Revised Design Specifications for Highway Bridges in Japan and Design Earthquake Motions

Transcription

1 Revised Design Specifications for Highway Bridges in Japan and Design Earthquake Motions Keiichi Tamura ABSTRACT The Design Specifications for Highway Bridges in Japan were revised in March The most important change regarding seismic design in this revision is the adoption of performancebased design criteria. Although we had similar design concepts in the previous specifications, they are systematically arranged in the newly revised specifications. Two levels of design ground motions, which are further classified into three kinds of ground motions, three levels of seismic performance criteria and corresponding limit states are prescribed. A new concept has also been introduced to the provision of design earthquake motion, and a site-specific ground motion can be conditionally employed for design practice. INTRODUCTION The first requirements for seismic design of highway bridges in Japan were included in the Details of Road Structures (draft), which were issued in 1926, following the 1923 Kanto earthquake. Since then, the seismic design regulations for highway bridges have been repeatedly revised, based on earthquake disaster experience and progress of research. Among them, the most comprehensive revision was made after the 1995 Hyogo-ken Nanbu (Kobe) Earthquake, which caused the worst damage to various structures including highway bridges since the 1923 Kanto earthquake [1, 2]. After this earthquake, the Design Specifications for Highway Bridges were revised in 1996, in which a number of new design techniques were incorporated [3, 4]. They include: 1) Seismic design force that represents destructive near-field ground motion caused by an inland earthquake, 2) Ductility design method that is applicable to bridge pier, foundation, bearing support and unseating prevention system, 3) Seismic isolation design, 4) Seismic design of unseating prevention system. The Design Specifications for Highway Bridges in Japan were revised in March 2002 [5]. This revision is not large in scale, comparing to the previous one in 1996, however, several important additions and modifications are included. Among them, the most important change regarding seismic design is the adoption of performance-based design criteria. This revision also allows the designer to develop a site-specific design ground motion, in addition to using the standard design ground motion or design spectrum. Keiichi Tamura, Team Leader, Ground Vibration Research Team, Public Works Research Institute 1-6 Minamihara, Tsukuba-shi, Ibaraki-ken Japan

2 This paper first summarizes the basic concepts and principles of current seismic design of highway bridges, where an emphasis is put on the new design philosophies introduced to the specifications. Also presented in this paper are the prediction of site-specific earthquake ground motion and a numerical example. BASES AND PRINCIPLES OF SEISMIC DESIGN Basic Concepts of Seismic Design A Bridge shall be designed to achieve the required seismic performance in accordance with the level of design ground motion and the importance of the bridge. In designing a bridge, the earthquake-resistant structural type shall be selected based on topographical, geological and site conditions, and both individual members and the bridge as a whole system shall be ensured to be earthquake-resistant. Highway bridges play key rolls for evacuation, rescuer, first aid, fire fighting and transporting relief supplies after an earthquake. Hence, it is most essential to secure the safety of bridge against an earthquake and minimize the influence of its functional deterioration on social activities. Reflecting such important rolls of bridge, a bridge shall be designed to achieve the required seismic performance according to the level of design ground motion and the importance classification. Principles of Seismic Design Design Ground Motion In designing bridges, two levels of design ground motions, i.e., ground motion likely to occur during the service period of bridge and destructive ground motion less likely to occur during the service period shall be considered. These design ground motions are termed Level 1 ground motion, and Level 2 ground motion, respectively. Level 2 ground motion contains the ground motion resulting from a large plate-boundary earthquake and that from an inland earthquake which occurs at short distance from a construction site. The former and latter ground motions are designated as Type I and Type II ground motions, respectively. These design ground motions are the same as adopted in the previous specifications that were issued in Type I ground motion is characterized by large amplitude and large number of cycles, while Type II ground motion has short duration and destructive strength. Importance of Bridge In the specifications, each bridge is classified either ordinary bridge or important bridge, according to the road classes, function and structure of the bridge. Table 1 indicates this classification. Seismic Performance

