Simple and complex modelling of seat-type abutmentbackfill systems

Ioannis G. Mikes, Andreas J. Kappos

Research output: Contribution to journalConference articlepeer-review

4 Scopus citations


The response of the seat-type abutment-backfill system under a dynamic excitation and its contribution to the structural system of the entire bridge is usually ignored in practice in Europe, since the designers prefer providing joint gap sizes larger than the required for the design earthquake. In the high seismic hazard areas of the US, various versions of Caltrans Guidelines prescribe a relatively simple way to account for the abutment - backfill interaction. However, the design of Caltrans abutments is based on the 'fully sacrificial' approach, wherein the backwall 'shears off' at an early stage, while in other countries the detailing of the deck-abutment interface is such that a plastic hinge forms at the base of the backwall which is detailed for ductile behaviour. In all cases, if assessment of the bridge safety beyond the design earthquake is sought (e.g. in fragility analysis), it is essential to properly account for the response of the bridge when the end joint is closed. This paper focuses on seat-type abutments with backwall hinging. In a practical context, a 'simple' model in this case consists of a spring-gap element that models the entire abutment-backfill system, while a 'complex' model includes explicit modelling of the abutment using beam-column elements, and of the backfill behind it using one or multiple soil springs. For dynamic response-history analysis, dashpots are also needed for modelling radiation damping. The issues of the number and the arrangement of the spring-dashpot systems and their nonlinear constitutive laws are addressed herein and several configurations are studied. SAP 2000 is used for analysing a typical overpass bridge with seat-type abutments and joints in both the longitudinal and transverse directions, for a number of spectrum compatible records. A series of pushover analyses of the 'complex' model are also carried out; their output can be used to define the (single) spring properties of the simple model. Interesting conclusions are drawn, both with regard to the spring configuration and to the difficulties in combining the various nonlinear elements in SAP 2000.

Original languageBritish English
JournalCOMPDYN Proceedings
StatePublished - 2021
Event8th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, COMPDYN 2021 - Athens, Greece
Duration: 28 Jun 202130 Jun 2021


  • Abutment
  • Backfill
  • Bridges
  • Seismic design


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