Tandem Lifting Calculations

TANDEM LIFTING (Multiple Lift )

When you are considering to lift an special piece in tandem lift with 2 cranes, some quick calculations are needed to know in advance what will be the weight at each crane will be working.

Knowing the length, weight and position of the cog (center of gravity) we can easy calculate the weight supported by each crane at different positions where the slings are placed.

General example

  

Tandem Lift using 2 Mobile boom cranes

        

      Any lifting operation which involves  more than one crane is classed as a complex lift. BS:7121 :2016 explains it as follows;

      " Complex lift: lifting operation that requires more than one crane to lift the load, or cranes using load enhancement equipment, lifting of persons or when the lifting operation is at a location with exceptional hazards. (Clause 3.11.3.)"

                    Load Sharing Calculation 

All Multiple crane lifts should be planned by a competent person ie; the Appointed person. 

By using a general formula we can calculate the load sharing of each crane while engaged in a tandem lift.

'''P''' is the weight of the piece (Tons)
'''F1''' is the weight soported by crane number 1 (Tons)
'''F2''' is the weight soported by crane number 2 (Tons)
'''l''' is the length of the piece (M)
'''a''' is the distance between the leftmost to the position of the sling from crane number 1 (M)
'''b''' is the distance between the rightmost to the position of the sling from crane number 2 (M)
'''c''' is the distance between the leftmost to the position of the cog (center of gravity) (M)
Equilibrium condition says that:
1.- The resultant force acting on the object is zero.
'''F1 + F2 = P''' so '''F2 = P - F1'''
2.- The sum of the moments acting on an object must be zero.
F1 x (c-a) = F2 x (l-b-c)
F1 x (c-a) = (P - F1) x (l-b-c)
F1 x (c-a) = (P x (l-b-c)) - (F1 x (l-b-c))
F1 x (c-a+l-b-c) = P x (l-b-c)
'''F1 = P x (l-b-c) / (-a+l-b)'''

Now u have the value of F1 to calculate F2 , Subtract "F1" from "P" (weight of load)
Work out the obtained data along with the cranes rated duty charts while preparing the lift plan.Take account of the radius, boom length and other contributory factors(See my Blog on Selection of cranes and calculating the Gross load).
The details obtained by using this method will allow  you to ensure that the whole lifting was performed well inside the cranes radius.


Disclaimer : The above stated information is solely for the purpose of reference; additional factors such as ground conditions,Proximity hazards all are to be taken into account for. The writer hold no responsibility to any misdeeds.