Steel Floor Joist Vibration

Resonance

• Vibrations traveling through a structure are mechanical waves -- disturbances that move energy rather than matter. One important property of these waves is the frequency, the number of waves that pass a given point over a given time interval. Just like other building materials, steel has certain resonant frequencies, frequencies at which it will oscillate more readily and absorb more energy than at other frequencies. If a structural component is made to oscillate at a frequency that matches its resonant frequency, it will vibrate more readily rather than damping the vibrations.

Steel Joists

• Steel joist flooring beneath a wood deck typically has a resonant frequency between 10 and 30 hertz. When occupants walk across the floor at the right pace, they can create vibrations that can potentially become keenly irritating. Usually this situation doesn't pose any serious risk of structural collapse, although under extreme circumstances -- like a crowd pounding on a dance floor, for example -- it could pose a potential problem.

Design

• It's best to evaluate the potential for vibration during the design process and not after construction has already begun. If you've already reached the stage where the steel frame has been designed by the manufacturer and the joists are being designed, solving the problem may prove more expensive. Increasing the stiffness of the floor is often the best approach. If practicable, adding new columns between those that already exist can help improve the situation.

Alternatives

• Adding decking or blocking is another way to stiffen the floor and reduce the effects of vibrations. Alternatively, damping can help minimize the problem. Structural components like drywall provide damping and cut down on vibration, as do specially designed devices like damper posts and mass dampers. In buildings that have already been constructed and furnished, furniture and other furnishings can sometimes help dampen vibrations depending on the shape of the room and where they are placed. Ultimately, however, the problem is easier to solve before construction rather than afterward.