What is crane Design Life Analysis

Although crane design classes have different designations in different design standards, one thing remains constant: each crane and hoist has an intended Design Life, which determines the amount of work it can safely perform before its useful life is over.

So why does this matter? Is it a matter of life or death? Actually . . . it is, when you think about avoiding a load drop, and its most dire potential consequences. But it’s also about getting the best return on investment from your crane through timely overhauls and modernizations.

No matter what job your crane is doing, it is a safe bet that it was purchased with a specific task in mind that determined its size, strength, durability and configuration. It might be single girder or double girder, rated for intermittent use or continuous severe duty 24 hours a day. It may have a single or double hoist. It could be a monorail system, or a jib with a chain hoist – the options are almost endless. But some things are constant: when a crane performs one work cycle moving a load from A to B, the structure experiences only one stress cycle, while the components of rotating machineries experience hundreds or even thousands of stress cycles in their various parts.

Structures and machineries are not designed equal

Bottom line, the crane structure and its machineries fatigue differently, and at different rates. Structures consume their design life in work cycles, while machineries consume their design life in running hours*. A girder is stressed only once per stress cycle; thus, the propagation of fatigue is relatively slow. Even when a fatigue crack does appear, the rate of failure is usually measured in years. But rotating machineries are different. Working constantly under loaded conditions, their components may be stressed tens of thousands of times during one movement. When a crack forms in these components, the time before it becomes critical is usually very short – think hours or days.

The failure interval for crane machinery is usually too fast for it to be reliably monitored with inspections. Inspectors can’t be on site every day. This is why we have to calculate the remaining Design Working Period (DWP), based on the utilization of the crane.

Design class determines the lifetime of the crane - or does it?

When a designer starts dimensioning a crane from fatigue point of view, the first thought is the class of utilization. This determines how many times a load will be lifted, and how many hours a machinery will work during the lifetime of the crane. Then, he or she considers the load spectrum, which determines what kinds of loads the crane will handle. And, loads may not always be at full capacity. Will a crane be used at full capacity nearly all the time, or will most lifts be small loads?

As these were only estimations during the design phase, this is where the detective work occurs in the Design Life Analysis. Through production records, process observations and interviews, a qualified and trained inspector researches how many actual cycles the crane has performed, how many loads it has handled and at what capacity to form a calculation of its current remaining design life.

Practical applications

“But wait,” you ask. “Isn’t that what crane inspections are for?”

Not really. Periodic inspections help identify risks and support compliance, but their frequency and scope are not typically enough to catch failures caused by fast fatigue crack propagation in machineries. The likelihood of such failure increases closer to the end of the design life of your crane.

Remember, cranes do not exhaust their life in calendar time, but in relation to work cycles and running hours. When a crane is nearing the end of its Design Life, a Special Assessment should be conducted to determine the actions to permit further usage of the crane.

If the subject is a chain hoist, the answer is typically simple – just replace it. But if it is an overhead crane whose structure is still viable, you can consider General Overhauls, modernizations or trolley replacements. If evolving production needs indicate that duty or capacity requirements have changed, or if the structure doesn’t have sufficient Design Life left, your most cost-efficient option may be to invest in a new crane.

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