-International cooperative research program between Nagoya Institute of Technology, Japan and Tongji University, China-

Photo 1-1 Progressive collapse of concrete elevated highway bridge in the 1995 Kobe earthquake (Pilz-typed bridge) ( Courtesy of Prof. Yozo Fujino of Univ. of Tokyo)

【Introduction】

In the current seismic design, the performance of elevated girder highway bridges is usually checked for the unidirectional design seismic accelerations applied in the longitudinal direction and that applied in the transverse direction, independently. However, in reality, the multi-directional components of seismic accelerations act simultaneously on bridges. This implies that unexpected damages or collapse may occur in the bridges that are designed based on the current seismic design code. Recent development in earthquake engineering has made it possible to predict the multidirectional components of regional ground motions by considering earthquake scenarios. Among the multidirectional components, coupling of the bidirectional horizontal ground motion components is considered to have a large influence on the ultimate behavior of elevated girder bridges. Therefore, it will be more rational to verify the performance of columns used as bridge piers by taking into account the bidirectional seismic acceleration components simultaneously. For this purpose, it is necessary to investigate the ultimate behavior of the columns under bidirectional horizontal seismic excitations and specify their ultimate limit state.

We have investigated both experimentally and analytically the ultimate behavior of thin-walled steel columns and CFT columns subjected to bidirectional horizontal seismic forces by employing a unique 3D loading system and an advanced nonlinear analysis that can assess the interface action between steel tube and in-filled concrete by precisely considering the cyclic local buckling behavior of the steel tube and the confinement of the concrete in-fill. In the above investigation, first, the strength and ductility of the thin-walled circular steel columns under bidirectional horizontal cyclic loads were compared with those under unidirectional cyclic loads and formulas were presented for the ease of the prediction of their strength and ductility. Second, an interaction curve expressed in terms of the two horizontal seismic force components acting at the top of the columns was proposed to specify their ultimate states. To confirm the validity of the research done up to the present, bidirectional shaking table test was conducted from 2008 to 2011 on 1/8 scale models of single thin-walled steel and CFT columns by using the shaking table of 6 degrees of freedom at Tongji University. This test was conducted as the first international cooperative research program between Nagoya Institute of Technology (NIT), Japan and Tongji University(TJU), China.

However, the overall ultimate and collapse behavior of elevated girder bridges are not only influenced by the columns used for the piers but also by the interaction between the piers, bearings and superstructures. Specifically, the large-scale progressive collapse behavior of the elevated highway concrete bridge(Pilz-typed bridge) that occured in the 1995 Kobe earthquake (Photo 1-1)was strongly influenced by this interaction. Therefore, in order to ensure the safety of the elevated-girder highway bridges under super-mega earthquakes like the 2011 Tohoku earthquake more precisely, it is essential to examine their overall behavior by taking into account the interaction between the piers and superstructures via bearings. For this purpose, some accurate numerical model is required to predict the ultimate and collapse behaviors of the elevated girder bridge systems under multi-directional seismic accelerations. To develop the accurate numerical model, experimental data is indispensable to calibrate it. Herein, a bi-directional shaking table test is carried out on 1/6.7 scale two-span continuous elevated-girder highway bridge models supported by the thin-walled steel columns or CFT columns via rubber bearings(Fig.1-1 ). The experiment is conducted by the shaking table array consisting of independent four tables at TJU as the second international cooperative research program between NIT and TJU. In this experiment, special emphasis is placed on the observation of how seismic inertia forces acting on masses installed on the superstructure are transferred to the columns via the bearings and the cross beams, depending on the damages of the columns.

【Objectives】

・Examine the effect of the performance of bridge piers and bearings on the ultimate behavior of the overall elevated-girder highway bridge systems under multi-directional seismic accelerations

・Develop a rational seismic-response control method by dampers or isolators

・Investigate a detailed mechanism of progressive failure of elevated-girder highway bridge systems by multi-directional shaking table test

・Develop an advanced and precise analysis method to assess the ultimate and collapse behaviors of elevated-girder highway bridge systems by considering the interaction between piers, bearings and superstructures

【Points to be remarked in the present shaking table test】

・One of the world largest continuous elevated girder bridge models are used in the multi-directional shaking table test.

・First shaking table test on the continuous elevated girder bridge models supported by thin-walled steel piers or thin-walled CFT piers via rubber bearings