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CSMIP 95-02

"Evaluation of the Response of I-10/215 Interchange Bridge Near San Bernardino in the 1992 Landers and Big Bear Earthquakes"

by G. Fenves and R. Desroches

March 1995, 132 pp.

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The response of the Northwest Connector at the I-10/215 Interchange in the 1992 Landers and Big Bear earthquakes was recorded by a strong motion instrumentation network. The Connector is a 2540 ft. curved bridge constructed in 1973 and retrofitted for improved earthquake performance in 1991. Although the peak ground acceleration was approximately 0.10 g at the site for both earthquakes, much less than the level of ground motion expected in a major earthquake, the strong motion data provides important information about the earthquake response of a typical curved freeway bridge. The objectives of this study are to: evaluate the importance of non-uniform support motion on the response of the bridge; determine the vibration properties of the bridge; determine efficacy of typical modeling and dynamic analysis techniques used in the design of bridges to predict the response recorded in the earthquakes; and examine the role of the intermediate hinges on the earthquake response of the bridge.

The input motion was fairly uniform in the two earthquakes. Consequently, the spatial variation of free-field input motion is not important for these cases. The earthquake response of the Connector is predominantly transverse. Bent 8 near the center of the bridge is the most heavily instrumented bent. The maximum transverse deformation of the column was 4.76 in. for Landers (drift=0.86%), and 2.98 in. for Big Bear (drift=0.54%). Pile cap rotation produced 10 to 15 percent of the column displacement in the transverse direction.

The strong motion records show the effects of pounding as evidenced by large acceleration spikes at the five intermediate hinges in the Connector. Using the processed displacement records, Hinge 7 had the largest opening: 1.41 in. for Landers and 1.70 in. for Big Bear. Several of the closing excursions have the high frequency oscillation associated with pounding. Accumulated crushing of the filler material in the hinge may have occurred in the second earthquake. In the transverse direction, the relative hinge displacement is constrained by a shear key. The shear keys were effective in limiting the relative displacement to a maximum of 0.61 in. The maximum relative transverse displacements, however, were greater than the nominal 1/4-inch gap, indicating that the shear keys likely suffered local crushing of filler material.

The most significant finding is the difference in the fundamental mode period of the Connector in the Landers and Big Bear earthquakes. The fundamental period lengthened from 1.56 sec in Landers to 1.75 sec in Big Bear. The other noticeable difference between the two earthquakes is that the damping ratio increased in the first and third mode, although it decreased in the second mode. The lengthening of the vibration periods and generally increased damping may indicate that the bridge "softened" in the Landers earthquake. Changes in the pile foundations appear to be the most likely cause, although reduction in stiffness of the steel jacketed columns is a contributing factor in the lengthening of the vibration periods.

Standard three-dimensional "stick models" are used in addition to a nonlinear model of the intermediate hinges. The gross flexural rigidity (EI) for the columns, increased 10 to 15 percent for the steel jackets, are then modified to match the vibration periods of the model with the identified periods of the Connector in the two earthquakes. For the Landers model, the modification factor is 1.05; for the Big Bear earthquake the modification factor is 0.85. The reduction of the column stiffness between the two earthquakes, along with the reduced rotational stiffness for the pile foundations, accounts for the lengthened vibration periods identified from the recorded motion. With these assumptions, the comparison between the model motion and recorded motion is good in many aspects. Although there are some differences, the comparison provides confidence that standard analysis techniques are adequate for design with proper assumptions about column and foundation flexibility. However, the modeling of hinges using gap elements does not provide accurate high frequency response due to pounding of adjacent frames.

The analysis shows that the most heavily loaded column was Bent 9 in the Landers earthquake, with a bending moment that was 73 percent of the ultimate flexural capacity. The nonlinear model showed the restrainers developed a maximum stress of 97 ksi, less than the yield stress. The hinge pounding included in the nonlinear model caused a moderate increase in some column forces compared with the linear tension and compression models which neglect pounding.