Advancing the seismic design of reinforced concrete bridge columns

Recent earthquakes near highly populated cities have reminded the world and engineers about the destructive capabilities of an earthquake event. This is especially true in the design of reinforced concrete bridge columns that need to survive an event to maintain life safety and provide operational capacity along major lifelines to provide immediate assistance to the regions affected. The research presented within this document examines the current state of practice and looks for ways to improve the design methodologies prior to an event that highlights unforeseen consequences. This is accomplished through a series of analytical and experimental studies that include: (1) the development of a new simplified method for the lateral load response of columns continuously supported on drilled shafts; (2) the behavior of concrete and soil when subjected to frozen conditions; and (3) the impacts of current methodologies for the establishment of transverse confinement reinforcement on the seismic response of reinforced concrete bridge columns.;A new approach is discussed within that defines an equivalent cantilever supported by a flexible base using a set of three springs for the establishing the lateral load response of columns continuously supported on drilled shafts in non-cohesive soils. This is an extension of the work presented in Appendix A in order to provide a consistent approach for both cohesive and non-cohesive soils that may be encountered near a bridge site. The effective height of the system was defined as the distance from column tip to the point of maximum moment to identify a critical design location for the establishment of transverse confinement reinforcement. The new method was found to adequately capture the response when compared with the experimental work of Chai and Hutchinson (2002) and analytical models produced in LPILE.;Controlled material tests were performed on concrete and soil to examine the effects of seasonal freezing on their respective behaviors. Testing was conducted in controlled environments to maintain the temperature throughout the entire loading protocols. In each set of tests, it was concluded that significant changes would occur to the engineering properties when subjected to subzero conditions.;• Materials testing on confined and unconfined concrete provided evidence that both the confined and unconfined compressive strength would increase as temperature decreased. The strain at the peak compressive strengths increased for the confined concrete and decreased for the unconfined concrete. Furthermore, the modulus of elasticity increased in both instances. These changes are important in ensuring an accurate moment-curvature response of the reinforced concrete sections used in a seismic design.;• Soil testing found that an increase in strength by a factor of 10 and 100 would occur at temperatures of -1 °C (30.2 °F) and -20 °C (-40 °F) when compared with the warm weather testing at 20 °C (68 °F). This is of importance as the upper levels of soil cause changes that result in the shifting of the maximum moment location and increasing of the foundation shaft and column lateral shear demands.;The final portion of these studies was an investigation into the required amounts of transverse confinement reinforcement. The study indicated that the variation in requirements could be as high as a factor of two to three depending on the approaches compared within the study. Furthermore, the information was extended to the impacts of the approaches on the curvature and displacement ductility capacity and demand. This study found that current equations may result in demand exceeding capacity and a new equation should be developed that takes into account the expected demand, axial load ratio, amount of longitudinal reinforcement, material properties and the ratio of the gross area to core area of the concrete cross-section.