Floors are in every building and, just like skyscrapers and bridges, they can be affected by vibrations.
If floors move too much, they can exert significant influence on a variety of spaces. Large floor vibrations, for instance, can make people in a space feel quite uncomfortable. In extreme cases, such as the stands moving significantly in an arena, the vibrations can even cause panic.
Lower levels of vibrations can also impact spaces, specifically with sensitive processes like medical surgery. Surgeons want to have vibrations that are lower than what they can perceive, as this eliminates floor vibrations as a potential distraction. However, vibrations even smaller than these can still cause issues – vibrations that are smaller than we can perceive can still disrupt the performance of sensitive research and medical equipment, such as MRIs.
Where floor vibrations are expected to be problematic, they can be mitigated by the addition of mass, stiffness, or damping to the structure. Of these properties, damping is likely the most difficult to understand, but it simply means the dissipation of energy. All structures inherently contain small amounts of damping; however supplemental damping can be added to the system, which can often dramatically reduce the vibrations of the structure. When dealing with floors, we often call the supplemental damping devices, floor dampers.
What types of spaces require floor damping?
Increasingly, designers are creating more unique spaces with long open spans in an effort to produce an open and specific aesthetic. Adding columns would stiffen up the floor, increasing the floor thickness would add mass, and increasing the beam sizes could add both mass and stiffness. These changes would mitigate floor vibrations but would also change the desired look. Incorporating floor dampers to the floor helps decrease the vibration but still allows architects to deliver the design they had in mind.
An excellent example of this is with a large open-span ballroom, which are susceptible to floor vibrations due both to their flexible, lightweight structures, and to the activities of the crowds that occupy them. Intended for large crowds dancing to a rhythmic beat, users of the space are often all moving at the same frequency, and thus are likely to excite the structure at its resonant frequency. In these cases, adding floor dampers can significantly mitigate the vibration and improve the floor’s performance.
Updates or extensions to a hospital or research facility often require mitigation of floor vibrations as well, especially when sensitive equipment is involved. Relocating sensitive medical equipment often requires measuring the existing space to determine what vibration levels are currently working for the equipment and designing the new floor accordingly. In these cases, mitigation is often best done by increasing mass or stiffness rather than by adding damping, as discussed below.
Finding the Right Equation: Adding a Damper, Mass, or Stiffness
When floor vibrations need to be reduced, adding damping – through supplemental floor dampers – to the system, is often the biggest “bang for your buck.” A floor damper is a type of tuned mass damper (TMD). It is an auxiliary mass added to the floor that is tuned to the natural frequency of the floor. When the floor moves the movement of the floor damper lags just behind the floor movement. Energy is transferred from the floor to the damper and dissipated through an energy dissipation device within the damper.
Floor dampers work best when the floor is responding predominantly in one – or just a few – modes. One example of this is a long span lightweight floor that responds to movement by moving up and down predominantly at the center of the floor. Adding a floor damper at that point can dramatically reduce the floor’s response to the movement.
In stiffer, heavier floors, the floors’ response is often best understood by determining the combined response of many modes acting together. In such situations, we are concerned with the response at many different points on the floor, and it may not be possible to put dampers at every significant response point on the floor. In these situations, adding mass or stiffness to the system can be necessary to mitigate the floor vibration.
How Much Vibration is Too Much?
In hospitals, research facilities, and other buildings where only very low levels of vibration can be tolerated, the vibration criteria used are typically quite specific. This is because specific targets must be met for the sensitive equipment to function properly.
When the vibration concerns relate to human comfort, rather than sensitive equipment, the vibration criteria are much more variable. This is because human tolerance for vibration depends on the expectations of the people experiencing the vibration and on the activities in which they are engaged. In many cases, there is room to have conversations with the architects and owners of a space regarding how they want it to be perceived and expect it to be used. The acceptable criteria regarding floor vibration are then determined based on these conversations.
Build It and Forget It: Is Floor Vibration Only Addressed in New Construction?
No – certain modifications to existing buildings can also cause floor vibration issues. One such example is the removal of mass from the floors, such as when offices go paperless. This involves removing heavy paper filing systems from the floors, which can result in additional movement and vibration from sources such as people walking around the office.
Additional movement on floors can also result in floor vibration issues in existing buildings. Adding a gym into an office building is a common example, as running on treadmills and, especially, synchronized bouncing from aerobics classes, can cause issues on a floor originally designed to support people sitting at desks.
Existing buildings can also be converted, such as an office building into a medical lab facility, which changes vibration criteria. Sensitive pieces of equipment, such as MRI machines and CT scanners, are very sensitive to floor vibrations. The floors in an office building would not have been designed to handle sensitive equipment, which is why vibration issues that require post-build mitigation strategies can arise during the building conversion process.
Learn more about mitigating floor vibrations in existing buildings in our post Disrupted Work: Floor Vibration In Existing Buildings. Alternatively, check out the previous chapter in our Damping Explained series here.