ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297024
50 | ASPIRE , Spring 2012 F H WA F H WA H ighway and transportation agencies are adopting innovative methods, technologies, and materials. They are using systematic preventive maintenance, bridge management systems, and asset management principles to balance demands and resources, while maintaining a high level of safety in highway bridges. This article summarizes a framework to achieve more resilient highway bridges. Resilient Highway Bridges Resilient concrete highway bridges have the capability to withstand unusual or extreme forces without collapse or loss of lives. They are able to recover from distress or major damage with minimal disruption to traffic and essential services. Three key factors affect the resilience of highway bridges: ductility, redundancy, and operational importance. • Ductility in a str uctural system is characterized by development of significant and visible inelastic deformations before failure. • Redundancy may be defined as the capability to continue to carry l o a d s a f t e r t h e fa i l u r e o f o n e o f its components. In other words, a redundant bridge system has multiple- load paths for distributing the loads when a component fails. • Operational importance relates to the consequences of loss of use of the bridge. Rapid emergency response is important for the survival of people and the security of the incident scene. T h e A A S H T O L R F D B r i d g e D e s i g n Specifications recognizes the significant effects of ductility, redundancy, and operational importance on the resilience of concrete highway bridges. The LRFD Specifications accounts for these effects on the load side of the limit states equation. It recommends the use of multiple load paths and continuous bridges, unless there are compelling reasons for not doing so. The National Cooperativ e Highway Resear ch Program (NCHRP) Repor ts 406 and 458 contain discussions and recommendations on quantifying redundancy in highway bridge superstructures and substructures, respectively. There is an ongoing N C H R P P r o j e c t 1 2 - 8 6 " B r i d g e S y s t e m S a fe t y a n d Re d u n d a n c y " for developing a methodology to quantify bridge system reliability for redundancy. The project is scheduled for completion in July 2012 with recommendations for updating the LRFD Specifications and the AASHTO Manual for Condition Evaluation. The FHWA publication titled Framework for Improving Resilience of Bridge Design, Publication No. FHWA-IF-11-016, introduces the fault-tree methodology for performing failure analysis during design. A bridge designer goes through a fault-tree analysis mentally in making sure that the design is devoid of weaknesses or trouble spots that could lead to bridge closure or failure. This is generally adequate for simpler and more common types of concrete bridges. For more complex bridges, it is desirable to perform a fault-tree analysis to systematically determine all potential contributing factors or events that could lead to a bridge failure. The contributing factors or events can then be considered and carefully addressed in the design. Fault-Tree Analysis A fault-tree analysis can be carried out using a fault-tree diagram. The diagram is a graphic model that shows parallel and sequential failure paths that can lead to an undesirable outcome; in this case a bridge failure. The fault-tree diagram is helpful in determining potential failure modes and their interactions. A fault-tree diagram is developed in a top-down direction. In this application, the top event is the failure of the bridge. The events immediately beneath the top event lead to the execution of the top resilience of Concrete Highway Bridges by M. Myint Lwin, Federal Highway Administration The new I-35W Bridge across the Mississippi River in Minnesota provides continuity throughout the bridge and state- of-the-art smart bridge technology that monitors bridge behavior in real time. Photo: FIGG Bridge Engineers. Portion of the girder bridge fault tree. Diagram: Federal Highway Administration. Bridge Failure Design/Operation Superstructure Inspection Construction Fabrication Substructure Girder/Beam Bearings Concrete Deck Events Events Events Book_Spr12.indb 50 4/3/12 9:18 AM