floor (joist) vibration

Other than a project or two where I looked at vibration from machinery, I never worried too much about vibration in floor / floor framing design. Typically, I never had to. Manufacturers of floor joists (e.g., prefabricated wood I-joists) worry more about vibration. The issue with floor vibration is typically one of perception … a vibrating floor is bothersome, or may seem unsafe. It’s an issue of `performance’. Does the floor perform to the satisfaction of the user, or occupant? (Despite the fact that it may be perfectly, or at least theoretically, safe!) Building codes demand that floor designs are safe (don’t collapse), with some demand on `serviceability’, e.g., that a sagging floor joists not crack basement ceiling gypsum attached to the their undersides, but `performance’ beyond that typically goes beyond building code scope. A floor joist manufacturer may provide various performance levels for the company’s products. Designing for vibration control typically involves use of joists with greater stiffness (E, I, or EI). More stiffness … less vibration … better performance. At a cost, of course … due to more wood (I), stiffer wood (E), deeper joists, and so on. While the building code may limit the live-load deflection of a floor joist (beam, girder, etc.), say, to which gypsum is attached, to L/240 (joist length divided by 240), or to which plaster is attached, L/360, a more stringent limitation (on deflection) of L/480, or L/600, might very well also provide better vibration control (better performance). Demanding a tighter deflection (L/480, L/600, L/720) is an easy way to, presumably, reduce vibration. Code requirements typically provide minimum requirements. Going beyond the code minimums, while increasing immediate construction costs, provides additional benefits. A stiffer floor will deflect (sag) less, whether or not it ever vibrates, and it will be safer, whether or not the added safety would ever be necessary.