Study on the seismic isolation of high-elevated rigid frame bridge with double deck

Transcription

1 Study on the seismic isolation of high-elevated rigid frame bridge with double deck H Otsuka, S. Kunki<% J.G.Park^, Y Suzuki and T Tsuchida ^^Kyushu University, HakozakiHigashikuHukuokashi, Japan, kuriki@civil doc. kyushu-u. acjp ^Construction Technique Institute Engineering Co., Ltd. Abstract The purpose of this paper is to investigate dynamic characteristics of a base isolated bridge for transverse direction and to study the effectiveness of base isolation. This bridge is rigid frame with double deck and its center of gravity is relatively high compared to its pier distance in transverse direction. It indicates that tensile reaction would occur at the isolator during strong earthquake. As a result of 3-dimensional time history analysis of a non-isolated bridge, this kind of bridge is unstable for an overturning in the transverse direction and must be prepared for overturning. But according to the survey of the suitable characteristics of the isolator, tensile reaction at the isolator can be reduced in practical terms, and other seismic responses such as bending moments and shear forces are also reduced remarkably. 1 Introduction The effect of base isolated bridge has already been confirmed in Hyogo-Ken Nanbu Earthquake amongst others. In most of the seismic isolation bridge constructed until now, the seismic isolator has been installed on the seismic isolator bridge between the girder and the top of bridge pier. The selected bridge model in this paper has comparatively long natural period, tall piers and heavy superstructure. For restricting moments of piers in the kind of bridge, it is effective to isolate all structure by installing the seismic isolator at the bottom of

2 28 Earthquake Resistant Engineering Structures piers. The only example of this type in Japan is realized in Han- Shin highway route.3 as earthquake disaster restoration A work at Kobe.' About dynamic response in V longitudinal direction of seismic A isolation bridge at pier base under strong earthquake, it has been "V studied by the part of authors/ However, it is clear that this kind of bridge is unstable for an Fig.l Bridge pier sectional view (in m) overturning in the transverse direction and must be prepared for overturning (Fig.l). Then, in this study, highelevated rigid frame bridge with double deck is analyzed by 3-dimensional nonlinear time history analysis, dynamic response characteristics during strong earthquake of this bridge is investigated. Parametric analysis changing characteristics of seismic isolator is also carried out. In comparison with the model that the bridge pier base is fixed, effectiveness and feasibility of base isolation is considered. 2 Analytical Model The analytical model used in this study is shown in Fig.2. In 4 span continuous bridge of 3 (m) length of bridge, 75 (m) span, height of bridge pier 21 (m), the member subject is made to be the concrete at footing, and they are all steel except for it. The model which installed the seismic isolator in the bridge pier base (position of the l(m) height from the ground level) and fixed model which made the combination condition of the bridge pier base to be rigid are made. The total number of node is also 273 on both models, and the total number of threedimensional beam element is 275. The basic soil spring is considered at the bottom of footing. In order to consider the effects of the inertial force of adjoining girders, the 231 (tf) added mass has been given in each junction at PI and P5 with both models. On the boundary condition, the girder of the fixed model is movable only in PI in longitudinal direction and fixed in transverse direction. At the other pier, the girder has been connected rigidly with pier. In the isolation model, the girder is made to be movable in longitudinal direction at PI and P5,but to be fixed in transverse direction at both end. The seismic isolatrors are installed at bottom of

3 Earthquake Resistant Engineering Structures 29 P2 P3 P4 P5 Fig.2 Analytical model for a selected bridge P2, P3 and P4, and each isolator is affected the dead load of 782(tf). In this study, the seismic isolator assumes lead rubber bearing (LRB). On the horizontal spring in longitudinal and transverse directions, have bilinear restoring force characteristics. The seismic isolator with characteristics are shown in and Table a Table. 1 Properties of isolators Khl(ttfm) Kh2(tffm) Kc(tf7m) Kt(ttfm) : Yield force ratio, KM: Elastic stiffness, Kh2 : Post-elastic stiffness, a= Khl/ Kh2, Kc : Compressive rigidity in vertical direction, Kt : Tensile rigidity in vertical direction