3 The seismic performance of bridge is categorized into the following three levels based on the seismic behavior of a whole bridge system: - Seismic performance 1; to secure integrity. - Seismic performance 2; to limit damage and secure rapid restoration of function. - Seismic performance 3; to prevent fatal damage. Table 2 shows the seismic performance objectives, which is prescribed by the combination of design ground motion and importance of bridge. The seismic performance is established from the three different standpoints, i.e., safety, serviceability and reparability, which are further classified into three levels, respectively, as summarized in Table 3. Safety is the seismic performance to prevent loss of human lives due to unseating of superstructure. Serviceability represents the performance to maintain the original traffic function after an earthquake and serve as a route for evacuation, rescuer, first aid, fire fighting and transporting relief supplies. Reparability represents the performance to be able to repair the damage caused by an earthquake. Requirement for Preventing Unseating of Superstructure An additional requirement is to prevent the unseating of superstructure due to unexpected seismic behavior of the bridge and ground failure. As Type II ground motion, those recorded in the 1995 Kobe earthquake that influenced the most destructive effects on structures were incorporated into the specifications, whereas even greater ground motions could occur. There still remain large uncertainties to predict such ground motion characteristics and reflect them to the seismic design of bridges. Furthermore, ground failure and unexpected response of structural members may cause unpredictable effects to the bridge structure. Even under such circumstances, it is intended to secure safety against the unseating of superstructure. DESIGN GROUND MOTION General The standard design response spectrum, which is presented later in this paper, may be used as design ground motion, whereas a site-specific design ground motion shall be developed when the ground motion at a construction site can be appropriately predicted, based on the information of past earthquakes, active faults, plate-boundary earthquakes, geological structure, local site condition, recorded ground motions, and so forth. Reflecting the recent research and development in the area of ground motion prediction, the site-specific design ground motion shall be established, if enough information is available to appropriately predict ground motion at the site. Otherwise, the standard design response spectra may be used as design ground motions. Standard Design Response Spectra Level 1 Ground Motion Level 1 ground motion in terms of acceleration response spectrum S may be determined from the following equations:

4 S=c Z c D S 0 (1) c D =1.5/(40h+1)+0.5 (2) where c Z is the zone factor (=1.0, 0.85, 0.7), c D is the modification factor by damping ratio h, and S 0 is the standard acceleration response spectrum. This standard response spectrum was established from attenuation relations of spectral acceleration, characteristics of past earthquake damage and ground vibration, and so on. Since ground motion characteristics and resultant structural damage are closely related the soil condition, the standard acceleration response spectra are defined for the three different soil classes that are given in Table 4. Table 5 and Figure 1 show the standard acceleration response spectra of Level 1 ground motion. Level 2 Ground Motion Similar to the case of Level 1 ground motion, Level 2 motion is also prescribed by acceleration response spectrum, and Type I and Type II ground motions, which are denoted by S I and S II, are expressed as S I =c Z c D S I0 S II =c Z c D S II0 (3) (4) where S I0 and S II0 indicate the standard acceleration response spectra of Type I and Type II ground motions, respectively. Table 6 and Figure 2 give these standard response spectra. Type I ground motion stands for ground motions in Tokyo by the 1923 Kanto earthquake, for instance. The acceleration response spectrum of this ground motion is estimated from attenuation relations and past experiences. The acceleration response spectrum of Type II ground motion, which represents ground motion generated by an inland earthquake at short distance, was developed by smoothing the response spectra that are computed from the ground motions records obtained in the 1995 Kobe earthquake. VERIFICATION OF SEISMIC PERFORMANCE General In order to verify the seismic performance of bridge, the limit states of individual structural members shall be first established, based on the limit state of bridge as a whole, which is described in the following section. Then, the seismic response of individual members shall be verified not to exceed the corresponding limit state, in which an appropriate method depending on design ground motion, structural type and limit state should be selected. In the specifications, standard static and dynamic verification methods are prepared for the bridges whose seismic behavior is uncomplicated and complicated, respectively. In addition to this, it is necessary to verify to prevent unseating of superstructure due to unexpected structural response and ground failure, for which the standard method is also provided in the specifications.

