1、56 2008 ASHRAE ABSTRACTThe experimental study presented in this paper is aimedat evaluating the seismic performance of integrated isolation/restraint (I/R) systems for heavy HVAC equipment. Earthquakesimulator experiments were conducted on a heavy centrifugalchiller supported by four I/R systems. Th
2、e peak response quan-tities of the equipment and the peak dynamic forces inducedinto the I/R systems were considered as the key components forthe seismic-performance evaluation of the I/R systems. The testplan included seismic and system-identification tests andincorporated different input motion am
3、plitudes and differentconfigurations of the restraint components of the I/R systems.The test results showed that limiting the displacement of theequipment by the impacts occurring in the restraint compo-nents of the I/R systems resulted in amplification of the accel-eration response of the equipment
4、 and excessive dynamicforces induced into the I/R systems. Throughout the experi-ments, the I/R systems experienced dynamic forces muchhigher than their static design capacity. The configurationvariables of the restraint component, particularly the gap size,were proven influential on the seismic per
5、formance of the I/Rsystems. Among different configurations of the restraintcomponent considered in this study, the configuration with thesmallest gap size resulted in the best overall seismic perfor-mance. INTRODUCTIONHeating, Ventilation, and Air-Conditioning (HVAC)equipment located on building roo
6、fs and inside penthouses areoften vibration isolated. Flexible isolator supports such as coilsprings prevent the noise, and vibration produced by theequipment to be transmitted into the building structure or intosensitive equipment installed in the building. While vibrationisolators are perfectly ca
7、pable of reducing the annoying oper-ation-induced vibrations, their performance in severe seismicevents is seriously questioned.Due to the flexibility of isolators, the natural frequenciesof the mounted equipment decrease. If the lowered naturalfrequencies of the mounted equipment match the response
8、frequencies of the building, quasi-resonance may happenduring an earthquake, causing the equipment to experiencedisplacements much larger than the isolator capacity. Conse-quently, the isolator may fail and the accelerated equipmentwithout supports will act as a massive free projectile. In addi-tion
9、 to the severe damage to the equipment, the penthouseenclosing the equipment and the floor system might beaffected after the equipment is shaken off its vibration isolatorsupports. Furthermore, the excessive relative displacement ofthe equipment results in breakage of the electrical wiring,ducts, an
10、d pipes connected to it. The flood resulting from abroken water line can render a structurally intact facility inop-erative after an earthquake (Ayres and Phillips 1998).Following the dramatic damage to vibration-isolatedequipment during the 1971 San Fernando earthquake, twotypes of seismic restrain
11、ts were introduced to protect vibra-tion-isolated equipment: lockout devices and snubbers.However, lockout devices, which functioned like seat belts,were soon proven impractical, unpredictable, and costly.Therefore, installation of snubbers became the predominantmethod of seismic protection of vibra
12、tion-isolated equipment. Snubbers (seismic stops) are essentially bumpers installedwith a practical clearance or air gap from the equipment to limitdisplacements of the equipment. The air gap is intended to keepExperimental Seismic-Performance Evaluation of Integrated Isolation/Restraint Systems for
13、 Heavy HVAC EquipmentSaeed Fathali Andr Filiatrault, PhDSaeed Fathali is a Ph.D. candidate and research assistant in the Department of Civil, Structural, and Environmental Engineering, Universityat Buffalo, The State University of New York, Buffalo, New York. Andr Filiatrault is a professor in the D
14、epartment of Civil, Structural, andEnvironmental Engineering, University at Buffalo, The State University of New York, Buffalo, New York.NY-08-0102008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part
15、 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 57the snubbers out of contact and prevent short-circuiting of thevibration-isolators during normal operation
16、 of the equipment.When the moving equipment hits the snubber, impact occursand the equipment bounces back to move within the acceptedrange of displacement. The snubber surface is often coveredwith a resilient material such as neoprene or natural rubber. Itshould be noted that the resilient surface i
17、s not meant to, anddoes not increase the energy-dissipation capability of snubbers.The resilient surface is used mainly to prevent the potentialdestructive strike between two hard surfaces. Essentially, snub-bers control the displacement response of the isolated equip-ment by changing the stiffness
