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Vibration must be taken into consideration when structural engineers design and build structures. Vibrations in a structure can occur in a number of different settings and situations including:
Vibrations of tall buildings in the wind
Vibrations of foundations supporting sensitive equipment in hospitals
Vibrations of floor structures including dance floors
Vibrations in other horizontal structures like bridges
Vibrations can negatively impact occupant comfort and safety, structural integrity or damage sensitive equipment. While entire structures can move and oscillate, floor vibrations are often what is noticed by a structure’s occupants. Dancefloors, sports stadiums, large crowds, even heavy filing cabinets in office spaces all can create very large loads that will have implications for the vibration effect.
Floor vibrations can become troublesome when the motions that cause them are large enough that they cause annoyance. In extreme cases, floor movement can even cause panic.
It takes 3 to tango
Basic vibrational theory comes down to three interrelated factors: the structure’s mass, stiffness and damping. Every structure has a natural frequency, that is, the frequency at which the structure resonates. Structures with low natural frequencies are more easily excited at resonance by human activity; therefore increasing a structure’s stiffness will make it more difficult to excite. Increasing the structure’s mass also makes it more difficult to excite as a large mass is harder to move than a small mass. Damping dissipates energy from the system. Floor structures have very low inherent damping so increasing the damping can dramatically reduce the floor vibration.
The Unique Challenges of Dancefloors
Dancefloors are particularly vulnerable to floor vibrations and that’s because often dancers are moving at a constant rate to the rhythm of music. This can cause the floor to resonate, which - if not designed properly - can make you feel a little like you’re dancing on a trampoline.
When the natural frequency or vibration of a dancefloor aligns with the frequency of excitation (the movement caused by dancing). This is known as a resonant response.
Ballrooms are especially susceptible to these challenges, as they tend to be large spaces, often with areas of the floor that are unsupported, and are built to accommodate hundreds of people at once. Good planning and foresight are vital to avoid problems – or at least the perception of problems - in the future.
Modeling the resonant response and dynamic interactions between crowd and structure
The best way to solve these challenges is to numerically model the vibrational response of the floor in order to understand the dynamic interaction between the crowd and the structure. When the mass of the crowd is significant compared to the mass of the structure, as is often the case in ballrooms, dancefloors and stadiums, it’s necessary to consider the dynamic interaction between these two systems.
A crowd adds significant mass and damping to the system which modifies the system’s dynamic properties. If the dynamic interaction between the crowd and structure is not considered, then it’s likely that higher than accurate accelerations will be predicted for the floor.
Reducing Bad Vibrations with Tuned Mass Dampers
One way to resolve bad floor vibration is using a tuned mass damper. A tuned mass damper is a passive mechanical system consisting of an additional mass that is connected to the structure using springs and dashpots (e.g., viscous dampers). When properly designed, motion from the structure naturally causes the tuned mass damper mass to oscillate out-of-phase with the motion of the structure, opposing the occupant forcing and thereby reducing the overall movement of the structure.
Determining if you need a damper through modeling is just the first step; understanding how your floor moves under normal and extreme conditions is vital to selecting the ideal type of damping system for your project.
Tuned mass dampers are also used in other structures including skyscrapers, bridges and telecom towers. Learn more about tuned mass dampers for floors to see if they’re the right solution to your floor vibration problems.