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Higher education facilities and medical laboratories today need to draw in world-class researchers. Cutting-edge laboratory equipment, functionality, adaptability, and aesthetic surroundings all play an important role.
Laboratory equipment has become progressively more sensitive to vibrations as magnification levels have increased, though the equipment type, sensitivity, and quantity vary with the use, location, and size of the laboratory. As a result, the structural design of lab spaces requires special attention.
For example, bench microscopes with a 100x magnification require a peak floor system vibrational velocity achieved with only minor structural adjustments when compared with a typical structural floor system. By contrast, microscopes with 400x magnification require a significant increase in structural stiffness that dictates a structural system depth increase.
Excessive floor vibration is, at a minimum, problematic for sensitive equipment and can even render the equipment useless. Sensitive equipment sets a limit on a floor system’s maximum velocity response with lower velocities requiring a stiffer and more costly floor system.
Proper consideration of structural floor vibration resulting from footfall is also critical to the design of laboratory facilities. Mechanical systems and some electrical equipment may cause vibration in a floor system but are normally isolated from vibration-sensitive areas and create vibration at higher frequency. Therefore, mechanical systems with higher frequencies generally do not pose the same problems created by footfall-induced floor vibration.
Factors to consider
To adequately deal with floor vibrations, the design team must accurately address three crucial factors during the schematic design phase: sensitive equipment and its vibration criteria, sensitive equipment location, and structural system type and framing layout.
Sensitive equipment and vibration criteria — Equipment vibration criteria are not limited to velocity. Equipment suppliers can use displacement, acceleration, or velocity for vibration criteria. Fortunately, all responses can be calculated using structural dynamics by knowing the structural properties and the footfall forcing function. The assumed forcing function is the largest unknown in the analysis and causes the results to vary over a wide range.
The easiest and least expensive way to provide very low velocity response is to place the equipment on a concrete slab on grade that is isolated from the surrounding slab and bears directly on bedrock.
Sensitive equipment location — Knowing where sensitive equipment will be placed is critical to both equipment performance and construction cost. An economical method to achieve reduced vibration velocity is to place sensitive equipment near a column or centered on a girder.
Unfortunately, typical conditions often require the equipment to be centered within a floor bay, which is also the location of the highest vibration velocity.
When dealing with multi-story laboratories, such as the 12-story Southwestern Medical School Phase V currently under construction, the lower floors have better vertical and lateral vibration characteristics when compared with higher floors. As the column length between the specified floor and the foundation grows, the floor system’s frequency decreases while its peak velocity increases. Both the reduction in frequency and the increase in peak velocity have negative impacts on vibration-sensitive areas.
In addition, the upper floors undergo larger lateral displacement and higher lateral velocity from wind loading. Lateral vibration, like the more commonly addressed vertical vibration, is problematic for sensitive equipment. To minimize lateral vibration, equipment should be located on the lower floors and the building’s lateral structural system should be planned and designed to maximize stiffness and minimize lateral deflection.
However, the future location of sensitive equipment is not always known during the design phase of a laboratory project. This condition dictates that laboratory areas be uniformly designed to stringent vibration limits.
Structural system type and framing layout — System planning is critical to accommodate the eccentricities of each project.
The University of Texas Southwestern Medical Center also incorporated a super-wide pan structural system with a 10-foot module to match the laboratory module. The beams are offset from the grids to facilitate MEP penetrations within the lab walls. This met the strict vibration criteria required for the research activities in the labs. It also provided a modular, predictable layout for economy and future adaptability.
In another case, the design of the Natural Science and Engineering Research Building of the University of Texas at Dallas encourages collaboration with a flexible laboratory space. Designers chose a dual structural system for this facility. Classrooms and administration spaces are on a one-way concrete slab with post-tensioned beams. A two-way concrete flat slab was used at the research labs. This maximized the economy for the different bay modules. Using concrete flat plate or waffle slab for larger spans that span in two directions is an extremely efficient method of meeting strict vibration criteria.
The selected structural system plays an obvious yet pivotal role in meeting any vibration criteria. Laboratory buildings are most commonly constructed with either steel or concrete framing. Sensitive equipment vibration limits often require the floor system depth to increase. In the case of steel construction, the increased depth adds to the total steel tonnage, as deeper steel shapes are also heavier.
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Increasing the depth of concrete framing is relatively simple and the increased structural depth can reduce the required reinforcement tonnage. A concrete structural system is typically more economical when meeting a given vibration criteria. In addition, concrete systems have greater internal damping when compared with steel systems. Damping is extremely beneficial for reducing a floor system’s vibration response — it not only reduces the vibration velocity but also the length of time the floor system vibrates following a footfall.
The structural column layout for either steel or concrete systems can assist in meeting a specified vibration criterion. Smaller column spacing in areas with vibration-sensitive equipment is beneficial if the architecture can accommodate it.
Designing a steel or concrete building for only code-prescribed gravity loads is not adequate for laboratories. Laboratory floor systems must have vibration characteristics that allow sensitive equipment to function without loss of resolution.
Vibration analysis
Once these parameters are set, a vibration analysis must be conducted. An iterative process, vibration analysis allows for an adjustment of slab, beam, and girder sizes. Numerous analysis procedures have been developed to determine an acceptable floor response to footfall. These procedures range from relatively simple minimum floor stiffness or minimum floor frequency to more complex time and history analyses assuming numerous walking speeds and walking paths.
Occasionally, highly sensitive equipment requires such a low vibration level that the structural system cannot be reasonably designed to obtain the specified criteria. Similarly, new laboratory equipment can be added to an existing floor system where the original design did not address vibration criteria. The likely solution to either situation is the installation of isolation tables under the sensitive equipment. Isolation tables are produced by several different manufacturers, including Kinetic Systems, Vistek Inc., and Newport Corporation, and table installation requires a site-specific design. Isolation tables greatly reduce the vibration level at the equipment’s base when compared with the floor vibration level immediately below the table. The effectiveness of an isolation table decreases as the floor structures frequency decreases. Structural systems designed for sensitive equipment have higher frequencies than structural systems not designed for sensitive equipment. Thus an isolation table’s effectiveness is enhanced when installed on a properly designed laboratory floor compared with one that was not.
William Stephen Price, P.E., is executive vice president and director of laboratory design at Datum Engineers, Inc., in Dallas. He has served as project manager for a variety of medical education research towers and laboratory facilities. He can be reached at stephen@datumengineers.com.
