RELIABILITY ANALYSIS OF REINFORCED CONCRETE BRIDGE DECKS SUBJECT TO FATIGUE
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Problem Statement
Deterioration of reinforced concrete bridge decks has been a major problem in the country. Both Federal Ministry of Works and Federal Roads Maintenance Agency records show that as at the year 2008, there were 1,696 Bridges that had been completed from year to year and were in use on the Federal Highways nationwide. However,as at date some of these bridges are requiring emergency intervention and some have even been shut to traffic. 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 (Mabsout et.al 2004).
It is not only an economic problem but also risk to commuters who traverse these bridges. Some of the deficiencies may include; slight damages to the deck surfaces, spalling of top concrete surface and the decrease in serviceability of the deck which may lead to danger to the public. As such there is the need to understand the behaviour of bridge deck under service conditions and develop a reliable way to check the serviceability of the deck.
In the Nigeria, most of the bridge decks are constructed as reinforced concrete slabs supported by precast prestressed beam girders. Such decks have traditionally been designed using the “empirical strip method”, based on a conventional beam theory, which assumed 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. It is believed that the decks designed by empirical method are more resistant to deterioration due to fewer sources of corrosion (fewer steel rebars). This empirical method has been adopted in the current AASHTO LRFD code (2010). In bridges, there are usually a large number of cycles of significant live load, and fatigue will almost always precede fracture. Therefore, controlling fatigue is practically more important than controlling fracture because this can result in increased displacement of the reinforcing bar relative to the concrete, known as slip (Rteil et.al, 2011).
Therefore, in this thesis, a reliability based analysis with a probabilistic view would be carried out to check the uncertainties in structural resistance and applied loads. Limit state function for flexural capacity of deck slab and beam as well as shear capacity in deck beam were developed and evaluated using FORM. The entire process was implemented through a developed MATLAB program.
1.3 Justification of the Study
In the past, fatigue failure has been primarily regarded as a design problem associated with civil engineering structures such as highways bridges, car parks and rail bridges which are subjected to reasonably well defined man-induced repeated loads. However, serious fatigue problems can also arise as the result of complex, naturally occurring, ill-defined patterns of repeated loads. For example, fatigue problems in concrete structures have arisen recently in relation to off-shore and marine construction(Karin et.al, 2010).
In RC slab damage process, microcracks occur and gradually develop, and at the final stage, large cracks occur and slab displacement increases sharply. Mid-span deflection caused by fatigue is thought to be a possible indicator for the degree of slab damage under the influence of overall progress of slab cracking.The fatigue lives of flexural members are typically a function of the stress range induced in the reinforcement (Braimah et.al,2006). It is practically hard to exactly interpret the loading histories of bridge deck.
The failure can occur even if the maximum stress is below the ordinary strength of the material. Some materials have a certain stress limit e.g. steel, which means that the stress variation below a certain level can be repeated infinitely many times without fatigue failure. Fatigue failure is characterized by fracture in a localized area of a structure which is exposed to cyclic loading (Karin et.al, 2010).
A single lane of a Class A highway bridge experiences an average daily truck traffic of over 1000, or 27 million trucks over a 75 year design life (CSA A23.3, 2010).Bridge decks directly sustain these repeated moving wheel loads and may be susceptible to fatigue damage (El-Ragaby et al. 2007). Fatigue life and fatigue strength of reinforced concrete elements areinfluenced by many factors including the material properties of the concrete and reinforcement, reinforcement ratio, transverse reinforcement, minimum and maximum values of repeated loading, range and rate of loading as well as environmental factors such as temperature and humidity. The response of a member subjected to fatigue loading is affected by both the strength of the materials and the interaction between the concrete and reinforcement (Higgins et al. 2006). It is generally agreed that the most important factor influencing fatigue behaviour is the applied load range,or rather the induced stress range in each component.
The results of this research can be usefully employed in practice only when appropriate fatigue design formats have been developed which provide adequate, but not excessive, margins of safety against fatigue failure. The safety margins and the associated design safety coefficients would allow realistically for inherent variability in loads, in the fatigue properties of the materials, and in the performance of members and systems under high cyclic loads.
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