For a standard same-elevation lift, we first determine the distribution of weight at each pick point. Then we divide the sling's length by the head height, and multiply that "load factor" by the pick point weight. Arriving at the sling's tension, we can determine if the proper rigging is being used.
A phenomenon occurs in rigging when we are faced with hoisting a load which has its pick points on two different elevations. We cannot use the simplified system described above but must use the long-hand method to determine sling tensions. If we attempt to use the simplified method to find sling tension in off-elevation picks, we discover that the lower elevation pic point and its sling are realizing more tension than anticipated. We can mistakenly overload the rigging gear if we do not predetermine the actual tensions to each sling leg.
Try your hand at determining sling tension for the following examples, using the ITI Bookstore's Master Rigger's Reference Card, Section 10 (pictured below, last). Workshop answers can be found below.
Happy trails to all my crane and rigging friends,
Mike Parnell
ITI-Field Services
P.S. This article was originally published in The Professional Rigger Newsletter.
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