When to use
– Lifting where the size or geometry of the load does not allow lifting with slings from 2 to 4 legs.
– Lifting with great weight, which would require very high capacity slings.
Behavior
– The force acting on each sling in a hyperstatic set will vary depending on the structural rigidity of the load;
– The value of the maximum force and the most loaded sling is not easy or direct to obtain, such as 1 and 2 leg slings;
– Depending on the structure, few slings may be heavily loaded and many slings may be low loaded or unloaded;
-This variation in force on the slings can overload them or overload the load structure, possibly leading to collapse.
Simple solution
– Consider that 2 to 3 legs are loaded and the others unloaded;
– Use all slings equal to the most loaded one;
– Check the integrity of the structure to be lifted by 2 or 3 points;
– Often makes lifting unfeasible. Impossible in this example of the domed ceiling.
Solution by non-linear structural analysis
– Use of cable elements and hook elements (“hook”);
– There is no point in doing linear analysis, as it is against security;
– Consideration of various combinations of difference in length and clearance between slings;
– More realistic, leading to safer lifts. Used in dome roof lifting.
Security risk
– If we divide the weight of the roof to be supported equally by all the slings, the force on the sling will be 3 to 4 times smaller than the force obtained by the non-linear structural analysis, therefore, against safety, which could lead to the rupture of the slings and/ or collapse of the structure.
This real case is an example used in our advanced rigging plan course and was carried out by Indústria Mecânica Zanuto in Goiás.
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