Operating in the Twilight Zone of Crane Capacities
by Ron Kohner
How many times have you heard a conversation like this? "We had a heavy piece to lift. The weight was over the chart, but we were in a tipping range, so I knew she'd get light before anything would break."
Sound familiar? -- Believe it's true? -- It absolutely is not!
By relying on tipping to signal a problem, an operator can overload a crane's structural components by a tremendous amount. In over 25 years in the crane and rigging industry, I have heard the above "logic" too many times to believe it constitutes an isolated misunderstanding. Rather, it would seem that many users are guilty of trying to second guess the crane manufacturer.
US-OSHA (Occupational Safety and Health Administration) requires that, "When ratings are limited by structural competence, such ratings shall be clearly shown and emphasized on the rating charts." There can be no doubt that the intent here is to alert crane operators to the structural limitations for their cranes. OSHA clearly recognizes the importance of structural limits. Their further requirements also state, "When loads which are limited by structural competence rather than by stability are to be handled, it shall be ascertained that the weight of the load has been determined within 10 percent before it is lifted."
That's clear enough. Who cannot imagine the potential disaster of a structural failure? Most operators agree -- you don't cheat on structural limitations. However, the rating chart does not tell us that it is safe for the crane to approach tipping without suffering structural consequences. It only indicates that no structural limitations are exceeded by the stability ratings shown on the chart. These are not ratings which will tip the crane! They are only a certain percentage of the load at which the crane will tip. If we load the crane beyond the manufacturer's ratings -- even in a stability-limited area of the rating chart -- we have no idea which structural components may be overloaded before the crane actually shows signs of tipping.
A chart may list a rating of 5,000 pounds limited by stability. In fact, the strength of the boom may only be 5,100 pounds. Under current federal stability regulations for a crawler crane, it would take 6,700 pounds to actually tip this crane. If the operator lifts "until she gets light," the boom is being overloaded by 1,600 pounds or 32 percent. There is a "twilight zone" between the stability ratings shown on a chart and the load it actually takes to tip a crane. A rating chart does not tell us what structural limits are violated when the crane is taken beyond the chart and approaches tipping.
A simple story helps to illustrate this problem. Imagine you and I have just built a crane. We have scratched our heads bald and gone cross-eyed, working at a computer screen to determine just how strong our crane is and what it can lift. It's a crawler-mounted crane, so OSHA regulations tell us that we can only rate the crane at "75 percent of the load, which produces tipping." We do some calculations to develop a rating chart. We also do some testing to verify that chart. Sure enough, the computer was right. Under carefully controlled test conditions, our crane begins to tip at a 20-foot radius with a 100,000 pound load.
Some simple math quickly tells us that if the crane tips with a 100,000 pound load, our rating for the crane (remember, it's a crawler) can be no more than 75 percent x 100,000 pounds or 75,000 pounds -- if tipping is the limiting factor. We dig through piles of computer printouts that predict stresses in our crane, and find that only two components are near their strength limits for this particular boom length and radius. Our boom can handle 76,000 pounds at a 20-foot radius. Rope strength limits the crane's capacity to 85,000 pounds. Our tipping rating of 75,000 pounds is, therefore, the lowest of all the factors. We publish our rating chart showing 75,000 pounds at a 20-foot radius, and we don't asterisk the rating because it is stability limited -- not structural.
We sell our first crane. A few weeks later, we are at a jobsite proudly watching our brainchild at work. The rigger hooks onto a large bridge girder to off-load it from a truck. Painted on the side of the girder is the weight: 97,000 pounds. We pull out our trusty tape measure and check the radius: 20 feet. We think back to our rating chart -- 75,000 pounds at 20 feet. Certainly, the operator will try to get closer before lifting. But no -- the engine revs and the crane begins to take a strain. We run screaming toward the operator. He's going to collapse our new crane!
Looking more than a little disgusted, the operator calmly explains to us that, yes, he knows the load is beyond the rating chart. He smugly points out that he's working in a tipping range on the chart, so logically, it is safe to overload the crane "until she gets light." He goes on to assure us that it should tip before anything will break. How wrong can he be!
We begin to sputter, "I designed this crane. The boom is only good for 76,000 pounds and the rope for 85,000 pounds." Both will be grossly overloaded if he tries to pick the 97,000 pound girder at 20 feet, relying only on the feeling of tipping to stop him.
The key here, is that the rating he sees on the chart is not the tipping capacity. It is only a certain percentage of the tipping load (in this case 75 percent) as the law requires. There is a "gray" area between the rated capacity listed on the chart and the load that will actually tip the crane. Many structural limits exist in this "gray" area. If he relies only on tipping to warn him of a problem, he can be asking for big trouble. He will substantially overload structural components of the crane. When you are relying on "the bubble in your butt" to warn you of tipping, only the Good Lord and the crane designer know what is being damaged.
Some will argue that there is a safety margin built into the boom and other structural components which should protect us during overloading. Yes, there is a margin. However, that margin does not belong to the user. It is there to protect against unforeseen problems beyond the user's control -- material variations, wind, soft ground, etc. If that margin is deliberately usurped at the start, nothing remains to prevent catastrophe when Mr. Murphy and his Law visit our work site.
A crane will certainly not collapse under every overload, but make no mistake, the overloading does damage. Project managers consider the operator a hero when he makes a lift beyond the chart. Six months later, when the boom on that crane collapses with a light load, no one can figure out why. Think about it. Did the manager and operator help cause that accident by pushing the crane near tipping in the past? Each overload should be viewed as cutting one strand of a rope. The damage is cumulative, and it is only a matter of time until the rope breaks.
OSHA mandates, "No crane shall be loaded beyond its rated load." That's pretty hard to misinterpret, but too many users think they know more, and are tempted to bend the rules. The next time you are tempted to do it, or even to ask someone else to do it, remember our little story. When you appear to be in a stability-limited range of a rating chart, a structural limitation may be only a few pounds away -- and you don't know it. Stick to the charts.