Assessing the effectiveness of reducing seismic vulnerability by a program of bridge pier wrapping

Multi-column bent bridges in the New Madrid Seismic Zone (NMSZ) are susceptible to potential earthquake damage due to deficiencies of lap-spliced reinforcement at column ends, insufficient shear capacity of columns and cap beam, and soft soil conditions. The objective of this research is to develop a methodology for describing the generic vulnerability to seismic loading of such columns and a methodology for describing the changes in that vulnerability if specific strategies are used to retrofit those columns.; Five steps are taken to achieve this objective: (1) Identification of the characteristics of representative bridges with multi-column bents through development of an inventory of Southern Illinois bridges on primary emergency routes designated by the Illinois Department of Transportation; (2) Completion of structural and vulnerability assessments for both as-built and retrofitted representative bridge bents by means of pushover and nonlinear structural dynamic analyses; (3) Prediction of the structural damage likely under NMSZ synthetic earthquakes in terms of severity (minor, moderate, major, collapse) and pattern (brittle, ductile); (4) Correlation of structural damage with loss of bridge functionality due to bridge pier damage; and (5) Mapping of how the cost of mitigation relates to the reductions in terms of cost of replacement or repair, in a repeat of the 1811–12 New Madrid or similar event.; The research shows that: (a) The Southern Illinois bridge inventory reported here is reasonably representative of the bridge inventory in the New Madrid Seismic Zone as a whole and has characteristics that differ markedly from those for inventories of bridges in California seismic zones; (b) There is likely to be significant damage to multi-column bents of bridges on primary emergency routes if an earthquake of 2% probability in 50 years strikes the New Madrid area; (c) Damage will include lap splice failure of columns, beam or column shear failure, rocker bearing failure, excessive ground displacement, and significant superstructure misalignment; (d) A program of bridge pier wrapping can significantly reduce bridge functionality losses and allow continuous uninterrupted traffic flow on over approximately 80% of the existing bridges. The damage likely to the remaining 20% requires mitigation actions additional to column wrapping and bearing retrofit.