Estimation of member forces in Fatih Sultan Mehmet (FSM) suspension bridge from ambient vibration records

Abstract

Suspension bridges are critical lifeline structures in transportation systems. Most suspension bridges, built in recent years, have monitoring systems to identify and track any changes in their dynamic characteristics. Such systems are useful for reducing the cost of maintenance during the life of the bridge, as well as assessing the structural safety for any extreme loading condition, such as those induced by large earthquakes and strong winds. The standard approach for analyzing data from the monitoring systems in suspension bridges has been modal analysis, where the dynamic response is approximated as the sum of modal responses, each defined by its modal frequency, damping ratio and the mode shape, which are identified from vibration records. Theoretically, modal analysis is appropriate for linear structures with mass and/or stiffness proportional viscous damping. Modal analysis is not appropriate for suspension bridges for several reasons. The dynamic behavior of a typical suspension bridge is not linear. Their size and flexibility make them geometrically nonlinear. In addition, their mass is not constant. They have time-varying mass due to moving traffic loads, which can be a significant portion of total mass in modern suspension bridges with lightweight steel decks. Also, all vibration records from suspension bridges show that damping identified, does not satisfy the requirements of classical modal damping (i.e., mass and/or stiffness proportional), and it is not a viscous type. Therefore, alternative methods for the analysis of vibration data from suspension bridges are needed. This study presents an alternative method for system identification of suspension bridges from their vibration records. Instead of identifying the modal properties of the bridge, the method aims to identify the forces in the principal bridge elements (i.e., main suspension, back-stay and hanger cables and towers). Based on some simplifying assumptions, this study first develops the equations that relate the element forces to the fundamental frequency of that element. The fundamental frequencies of the elements are identified from the ambient vibration records taken on the element. Using the equations developed, the forces in each element are calculated, and crosscheck to confirm that they satisfy the boundary conditions at element junctions. The methodology is tested by using the vibration records from one of the suspension bridges in Istanbul. Currently, Istanbul has three suspension bridges over the Bosphorus, and all installed with real-time monitoring systems. These bridges connect Asian and European parts of Istanbul, and are the critical lifelines for the city. The bridge used for the test is the second Bosphorus Bridge, known as the Fatih Sultan Mehmet (FSM) Bridge, which is between the first and the third suspension bridges on the Bosphorus with a daily traffic load of approximately 200,000 vehicles. The bridge is being monitored with a realtime Structural Health Monitoring (SHM) system composed of 44 channels of acceleration sensors. The forces in the members of the bridge are calculated by the methodology presented in this study. The results are compared to those from previous investigations (e.g., field tests, analytical models and design calculations), and are found to be consistent. The study shows that the fundamental frequencies of members identified from ambient vibration records provide a simple means to estimate the forces in the elements of suspension bridges.