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ELABORATE SIMULATION AND PREDICTION OF SEISMIC BEHAVIOR OF A TRUSS BRIDGE

In this study, the time-history analysis procedure is elaborately carried out for a truss bridge by identifying the actual structural vibrating characteristics measured by on-site tests and employing the input ground motions at different sites converted from the motions recorded at other observation spot. This procedure is first validated by simulating the seismic behavior of a deck-type truss bridge subjected to a moderate earthquake (M5.7) happened near the bridge in 2002. Then an extreme earthquake (M7.5) is assumed on the basis of ground motions recorded in the moderate earthquake, and the corresponding seismic responses of the bridge are predicted, by consistently estimating the ground motions at hypocenter with assumption of the ground faults and formulating ground motions at the site with consideration of spreading procedure from the hypocenter. INTRODUCTION After the 1995 Kobe earthquake, many research efforts in Japan are attracted on the improvement of seismic design methodology based on dynamic analysis, and consequently some design procedures as well as practical analysis programs are proposed or improved for steel bridges (e.g., References [1]-[3]). Furthermore, in the latest Japan road design codes updated in 2002 (JRA 2002 [4]), the dynamic analysis is required for many practical design cases. For such dynamic analysis, structural data such as stiffness and masses are formulated on design drawings or design statements. Moreover, the damping is assumed on this basis and the standard seismic ground motions specified in codes are employed as the input earthquakes. However, by using such analysis model and input ground motions, the responses obtained from the dynamic analyses could be quite different from the real responses of actual structures subjected to actual ground motions spread from the hypocenter. The reasons lie in that the measured characteristics of actual structure usually differ from those of the analysis model as mentioned above, which based on the nominal design data, and that the input ground motions are significantly influenced by the oscillating mechanism of hypocenter, the characteristics of spreading 1 General Manager, Yamato Engineering Inc., Nagoya, Japan. Email: nonaka@yamato-eng.co.jp 2 Professor, Miyazaki University, Miyazaki, Japan. Email: harada@civil.miyazaki-u.ac.jp 3 Engineer, Yamato Engineering Inc., Nagoya, Japan. Email: zheng@yamato-eng.co.jp 4 PhD Student, Miyazaki University, Miyazaki, Japan. Email: hongze@civil.miyazaki-u.ac.jp path, and especially the ground surface characteristics around the structure location. Therefore, it is difficult for the present dynamic analysis procedure to accurately establish each term of the motion equations (i.e., structural stiffness, masses, damping and the input ground motions). In this study, by identifying the characteristics of actual structure through tests and applying the actually observed ground motions, an elaborate procedure of dynamic analysis is carried out to predict the seismic behavior of the studied bridge. Such predictions are obtained by consistently considering the oscillation of the hypocenter and the ground motions spreading from hypocenter to structural location. The calculated results are compared with those from the conventional dynamic method. Such elaborate structural model data and input ground motions are formulated through following steps. First, the analysis mode is formulated for a truss bridge [5], where the structural stiffness and mass are more accurately modified by using dynamic characteristics measured in tests. Then, the input ground motions are built for an assumed severe earthquake, which is based a moderate earthquake happened in 2002 at Hyuganadaoki, a place near the studied bridge. To build the ground motions, the empirical Green’s function is employed to simulate ground motions of assumed severe earthquake from recorded medium earthquake, and the ground motions of an assumed hypocenter is estimated by using the H/V spectrum ratio in the microtremor condition [6]. Moreover, the phase difference as well as material geometrical nonlinearity are taken into account. The bridge is an upper-deck steel truss bridge, which has complex seismic behaviors. STUDIED BRIDGE AND 2002 HYUGANADAOKI EARTHQUAKE The layout and general information of the studied upper-deck truss bridge is presented in Fig. 1 and Table 1. This bridge was built in Miyazaki-ken of Japan in 1981, based on the 1972 Japan design codes. A moderate earthquake happened on Nov. 4, 2002 and its hypocenter was the Hyuganadaoki area of Miyazaki-ken of Japan, which is near to the location of the bridge. Before it, an earlier severe earthquake (M7.5) also happened in this area in 1968 and the recorded ground motions are adopted in JRA (2002) as one of the Level-2 standard design ground motions. Figure 2 shows the ground accelerations recorded in 2002 Hyuganadaoki earthquake at an observation point nearest to the studied bridge. (Unit :mm) (a) Layout (b) Cross Section Fig. 1 Studied Bridge Table 1. General Information of the Studied Bridge Bridge Type Three Span Deck-type Truss Bridge Length of Bridge 166.0 m Spans 37.5m + 90.0m + 17.5m Width of Deck Section 9.75m Region Modification Factor 0.85 -60 -40 -20 0 20 40 60 0 5 10 15 20 25 30 35 40 Tim e (s) A c c e l e r a t i o n ( g