Deflection calculation on bar bundles when lifting 2

[MechEng..]

HI,

need some help to calculate the deflection curve.

18 M in length each bar is 24 dia bar bundle has 31 bars,weights 2T metric, has 9 -equally placed straps-how to calculate the deflection when the bar bundle is been lifted by a 10T crane or by a forklift.

Strap strength is 17KN per strap.

Need some help in calculating this.

 

[CANPRO]

SRE - when you lifted the rebar of this length/weight on site, did you typically use a lifting beam, or just rig it with a couple of slings to the crane hook?

Mecheng described two cases - lifting the bundle with a forklift and lifting with a 10T crane. Lifting with a forklift will create two fixed lifting points - what is the spacing of the forks? The second scenario is the 10T crane, which is not a very large crane. If you lift the bundle by attaching two slings at 3.7m from the ends (0.207*L) and set the slings at 60deg to the horizontal, you need around 9.2m to the crane hook, which may be too much for the 10T crane. How far does your 10T crane have to reach? I suspect you won't get much range with a 10T crane lifting a 2T load.

 

[robyengIT]

1 - I agree with SlideRuleEra and IDS if you consider 2 lifting points (the bars form almost a solid cylinder). Furthermore I suppose that the bundles are delivered to field with 9 straps, i.e. 9 tie points. If so you should handle the bundles as the still mill does.

2 - If not you should provide a spreader beam with 4 or 5 lifting points, adjustable in length and position all along the beam, so that to reduce the deflection (for me in this case you don't need anymore the straps). Such a beam can be used either with the crane or with the fork lift

3 - You can use the formulas for continuous beam

4 - Why not to ask the still mill to know what they do ? If they don't handle properly your bars are already over-stressed.

 

[SlideRuleEra]

CANPRO - Always used slings. Don't recall any of our competitors using lifting beams either. On a bridge job site a crane, usually a crawler not a truck crane, has too many varied things to do during a day... slings are general purpose and quickly rigged, lifting beams not so much.

When lifting loose bars, the two loops in the sling act as temporary ties. During the lift, rebar deformations near the pick points "lock" together to give a fair amount of group action to the bars in the bundle. If a lifting beam were used the loops from it would do the same thing.

http://www.SlideRuleEra.net

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[CANPRO]

Thanks for the info SRE. I've seen it done both ways. Usually no lifting beam on site and usually lifted with a tower crane or a large mobile crane where hook height isn't an issue. I've seen it done with a lifting beam in the shop where hook height was limited by the overhead crane and a proper sling angle couldn't be achieved.

 

[drawoh]

...

Assuming the bars are melted down to form a solid cylinder of 136 mm diameter: 27 mm
Assuming 24 mm diameter bars free to slip with no friction: 8600 mm

Is this a good assumption? Unless the bundles are assembled very tightly, what you have is thirty one bars bending individually. Definitely, this affects your bending analysis.

--
JHG

 

[rb1957]

if lifting with slings, do you have to take precautions to stop the sling slipping along the bundle ? Possibly not if it sags significantly around the lift. is there a recommended angle ?

another day in paradise, or is paradise one day closer ?

 

[SlideRuleEra]

rb1957 - Other than keeping the sling leg angle with horizontal as high as practical (the 60^o that CANPRO mentioned is a good compromise), no special precautions for longitudinal slipping. The rebar deformations are handy for preventing slippage - the wire rope sling lodges very nicely on the deformations. Even at a 45^o angle, we did not have slippage.

Unless the bundles are assembled very tightly, what you have is thirty one bars bending individually.

I though exactly the same thing... then realized that is not really true for rebar (been a long time since I was a bridge contractor). The "deleted" post above is after I realized my mistake. The reason the bars don't act individually is, again, the deformations. Even in a loosely bound bundle, the rebar are incredibly well "wedged" together. For example, no way one bar could be pulled out of a loose bundle.

IMHO, the assumption by IDS is pretty reasonable. What I question: Is a tightly bound bundle of rebar is anywhere close to circular? Another time where the deformations come into play. They are going to make a circular shape fairly hard to achieve. If not circular, the properties (I, S, etc.) of the bundle vary depending on how it is oriented.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[WARose]

What I question: Is a tightly bound bundle of rebar is anywhere close to circular? Another time where the deformations come into play. They are going to make a circular shape fairly hard to achieve. If not circular, the properties (I, S, etc.) of the bundle vary depending on how it is oriented.

Agreed. That's why in the deflection quote I gave above, it is based on the "I" of 31 bars....without considering the benefit of the "Ad²" term in the parallel axis theorem. You really don't know what that "d" is going to be.

 

[IDS]

Is this a good assumption? Unless the bundles are assembled very tightly, what you have is thirty one bars bending individually. Definitely, this affects your bending analysis.

The solid bar is the lower bound deflection assumption. Bars bending individually is the upper bound, which I also gave the deflection for. We can be sure that the actual deflection will be much less than the upper bound, but there is no simple way to know exactly how much less, other than measuring it. But if the upper bound deflection is OK we don't need to know the exact deflection anyway.

What I question: Is a tightly bound bundle of rebar is anywhere close to circular? Another time where the deformations come into play. They are going to make a circular shape fairly hard to achieve. If not circular, the properties (I, S, etc.) of the bundle vary depending on how it is oriented

Sure, but if the cross section is roughly circular the I value including voids is going to increase, and lifting the bars is going to tend to lengthen the vertical axis, which will also increase the I about the horizontal axis, so I think the solid bar assumption is a conservative estimate of the upper bound stiffness.

So if we have two lift points at say 3.75 m from the end, the deflections will be very small and we just need to check the stress.

For a single bar the maximum (unfactored) stress would be 184 MPa, and the maximum stress in bars acting as a group would be much less than that.

As for the number and strength of the tie points, I don't think there is any simple way to calculate that. As was suggested earlier, I'd ask the supplier what they do.

Doug Jenkins
Interactive Design Services

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