Conpresion spreader beam 5

[kinno]

I am currently working in china and this problem has arised, i have never seen anything like this before and was hoping somebody could help me.The contractor has ordered the wrong lenghth slings for the load to be lifted and intends to insert a compression spreader beam between the two slings to widen his angles.I have said no to this problem but they are insisting it ok.please find attached drawing of this problem

 

[msquared48]

And the problem is?...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[kinno]

Hi Mike. have you seen the drawings? would there be any extra stresse on the slings at the change of angle? How would the slings be certain of not sliping once the load had been raised?

 

[desertfox]

Hi kinno

What is the angle the slings are being used at? on page3 of your attachment you show two slings or wires going to the lifting beam eyes, the two main slings then appear to go over the ends of the lifting beam putting it in compression but there is no included angle or load rating given for these slings which, if its the main slings your concerned about you need to know these values and yes once the slings move from vertical they will see an increased load but that depends on there angle.
If the main slings are sat in a grooved saddle as they pass over the ends wouldn't that prevent them from slipping?
Can you explain how you get the horizontal force components in your table, I presume these are calculated from the sling angle but I tried working back without success.
Also not sure why the compressive strength you have used is nearly double the yield strength for the material.

desertfox

 

[paddingtongreen]

I would prefer to see clamps at the strut ends. If there is any slope on the beam, the resultants will try to rotate it even more out of level. The friction between the rope and the strut should be enough to hold it for small angles.

If the load is not carefully balanced between the ropes, the same thing will happen.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[msquared48]

Yes, I looked at the drawings.

If the angle of the slings is increased, you are correct that the load in the sling will increase - it will have to not only due to the geomentry, but to put load into the spreader beam.

Further thought - how is the spreader beam going to be secured to the sling to that it will not slide does the sling? Hopefully, that connection will not compromise the capacity of the sling too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

If P is the lifted load, the horizontal force H in the spreader beam is P/2tan[θ]

If P = 240, L = 3200, [θ] = 76^o then H = 240/2tan(76) = 29.9 (say 30T)

If the wrong sling length is used, [θ] will change. You need to calculate the revised angle and recalculate H. If [θ] = 60^o, H = 69T, more than twice the previous value.

BA

 

[kinno]

Hi Guys
Thanks for the responce.To answer some questions you have asked.No one has given me the sling size yet. the contractor has asked if he could use this method? i am very reluctant to use this method, the beam is home made uncertified, i could load test it and the two padeyes but to what? as its not acctualy lifting the load? the largest load is 240 ton tower.if it was lifting it would be 1.1 of the proof load but its not.as soon as i hear what the sizes of the slings are i will get back to you all.Thanks for your help.

 

[chicopee]

Say no as there is a great chance for the compression member to displace as pointed by Kinno. Better off to weld structurally sound eyelets on the ends.

I have been in the bridge construction and heavy structural steel industry for over ten years and we have never use such stupid system.

Remember that in the construction industry it is advisable not to have the sling legs exceed 45 degrees from the horizontal.

If you say yes to their proposal and a serious accident occurs, you will be prosecuted by the Chinese government. If the contractor still insist on the proposal, have it in writting that you are against their proposal.

 

[chicopee]

Oops, "...not to have the sling legs at less than 45 degrees from the horizontal."

 

[dhengr]

This really doesn’t seem like an unreasonable request from a rigging contractor, and the spreader beams are a common lifting means to force only a vertical pick (with no lateral force component) on a tower at a specific tower diameter, equal to the length of the spreader bar. I wonder if you haven’t slightly misunderstood the contractor’s comment about his sling length, because at the angles and spreader bar lengths that your sketches show, you won’t be changing the height of the main hook much, whatever the straight sling length is. The questions is, does he have enough max. boom tip height, for the sling length and tower height, and the spreader doesn’t change that much.

If lifting equip. testing is required isn’t it the responsibility of the rigger to have his equipment certified to whatever code you are working under? The idea that you might be sued by the Chinese gov’mt. if something goes wrong should be discussed with your own company. That should have been their concern when they took the job, and of course, they should employ competent engineering to manage this project, thus you should be safe in your engineering and means of work. It seems to me that you really have most of the info. you need to do the engineering required to approve or reject these spreader bars, and the rigger should provide any other info. needed to get his equip. approved. In fact he should provide calcs., etc. to prove his design. The round pipe sections (325 x 24) as the spreader bar should not be a problem, but the end fittings could be improved. The welding and bearing stresses in the end fittings should be checked.

Are these spreader beams already made, or are they to be made to the sketches you provided? For what they want to do, I would redetail the end fittings as follows: 1.) vertical end plate saddles with thickness a little larger than the sling dia., shaped to allow the sling to angle up to the main hook in side view, and shaped through its thickness (in plan view) essentially to match the inside of the U-bolt of a wire rope clip (a Crosby clip), and two side plates to retain the sling in the saddle. 2.) The spreader pipe should be slit vertically (vert. saddle pl. thickness +) to receive this saddle plate, and long enough for sufficient weld to transfer the spreader compression loading. 3.) Then, on the center line of the pipe I would weld two threaded rods to the vertical saddle plate, one each side, to mate with the saddle half of an appropriate sized wire rope clip, thus being able to clamp the sling to the spearer beam without harming the sling.

 

[csd72]

There is a fundamental issue here that has been overlooked.

A continuous sling past a corner beahaves differently from 2 separate slings shackled top and bottom.

If you look at the force components of the two legs of the sling then you get a force resultant that has a downward incline. i.e. there is a vertical point load applied at the ends of the spreader beam which needs to be resisted in bending and in tension in the cables.

If the lower sling is vertical and the top is at 45 degrees then this reaction is inclined at 22.5 degrees. Not a minor factor!

 

[paddingtongreen]

@csd72, that is what I was getting at, but it gets worse if the load is not precisely balanced, and/or the beam was not level to start with. Either tends to put the beam at an angle to the horizontal, which rotates the resultants in opposite directions and tends to increase that angle. If there is not sufficient friction to prevent this from starting, the beam could end up being nearly vertical.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[csd72]

No I think we are talking about 2 different things.

I am referring to the force diagram for a theoretcal balanced and level load and how the force vector diagram shown is not correct.

You are refering to the rebalancing effect of an unbalanced load.

As you have pointed out, if the friction is sufficient then neither is a problem. But friction is very unpredictable and can momentarily be zero if the sling vibrates or shifts.

 

[dhengr]

That’s exactly why I suggested the wire rope clips at the end of the spreader beam, to lock the sling and the beam together to prevent this type of potential movement, maybe he needs two clips at each end. Then if an unbalanced load causes the beam to rotate a few degrees, that should be taken care of too. My item 3.) in my last para. in my 2JAN11 post above. Of course, the slings and the spreader beam should be set up in the first place so they are equal in length and angles and so the beam is level.

 

[csd72]

dhengr,

If you are going to do that then why not put a decent set of loadbearing wire ropes and get rid of the top section of the sling altogether?

The difference between the stronger materials is very small but it will give a much more predictable behaviour.

 

[paddingtongreen]

Because the contractor bought the wrong length ropes. It looks as though they are a tad long and cannot clear the foundation with the load.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[csd72]

Oh, yes I have been there before.

 

[NLENG]

If the spreader bars are uncertified and there is no traceability of material certificates, don't use them.

I am confused in the calcuation how the compression strength of the material is 410 MPa and the yield strength is 245 MPa. ???

 

[BAretired]

NLENG,

Your second sentence does not seem to relate to anything in this discussion. Please elucidate.

BA