May 31, 2008

Understanding What Really Happened to that Crane in New York


This has been in the news recently that a crane "collapsed" in New York while on a construction site. I believe the mass media has mis-interpreted the situation and has quite a few facts wrong. As an engineer, I am going to put my two cents in here.



A crane is known as a cantilever- an edge or beam that sticks out from a base. Some news agencies are reporting that the crane fell, and some are reporting that the crane snapped "in half like a toothpick" and I am more inclined to believe that the crane actually did not fall onto a building, because witnesses claimed that the crane snapped. When that crane snapped, it took a few pieces of the building with it. So why did this happen?



A cantilever's beam supports a certain amount of weight, and the base to the cantilever is secured and is engineered to support a certain amount of weight. Let's arbitrarily play with numbers here and say that the maximum lifting weight for the crane is about 500 pounds- which means the crane cannot lift anything more than 500 pounds. As the crane extends towards the sky, the same weight will produce more stress on the members of the base. The problem is very simple- and it is a problem of momentum and torque. When a crane lifts an object, there is considerable stress on where the crane broke in two. If you catch my drift and know what I am talking about here- the crane lifted more than what the maxiumum capacity would allow. The incident was caused by the workers failing to follow proper precautions when operating the machine. In short, it is the operator's fault. When weight is lifted by the crane, that weight wants to cause a rotational motion with the cantilever- but the supports in the base will not let it.



Even if the crane did "fall" on a building, the cause is the same- they were lifting something heavier than the recommended maximum lift capacity. I am not inclined to believe that the crane fell unless they were lifting some ridiculously heavy material and were not set up properly, thus causing what is known as impending motion- and eventually tipping.



The media, as usual has blasted this and has not understood the situation in it's entirety. I think the mayor also is out of line citing that what happened is "unacceptable", and I think he should speak his mind rather than trying to appease the public by giving them an answer they want to hear. Instead, he could have said he would investigate into the practices of workers and worker safety. A lot of the time the problem is caused by a problem "down the line", which means it could have been a supervisor's order to lift more than what was recommended, if that is the case then that person is at fault and could face charges.



A "collapse" is comletely different then something snapping in two. Case and point: a collapse is the fall of a structure to the ground. Since weight produces a rotational motion about some point on the crane, in which that point is the point where the most rotational force is applied, then a cantilever can never "collapse" unless the base was poorly constructed. Reports cited that the cranes were inspected- and I say that there was a 0% chance of the crane being defective. I've said before that inspection can be at fault here, but after looking at the crane I would say inspections prior to this incident were probably spot on.



Folks need to get their facts straight before blowing things out of proportion- goes to show how accurate the media really is. I will keep an eye on what the safety report finds- and I bet a ham dinner I am right, or close.
edit- I get my ham dinner and turkish delight! There was a bribery scandal with one of the supervisors (or inspector?) on duty that passed things along to get things done. Regardless, the crane collapsed having a weakpoint and lifting too much......

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