One of the challenges in designing any space for human occupancy is creating and maintaining a comfortable environment, including vibration reduction in the floor. You may have been sitting at your desk when someone walked down the aisle on the other side of the partition, and you felt every footfall.
Floor vibrations can be damaging in other ways. In areas where vibrations can disrupt various types of testing or manufacturing, reducing vibration is crucial for production.
As it happens, vibrations in steel buildings are inherently minimized, as steel buildings meet most vibration performance requirements without the need for special modification.
Demand for buildings that are quickly and easily constructed, have large uninterrupted floor spaces, and remain flexible in final use continues to grow. Steel buildings meet all of those demands with competitive costs.
Steel is a lightweight material that can support the same loads as other building materials. It can be customized to any design and reduce waste material.
Steel is ideal for large, open-span construction offering excellent dynamic performance, including reduced vibration. Stiffer beams and a large mass of long-span floor plates reduce the magnitude of a vibrational response; in other words, the floor vibrates less in response to activity.
A long span floor creates a large space for vibrations to work through. With steel floor systems, the vibration is damped fairly quickly before it can travel very far.
What are vibrations?
The technical definition of a vibration is "an oscillatory motion experienced by a building and its occupants during the course of normal daily activities." That's a long-winded way of saying the building shakes as people, machinery, and traffic move around it.
You can visualize a vibration by dropping a pebble into the water. If you were to drop two pebbles so that the ripples meet on their journey outward, you would see that some of the ripples are disrupted and dampened when they overrun each other while other ripples are magnified.
The outward ripples behave the same way vibrations do through stiff materials, including disruption and magnification. When applied to the large, open-span floor like that mentioned above, vibration acts like a pebble in a very large lake. After a few feet, the ripples are damped and stop.
Normally, vibrations are up and down, or vertically oriented. However, horizontal vibrations are also possible. Depending on the severity, vibrations can be a nuisance to the people in the area or they can damage fixtures, fittings, and building structures.
The severity depends on the source and duration of the motion and the design and layout of the building. Vibrations are measured by frequency and amplitude.
What causes vibration?
Dynamic loads (meaning footfalls, dropped loads, and moving machinery) are applied directly to the floor, causing it to shake. In fact, the human activity of walking is the most common source or vibration. If that activity is more vigorous, as when people jump, run, or dance, the vibration is more severe.
Machine vibration, even that annoying printer in the middle of the cube farm, is another source. In manufacturing and other areas where large equipment is used, the vibrations are typically dampened at the source by using isolating mounts or motion arresting pads. The same thing is done for buildings in seismically active regions to dampen the shaking of an earthquake.
Floors act as a continuous structural system; it has its own natural frequency and reacts as a single entity to vibrations. You can estimate the natural frequency of an entire floor by using a specific equation that considers the actual loads, both permanent and 10% of non-factored imposed loads.
For example, a commonly used metric in estimating human induced vibration is that of the frequency of a walking pace: 1.8 to 2.2 Hz.
Damping is the loss of mechanical energy in a system. For our purposes, damping is how vibration is slowed and stopped.
Damping occurs at connections where there is friction, and with non-structural components such as partitions, furniture, and fit-out. As the damping persists, the vibration slows and stops. The amount of damping determines the duration of the vibration.
If you have a lot of heavy furniture and file cabinets in the area, the vibration from walking and other activity will lessen. However, today the trend is toward open offices and lighter furniture. Technology has gotten rid of some paper, so file cabinets are scarce. It all means vibrations are not damped much by the interior decoration.
When a continuous force is applied to a system with the same frequency as that of the system, each successive load cycle will add to the response, causing the amplitude to increase.
This is fancy talk for, “If the floor frequency is 2.2 Hz and the frequency of a vibrating machine resting on the floor is also 2.2 Hz, the shaking will get worse.”
If resonance develops unchecked, the amplitude of the vibration can grow to exceed the level of response from a single load cycle (like a foot-step or dropped load). When the resonance becomes severe enough, structural damage can occur.
Most buildings have sufficient damping to keep this from occurring, keeping everything in a so-called steady state. However, the steady state may still exceed design if it hasn’t been factored in. Resonance can be avoided in most floors by designing the floor to a natural frequency of 3Hz.
Vibration is treated as a serviceability issue related to the comfort of the occupants or damage to equipment. The factors impacting human perception and tolerance include:
- Type of activity
- Time of day activity occurs
- Type of environment
- Direction, amplitude, frequency, and source of vibration
- Level of damping
- Duration of exposure
The lowest perceptible vibration depends on the direction of the vibration relative to the direction of the human spine. The more perpendicular to the spine the vibration is, the more perceptible it is.
Vibration levels can be assessed manually using the critical Root Mean Square acceleration and response factor along with other factors found in SCI publication P354. Manual assessment tends to overestimate the vibration response, adding cost to design.
Now, there is a web-based tool that allows designers to make immediate assessments of dynamic response of a floor system. You can find it at http://www.steelconstruction.info/. Look for the Floor Response Calculator.
Reducing the chance of floor vibration in a steel building is best done during the design phase before the floor is ever placed. Increasing the mass or stiffness can be designed into the floor solution in anticipation of higher than normal vibrational activity. Corridor length should also be considered. Short corridors have more damping action.
It is expensive and difficult to modify an existing floor to reduce vibration problems. It would require major changes in floor mass, stiffness, and damping.
Steel buildings are exposed to vibration all the time from numerous sources. Reducing vibration is a serviceability issue for the comfort of the people in the building and the security of property.
Steel buildings are eminently suited to designs that include open-spans with floors that are cost-effectively designed to mitigate vibration.