system to lift PPC I Beams. These beams are going to be lifted by a crane. I am worried that the compression induced by the crane lifting the beams (due to the cables being at a 60 degree angle) may cause a buckling failure in the PPC I beam. I was wondering if there was anything (preferably in ASSHTO LRFD) that talks about this situation, and how to analyze this situation.
Thanks.
Thanks.






RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
To civilperson:
If the PPC beam is lifted at the lifting loops, the negative moment will be the same whether the beam is being lifted 100% vertical, or at any angle (such as 60 degress in my case). Therefore a spreader beam will only help in reducing the compression of the PCC beam and will not add negative moment to the cantilevered section of the PCC beam.
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
I am afraid that if I add an aditional sling under the midpoint of the beam there will be a region of negative moment at that around point, these PPC beams are not designed for a negative moment at this point. Actually these beams are not designed for any negative moment, they are designed for a simple span.
Also, it would be a lot of hassle to add a lifting loop at this point(if it would even work). I would have to get approval from the design engineer and the dot.
RE: Lateral Stability of PPC Beams
(1) You will need to model any pre-camber in vertical, and perhaps horizontal, directions.
(2) You will also need to allow for imperfect shape (in vertical and horizontal directions).
(3) You will need to model the actual lifting points in their correct locations, perhaps again allowing for some construction imperfections).
(4) Obviously, you have to include the lifting cables in your model.
(5) Your analysis will have to allow for geometric nonlinearity, since that is its point.
(6) You will have get your indication of the safety margin by factoring up your self weight until the analysis predicts the beam will buckle.
(7) You then need to convince yourself and others of the adequacy (or inadequacy) of that safety margin.
In my case, the software I was using was unable to solve the problem statically because of the close-to free-body movements involved in the analysis. So I had to solve it as a dynamic problem in the time domain, applying the load gradually then running the analysis until the resulting movements had died out. Since I was interested only in the final (steady) deflection, I used an exceptionally high damping in my model so that the steady-state condition would be reached quicker. But hopefully your software will be able to solve the problem statically.
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
Is it possible that these beams that you have seen lifted from the center were designed as 2 span beams, therefore designed for the negative moment at this point???? I find it hard to believe that a conc. beam that is not designed for a negative moment can be lifted at the midpoint. Didn't you state earlier "Most Precast Prestressed Concrete girders have low tolerance to added negative moment" and now you say that they can be lifted at the midpoint. Isn't that counter intuitive?
What I am looking for is something in some code (AASHTO, ACI, Etc.) that deals with lateral stability. That way I can check and see if the contractor can lift the beam the way he would like to.
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
How does your telling the contractor it can be done but that you have no earthly knowledge of how to do it make you a truthful and ethical engineer?!
As engineers we can analyze a great many things and show on paper that ideas will work, but that doesn't mean those ideas are good ideas. Spreader beams have been used for many years with much success.
Using FEA with more assumptions on top of numerical approximations and then indiscriminately modifying the damping doesn't help either.
Regards,
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RE: Lateral Stability of PPC Beams
Don't you think that would be an important thing to point out? Why did you even say that then?
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
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RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
Regards,
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RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
ANY OTHER SUGGESTIONS? PLEASE NO MORE TALK ABOUT SPREADER BEAMS, I KNOW OF THIS OPTION.
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The contractor asked me if the pcc beam could be lifted with out a spreader beam. Therefore I am going to do everything I can to answer that question. However if I cannot tell him a definate yes or no, then I will tell him that I was not able to determine that. Who ever said that I told the contractor that it could be done? I never said that. He asked me if I could do that, and I said that I would do some research and see what I could find. And yes spreader beams have worked in the pass and will continue to in the future, however there is nothing wrong with thinking outside the box and trying something new, is there? If ppc beams can be lifted without spreader beams in some cases and can save someone some money, why not do it?
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
I have designed girders for spans of 120' using 72" bulb tees and have seen them successfully erected with no spreader beams. In that particular case, the beams were lifted by two crawler cranes one at each lift point and walked in and set down. It was a very effective and efficient operation given that there were six beams in cross section and 10 spans. It did take time for site preparation and coordination was a must.
The lift was only about 50tons and something well within range of the crawler cranes. Yet the big issue was handling....if we would have used a single crane and if we would have spliced a spreader beam to facilitate lifting, what control would we have over the beam pivoting about the load line? Little to no control at all. Even if you ganged up on the beam's taglines with several workers it would be like the tail wagging the dog.
So if your spreader beam is too large from a practical standpoint perhaps you should be thinking outside a bigger box.
Regards,
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RE: Lateral Stability of PPC Beams
Using 2 cranes has been considered and is going be done (for part of the length that the beam is going to be moved). However there is a point where they want to handle the beam with 1 crane.
I appreciate the other suggestions, but that is not the info that I am looking for.
From now on, please only post messages that answer the initial question.
Thanks
RE: Lateral Stability of PPC Beams
RE: Lateral Stability of PPC Beams
Unless of course, you'd like for someone to give you the formulas and answers lock stock and barrel. Otherwise put the pen to paper and to quote "I am going to figure out how and do this for them. "
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