EVALUATION OF PERFORMANCE OF MULTI-SUSPENDED PENDULUM ISOLATION (MPI) SYSTEM WITH SPATIAL TRUSS STRUCTURE

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1 EVALUATION OF PERFORMANCE OF MULTI-SUSPENDED PENDULUM ISOLATION (MPI) SYSTEM WITH SPATIAL TRUSS STRUCTURE Ryosuke NARITA 1, Tej KOJIMA 2, Haruo KURAMOCHI 3, Takash TORIYA 4, Naom KITAYAMA 5, Mcho KURAMOCHI 6 And Tsuyosh OHTSUKA 7 SUMMARY Ths paper s concerned wth a new sesmc solaton devce, called Mult-suspended Pendulum Isolaton (MPI) system wth spatal truss structure, newly developed by author. MPI system s accordng to the prncpal of sngle pendulum, and s consstng of man pendulum, several secondary pendulums and base flame structure. Man pendulum to support superstructure, s multsuspended by several secondary pendulums and last secondary pendulum s suspended by base flame structure mounted on ground, therefore MPI system provdes horzontal flexble nterface connecton between superstructure and ground. Snce MPI system s based on the prncpal of sngle pendulum, predomnant natural perod of MPI system s accordng to total length ( l ) of respectve suspenson and ndependent from weght of superstructure, gven by followng equtaton; T = 2π l g In MPI system, all pendulums are suspended vertcally wth respectve suspenson members and ths gves all pendulums to oscllate parallel wth ground, therefore the force nduced by weght of superstructure s vertcal one to all pendulums, as well as to ground. In addton to ths, spatal truss structure s appled to all secondary pendulums, force transmtted to those pendulums are only axal force, by means of compresson or tensonal force. As a result of expermental oscllaton tests, acceleraton recorded at man pendulum s reduced 1/27 for El Centro earthquake ground moton and 1/6 for Hyougoken-nanbu (Kobe) earthquake ground moton. In addton to these, MPI system has certan decay of oscllaton due to multsuspenson and shape of pendulums, havng.22 of logarthmc decrement (.35 of dampng rato) recorded by trple suspenson expermental model. INTRODUCTION Snce Hyougo-nanbu (Kobe) earthquake n 1995, many dscussons have been held regardng revew of buldng structural analyss and current Buldng Code contents, and durablty of buldngs aganst earthquake has become most concerned subject n publc. Durng Kobe earthquake, two buldngs wth sesmc solaton system were recorded no damages on nether structures nor buldng nternal equpment, and after Kobe earthquake more than four hundreds buldngs wth sesmc solaton system have been constructed or under constructon n Japan, comparng wth about forty buldngs before ths earthquake. Therefore sesmc solaton system has been consdered one of most deal method to have buldng durablty especally for low-rse buldngs, because strengthenng strategy nevtably brngs larger masses and response acceleraton of stff structured buldngs s much greater comparng wth one of buldngs wth sesmc solaton system. There are several sesmc solaton methodologes beng under developng, however these are classfed nto two categores; natural perod controlled type and base-solaton one. Rubber bearng methodology s one of natural NEC System Integraton & Constructon Co., ltd., Tokyo, Japan,e-mal:NARITA_ @ala.nesc.nec.co.jp NEC System Integraton & Constructon Co., ltd., , Hgashshnagawa, Shnagawa-ku, Tokyo, Japan DPS Brdge Works Co.,ltd, , Ktaootsuka, Toshma-ku, Tokyo, Japan. DPS Brdge Works Co.,ltd, , Ktaootsuka, Toshma-ku, Tokyo, Japan, e-mal:t_torya@dps.co.jp NAO Structural Engneerng Insttute, Ushyama-cho, Fussa cty, Tokyo, Japan Kogakun Unversty, , Nakano-cho, Hachouj cty, Tokyo, Japan Kogakun Unversty, , Nakano-cho, Hachouj cty, Tokyo, Japan

