Cost Optimal Design of a Bridge Pier Foundation

Abstract

A reliability analysis of a pylon foundation for a suspension bridge design is illustrated aiming at the establishment of utility-based design principles. The structure is analyzed in the final situation, that is for a consolidated and undrained condition. It is thus assumed that full drainage and consolidation takes place during erection of the towers and installation of cables and deck structure. The possible failure is caused by a loading of such a short duration, that no further drainage takes place. The loading acting in addition to the self weight can be caused by wind, high traffic loading or ship impact.

The end-goal is the specification of an optimal set of partial safety factors for a traditional deterministic design. To arrive at such partial safety factors, models for loading and strength are formulated. These models contain random elements, allowing that a reliability analysis with an associated sensitivity analysis can be carried out for the relevant failure modes. The soil strength is modelled as a random field and the statistical uncertainty in estimating the soil parameters from a limited number of soil samples is included.

The desired reliability level is computed by minimizing a total expected cost expression, including construction cost and expected failure cost. Three design parameters are considered: the length and width of the foundation and the type and level of ballast. The partial safety factors are derived for a selected deterministic checking equation and are chosen to give the same structural dimensions as for the foundation with the desired reliability level.