EFFECT OF NEAR FIELD GROUND MOTIONS ON FORCE REDUCTION FACTOR AND RESIDUAL DISPLACEMENT

Transcription

1 EFFECT OF NEAR FIELD GROUND MOTIONS ON FORCE REDUCTION FACTOR AND RESIDUAL DISPLACEMENT Tokyo Institute of Technology Ken KIJIMA Gakuho WATANABE Kazuhiko KAWASHIMA USA-Japan Summer Student Symposium 6/11 25 at Tokyo Univ

2 DUCTILITY DESIGN Reduce the acceleration response of bridge, admitting the damage at plastic hinge Calculate the elastic force, divide it by Force Reduction Factor (R factor), get the inelastic force Inertia force of girder Plastic hinge

3 EL F R F DUCTILITY DESIGN High ductility Can decline the force demand Relationship between strength and ductility is trade-off Elastic restoring force EL Inelastic force demand F R NL = Force Reduction Factor R F Y NL F Y NL F Y = O uy u y u EL u NL unl u

4 DUCTILITY DESIGN If the ductility is high, R factor can be set excessively? Residual displacement is generated after an earthquake! u r Residual Displacement Displacement Residual Displacement ur Time

5 PROBLEM AND OBJECT OF RESEARCH Kobe earthquake(jpn), Northridge earthquake(usa),chi- Chi earthquake(twn),etc, are observed in recent years. Problem Present seismic codes don t include the effect of nearfield ground motions Object Compared with conventional middle-field and far-field ground motions, research the effect of near-field ground motions on R factor and residual displacement

6 MODELING AS SDOF M Response of Displacement K Modeling of Plastic Hinge Restoring Force k 1 k 2 Displacement Input Ground Motions Takeda degrading stiffness model Post-yield stiffness k 2 =

7 DEFINITION OF R FACTOR Force Reduction Factor R EL F R F ξ ξ EL NL R = : : ( µ, ξ, ξ, T ) F EL EL FR NL Y NL ( ξ ) EL, T ( µ, ξ, T ) NL NL F Y O uel Linear Viscous Damping Ratio Nonlinear Viscous Damping Ratio T : Natural Period µ : Ductility ξ EL 5 % ξ = = NL u max 2% u

8 ESTIMATE FORMULA Watanabe and Kawashima T a R ( T, µ ) = µ + + bt ae a Natural period where ( 1) 1 1 R = µ 1/b Period between P and Q Force Reduction Factor Parameters have physical meaning µ 1 P a 1/b Q Period (s)

9 DEFINITION OF RESIDUAL DISPLACEMENT RATIO Residual displacement and residual displacement ratio Residual displacement Residual Displacement Ratio Peak displacement u p u r Period (s) Residual Displacement Ratio r = Residual displacement ratio response spectrum r u u r p

10 Acceleration (gal) INPUT GROUND MOTIONS Near-field ground motion Record of 4km 2components 12 Middle-field ground motion Record of 4 2km 2components Far-field ground motion Record over 2km Time(s 14components

11 Near-Field Ground Motions Duzce1999 ChiChi 1999 Northridge1994 Erzincan 1992 JMA Kobe 1995 Tabas1978 Landers 1992 Loma Prieta 1989 Cape Mendocino

12 RESPONSE OF REPRESENTIVE NEAR-FIELD GROUND MOTIONS Tabas earthquake Iran, 1978 Displacement (m) Acceleration (m/s 2 ) Restoring Force-Displacement Input ground motion 8 EL 7. F R 3.7 NL F Y Response of acceleration decrease Linear response Nonlinear response -8-1 Response of displacement Displacement (m) increase.2 EL FR 7. R = = NL Residual displacement FY -.4 ur rr = =.19 Time (s) u.37 Restoring Force / Mass (m/sec2) max

13 SIGNIFICANT SCATTERING OF R FACTOR Difference when classified as types of ground motions Force Reduction Factor Middle-field Near-field Period (s) Mean value Mean value Standard deviation Far-field

14 COMPARISON OF NONLINEAR FITTING AND AVERAGE Force Reduction Factor 15 1 Near-field Middle-field 15 1 Proposed µ=2 Average µ=4 µ=6 µ=8 Far-field Period (s) Nonlinear fitting represents the general trend of mean value of R factor

15 Force Reduction Factor EFFECT OF DISTANCE ON R FACTOR Far-field Middle-field Near-field Far-field Middle-field 1 Middle-field Near-field Near-field R factors 2 depend on the types of ground motion. R factors for near-field ground 1 2motion 3 are 4smaller than 1middle 2 and 3far-field 4 ground motions µ=2 µ=4 µ=6 µ=8 Period (s) Far-field Middle-field Far-field Near-field

16 EFFECT OF DISTANCE ON RESIDUAL DISPLACEMENT Residual Displacement Ratio Near-field Middle-field Far-Field Near-field Middle-field Far-field Near-field Far-field µ=2 µ=4 Middle-field.3 Near-field Middle-field.5 Far-field Residual displacement ratio depend on the types of ground motions. The residual µ=6 displacement ratios for near µ=8field Period (s) ground motions are larger than middle and far-field.

17 R FACTOR VS. RESIDUAL DISPLACEMENT Residual Displacement (m) Force Reduction Factor 1 Mean Value Mean Value µ=2 µ= µ=6 µ=8 Standard Deviation

18 CONCLUSION R factors depend on the types of ground motions. R factor for near-field ground motions are smaller than middle and far-field ground motions The residual displacement ratios depend on types of ground motions. The residual displacement ratios for near-field ground motions are larger than middle and far-field ground motions. There is not an apparent relation between R factors and residual displacements. It seems that residual displacement are independent of R factors.