2 perod controlled and now a days, lead rubber bearng (LRB) s most commonly appled methodology n Japan for sesmc solaton system, however ths has dsadvantages stll reman to be solved; ts natural perods beng related wth weght of superstructure. Mult-suspended Pendulum Isolaton (MPI) System s new solaton system developed by author recently, and n ths system, solaton am can be establshed by supportng superstructure by man pendulum, of whch s multsuspended by several secondary pendulums. All pendulums are vertcally and respectvely suspended, and rested on the ground, therefore MPI system provdes horzontally flexble connecton between superstructure and ground. And the prncpal of pendulums s referred for ths MPI system so that the natural perod of MPI system and superstructure wll be predetermned and accordng to the total suspendng length of pendulums and ndependent from a weght of superstructures. In our study, we am to develop new base-solaton system of whch has predetermned natural perod ndependent from a weght of superstructure, and ths paper descrbe the prelmnary expermental results through oscllaton tests by snusodal wavemoton and several earthquake wavemoton. MULTI-SUSPENDED PENDULUM ISOLATION SYSTEM Mult-suspended Pendulum Isolaton system s consstng of man pendulum, secondary pendulums, supportng base structure and suspendng members. Superstructure s mounted on man pendulum (P-1) of whch s suspended by secondary pendulum (P-2), octahedron shape of spatal truss frame, and P-2 s suspended by next secondary pendulum (P-3). P-3 s same shape as P-2 and suspended by next secondary pendulum (P-4). Same skm repeats and last secondary pendulum (P-n) s suspended by supportng base structure mounted on ground, provdng all connecton between pendulum and suspendng members s pvot. For expermental oscllaton tests, steel made expermental model (trple-suspended pendulum) s manufactured and ts features are shown on Fgure 1 and 2. In general, t s understood that oscllaton system of suspended structure s accordng to smple pendulum and shows followng behavour,.e. 1) cycle oscllaton by gravty, 2) regardless ampltude, a natural perod to be (Unt:mm) Fgure 1: Photograph of expermental model used for expermental oscllaton tests constant when ampltude s small. Theory of MPI system s shown on Fgure 3, and total suspenson length of MPI system s gven by sum of respectve suspenson length. Ths mult-suspenson theory s maxmse total suspenson length wthn certan lmted heght. MPI system s conssts of several pendulums, as lumped masses and whose postons are ndependent of one another, therefore ths oscllaton system s consdered as mult-degree-of-freedom (MDOF) system and the oscllatory moton of MPI system s governed by followng equtaton; 2

3 5 Horzontal member Dagonal member 5 Superstructure Upper structure Base frame Connecton bolt Axal member Pvot hnge Unversal jont Horzontal member Dagonal member Base frame Suspenson member Base frame Base plate 5 Base plate 25 Foundaton (a) Plan (b) Elevaton Fgure 2: Expermental model (Steel made, trple-suspended pendulum) P1 : Man Pendulum P2 : 1st Sub-Pendulum P3 : 2nd Sub-Pendulum L1 P1 L1 L2 P1 P2 L1 L3 L2 P1 P2 P3 (a) Sngle suspenson (b) Double suspensons (c) Trple suspensons Fgure 3: The comparson of Mult-Suspended Pendulum Base Isolatons m1 m m mn x1 C1 x 1 K1 x1 m n x + C x + K x x m n x n C n x n K n xn x 1 n 1 M1 = M xg M n The term K ( x x 1) s a horzontal force to produce the dsplacement x, and ths s gven by followng equtaton; K ( x x 1) = m g tanθ (2) Where can be approxmated; θ s rotaton angle of th pendulum and g s gravty acceleraton, when small value of θ s gven, K 3

