What Does "Capacity" Really Mean?

Crane Capacity Vs. Crane Rated Capacity

Since I wrote the last post, I've been concerned that I glossed over a very important point. One of the risks of writing about something you've been involved with all your life is that u take for granted the use of "insider" terminology. I pride myself in avoiding this writer's roadside landmine most of the time, but I think it got me on my last post.

Crane Capacity

This seems like an obvious and easily defined term, but frequently the obvious answer is not necessarily the correct answer. In the early part of the 20th Century, engineers had to deal with two factors that we now take for granted. Steel producers couldn't manufacture steel with the production uniformity commonly assumed today. Further, computers with tools like finite element analysis were not even thought of. As a result, engineers added in "safety factors." 

Safety Factors on Top of Safety Factors

In the original crane industry specifications, a 5:1 safety factor was mandated. This safety factor was on top of the safety factor the steel producers and that used by prudent engineers. It's little wonder that experienced crane operators determined that their cranes could in fact lift far more that the posted limit. 

Enter CAD (Computer Aided Design)

We now have precision production and engineering. The steel producers measure all the inputs and outputs, producing steel that precisely meets their spec... no more, no less. Engineers can now cut out excess steel (and therefore costs) using CAD to produce exactly what's required, again, no more and no less. In other words, all of the redundant safety factor has been removed.

The Weakest Link

It is important to remember that rarely are the cranes independent of a building as a supporting structure. If I remember correctly, metal building specs only require a 2:1 safety factor in the design of their structures. To me this provides a frightening mental picture of a severely (5x) overloaded crane, not equipped with overload protection, in a metal building engineered with a 2:1 safety factor!!!

Crane Rated Capacity

 So this is why, in the late 1980's, the crane industry started using the term "Rated Capacity." The Rated Capacity of all cranes, new and old, should be determined and marked appropriately. All new cranes should be equipped with overload protection and all old cranes should be retrofitted with overload protection. 

Without overload protection, you may wake up one day to collapsed building, laying on the ground in pieces, with a fully intact crane ready to make another dangerous lift.