The Collapse of the Hanshin Expressway (Fukae) Bridge, Kobe 1995: Soil-Foundation-Structure Interaction, Reconstruction, Seismic Isolation

Abstract

The collapse of 18 spans (total length 630 m) of the Hanshin Expressway Route 3 elevated highway bridge in Fukae during the 1995 Kobe earthquake is investigated. The overturned concrete deck was monolithically connected (“piltz” form) to 3.1-m-diameter circular-column piers, founded on 17-pile groups in alluvium sand and gravel. The collapse has been attributed by many research engineers to inadequate structural design, stemming from insufficient and prematurely-terminated longitudinal reinforcement, inadequate hoop anchorage, and (for the large intensity of shaking) insufficient shear capacity. The importance of other factors has been largely ignored. This study presents evidence in the form of a parametric study of the inelastic response of the bridge-foundation-soil system, showing that the role of Soil-Foundation-Structure Interaction (SFSI) was significant and decisively detrimental.

The extreme distress of the structure also severely affected the piles, which were found to have suffered flexural cracking near their top; lateral pile load tests confirmed the decline of their structural stiffness and strength by about 50%. But the stiffness of the pile-soil system apparently only marginally decreased and, thanks to the elaborate seismic isolation bearings introduced in the new design, no changes to foundations were deemed necessary. We show that even a much simpler friction-based seismic isolation system would have saved the original bridge with rather minor acceptable damage.