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Concrete In Australia : March 2008
Bridge Failure due to poor durability Considering major sub-tree of the entire fault tree Superstructure Deterioration Substructure Deterioration Deck Deterioration Girders Deterioration Abutments Deterioration AB C Piers Deterioration D Bearings Deterioration E Figure 1. Top-level fault tree events. events in the fault tree were evaluated by simulation of scour equations presented in other researches. He also stated that it is possible to examine a more complex fault tree that would lead to a bridge failure. Another fault tree model of bridge deterioration has been developed to calculate the probability of bridge deterioration by LeBeau and Wadia-Fascetti (2000). The probabilities of basic events were obtained by assigning questionnaires to seven bridge engineers and inspectors. A comparison between the effi ciency of different rehabilitation alternatives also has been evaluated. However, the probability of deterioration is obtained under a number of assumptions on bridge structures and exposure environment is not considered in this model. Proposed method Development of the complete fault tree for a bridge A reinforced concrete bridge is comprised of superstructure and substructure which can also be further divided into several components, as shown in Figure 1. The top event of this fault tree is defi ned as bridge failure due to poor durability. Each of the components of failure A-E can be decomposed further. This is an overall frame of the fault tree model. After fully examining each component of failure, the overall failure risk of a bridge can be assessed. The fault tree method concerns all the possible conditions that could lead to the occurrence of major distress mechanisms, the output risk ratings can be regarded as a prediction of the performance of the bridge or bridge components during service life. The model also can be used to rank the risk of failure of a number of bridges based on suffi cient construction and inspection data. Parameters needed to establish a risk ranking of bridges To analyse the failure risk of a bridge or a selected bridge component, information on the elements being assessed is needed. This will require information on: • major failure modes; • the likelihood of the factors that contribute to each of the major failure modes • the consequences of each of the failure modes. Following sections will describe the method to acquire these parameters respectively. Chloride attack ASR I Piles Deterioration At this stage, the research mainly focuses on the sub-tree related to pier deterioration for several reasons. Piers are crucial components in a reinforced concrete structure. They are usually located in the tidal, splash or submerged zone which is directly exposed to an aggressive environment. Thus the problem of pier deterioration is considered as a major issue. By examining the pier branches, the analysis of pier conditions can be accomplished which might reflect the durability of the bridge at a certain extent. Other components failure can follow the same path to composition and analysis to get an overall risk of an entire bridge. Figure 2 shows the sub-tree of piers mentioned in this research. Piers Deterioration Headstocks Deterioration Columns Deterioration Pilecaps Deterioration Piles Deterioration FG Figure 2. Sub-tree of piers. Application of the proposed method Fault tree for failure of piers Based on literature review and case studies, the following distress mechanisms were identified: • chloride induced corrosion • alkali-Silica reaction • carbonation • stress corrosion of reinforcement • plastic shrinkage. These major distress mechanisms were selected as key failure modes because they obviously indicate defi ciencies in material durability of reinforced concrete bridges. They can often lead to cracking, spalling, honeycombing of concrete and signifi cant reduction of structural safety (Venkatesan et al 2006). Figure 3 presents the fault tree for piles deterioration. H I Carbonation Figure 3. Fault tree of piles malfunction. Concrete in Australia Vol 34 No 1 51 Stress Corrosion Plastic Shrinkage