18、of the support rather thandissipating energy. Snubbers and isolator supports can beinstalled separately or can be integrated into isolation/restraint(I/R) systems. Usually, at least four integrated I/R systems areused to support a piece of equipment.Observations following earthquakes in the past thr
19、eedecades and particularly following the 1994 Northridge earth-quake, indicated that vibration-isolated equipment protectedby snubbers fared far better than unrestrained vibration-isolated equipment. However, the overall performance ofrubber snubbers was inconsistent at best. In many cases, thesnubb
20、ers of the equipment mounted on the roof or upper levelof buildings were broken or their anchor bolts were shaken off(Reitherman and Sabol 1995).The repeated damage pattern to vibration-isolated equip-ment in recent earthquakes and the need for a more realisticassessment of equipment response amplif
21、ication and dynamicforces induced into the restraints have been highlighted byresearchers (Filiatrault et al. 2002; Gates and McGavin 1998).However, the basic research work in this area has remainedsparse, and the available codes and guidelines are mainlybased on experiences, engineering judgment, a
22、nd intuitionrather than on systematic experimental and analytical results.Restraints are traditionally designed for a conservative equiv-alent static load (Meisel 2001; Tauby et al. 1999). Based on theanalytical study conducted by Iwan (1978) it is believed thatthe maximum amplification of the accel
23、eration response forequipment mounted on isolation/restraint systems is betweenthree and four (Lama 1998).The actual dynamic forces induced into the restraints andamplification of the equipment acceleration response result-ing from severe impacts between the equipment and the snub-ber are not clearl
24、y addressed yet. Furthermore, there is noguideline for selecting properties of the snubber resilientsurface (commonly rubber). The only available requirement isthe minimum thickness of the resilient surface (ASHRAE2003). Effects of the snubber properties (thickness and hard-ness of the resilient sur
25、face and the gap size) on intensity of theimpacts between the equipment and the snubber, on the result-ing dynamic forces and on the amplified equipment responsesare still unknown.The experimental research presented in this paper isaimed at evaluating the seismic performance of an integratedI/R syst
26、em typical of commercially available systems forseismic application. The I/R system consisted of coil springsand rubber snubbers that restrained displacements of themounted equipment within an adjustable gap size in the hori-zontal and vertical direction. The heavy HVAC equipmentused as test specime
27、n in this study was a centrifugal liquidchiller. Using a six-degree-of-freedom earthquake simulator,several series of system-identification and seismic tests wereconducted on the test specimen supported by four I/R systems.The test plan incorporated different input motion amplitudesand different con
28、figurations of the restraint components of theI/R systems. Modal properties of the isolated test specimenwere established based on its response under pulse-typesystem-identification tests. The peak acceleration response atthe center of mass of the test specimen, the peak relativedisplacement respons
29、e measured at four points on the testspecimen, and the peak dynamic forces induced into the I/Rsystems were the three key components for the seismic-perfor-mance evaluation of the I/R systems. The seismic tests resultswere analyzed to determine variations of the three responsequantities with the inp
30、ut motion amplitude, to compare thepeak dynamic forces induced into the I/R systems with theirstatic design capacity, and to investigate the sensitivity of theseismic performance of the I/R systems to the change in theconfiguration variables of their restraint components.TEST SPECIMENThe equipment u
31、sed as a test specimen in this study wasa heavy centrifugal liquid chiller. Chillers are one of theimportant HVAC equipment in buildings, which are oftenmounted on vibration isolators. Centrifugal chillers areutilized for cooling of large buildings with centralized air-condoning systems. They use a
32、compression, expansion,evaporation, and condensing cycle to transfer heat from onewater loop to another water loop. The full system typicallyemploys chillers, pumps, cooling towers, and air handlers totransfer heat from the building air to the outside air. As shownin Figure 1, the test specimen inco
33、rporated seven majorcomponents: an evaporator, a condenser, a compressor, amotor, a control center (CC), a variable speed drive (VSD),and an oil sump (OS).The test specimen overall dimensions were 4.88 m 2.11 m (192 in. 83 in.) in plan, and 2.87 m (113 in.) in height.The test specimen was mounted on
34、 the I/R systems under itsheaviest condition (i.e. filled with water and refrigerant liquid)with 11997 kg (26450 lb) mass. The coordinates of the centerof mass of the test specimen with respect to the coordinatesystem defined in Figure 1 were 2.35 m (92.7 in.), 1.02 m(40.2 in.), and 0.97 m (38.2 in)
