Isolation as a Seismic Design Strategy for Bridges in the New Madrid Seismic Zone

The seismic hazard in the New Madrid Seismic Zone (NMSZ) is primarily from the New Madrid Fault System (NMFS), which produced a series of large earthquakes in 1811-1812. Estimates of the magnitude of these earthquakes and the event magnitude appropriate for structural design in the New Madrid Seismic Zone vary greatly and have been the subject of much debate.

Current bridge design practice in the region relies primarily upon controlled damage by plastic hinging in columns/piles at piers to prevent collapse. Abutments are typically modeled with linear springs to represent piles and backfill stiffness. The usefulness, after a major seismic event, of structures designed by this method lies in doubt due to the potential for high residual displacements.

Isolation has been selected as a viable design alternative for major structures since the 1970’s at least. The application of isolation to bridges in the New Madrid Seismic Zone has been limited, with the most notable example Interstate 40 over the Mississippi River (the Hernando Desoto Bridge). This was a retrofit project incorporating both Lead-Rubber Bearings (LRB) and Friction Pendulum System (FPS) bearings as isolators.

The feasibility of isolation as a design strategy for bridges in the New Madrid Seismic Zone is evaluated using non-linear response history analysis. Acceleration time histories from actual events are selected, modified, and used for the analysis. Synthetic motions are generated for target design spectra and used for analysis as well. Various options for target spectra are discussed. Currently implemented simplified procedures are evaluated. Alternative, direct displacement based design procedures are explored. Various bridge types commonly to the region are discussed. Bridge types selected for study include four pile bent bridges as well as 2 and 3-span grade crossings with multi-post, reinforced concrete bents supported on friction pile caps.

Isolation is proposed as an alternative which should be considered taking into account all factors, including economy. The proposition is made that isolation may be both effective and economical for certain bridges in the New Madrid Seismic Zone. To this end, the economic benefits which could potentially offset the cost of isolation bearings are explored.