Keywords: soil-structure interaction, bridge pier, base isolation, seismic response.

In recent years, several research programs on the seismic response of bridges have resulted in the development of both simple analytical approaches and elaborated design methods [1,2]. The use of isolators in the design of bridge piers tends to be established as one of the main methods to reduce seismic hazard in bridge structures. It is also well recognized that soil-structure interaction (SSI) could play a significant role on structural response. However, common practice usually does not account for the effects of SSI on the seismic behaviour of base-isolated bridges. The coupled effect of SSI and base isolation on bridge structures has gained the interest of researchers during the last years and pointed out the discrepancies occurring from neglecting the role of SSI phenomena on base isolated bridges [3,4,5,6].

The present study presents a methodology to assess the effect of SSI on base-isolated bridge piers founded on either an elastic soil layer overlying rigid bedrock or an elastic halfspace. A simplified model is used in which the pier, its foundation, the base isolation and the soil are studied as a four-degree-of-freedom (4-DOF) system [3,7]. A harmonic ground motion is assumed for the formulation of the equations of motion in the frequency domain. Assuming a massless foundation for the initial 4-DOF system, an equivalent fixed-base two-degree-of-freedom (2-DOF) system is developed [7]. Closed-form expressions of the modal and damping characteristics are derived, followed by a parametric study involving some of the pertinent system parameters.

The study concludes that the effects of SSI are more pronounced on the modal properties of the system, especially for the case of squat and stiff base-isolated piers, with relatively small mass ratios, founded on soft soil. The isolation mode dominates the response of the overall system, while the damping characteristics are not significantly influenced by SSI. The slenderness of the structure appears to be more significant in affecting the damping ratio compared with the soil stratum thickness. For preliminary design of base-isolated bridge piers, the methodology could serve as a means to assess the effects of SSI.

1

C.B. Crouse, B. Hushmand, G.R. Martin, "Dynamic soil-structure interaction of a single span bridge", Earthquake Engng and Struct. Dyn., 15, 711-729, 1987. doi:10.1002/eqe.4290150605

2

C.C. Spyrakos, "Assessment of SSI on the longitudinal seismic response of short span bridges", Engng Struct., 12, 60-66, 1990. doi:10.1016/0141-0296(90)90038-T

3

A.G. Vlassis, C.C. Spyrakos, "Seismically isolated bridge piers on shallow soil stratum with soil-structure interaction", Computers and Structures, 79, 2847-2861, 2001. doi:10.1016/S0045-7949(01)00105-5

4

C.C. Spyrakos, A.G. Vlassis, "Effect of soil-structure interaction on seismically isolated bridges", Journal of Earthquake Engineering, 6(3), 391-429, 2002. doi:10.1142/S1363246902000851

5

N.P. Tongaonkar, R.S. Jangid, "Seismic response of isolated bridges with soil-structure interaction", Soil Dynamics and Earthquake Engineering, 23, 287-302, 2003. doi:10.1016/S0267-7261(03)00020-4

6

M. Dicleli, S. Albhaisi, M.Y. Mansour, "Static soil - structure interaction effects in seismic-isolated bridges", Practice Periodical on Structural Design and Construction ASCE, 10(1), 22-33, 2005. doi:10.1061/(ASCE)1084-0680(2005)10:1(22)

7

J.P. Wolf, "Dynamic soil-structure interaction", Englewood Cliffs, NJ: Prentice-Hall, 1985.

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