5 Limit State of Bridge against Seismic Performance 1 The limit state of bridge against seismic performance 1 shall be appropriately determined so that the seismic response of whole bridge system remains within the elastic range. This limit state is determined to maintain function of bridge after an earthquake and limit structural damage to be minor. Corresponding to this limit state of the bridge as a whole, the limit states of individual structural members may generally be established so that seismic response of individual members remains within the elastic range. Limit State of Bridge against Seismic Performance 2 The limit state of bridge against seismic performance 2 shall be appropriately determined so that the plastic deformation is limited to the structural members that are allowed to be plastic and secure reparability. This limit state is determined to ensure rapid restoration of bridge function after an earthquake. As the structural members that are allowed to be plastic, members that can reliably absorb energy and be presently repaired shall be selected. The designer shall appropriately combine members that are allowed to be plastic and properly establish the limit states of individual members. For general bridges, bridge piers may be regarded as the structural members that can absorb energy and be easily repaired. In case of the bridges equipped with isolation bearings, the limit states of individual members shall be established so that energy can reliably be absorbed at the bearings. Limit State of Bridge against Seismic Performance 3 The limit state of bridge against seismic performance 3 shall be appropriately determined so that the plastic deformation is limited to the structural members that are allowed to be plastic and plastic deformation does not exceed the plastic deformation capacity. This limit state is determined to prevent fatal damage or collapse of bridge. As the structural members that are allowed to be plastic, members that can reliably absorb energy shall be selected. The designer shall appropriately combine members that are allowed to be plastic and properly establish the limit states of individual members. Verification Method of Seismic Performance The seismic performance of bridge shall be verified by an appropriate method, based on design ground motion, structural type and limit state. For the bridges whose seismic behavior is uncomplicated, the static verification method prescribed in the specifications may be employed as a verification method of seismic performance. The conventional seismic coefficient method and ductility design method are applicable as static verification methods. In case of the bridges that have complicated seismic behavior, the dynamic verification method in the specifications may be applicable to verify the seismic performance. The bridges that have complicated seismic behavior generally correspond to the following cases: 1) The principal vibration mode is definitely different from that assumed in the static verification method of seismic performance. 2) Two or more vibration modes which dominate the seismic response of bridge exist.

6 3) Plural plastic hinges are assumed to be formed in the verification of seismic performance against Level 2 ground motion or location of plastic hinges cannot be identified. 4) The applicability of energy-constant rule based on the nonlinear characteristics of structural member or whole bridge system is not sufficiently confirmed. Figure 3 illustrates the standard procedure of seismic design. DEVELOPMENT OF SITE-SPECIFIC DESIGN GROUND MOTION Ground Motion Prediction Method As previously mentioned, in the newly revised design specifications for highway bridges, a site-specific design ground motion shall be developed when the ground motion at a construction site can be appropriately predicted, based on the information of past earthquakes, active faults, plate-boundary earthquakes, geological structure, local site condition, recorded ground motions, and so forth. This provision does not restrict methods of ground motion prediction, and the recorded ground motion data and various techniques, such as attenuation relations, semiempirical and theoretical ground motion simulation methods may be used for this purpose. The semi-empirical ground motion synthesizing technique, in which ground motion record from a small event is used as a Green's function, has been applied to ground motion prediction [6, 7]. This technique has an advantage of automatically incorporating the complicated earthquake source mechanism and seismic wave path effects into calculation. On the other hand, this method requires an actual small event record, and it is obvious that an appropriate record is not always available at a construction site of new structure. To compensate this disadvantage, a stochastic Green's function technique has been proposed, in which the stochastically simulated small event motion is used as a Green's function [8, 9]. As an example of site-specific ground motion prediction, ground motion predicted by a stochastic Green's function technique is presented in the following section. Ground Motion Prediction for the Kanto Earthquake We herein simulate ground motion at Kannonzaki, which is located at the mouth of Tokyo Bay, from a Kanto earthquake [10, 11]. Kanto earthquake recurrently occurs off the coast of Tokyo, and the latest one occurred in 1923 caused destructive damage to Tokyo metropolitan area. We assume the fault plane model proposed for the 1923 Kanto earthquake, and systematically change the location of hypocenter and asperities to examine the effects of uncertainties of source parameters on the calculation results. This is because it is hardly possible to predict the location of these quantities for a future earthquake, which is essentially important for seismic design of structures. Figures 4 and 5 indicate the fault plane model and the changed locations of hypocenter and asperities. The length and width of the fault plane are assumed to be 130km and 70km, respectively. The following numerical results correspond to those estimated on the outcropping layer with shear wave velocity v S =700m/s. Figure 6 shows the synthesized acceleration time history that has the largest peak acceleration among the computed results and the corresponding velocity time history. The 5 % damped acceleration response spectra obtained from all the numerical simulations are plotted in Figure 7. We see from this figure that the spectral amplitude varies about four times for an

