Well-designed buildings minimize vibration

ERIC E. UNGAR and JEFFREY A. ZAPFE

High-precision optical equipment, such as that used for research in microbiology, nanotechnology, precision metrology, or the production of advanced integrated circuits, requires environments with minimal vibration (see Fig. 1). Optical systems sensitive to vibration are typically mounted on isolation tables or cradles, or even on isolated "islands" that are supported on air springs. Such isolation systems provide limited vibration protection, particularly at low frequencies, even if these systems include sophisticated and costly active controls.

Good vibration isolation is typically obtained by the use of soft, resilient elements such as air springs, isolation tables, and the like. These methods tend to move excessively as items mounted on them are being adjusted, often prompting equipment users to disable the isolation systems during adjustments. The users then may need to wait for considerable time while the isolation systems return to equilibrium. It is therefore advantageous to house sensitive equipment in buildings that are designed a priori to provide environments with minimal vibration.¹

Sources of vibration

The major sources of vibration in buildings consist of external environments, internal activities, and internal machinery. External sources include ambient vibrations or "micro-tremors," road and rail traffic, and nearby construction and machinery. Even overflying aircraft can induce potentially disruptive vibrations. Internal activities of concern include personnel moving about, service and repair work, the use of internal vehicles such as carts and forklifts, and production work. In-building machinery includes mechanical and electrical equipment, such as that associated with heating, ventilating, and air-conditioning, as well as elevators. Some of this machinery produces vibrations that are transmitted to the building directly, as well as fluctuating pressures that can induce vibrations in building components. Vibrations can also be generated by experimental and process equipment.

The most severe vibrations associated with road traffic typically result from heavy vehicles moving rapidly along roads with irregular surfaces. Therefore, heavy vehicles should be kept away from sensitive facilities, speeds near such facilities should be controlled, and street surfaces should be kept smooth. Speed bumps or expansion joints should not be allowed near a facility, and potholes and misaligned slabs should be eliminated. Furthermore, loading docks should be constructed with soft bumpers to curtail hard impacts of trucks against the building.

There are no practical means for attenuating low-frequency vibrations that propagate along the ground. Trenches, sheet pilings, slurry walls or other underground structures of feasible sizes, and geotechnical soil modification (for example, grout injection) have no significant effect on these vibrations. The same is true of such aboveground structures as berms and heavy walls. The reason is that the wavelengths associated with the most disturbing vibrations that propagate in the ground are long—of the order of 100 ft or more—and discontinuities that extend over only a fraction of a wavelength have little effect on propagating waves.

External vibrations

Appropriate design of the foundations of a building or of the sensitive parts of a building can reduce intruding vibrations. For example, when vibrations of bedrock are considerably less severe than those of the soil near the ground surface, it is helpful to support the building on columns that extend to bedrock and that are provided with sleeves to separate them from vibrations in the soil. On the other hand, mat foundations or spread footings are preferable in areas where the surface soil vibrations are less severe than those of the lower strata.

In some cases, it is possible to "tune" the column footings. That is, the footing areas can be selected so that the natural frequency associated with the flexibility of the soil under a footing and with the mass carried by that footing is relatively low, resulting in attenuation of intruding vibrations above that frequency. In areas where footing design cannot provide sufficient attenuation, selected parts of a building can be supported on resilient elements. For example, the bases of columns can be supported on neoprene bridge-bearing pads, on air mounts (pneumatic springs), or on steel spring assemblies. (The use of neoprene pads at column bases resulted in successful operation of a microelectronics facility constructed a decade ago directly above a subway line in Cambridge, MA.) Of course, care must be taken to ensure the structural stability of such arrangements.

Massive concrete slabs, placed on the soil near spread footings, can be used to attenuate vibration in limited frequency ranges. The masses of the slabs, supported on the resilient soil, in effect act like a classical spring-mass "tuned absorber" to attenuate the vibrations that act on a footing at the absorber's natural frequency.

Vibration from footfalls and internal vehicles

Footfall-induced vibration usually can be kept within acceptable limits by the use of relatively stiff structures, generally with comparatively short spans.² It is often advantageous to use structural separatings to isolate areas that house sensitive equipment from those that carry considerable foot or cart traffic. Such a separation can consist of a structural joint, which can include resilient seals, but which should not be bridged by beams or the like. It may be appropriate to provide "walk-on" floors that are supported only at columns without making contact with the floor structure that supports the sensitive equipment.

Vibration from in-building vehicles can be further limited by avoiding vertical discontinuities, such as speed bumps, nonaligned joints, and rapid changes in floor slopes. Where vehicles need to traverse a joint, a joint of the interlacing-fingers type should be used in order to smooth the transfer of a vehicle's weight across the joint. It is also helpful to skew such joints, so that only one wheel of the vehicle crosses the joint a time. It is advantageous, of course, to use soft pneumatic tires on all vehicles used in a building.

Vibration from machinery

The good news is that most machinery vibration problems can be addressed successfully by providing the machinery with suitable vibration-isolation systems, a wide variety of which are commercially available. The somewhat bad news is that efficient performance of these isolation systems requires that they act on relatively stiff structural supports. Therefore, machinery that generates considerable vibration is often placed in a building's basement, on a heavy soil-supported slab. This, however, can lead to conflicts with the requirements of sensitive equipment located in the same basement. Unfortunately, joints in the basement slab do little to limit the transmission of vibrations, which tend to propagate to a considerable extent around the joint via the soil below. It is important to consider that machinery isolation generally involves not only mounting the machinery on resilient isolators, but also using isolation means to ensure that the propagation of vibration along piping, ducts, and conduits is kept to a minimum.

Where sensitive equipment is located in a basement, it makes sense to confine vibrating machinery to a rooftop penthouse or to an upper floor. If machinery is located on the roof or on an above-grade floor, it should be placed near columns, where the floor structures are relatively stiff, or it should be supported atop dunnage (auxiliary structural framing) that is connected only to columns.

Because airborne noise and air pulses can induce significant vibrations in above-grade floors, it is important to limit the noise from air-handling equipment that reaches a sensitive area. This typically requires that the air velocities in ducts be kept relatively low, that the airflows be kept smooth, and that suitable mufflers be provided. Sudden pressure changes, such as those that can result from the opening of doors between pressurized and unpressurized areas, should be limited, for example, by use of vestibules with air-tight doors.

Early planning for vibration control and good design from the beginning of a project are important. Ideally, a collaborative effort is required that includes the facility owner, the users, the project architect, the geotechnical, structural and mechanical engineers, as well as vibration and noise-control specialists.

REFERENCES

1. E. E. Ungar et al., Sound and Vibration (July 1990).
2. T. M. Murray et al., Steel Design Guide 11, American Institute of Steel Construction, Chicago, IL (1997).
3. E. E. Ungar and C. G. Gordon, Shock and Vibration Bulletin 53, Naval Research Center, Washington D.C.

Eric E. Ungar is chief engineering scientist and Jeffrey A. Zapfe is a senior consultant at Acentech, 33 Moulton St., Cambridge, MA 02138; e-mail: [email protected] and [email protected].