Now, That's Service

When an engineer determines the size of the beams he's putting into a building, the deciding factor is not always the amount of weight put on the beam. A beam may be more than strong enough to carry the weight put on it, yet still bend a quite a bit with that weight. This bending, called deflection, is especially apparent in wood beams. Deflections of even an inch are very noticeable to humans. Furthermore, when people walk across a floor that deflects and bounces a bit with their footsteps, they tend to get a little freaked out, even if the floor is perfectly safe and in no danger of failing. Thus, engineers usually make their beams bigger than they need to be in order to decrease deflection and make the inhabitants of the building feel more comfortable. They call this serviceability.