Evaluation of Seismic Performance of The White Bridge in Ahwaz and its Retrofitting Methods

Arch bridges have a desirable behavior under gravity loads. They transfer the gravity loads of the deck, through the axial performance of arches. But the performance of these bridges, under lateral forces requires special analysis. Seismic forces are known as the most important lateral forces in most of the engineering structures. In this study, the seismic performance of a steel arch bridge has been evaluated, using a finite element model. The "White Bridge" in Ahwaz, in south east of Iran is selected as the case study.In part 1, a general description of the paper is provided.In part 2, different performance levels from different bridge codes and specifications have been studied. Performance levels are different conditions that are expected to occur, after earthquakes of different return periods, for bridges of different levels of importance.Part 3 is a review on rehabilitation of the bridges. Bridge rehabilitation, contains three main steps of determining the bridges critical in need of repair, evaluating the structure and site conditions of the bridges and planning a procedure to perform the rehabilitation.In part 4, the case study has been introduced. Ahwaz “White Bridge” is a pass-through steel arch bridge which has been constructed on the Karoon River in Khuzestan, Iran in 1936. The bridge has two main arch spans with 130 and 136 meters length and is one of the largest and oldest engineering designed bridges in Iran. The steel arches, have I shape sections and the deck, is a composite slab which has been rehabilitated in 1999. Two ends of the arches are pinned to the supports and the abutments are huge concrete boxes, filled by rock fill.Part 5 describes the finite element model of the structure, which is generated by SAP2000 software to model the performance of the bridge under seismic forces. Arches, hangers and braces, have been modeled by "Beam" elements and the deck and abutments, has been modeled using "Shell" elements. Dead load, is applied to the shell elements of the deck and live load, is applied to the longitudinal beams of the deck, based on Iran "139-bridge loading code". Seismic analysis has been performed using the response spectrum method, based on the first 12 mode shapes of the bridge. Accelerations in the three longitudinal, transverse and vertical directions have been applied based on Iran Earthquake "Seismic design of bridge" specifications.In part 6, results of the response spectrum analysis have been studied. Regarding the modal analysis, first deformation mode shape of the structure is longitudinal with a period of 2.87 seconds. Response spectrum analysis in longitudinal direction results in allowable displacements and stresses in different parts of the bridge. In vertical direction, same results are obtained, but in transverse direction, displacements and stresses in some parts of the bridge, are greater than the allowable quantities. Especially at un-braced parts of the arches (traffic route), stresses may exceed the yield limit and plastic hinges may be formed. Evaluating the transverse behavior of the bridge requires a non-linear analysis. In this study inelastic static analysis, pushover method, has been utilized.Part 7 describes the pushover analysis procedure. To evaluate the behavior of the bridge under transverse horizontal loads, point loads of certain magnitudes are applied to deck nodes and stresses in different parts of the slab and steel structure is monitored. The quantities of the loads is increased step by step and in each step, elements having stresses larger than yield, buckling and fracture limits, have been omitted. This process has been repeated until the structure becomes globally instable. Instability occurs when applied load is greater than what is applied to the structure in transverse response spectrum analysis. So, the bridge may not become unstable after a design earthquake.In part 8, results of the response spectrum analysis, have been verified through comparison with a time history analysis, which had performed using Naghan earthquake accelerations, scaled for the site specific conditions. Nodal displacements in different parts of the structure in the three directions are almost equal for two response spectrum and time history methods. Regarding the results of different analysis methods which have performed, In part 9, seismic performance of the bridge has been compared with the performance levels of different codes and specifications. It can be seen that at the existing conditions, the bridge performance is not acceptable and probable damages would require fundamental and long time rehabilitation.Part 10 briefly demonstrates the strengthening method which is proposed to improve the seismic performance of the bridge to acceptable levels.The strengthening plan is concentrated on the non braced parts of the arches and this stresses that traffic area of the arch bridges is the most vulnerable part in an earthquake.