Stability of existing bridges improved by structural integration and nailing

Abstract

To examine whether and how the seismic stability of existing bridges can be substantially improved by integrating the girder, the abutments and the backfill, a series of shaking table tests were performed in 1 g. The tested small bridge models are (1) a conventional-type comprising a girder, supported by a pair of gravity-type abutments (without pile foundation) via bearings (fixed and movable), and unreinforced backfill, (2) the girder and the abutments of the above are integrated (without using bearings), (3) the backfill of the above is reinforced with two layers of large-diameter nails connected to the abutment top and the toe or the heel of the abutment footing and (4) the bottom nails of the above are replaced with longer ones connected to the toe of the abutment footing. Their dynamic behavior was analyzed as a damped single-degree-of-freedom system. The dynamic stability of the bridge was found to increase with an increase in (i) the dynamic strength against the response acceleration, (ii) the initial stiffness, (iii) the dynamic ductility (i.e., a smaller decreasing rate of stiffness during dynamic loading) and (iv) the damping ratio. When factors (ii) and (iii) are high enough, the natural frequency of a bridge can be kept much higher than the input frequency, and thus, the response acceleration can be kept low. All these factors can be improved by integrating the girder, the abutments and the backfill together with part of the supporting ground. In a series of static model tests, lateral cyclic displacements, caused by the seasonal thermal deformation of the girders with prototypes, were applied to the top of a small abutment model. The active failure in the backfill and the detrimental effects of large passive pressure, both developing due to the dual ratchet mechanism, can be effectively restrained by reinforcing the backfill and supporting the ground with nails connected to the top and the bottom of the abutments.