Direct Displacement Based Seismic Design of Continuous Curved Bridges

This dissertation aims to contribute to the development of AASHTO Guide Specifications for LRFD Seismic Bridge Design (AASHTO Guide Spec.) and Direct DisplacementxxxBased Design (DDBD) approach for Reinforced Concrete (RC) curved bridges.;AASHTO Guide Spec. provides guidelines for the seismic design of bridges and is recommended for the entire United States. The design philosophy of this code is displacement-based in nature and is preferred over the traditional force-based design method. In order to determine the seismic demand on the curved bridge; AASHTO Guide Spec. propose that curved bridges may be analyzed as if they are straight, provided the bridge is regular. This hypothesis is evaluated through a large parametric study that consists of curved bridges with subtended angles varying from 0 to 180 degrees and having the total arc length equal to the length of the equivalent straight bridge. Other parameters considered are the number of spans (four and six) and abutment restraints (nine different cases), along with several pier height and span length configurations. The evaluation process includes the seismic design of the equivalent straight bridges using the DDBD procedure. The resulting designs are then analyzed with Inelastic Time History Analysis (ITHA) using both straight and curved bridge geometries for 7 spectrum compatible time histories. It is observed from the comparison of equivalent straight and curved bridge ITHA results that deviation in the displacement response of the curved bridges from the equivalent straight bridge increases as the subtended angles become larger. However, variables such as span length, pier height, and number of spans are found to be less important for the bridges considered in this study. It is also found that the type and degree of abutment restraints are critical parameters in controlling the response of the bridge and should be incorporated to the AASHTO Guide Spec. to decide if the curved bridge can be analyzed as an equivalent straight bridge.;In DDBD procedure, the inelastic system is modeled by an equivalent linear system using effective stiffness and Equivalent Viscous Damping (EVD) at peak response which is consistent with the concept of performance-based design. Recently, the design approach is documented in a draft code known as 'DBD12: A Model Code for the Displacement-Based Seismic Design of Structures' for wide range of building systems and bridges. With an effort to extend the DDBD approach for curved bridges, this part of the dissertation investigates the issues related to the DDBD procedure of straight bridges when implemented for curved bridges. It is observed that, unlike straight bridges, the seismic response of curved bridges is coupled in both lateral directions. Hence, modifications are proposed to extend the current DDBD procedure of straight bridges to curved bridges. The extended DDBD method is evaluated through a parametric study that consists of 6 case study bridges having 4 and 6 spans with different pier heights configurations. Furthermore, two superstructure geometries each with subtended angles of 30, 60, and 90 degrees are considered. Each case study bridge is designed using extended DDBD method of curved bridges in both longitudinal and transverse directions. For verification of the design procedure, ITHA is performed by using seven spectrum compatible time histories, and the results of DDBD are compared with ITHA. The results show that extended DDBD approach for curved bridges is capable of predicting the seismic response in both longitudinal and transverse directions (with few exceptions). Furthermore, the results indicate that superstructure to substructure stiffness and pier stiffness irregularity are important parameters that cause an increase/decrease in the deviation between target displacements (predicted by DDBD) and average ITHA displacement envelope.;This dissertation also investigates the effect of different parameters on EVD for short period (effective period less than 1 second) single-degree-of-freedom (SDOF) systems. EVD is used to idealize the nonlinear system as an equivalent linear system in DDBD. Past research indicates that ductility, hysteretic model type, and effective period are the primary factors that affect the EVD for short period SDOF systems. However, it is investigated by the authors that EVD is also a function of width of the constant acceleration region of the acceleration response spectrum and is significantly affected by the post yield stiffness ratio of the hysteretic model. This investigation is conducted for Modified Takeda Degrading Stiffness hysteretic model using large number of ground motions. New expression for EVD is proposed which includes the effect of width of the constant acceleration region of design spectrum and post yield stiffness ratio, as well as effective period and ductility. The proposed damping model is compared to two existing models. The results indicate that significant improvement is achieved in predicting the peak displacement using the proposed damping model when compared to existing models.