With the continual deterioration of the US infrastructures, the matter of safety is consistently in question. Records show that 80% of the entire US transportation infrastructure, such as bridges, roads, railways, etc. is either deficient or functionally obsolete. This thesis focuses specifically on the seismic capacity and demand of bridge structures at different levels of deterioration that may occur over time. This thesis also relates the design demand and capacity of the original and deteriorated bridges. The purpose is to show that the existing conditions of deteriorated bridges are more critical under earthquake loading than expected. A simple bridge design was constructed using design guidelines from Caltrans. Accelerograms were taken from recorded earthquakes within the state of California. The model was developed in SAP2000 and nonlinear time-history analyses where performed for multiple earthquake motions to demonstrate the demand and the capacity of the bridge. Deterioration was predetermined as a 10, 25, and 50 percent reduction in the diameter of the longitudinal reinforcement bars and therefore, 15, 37.5, and 75 percent reduction in the diameter of the lateral reinforcement bars. The results did confirm that with deterioration, even as little as ten percent, the bridge was no longer able to provide the displacement capacity that it was originally designed for and failed when the displacement demands were larger than this reduced capacity. Current methods utilize only dead and live loads to determine the maximum load-carrying capacity of a structure; deterioration is not explicitly accounted for. The contribution of this thesis is a set of first-of-a-kind nonlinear time-history analyses that examine the two dimensional behavior of columns at critical sections, such that evaluation of the columns' reduction in rotation capacity due to deterioration under seismic loading can be formalized.