Athanasios Agalianos

Comparative Assessment of Two Rocking Isolation Techniques for a Motorway Overpass Bridge

Rocking isolation of structures is evolving as an alternative design concept in earthquake engineering. The present paper investigates the seismic performance of an actual overpass bridge of the Attiki Odos motorway (Athens, Greece), employing two different concepts of rocking isolation: (a) rocking of the piers on the foundation (rocking piers); and (b) rocking of the pier and foundation assembly (rocking footings) on the soil. The examined bridge is an asymmetric 5-span system having a continuous deck and founded on surface foundations on a deep clay layer. The seismic performance of the two rocking isolated bridges is comparatively assessed to the existing bridge, which is conventionally designed according to current seismic design codes. To that end, 3D numerical models of the bridge–foundation–abutment–soil system are developed, and both static pushover and nonlinear dynamic time history analyses are performed. For the latter, an ensemble of 20 records (10 ground motions of 2 perpendicular components each) that exceed the design level are selected. The conventional system collapses in 5/10 of the (intentionally severe) examined seismic excitations. The rocking piers design alternative survives in 8/10 of the cases examined, with negligible residual deformations. The safety margins of the rocking footings design concept are even larger, as it survives in all cases examined. Both rocking isolation concepts are proven to offer increased levels of seismic resilience, reducing the probability of collapse and the degree of structural damage. Nevertheless, in the rocking piers design alternative high stress concentrations at the rotation pole (pier base) are developed, indicating the need for a special design of the pier ends. This is not the case for the rocking footings concept, which however is subject to increased residual settlements but no residual rotations.

Simplified method for the assessment of the seismic response of motorway bridges: longitudinal direction—accounting for abutment stoppers

This paper investigates the effect of abutment stoppers on the seismic response of motorway bridges in the longitudinal direction. A rigorous 3D finite element model of a representative overpass bridge, including the entire bridge–foundation–abutment–soil system, is developed and used as a benchmark. The effect of abutment stoppers is shown to be significant, and must therefore be considered for proper simulation of the seismic response of such bridges. Subsequently, the resistance mechanism of abutments triggered when the bridge deck collides on the stoppers is examined. The model is first validated against theoretical solutions. The abutment cantilever wall is subjected to slightly—but crucially—different loading when the deck collides on the stoppers: the loading is applied at the top of the abutment without any rotational restraint. To gain insights on the key parameters affecting the abutment resistance to such passive loading at the top, a dimensionless analysis and a comprehensive parametric study are conducted, employing an equivalent 2D model of the abutment. The latter is validated against the results of a rigorous 3D model. Based on the results of the parametric study, a simplified model accounting for the effect of abutment stoppers is developed. Its efficiency is assessed on the basis of slow-cyclic pushover and nonlinear dynamic time history analyses, using the full 3D model as a benchmark. Overall, the extended simplified model is shown to offer a reasonable approximation (excellent for cohesive soil) of the seismic performance of typical motorway bridges in the longitudinal direction.

Seismic performance comparison of two rocking isolation alternatives for an overpass bridge

Rocking isolation of structures is evolving as an alternative design concept in earthquake engineering. The present paper investigates the seismic performance of an actual overpass bridge of the Attiki Odos motorway (Athens, Greece), employing two different concepts of rocking isolation: (a) rocking of the piers on the foundation (rocking piers); and (b) rocking of the pier and foundation assembly (rocking footings) on the soil. The examined bridge is an asymmetric 5-span system having a continuous deck and founded on surface foundations on a deep clay layer. The seismic performance of the two rocking isolated bridges is compared to that of the existing bridge, which is conventionally designed according to current seismic design codes. To that end, 3D numerical models of the bridge–foundation–abutment–soil system are developed, and both static pushover and nonlinear dynamic time history analyses are performed. For the latter, an ensemble of 20 records (10 ground motions of 2 perpendicular components each) that exceed the design level are selected. The conventional system collapses in 5/10 of the (intentionally severe) examined seismic excitations. The rocking piers design alternative survives in 8/10 of the cases examined, with negligible residual deformations. The safety margins of the rocking footings design concept are even larger, as it survives in all examined cases. Both rocking isolation concepts are proven to offer increased levels of seismic resilience, reducing the probability of collapse and the degree of structural damage. Nevertheless, in the rocking piers design alternative high stress concentrations at the rotation pole (pier base) are developed, indicating the need for a special design of the pier ends. This is not the case for the rocking footings concept, which however is subject to increased residual settlements but no residual rotations.