4 21 Earthquake Resistant Engineering Structures 3 Analytical Condition Three-dimensional time history analysis by direct integral method considering the dynamic nonlinear of seismic isolator is carried out. The numerical integration used is Newmark ]8 method (]3 =.25), and the integral interval is.1 (sec). Result obtained by dead weight analysis are adopted as initial internal forces nd displacement. As an input acceleration time history waveform, type I ( largescale earthquake of the plate boundary type ) and type n ( the inland near source earthquake ) are used. Each time history waveforms are shown in Fig.3 and Fig.4, these are fitted for the acceleration response spectrum defined in the specification of highway bridge in Japan. 8 6 x-^ \c9 o o* -2 «< i Max. =-433(gal) Fig.3 Type I waveform ; (gal) [H -6 r -8 I Fig.4 Typell waveform 4 Parametric study and discussion 4.1 Analysis for comparison For 18 models shown in Table. 1, by inputting the accelerogram shown in Figure.2 and Figure.3, parametric analysis is carried out. The responses which obtained at P3 are compared. The comparison of largest tensile force at isolator, the largest bending at the bridge pier column, the largest displacement at the bridge pier top, and relative displacement between top of and bottom the pier is shown in Fig.5,6,7, and 8 respectively. Generally, each response decreases according to the decreasing a value. Especially, the largest tensile axial force become almost zero at a =. In addition, each response also varies by almost zero. According to the increase of, the response changes from reduction to increasing at the vicinity of =.2. It seems to be because it become to the almost elastic response, when become large, and the plastic deformation occupy for the

5 Earthquake Resistant Engineering Structures 211 ^ 1 p "ys ^ ^ 1 g "y; - 4.= 2 5 o (a) Type I (b)typeh Fig.5 The comparison of Maximum tensile force Fig.6 The comparison of Maximum bending moment 7 _ 6 #1=5 8 "^ "o 5 4 ig^o I 1 2 c:: ^ (a) Type (b)typed Fig.7 The comparison of Maximum horizontal displacement 7 4.> Jso "o ex P/-O6U on (a)typel (b) Type II Fig.8 The comparison of Relative horizontal displacement.5

6 212 Earthquake Resistant Engineering Structures 25 g 2 c 15 J 1 OJ < 5 PI P2 P3 P4 (1) Acceleration at pier top PI P2 P3 P4 P5 (2) Horizontal displacement at pier top PI P2 P3 P4 (3) Bending moment at pier column P o^ p MO 1 6 _o WO CO 2 PI P2 P3 P4 (4) Shear force at pier column P PI P2 P3 P4 F (5) Bending Moment at upper pier 9 8 Q 7 ^ 6 P 5 Pi P2 P3 P4 (6) Shear force at upper pier P2 P3 (7) Maximum axial force at isolator General Explanations Fixed Model (Type I) # Fixed Model (Type H) D Isolated Model (Type I) O Isolated Model (Type H) Fig.9 The comparison of responses between best isolated model and fixed

7 Earthquake Resistant Engineering Structures 213 most part, so hysteresis damping is decrease when become small. In case of type n, The horizontal displacement at the top of P3 monotonously decreases with the decrease of a, and it becomes a result of suppressing the horizontal displacement by the rotation. However, in case of type I the clear tendency is not coming out, and that it comes out that horizontal displacement depends on input waveform vibration. Examining all responses presented in these figures one isolated model with (=.12 and ct =) are determined to be the best model because which showed the best response in tensile reaction of the isolator, bending moment of pier column, relative displacement of pier top. The comparison of responses of best isolated model and fixed model are shown in Fig.9. Tensile force occurs at isolator at P3, but its magnitude is very small. And, maximum response acceleration, displacement, bending moment and shear force of isolated model are also reduced remarkably at anywhere of bridge. This improvement of the response comes from the following reasons. First, this structure becomes choosing the optimum and a. Secondly, input seismic excitation is effectively reduced from comparatively low level of seismic input by the hysteresis damping. Thirdly, the rotation of the superstructure is suppressed, since the horizontal deformation by the seismic isolator with the small stiffness is large. 4.2 Analysis considered vertical component of input vibration It is considered that tensile force at seismic isolator would be influenced by the vertical component of seismic excitation. Therefore, supplementary analysis adding the vertical component is carried out. Other analysis condition is not C.f)(\ 4 -i 2 *> o 7 -> " one / \ 1 - -, ^^jllj^. ; : ; * "WWjWf fpjw"?^ : -,,,, i,,,, i,,,, i,,,, i,,,, 1,,,,: Fig. 1 Type I waveform (vertical component) 8 6.[Max. =-396(gal) z 4 2 o Fig. 11 TypeE waveform (vertical component)