4 Xg L1 θ1 m1 L2 θ2 X1 m2 L3 θ3 X2 X3 Xn m3 Ln θn mn K m = g (3) l Of whch, m : Lumped mass of th pendulum M : Sum of lumped masses (1 st to th ) C : Dampng coeffcent of th pendulum x : Dsplacement of th pendulum x g : Ground dsplacement K : Sprng constant of th pendulum (Restorng stffness) : Suspended length of th pendulum l Accordng to equtaton, the oscllaton of MPI system has n th (n) order of natural crcular frequences ( ω ) and 1 st order of natural crcular frequences (, when small value of θ s gven. Accordngly 1 st order of natural perod ( T Fgure 4: MDOF oscllaton model = 2π l g ω ) can be approxmated, ω = g l T ) of MPI system s gven by followng approxmaton; In MPI system, all pendulums are suspended vertcally wth respectve suspenson members and ths gves all pendulums to oscllate parallel wth ground, therefore the force nduced by weght of superstructure s vertcal one to all pendulums, as well as to ground. In addton to ths, spatal truss structure s appled to all secondary pendulums, force transmtted to those pendulums are only axal force, by means of compresson or tensonal force. Accordng to above mentoned, the features of MPI system are summarzed herewth; 1) To have ts natural perod ndependent weght of superstructure 2) To predetermne ts natural perod by total length of suspenson 3) To restore the dsplacement by means of gravty 4) To have flexblty to horzontal dsplacement 5) To have nherent dampng EXPERIMENTAL TESTING METHOD AND RESULTS (4) Table 1: Dmenson of expermental models Prototype Model Expermental Model Number of suspenson Sngle Double Trple Trple Materal Alumnum Steel l Length of l suspenson (mm) l Total l m Weght of m Lumped Mass (kgf) m Total m Natural perod calculated 1.85 seconds 1.64 seconds 4

5 To evaluate and verfy fundamental performance of new MPI system, followng expermental oscllaton tests has been performed wth steel made expermental model and alumnum made prototype model, all dmensonal features are shown on Table 1; And for oscllaton test, shakng table engaged s one of sngle axal wth maxmum dsplacement of ±75.mm and manpulaton of oscllaton to be by means of ampltude. Resonance of mult-suspenson and MPI system: To verfy and compare resonance frequency of systems, followng wave-moton s appled to shakng table and response acceleraton of each system s recorded. Accordng to Fourer analyss of acceleraton spectra as shown on Fgure 5 and 6, predomnant resonance frequences are verfed and known, as lnear natural frequency of systems (recprocal s natural perod). Input wave-moton Snusodal (.2Hz to 5.Hz, contnuous) Ampltude 1. % (Maxmum dsplacement ±7.5mm) Weght appled For Prototype model.482 kgf For expermental model kgf 5 Fourer ampltude (gal sec Sngle Pendulum Double Pendulums Trple Pendulums Perod (sec) Fgure 5: Fourer Spectrums ( Prototype Model ) 12 Fourer ampltude (gal sec) Perod (sec) Fgure 6: Fourer Spectrum (Expermental Model) 5

6 Response acceleraton by actual earthquake moton: To verfy the effectveness of MPI system aganst earthquake, ground moton of followng earthquakes s appled to shakng table as nput wave-moton and tme hstorcal acceleraton s recorded and results as shown on Fgure-7, 8, 9 and 1. Input wave-moton Ampltude Weght appled El Centro 194 NS components Hyougokenn-nanbu (Kobe) 1995 NS components Hachnohe 1968 NS components Kushro 1961 NS components 2. % (Maxmum dsplacement ±15.mm) kgf Acceleraton(Gal) Tme(Second) Base Man Pendulum Fgure 7: Acceleraton response tme hstores of ElCentro 194 NS components 6 A cceleraton (Gal) Tme(Second) Fgure 8: Acceleraton response tme hstores of Hyogo-k en Nanbu(Kobe) NS components Acceleraton(Gal) Tme(Second) Fgure 9: Acceleraton response tme hstores of Hachnohe 1968 NS components 6

