In this report the development and validation of a simple yet dependable method to estimate the seismic response of freeway overcrossings is presented. The proposed method adopts the sub-structure approach to address the issue of soil-foundation-superstructure interaction. The various steps of the method are validated with scarce historic records and are compared with the results obtained by other investigators.
Recognizing that soil-structure interaction affects appreciably the earthquake response of highway overcrossings, Chapter 2 concentrates on the calculation of the kinematic response functions and dynamic stiffnesses of the approach embankments. It is shown that the shear-wedge model yields dependable estimates for the amplification functions of typical embankments. The shear-wedge model is extended to a two-dimensional model in order to calculate the transverse static stiffness of an approach embankment loaded at one end. The formulation reveals a sound closed-form expression for the critical length, Lc, that is the ratio of the transverse static stiffness of an approach embankment and the transverse static stiffness of a unit-width wedge. It is shown through examples that the transverse dynamic stiffness (“spring” and “dashpot”) of the approach embankment can be estimated with confidence by multiplying the dynamic stiffnesses of the unit-width wedge with the critical length, Lc. The study also shows that the values obtained for the transverse kinematic response function and dynamic stiffness can also be used with confidence to represent the longitudinal kinematic response function and dynamic stiffness, respectively.
The dynamic stiffness of piles and pile groups is revisited in Chapter 3 where an existing methodology is employed to determine the group effect. Chapter 4 concentrates on the computation of bridge response quantities. The analysis is conducted in the time domain using either an elementary stick model or a more sophisticated finite element formulation to discretize the bridge super-structure. All dynamic stiffnesses of approach embankments and pile groups are approximated with frequency-independent springs and dashpots that have been established in chapters 2 and 3. A real eigenvalue analysis confirms the one-to-one correspondence between modal characteristics obtained with the three-dimensional finite element solutions and the result of the simpler stick-model idealizations. A complex eigenvalue analysis reveals modal damping values in the first six modes of interest and shows that realistic damping ratios assume values much higher than those used by Caltrans. The efficiency of the proposed method is validated by comparing the computed time response quantities with records from the Meloland Road and the Painter Street overcrossings located in southern and northern California, respectively. The proposed procedure allows for inexpensive parametric analysis that examines the importance of considering soil-structure interaction at the end abutments and center bent. Results and recommendations presented by past investiga- tions are revisited and integrated in comprehensive tables that improve our understanding of the dynamic characteristics and behavior of freeway overcrossings. The study concludes with a step- by-step methodology that allows for a simple, yet dependable dynamic analysis of freeway overcrossings that involves a stick model and frequency-independent springs and dashpots.
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