8 214 Earthquake Resistant Engineering Structures Table.2 Influences to largest responses by vertical component Response Ace. (gal)* I Disp. (cm)* I Bending (tf-m) Shear Force(tf) Tensile force (if) Position Superstructure Superstructure Seismic Isolator Pier Column*2 Center of girder Top of Pier Column Bottom of Pier Column Sesmic Isolator Type I Only Simalu Transverse -taneous Input Input Ratio (%) Only Transverse Input Type II Simalu -taneous Input Responses in trasverse direction, *2 Compared in largest response in the whole of pier column Ratio (%) = ^ c. 1 "^ 5 CC o Z :Max.= 1.9 (cm) 15 "B ^ c. 1 M ^ 5 CCo 'Z r-wyyv^v^vv^^ Max. = 3.9 (cm) Fig.12 Time history of vertical disp. (Type I,simultaneously) Fig.13 Time history of vertical disp. (Typel!, simultaneously) Horizontal disp. (cm) Fig. 14 Horizaontaldsip. - Verticaldisp. (Type I, simultaneously) Horizomtal disp.(cm) Fig. 15 Horizontal disp.-vertical disp. (Type n,simultaneously)

9 Earthquake Resistant Engineering Structures 215 change at all. The best isolated model is selected for this study. The analysis is carried out using type I and type n seismic forces, and the effect of the vertical component is examined. Input time history waveform of vertical component shown in Fig. 1 and Fig.ll. These are the actual waveform of observation point corresponding to the horizontal excitation used in this study (see Fig.3 and Fig.4). The effect of the vertical component on the each largest response is shown in Table.2. The time history of vertical displacement are shown in Fig. 12 and Fig. 13,and horizontal displacement vertical displacement ratio are shown in Fig. 14 and Fig. 15. As a result, it is confirmed that most response increased. Especially, tensile force at seismic isolator becomes 1.55 times in the case of typee, and 1.75 times in the case of type I. So, the effect of the vertical acceleration on largest tensile force at seismic isolator can not be disregarded. However, tensile force at seismic isolator is largest 2.45(kgf/cnr) even in the case of the type E in simultaneous input. Any problem would not seem to occur, because that the stress-strain relation also keeps the linearity for the short period tensile stress up to about 5.(kg/cnf)\ 5 Conclusion In this study, high-elevated rigid frame bridge with double deck is modeled as three-dimensional framed structure, and it clarified the dynamic response characteristics during strong earthquake are investigated using numerical results. In addition, by changing the properties of seismic isolator to be a parameter, the effect of those differences on the response characteristic are. The conclusions obtained here are shown in the following. (1) Natural Period is extended from.45(sec) to 2.66(sec) by the adoption of appropriate seismic isolator in the best isolated bridge. As the same time, the largest response acceleration in the superstructure is reduced from 197(gal) to 473 (gal) in case of type n, and is reduced from 1119(gal) to 475(gal) in case of type I (2) In comparison with best isolated model and fixed model, largest response internal force of the primary member is drastically reduced in best isolated model. In case of type I The largest bending moment of the best isolated bridge at P3 bridge pier is reduced to 54% compared to fixed model. The maximum shearing force is reduced to 49%. In case of type n, the largest bending moment is reduced to 28%. Similarity, the maximum shearing force is reduced at 26%. (3) In the isolated bridge displacement increase. In case of type I, the largest response displacement in P3 bridge pier is increased from 5cm to 83cm, and it

10 216 Earthquake Resistant Engineering Structures increased from 1cm to 71cm in case of type n in the best isolated bridge. (4) Though the negative reaction occurs in the seismic isolator in the base isolated bridge, however, the negative reaction is greatly reduced to the degree ( 38tf ). Therefore, the construction of base isolated bridge actually possible by selecting the appropriate characteristics of seismic isolator. In addition, the vertical displacement arises only 2.4cm, and the influence for of the bridge during the earthquake seems to be small. (5) The whole structure is displaced horizontally, because the shearing deformation of seismic isolator is excellent, when the comparatively low values for horizontal secondary stiffness of seismic isolator is adopted. Therefore is possible that it suppresses the restoration of the frame about bridge axis, and that tensile force in the seismic isolator low value. (6) Tension force of the seismic isolator increases in case of simultaneously excitation. However, the tensile force remains in the safety range insured by the experimental data. References 1. H.Hayashi, S.Kawakita,: An Example of disaster restoration medial work at Hanshin Expressway routes in Benten section, Journal of Bridge and Foudation, Vol.3, No8.pp48~52, J.G.Park, H.Otsuka, A.Koyama and Y.Suzuki: Dynamic Characteristic of Isolated Bridges Considering Different Location of seismic Isolation Journal of Structural Engineering, Japan Society of Civil Engineering, Vol.44, pp743~751, M.Uryu, T.Nishikawa,: Study on Stiffness, Deformation and Ultimate Characteristic of Base Isolated Rubber Bearings (Vertical Characteristics), Journal of Structural and Construction Engineering, Architectural Institute of Japan, No.477, pp31~37,