When you think about deflection, you may think of one object bouncing or careening off another. Engineers think about a different kind of deflection: a physical distortion of an object because of load on the object. Beams are vital members in most structures and must be engineered with care to avoid deflection problems.
Forces and Deflection
The purpose of a beam is to distribute one or more small point-loads or concentrated forces to vertical members supported by solid foundation. Generally, the longer the span and greater the load it must carry, the larger the beam must be. It is not difficult to figure out how much a beam will flex if it is simply spanning two points. But engineers have to consider some very complex forces, including compound and multiple forces, then calculate, given the beam's properties, how it will respond. Incorrect engineering can lead to poor performance and even structural failure.
A very common problem sometimes caused by beam deflection is floor bounce. It is just what it sounds like: the floor literally bounces. The problem is, a support beam is not rigid enough, so walking on it causes excessive deflection on a down-step and a spring when the force is released. A degree of floor bounce from beam deflection is structurally acceptable; too much and it doesn't inspire confidence to walk on.
Wall and Ceiling Cracks
Besides the uneasy feeling of walking on floors that have excessive bounce, the bounce is likely to cause cosmetic damage in the structure. Drywall, plaster and finish woodworking aren't especially flexible. So, excessive beam deflection can crack drywall and plaster. Fixing the plaster caused by beam deflection does not solve the problem. If cracks are caused from the beam settling, you can patch the cracks. But if the crack is caused from variations in load and subsequent beam deflection, you can bet that if you fix the plaster, it will only crack again.
It is easy to take the structural security of a home for granted. But structures fail on occasion and beams are sometimes the source. Every beam has a deflection threshold. At some point, it will fail. So, the beam has to be engineered so that no circumstance ever exceeds that threshold, even for a second. Several tragic and notorious cases of beam deflection and ultimate failure have resulted from large numbers of people on decks. It is possible that the beams were able to carry the weight of the people, but the load was compounded by their motion.
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