2 Reasons for Design Factors in Rigging Equipment

As you know in ASME B30.9 SLINGS and other industry documents that highlight sling capacities, there are stated design factors. Also in B30.26 RIGGING HARDWARE which covers shackles, swivel hoist rings, eye bolts, rigging blocks and the like, design factors between 4 and 5 are listed.

The primary reason for design factors is to account for some measurable amount of wear to the sling or component during its lifespan. This wear will reduce its overall minimum breaking force (ultimate strength). This may occur in the form of metal loss in links of a chain sling or in the bow or pin of a shackle. The respective manufacturers have pre-calculated the amount of loss they can live with before calling for the equipment's removal from service.

The second reason is unexpected overload which may occur during dynamic lifting. A crane that lifts a load quickly can temporarily cause an increased tension to the rigging, the crane block, hoist rope and crane structure. Sometimes an unexpected overload can happen if the person rigging doesn't account for a sling's change of capacity due to hitch type and angle from horizontal. Two slings used in basket hitches (90 degree) then converted to choker hitches (30 degree, each) on the same load might show a change of capacity of -60%. If the original load were applied, the new "in field" design factor would be 2 instead of 5. If lifted or lowered quickly, the sling(s) can fail due to a miscalculation of tension-to-capacity and "shock load".

Design factors in rigging have saved more folks than we will ever know. Damage we can generally spot, so the first issue can be more easily avoided. Without obtaining or estimating the load's weight in advance, the hitch method, the rigging capacity and the angle from horizontal, the rigger can unexpectedly overload the rigging gear and experience a failure without notice.

A solid foundation and continuing education in rigging principles can help overcome these challenges and lead to ongoing success!

 

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Mike Parnell

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