Building vibration can impact user experience, and is largely preventable during design, or potentially mitigated later, with help from a vibration consultant.
Building Vibration – When should I hire a consultant?
We often receive questions from people wanting to know whether they need to address building vibration for their specific application. Building vibration is a broad topic with too many potential causes and remedies to be covered in a blog post. However, here are some guidelines that can be used to determine whether hiring a vibration consultant is warranted.
When Should I Hire a Building Vibration Expert?
Generally, hiring a vibration consultant is a good idea under the following circumstances:
- There is vibration-sensitive equipment operating in the building or there are vibration-sensitive activities that occur in the building. Sensitive equipment may include medical imaging equipment such as MRIs, precision manufacturing or measurement equipment, photo-lithography equipment, high-powered microscopes, or other highly sensitive optical equipment. Sensitive activities may include medical procedures, audio/video recordings or performances, or precision testing or manufacturing activities.
- The building is located close to a significant source of vibration. These sources may include freight rail lines, commuter rail lines, subways, elevated trains, roadway traffic, heavy industrial activities, or construction/demolition.
- The building includes equipment or facilities that generate significant vibration levels. Problematic vibration sources within a building could include large generators, large air handling equipment or other mechanical equipment, weight training facilities, pools on upper stories of a building, manufacturing equipment such as presses and shakers, movement of heavy pallets or carts on upper stories of a building, or helipads.
If the project does not fall into one of the categories above, there is a good chance that following standard design guidelines will be sufficient to mitigate vibration within the building.
Common Building Vibration: Footfall and Mechanical
To mitigate footfall vibration issues, the structural deflection of the building elements should be designed per the applicable criteria (L/360, for example) based on code requirements and other guideline documents, such as the AISC Design Guides. Designing the floor system to meet deflection criteria for uniform (distributed) loading, such as the L/360 criterion previously mentioned, is typically adequate to mitigate footfall vibration issues for typical concrete construction. However, to minimize occupant complaints in lightweight wood construction, additional deflection criteria may need to be used for point loading, which may require the use of thicker subfloor materials than would be required to meet deflection criteria for uniform loading. Mechanical equipment should also be vibration isolated per the guidelines published in the ASHRAE Handbook – HVAC Applications.
I’ve Decided to Hire a Vibration Consultant. What is the Process?
Typically, the steps involved to address building vibration issues are to determine the allowable vibration limits for the specific application, quantify the vibration levels in the building (without mitigation), calculate the amount of vibration reduction that is needed, and determine the appropriate mitigation methods to achieve the required reduction. Each of these steps is described in more detail below.
Determine allowable building vibration limits
The allowable vibration limits can vary significantly depending on the specific application. For vibration-sensitive equipment, the equipment manufacturer can often provide allowable vibration criteria. For more general applications, a set of generic facility vibration criteria have been adopted in ANSI Standard S2.71 and ISO Standard 2631-2. These criteria are also described in the Noise and Vibration Control chapter of the ASHRAE Handbook – HVAC Applications. The generic facility vibration criteria address continuous vibration sources and are based on 1/3-octave band rms velocity, primarily in the frequency range of 8-80 Hz.
Construction or blasting
For vibration from construction activities or blasting for mining activities, vibration amplitudes may be high enough to cause damage to nearby structures, especially historic buildings. For these activities, allowable vibration limits are often expressed in terms of peak particle velocity (PPV). This metric is useful in determining the damage potential for vibration levels, as it relates to stresses that are imparted to a building from a vibration source.
Quantify the vibration levels in the building
For buildings that have already been built, quantifying building vibration levels typically involves conducting measurements in the building while the vibration source is active. For multi-story buildings, it is important to collect data from sensors mounted both near support columns and at the mid-span of the floor on above-grade floors of the building. These measurements characterize the range of vibration amplitudes that can occur at different locations in the building. The response of an above-grade floor can be visualized to respond like a trampoline, which deflects more at its center than it does at its perimeter. Similarly, the maximum vibration levels of an above-grade floor are typically found at the mid-span of the floor, where the floor is most flexible and the resonant response of the floor can act as a natural amplifier of vibration. Measurements near the building columns typically indicate the lower limit of vibration levels that can be achieved by stiffening the floor system.
For buildings that have not yet been constructed, vibration measurements are often taken at the proposed building site to characterize the ground vibration levels due to existing vibration sources, such as nearby rail lines or roadways. These measurement results can be used to predict the vibration levels inside the building by applying correction factors that account for how much vibration will be transmitted from the ground to the building foundation based on the type of foundation planned. The predictes are further refined by applying additional correction factors to account for the way vibration will travel through the building and the resonant response of above-grade floors.
If the project includes a light rail line that has not yet been constructed, measurements can be conducted to predict the vibration levels at different distances from the proposed light rail line. This is accomplished by generating impacts at multiple locations along the proposed railway line, measuring the response at different distances from the line, and using the measured data to characterize the ground vibration propagation characteristics at the site. The ground vibration propagation model is used in conjunction with information about the building response and the energy imparted to the ground from the light rail vehicles to predict the vibration levels within a building. These measurement and analysis techniques are laid out in the FTA Transit Noise and Vibration Impact Assessment Manual. Similar measurements can be conducted to characterize the ground vibration propagation characteristics near the building site to predict vibration levels from construction activities.
Calculating the vibration reduction needed and determining appropriate mitigation
Once the allowable and existing/predicted vibration levels are known, the amount of vibration reduction needed at different frequencies can be quantified. With this information, a specific mitigation strategy can be designed. Whenever possible, it is best to reduce vibration levels at the source. For vibration sources inside a building, this often means installing appropriate vibration isolation components on the equipment that is generating the vibration. However, in some cases, it may not be feasible or even possible to mitigate vibration at the source. For instance, there may be an external vibration source outside the control of the building owner, such as an existing railway or roadway near the site. In these cases, the mitigation will need to be in the building, itself. This could involve something as simple as moving the sensitive equipment farther away from the vibration source or installing vibration-sensitive equipment on an isolation table. On the other end of the spectrum, whole-building vibration isolation systems can be installed to isolate an entire building from a nearby vibration source.
Hire a Building Vibration Consultant Early
If your application requires it, we urge you to engage a vibration consultant, like ABD Engineering & Design, as early as possible in your project. It is typically much easier and more cost-effective to mitigate vibration in the design phase of a building rather than after the building has been built.