7 Acceleraton (g Tme (Second) Fgure 1: Acceleraton response tme hstores of Kusro 1961 NS components Dampng effect of mult-suspenson system: To verfy and compare dampng effect of mult-suspenson system, followng wave-moton s appled to shakng table for approx. 2 seconds and decay of oscllaton of each systems (sngle suspenson, double suspenson and trple suspenson) are recorded and results as shown on Fgure 11. Input wave-moton Snusodal (Frequency.6Hz ) Ampltude 1. % (Maxmum dsplacement ±7.5mm) Weght appled.482 kgf Acceleraton (gal) Sngle suspenson Double suspensons Trple suspensons Tme (Second) Fgure 11: Comparson of dampng effect EXPERIMENTAL TEST RESULTS AND FINDINGS Resonance of mult-suspenson and MPI system: Accordng to expermental results as shown n Fgure 5, common resonance frequency for three prototype models s observed.56hz (1.8 seconds as fundamental natural perod). Ths s almost same as one calculated accordng to total length of suspenson as shown on Table-1, and ths s consdered as 1 st order of natural perod of system. Besde above, there are another resonance are observed on double and trple suspenson,.4 and.45 seconds respectvely, and whch are consdered as 2 nd and /or 3 rd order of natural perod of system. Steel made expermental model has also resonance frequency of.61hz (1.64 seconds as natural perod) as almost same as one calculated accordng to the total length of suspenson. Response acceleraton by actual earthquake moton: Comparson of measured response acceleraton at man pendulum and nput acceleraton to expermental model s shown on Table2. Dependng upon type of earthquake, decrement of response acceleraton at man pendulum s varable, 1/6 for Hyougoken-nannbu (Kobe) pattern to 1/27 for El Centro pattern, however effectveness s sgnfcant for all type of earthquake. Table 2: Peek value of Acceleraton Type of Earthquake Peak value of Acceleraton (gal) Input (Base) Man Pendulum Reducton Rato El Centro 194 NS /27 Kobe 1995 NS /6 Hachnohe 1968 NS /9 Note) Level of nput acceleraton to be controlled by means of shakng table ampltude. 7

8 Dampng effect of mult-suspenson system Accordng to tme hstorcal of oscllaton decay shown on Fgure 12, dampng rato and logarthmc decrement are calculated and shown on Table 3. Table 3: Dampng rato and logarthmc decrement Type of suspenson Sngle Double Trple Dampng Rato (h) Logarthmc Decrement ( In d ) CONCLUSION The object of ths paper s to evaluate and verfy the fundamental behavors and characterstcs of multsuspenson system of whch wll be appled for development of Mult-Suspended Isolaton (MPI) System. Accordng analyss and expermental results of mult-suspenson system, followngs are summarsed, 1) 1 st order of natural perod ( T ) of MPI system s gven by followng approxmaton and ndependent of weght of superstructure; T = 2π l g 2) Oscllaton decrement due to mult-suspenson system and shape of pendulum, and those of trple suspenson s; Dampng rato.35 Logarthmc decrement.22 And as a result of expermental oscllaton test by actual earthquake ground moton, sgnfcant effectveness of sesmc solaton s verfed as, 1/6 for Hyougoken-nannbu (Kobe) pattern to 1/27 for El Centro pattern. It s concluded that newly developed Mult-suspended Isolaton (MPI) system s prelmnary verfed as effectve as sesmc solaton devce and of whch has 1 st natural perod ndependent of weght of superstructure and decay of oscllaton by tself. For our further development, t s understood that followng lsted subjects to be clarfed; 1) Resonance due to predomnance order of n th natural perod 2) Effectveness of oscllaton decrement due to mult-suspenson - Relaton wth suspendng order - Respectve length of each suspensons 3) Effectveness of oscllaton decay due to shape of pendulum 4) Relaton between dsplacement and respectve lumped masses (4) REFERENCE MASAKI S., KURAMOCHI M. and KURAMOCHI H.(1988), A Study of a Natural-Perod Changng Type of Sesmc Force Reducton Devce (Part 1 A Test of a Model Devce Usng Shakng Table), Conference of Archtectural Insttute of Japan, pp KITAYAMA N., KOJIMA T., NARITA R., KURAMOCHI H., TORIYA T., KURAMOCHI M. and OHTSUKA T. (1999), Research on the Development and Performance of Mult-Lad Pendulum base Isolaton Systems (Part 1. Sesmc solaton performance aganst standng wave s shape), Conference of Archtectural Insttute of Japan 8