Seismic Assessment and Retrofit of Existing Multi-Column Bent Bridges





Introduction

Background

Concrete bridge design inadequacies were revealed in the 1971 San Fernando Earthquake

as well as the more recent 1989 Loma Prieta and 1994 Northridge earthquakes. Even though

these earthquakes occurred in California, similar design issues are present in Washington State

concrete bridges designed prior to 1975. The two main column inadequacies are light

confinement reinforcement and short lap splices. Concrete columns were specified with No. 3 or

No. 4 transverse reinforcing hoops spaced at 30.5 cm (12 in.) on center. To increase the capacity

of the columns at large displacements, current standards require minimum transverse

reinforcement of No. 3 spirals spaced at 10.2 cm (4 in.) on center (AASHTO, 2005). Inadequate

lap splice lengths varying from 20d b to 45d b located in the plastic hinging regions of the bridge

columns are not sufficient to resist lap splice failure. Current design methodology requires the

lap splice to be a length of 60d b

and located at the mid-height of the column (AASHTO, 2005).

Other pre-1975 bridge design inadequacies may include inadequate girder seat lengths at the

abutments and bents, inadequately designed column and abutment foundation footings,

inadequate number of girder stops, and poor joint detailing.



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