Reliability analysis of a reinforced concrete deck slab supported on steel girders
Analyse de fiabilité d'un bloc de béton armé sur des poutrelles d'acier
Résumé
The premature deterioration of concrete bridge decks is a multi-billion dollar problem in the United States. In December 2003, the Federal Highway Administration estimated that approximately 27 percent of the 592,000 nation's bridges are considered structurally deficient or functionally obsolete. It would cost about 80 billion dollars to bring all of the nation's bridges to an acceptable and safe standard by either rehabilitation or replacement. Moreover, according to the data of the “national bridge inventory” obtained from the U.S. Department of Transportation, it is estimated that deficiencies occur mostly in the decks in more than half of the bridges in United States. Not only bridge deck deterioration is an economic problem; it is also a risk to those who traverse the structure. Forms of deterioration can range from slightly damaged deck surfaces, causing unpleasant sights and decreasing bridge deck serviceability, up to spalling of large pieces of concrete that reduces the structural integrity and it can be a danger for the public. Therefore, there is a compelling need to understand the behavior of bridge decks under service load and develop a reliable procedure to assess the serviceability of the deck, which will then serve as a decision-making tool for the rehabilitation or the replacement of the decks. In the United States, most of the bridge decks are constructed as reinforced concrete slabs supported by steel or precast prestressed girders. Such decks have traditionally been designed using the “strip method”, based on a conventional beam theory, which assumes that the slab is continuous over fixed supports. As a result, the top part of the slab is reinforced with steel bars to resist the negative moments, and the bottom part of the slab is reinforced with steel bars to resist the positive moments. Temperature and shrinkage reinforcement is added orthogonally at the top and at the bottom. When cracks occur in concrete, the top reinforcement can be subjected to environmental agents and aggressive chemicals; such as deicing salt, and it can start to corrode. The corrosion can result in a lateral expansion of the steel bars, leading to spalling of concrete cover and subsequent formation of potholes. Previous research in the United States and mainly in Canada showed that the flexural capacity of bridge decks can be increased by the presence of in-plane compressive forces, created when the deck is restrained by supports that cannot move laterally. This phenomenon is referred as “arching action” and is the basis of the empirical design provisions of the Ontario (Canada) Bridge Design Code (1993). This empirical method has been adopted in the current AASHTO LRFD code (2005), and it is referred to as isotropic reinforcement. According to the empirical method, arching action requires less steel reinforcement than that required by the strip method of AASHTO LRFD code (2005). Therefore, it is believed that the decks designed by empirical method are more resistant to deterioration due to fewer sources of corrosion (fewer steel rebars). At the present, there is no assessment method available to evaluate the serviceability and durability of bridge decks. Therefore, in this dissertation, a procedure for bridge decks evaluation is developed, which is focused on evaluation and comparison of bridge decks performance for the two aforementioned design procedures. A reliability based method associated with a state of the art nonlinear finite element analysis, calibrated using field tests, is developed in order to understand the structural behavior of the deck and to assess its performance.
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