Lateral Stability of PPC Beams 1

[DWHA]

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.

 

[swearingen]

Require that the lift points lift vertically. Most contractors with lift capability have spreader bars that take the compression, leaving vertical lifting lines from the ends keeping loads in the lifted object vertical.

 

[DWHA]

I should have stated this in the original post, but we were hired by the construction company and they want to know if they can lift the beam without the use of a spreader beam. It would be very difficult to deliver a spreader beam of the length needed to the site.

 

[civilperson]

A fractional length of spreader beam which increases the cable angle toward vertical will help. Most Precast Prestressed Concrete girders have low tolerance to added negative moment, (tension in the top from the cantilever length beyond the lift points), so a spreader beam is necessary. Spreader beam can be fabricated from smaller components as a space truss.

 

[DWHA]

I appreciate the suggestions, however I would like to analyze the PPC beam with out the use of the spreader beam. The construction company asked if it was possible to lift the PPC beam with out the use of a spreader beam. I am going to figure out how and do this for them. I could take the easy way out and tell them to use a spreader beam, however they asked if it was possible to lift it with out one. So for me to say it is not possible with out doing an analysis makes me a liar and an unethical engineer.

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.

 

[swearingen]

One thing that may help is to have them strap a sling under the PPC at the mid-point and over the crane hook to take out some of the P-delta effect. You could also have them install another loop in the center to do this as well. The compression will still be there, but your Euler effect won't be able to diverge.

 

[DWHA]

Swearingen
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.

 

[Denial]

I had an almost identical problem to this about a year ago. I ended up doing a finite element analysis of the lifting situation. Particular points.

(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.

 

[civilperson]

Lifting the PCC beam from locations other than the built-in lifting loops is possible using straps and other rigging. The non-use of spreader beam encourages this novel lift location which may cause larger cantilever lengths. I have seen beams handled from the center only by forklifts and excavators.

 

[DWHA]

Civilperson
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.

 

[civilperson]

I did not say handled successfully, the components were damaged and replaced.

 

[Qshake]

DWHA - Since you brought up the logical process here allow me to extend your note on "saying that it is not possible without running an analysis makes me a liar and an unethical engineer".

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,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[DWHA]

Civilperson
Don't you think that would be an important thing to point out? Why did you even say that then?

 

[civilperson]

My example of novel lifting locations was a cautionary one, example of how NOT to proceed. I was continuing my earlier advice to use a spreader beam and the perils of jury rigging lift apparatus such as you were considering. I had already cautioned against inducing negative moment which as aa engineer you readily understood in that you equated the negative moment at the lifting loops to using a spreader beam with not using a spreader beam.

 

[Denial]

Qshake. The original post asked how to analyse the situation: that was the question I answered. As for FEA assumptions and numerical approximations, I believe that the situation is perfectly amenable to analysis. And as for "indiscriminately modifying the damping", I suggest you read my post more carefully before indiscrimately accusing me of being indiscriminate.

 

[Qshake]

Denial- I read it as long as necessary. The only question that remains is how do you justify the use of "exceptionally high damping"?

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Denial]

As a numerical ruse to accelerate the time history solution to converge upon the static steady-state solution that I was seeking. Under these circumstances the value of damping used makes no difference to the final answer, merely to the time taken to reach that answer.

 

[Qshake]

The ruse is what I expected but I don't understand the use of the dynamic method to solve a geometrically non-linear problem.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Denial]

The static solver in the FE software was not able to handle the fact that the structure was a mechanism. Use of the dynamic solver overcame this problem. This quirk is probably unique to the particular software I was using, but I felt I should mention it anyway. (No names, no packdrill.)

 

[TampaBridgeDesign]

Anyone willing to share sample plans for securing the first beam. Once the second beam is in place then you can tie the two together with temporary x-frames, etc and you shouldn't have any problems. Sure there are lateral stability issues during lifting but these issues are still present due to wind loading until that second beam is in place.