7 arbitrary natural period due to uncertainty of hypocenter and asperity locations. The thick line in this figure indicates the spectral level that has a 90 % probability of not being exceeded for each natural period. This spectral line exceeds 1G over the wide natural period range 0.1<T<2 (s), and reaches 2G for 0.1<T<0.6 (s). Although further detailed study is necessary, the presented result seems to be consistent with ground motion characteristics from a large earthquake. CONCLUDING REMARKS The Design Specifications for Highway Bridges were revised in March The major modification in this revision is the introduction of performance-based design criteria. This paper presented the basic concepts and principles of seismic design of highway bridges including design ground motion, seismic performance and limit states. In the revised specifications, a sitespecific ground motion can be conditionally employed as design ground motion. To predict a site-specific ground motion, we adopted a stochastic Green's function technique. The ground motion at the mouth of Tokyo Bay was simulated, in which a Kanto earthquake was assumed. Based on numerical results, effects of source parameter uncertainties on the simulated ground motions were examined. ACKNOWLEDGEMENT The draft of Design Specifications for Highway Bridges, Part V Seismic Design was developed by the Working Sub-Committee on Seismic Design (Chair: Dr. Shigeki Unjoh) and approved by the Committee on Bridges (Chair: Dr. Shoichi Saeki) under the auspices of the Japan Road Association. REFERENCES [1] Japan Society of Civil Engineers (1996). The 1995 Hyogoken-Nanbu Earthquake Investigation into Damage to Civil Engineering Structures. [2] Public Works Research Institute (1997). "Report on the Disaster Caused by the 1995 Hyogoken Nanbu Earthquake", Journal of Research, Public Works Research Institute, Vol.33. [3] Yokoyama, K. and Unjoh, S. (1997). "Seismic Design and Retrofit of Highway Bridges in Japan", Second National Seismic Conference on Bridges and Highways, Sacramento, CA, U.S.A. [4] Japan Road Association (2000), Design Specifications for Highway Bridges, Part V Seismic Design (English edition). [5] Japan Road Association (2002), Design Specifications for Highway Bridges, Part V Seismic Design (in Japanese). [6] Hartzell, S. (1978). "Earthquake Aftershocks as Green's Functions", Geophysical Research Letters, Vol.5, No.1. [7] Irikura, K. (1983). "Semi-Empirical Estimation of Strong Ground Motions during Large Earthquakes", Bulletin of Disaster Prevention Research Institute, Kyoto University, Vol.33, No.298. [8] Boore, D. M. (1983). "Stochastic Simulation of High-frequency Ground Motions based on Seismological Models of the Radiated Spectra", Bulletin of Seismological Society of America, Vol.73, No.6. [9] Kamae, K., Irikura, K. and Fukuchi, Y. (1991). "Prediction of Strong Ground Motion based on Scaling Law of Earthquake", Journal of Structural and Construction Engineering, Architectural Institute of Japan, No.430 (in Japanese). [10] Yasuda, M. et al. (2000). "Seismic Design Ground Motions for Strait-crossing Projects in Japan", Thirty-second Joint Meeting, U.S.-Japan Panel on Wind and Seismic Effects, UJNR, Tsukuba, Japan. [11] Tamura, K. and Kataoka, S. (2000). "A Procedure for Setting up Level-2 Earthquake Motions for Seismic Design of Bridges", Sixteenth U.S.-Japan Bridge Engineering Workshop, U.S.-Japan Panel on Wind and Seismic Effects, UJNR, Lake Tahoe, NV, U.S.A.

