Answer to Quiz No. 5

SETTING A STACK SECTION

Traditionally, stack sections up to 30 tons are set using a method where the hoist line runs over the jib point sheave, then down and around the sheaves on the spreader bar and finally dead ended up on the boom tip.

Is it possible to set a 60 ton stack section rigged as shown if there is adequate crane capacity, jib capacity, crane stability and head room?


The answer is YES and NO..

YES, because the stack section can be lifted safely as shown. And NO because the problem is that when it lifts off the ground, it will hang at approximately a 75 degree angle and would be hard to set on top of an existing stack. Also the lugs would have to be designed to take the resulting bending..

If the stack section lifted off the ground plumb, the vertical reaction at each lift lug would be 30 ton and the jib would be over loaded..

But due to the equalization of the sheaves, the tension in each part will be the same and as the section is being lifted, the bottom of the stack on the inside will raise first and the CG will start to move toward a equalized loading position between the hoist lines on the boom and the hoist line on the jib..

Before the bottom of the stack on the out side clears the ground, the CG will reach the equalized loading position where it is approximately 2.5' from the vertical extension of the boom's hoist lines and 7.5' from the jib's line. This position of the CG will produce a vertical reaction at the left lug of 45 tons and a vertical reaction at the right lug of 15 tons and the system is then in equilibrium with 15 tons per part..

Note that until the CG is in the equalized position, the support will carry any excess load on the outer lug over 15 tons. When the CG is in the equalized position, then the stack will lift off the support without overloading the jib, but will be hanging at approximately a 75 degree angle with the bottom tilted toward the crane..

If the crane could physically walk forward until the low edge of the top stack section could be lowered down on the far side of the existing stack, dogged off to prevent horizontal movement toward the crane, stabilized with chain hoists against horizontal rotation, and then slowly lowered as the crane was walked backwards until the top stack section was resting solidly in a plumb position, the above rigging hook up would work. But, as they say on TV, don't do this at home as it is a very complicated procedure, one for a Rigging Engineer to design and execute. 


    

    

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