35、 in the x, y, and z direction,respectively. The longitudinal, transverse, and vertical direc-tions of the test specimen were associated with the x, y, and zdirection, respectively. The eccentricities between the centerof mass and the geometric center of the four corner supportsof the test specimen w
36、ere 33 mm (1.3 in.), 84 mm (3.3 in.), and970 mm (38.2 in.) in the transverse, longitudinal, and verticaldirections, respectively.58 ASHRAE TransactionsINTEGRATED ISOLATION/RESTRAINT SYSTEMThe integrated I/R system used for this experimentalstudy is typical of commercially available systems for seism
37、icapplication. The isolation and restraint components of thestudied I/R system are oriented orthogonally with respect toeach other. The two components are integrated into an I/Rsystem unit by bolting the top and bottom plate of the restraintcomponent to the top and bottom plate of the isolation comp
38、o-nent. The assembled I/R system is about 281 kg (620 lb),292 mm (11.5 in.) tall, and 660 mm 660 mm (26 in. 26 in.)in plan. Four I/R systems were installed under the four cornersof the test specimen. Figure 2 shows one of the four integratedI/R systems supporting the test specimen.Isolation Componen
39、tConfiguration. The isolation component of the I/Rsystem considered in this study consists of two sets of nestedcoil springs embedded between two parallel rectangular steelplates. Figure 3 shows the isolation component of one of thefour I/R systems supporting the test specimen. Nested coilsprings wi
40、th different geometry and stiffness were used toprovide the vertical stiffness required for supporting the testspecimen. The 343 mm (13.5 in.) wide horizontal clearancebetween the two sets of coil springs is provided for installationof the restraint component of the I/R system. The verticaldistance
41、left between the top and bottom plate after mountingthe equipment (when the springs are compressed) is importantbecause the restraint component should fit and function prop-erly between the two plates. The required distance between thetop and bottom plate is adjusted by the two leveling bolts thatpa
42、ss through the load plates on top of the springs. The nutwelded at the end of the leveling bolt provides the propercontact area with the top plate.Design. Coil springs are commonly used as isolationcomponent of I/R systems. They are designed and constructedfor a required axial stiffness or a target
43、vertical deflection.Typically, the required axial stiffness or target vertical deflec-tion is selected based on the mass and the operation-inducedforces of the equipment without any seismic considerations.After selecting the coil spring for a required axial stiffness orFigure 1 Components of centrif
44、ugal liquid chiller used as test specimen.Figure 2 Assembled isolation/restraint system installedunder test specimen (A: isolation component, B:restraint component).Figure 3 Isolation component of isolation/restraint system.ASHRAE Transactions 59a target deflection, the lateral (horizontal) stiffnes
45、s of thespring can be calculated. The lateral stiffness of a coil springis a function of several parameters including its axial stiffness,geometry, uncompressed and compressed length, and endconditions (Yao and Lien 1998). For this study, two sets ofnested coil springs were selected for a 51 mm (2 i
46、n.) targetvertical deflection after mounting the test specimen. The isola-tion component of the I/R systems was unchanged throughoutthe experiments.Restraint ComponentConfiguration. Figure 4 shows the restraint componentof the I/R system. The restraint component consists of a topand a bottom thick r
47、ectangular steel plate. A piece of steel pipeis welded to the center of each plate. When the pipes arealigned coaxially, their different diameters allow the top pipego through the bottom pipe with a 44 mm (1.75 in.) thick cylin-drical air gap. Part of the cylindrical air gap is filled by a tubu-lar
48、rubber pad fitted inside the bottom pipe. Two threaded rodsare welded to the sides of the top plate. Each rod has a coupleof nuts. Two pieces of thick steel angle are welded to the sidesof the bottom plate. A thick plate with a hole in its center iswelded on top of each of the two angles. One rubber
49、 grommetis fitted into the hole of each plate. When the top and bottompipes are aligned coaxially, the rods of the top plate of therestraint component pass through the center of the rubbergrommets. On each side of the rubber grommet, a thick steelwasher interfaces the nut and grommet surface.Restraining Mechanism. In the horizontal direction, thetop and bottom parts of the restraint component can movefreely relative to each other within the cylindrical air gap leftbetween the top pipe and the tubular rubber pad inside thebottom pipe. In other words, the restraining mechanism in the