8 Bridge class Class-A bridges Class-B bridges TABLE 1. IMPORTANCE CLASSIFICATION Bridges included - Bridges other than Class-B bridges - Bridges on national expressways, urban expressways, designated city expressways, Honshu- Shikoku bridge highway and general national highways - Double-section and overpass bridges on prefectural highways and municipal roads, and other bridges and viaducts that are important in view of regional disaster prevention plans, traffic flow volume, etc. TABLE 2. DESIGN GROUND MOTION AND SEISMIC PERFORMANCE Class-A bridges (Ordinary bridges) Class-B bridges (Important bridges) Level 1 ground motion Secure integrity Level 2 Type I ground motion Prevent fatal damage Limit damage and secure ground motion Type II ground motion rapid restoration of function Seismic performance criteria Seismic performance 1 Seismic performance 2 Seismic performance 3 Safety TABLE 3. SEISMIC PERFORMANCE CRITERIA Serviceability Reparability Short-term Secure preearthquake function Secure safety against collapse Secure safety against collapse Secure safety against collapse Secure rapid restoration of function Need no repair for restoration of function Emergency repair enables restoration of function Long-term Need minor repair Possible to perform permanent repair easily TABLE 4. SOIL CONDITION CLASSIFICATION Soil classification Natural period (s) Geological description Group-1 T G <0.2 Rock or shallow soil deposits Group T G <0.6 Diluvium or alluvium Group T G Soft alluvium Soil classification Group-1 Group-2 Group-3 TABLE 5. LEVEL 1 STANDARD DESIGN RESPONSE SPECTRA Spectral acceleration S 0 (cm/s 2 ) at natural period T (s) S 0 =431T 1/3 for T<0.1 S 0 =200 for 0.1 T 1.1 S 0 =220/T for 1.1< T (S 0 160) S 0 =427T 1/3 for T<0.2 S 0 =250 for 0.2 T 1.3 S 0 =325/T for 1.3< T (S 0 200) S 0 =430T 1/3 for T<0.34 S 0 =300 for 0.34 T 1.5 S 0 =450/T for 1.5< T (S 0 240) TABLE 6(1). LEVEL 2 STANDARD DESIGN RESPONSE SPECTRA (a) Type I ground motion Soil classification Spectral acceleration S I0 (cm/s 2 ) at natural period T (s) Group-1 S I0 =700 for T 1.4 S I0 =980/T for 1.4< T Group-2 Group-3 S I0 =1505T 1/3 for T<0.18 (S I0 700) S I0 =1511T 1/3 for T<0.29 (S I0 700) S I0 =850 for 0.18 T 1.6 S I0 =1000 for 0.29 T 2.0 S I0 =1360/T for 1.6< T S I0 =2000/T for 2.0< T

9 TABLE 6(2). LEVEL 2 STANDARD DESIGN RESPONSE SPECTRA (b) Type II ground motion Soil classification Spectral acceleration S II0 (cm/s 2 ) at natural period T (s) Group-1 S II0 =4463T 2/3 for T<0.3 S II0 =2000 for 0.3 T 0.7 S II0 =1104T -5/3 for 0.7< T Group-2 S II0 =3224T 2/3 for T<0.4 S II0 =1750 for 0.4 T 1.2 S II0 =2371T -5/3 for 1.2< T Group-3 S II0 =2381T 2/3 for T<0.5 S II0 =1500 for 0.5 T 1.5 S II0 =2948T -5/3 for 1.5< T 500 Response Spectral Acceleration(cm/s 2 ) h=0.05 Group1 Group2 Group Natural Period(s) 2 3 Figure 1. Standard acceleration response spectra of Level 1 ground motion Response Spectral Acceleration(cm/s 2 ) h=0.05 Group1 Group2 Group3 Response Spectral Acceleration(cm/s 2 ) h=0.05 Group1 Group2 Group Natural Period(s) Natural Period(s) (a) Type I ground motion (b) Type II ground motion Figure 2. Standard acceleration response spectra of Level 2 ground motion

10 Start No Is seismic response to Level 1 ground motion complicated? Yes Determine lateral seismic force coefficient and inertia force Compute response values by dynamic analysis Determine allowable values (Allowable stress, etc.) Compute sectional forces and displacements by static analysis Verification of seismic performance for Level 1 ground motion No Is seismic response to Level 2 ground motion complicated? Yes Determine lateral seismic force coefficient and inertia force Compute sectional forces and displacements by static analysis Compute response values by dynamic analysis Determine allowable values (Horizontal capacity, allowable displacement, etc.) Verification of seismic performance for Level 2 ground motion Design of unseating prevention system End Figure 3. Standard procedure of seismic design

11 Figures 4. Fault plane model of Kanto earthquake Asperity Kannonzaki Hypocenter N Figure 5. Locations of hypocenter and asperities

12 Figure 6. Simulated ground motion Figure 7. Acceleration response spectra