SEISMIC FRAGILITY OF THREE-SPAN REINFORCED CONCRETE SLAB BRIDGES DERIVED FROM PUSHOVER AND RESPONSE SPECTRA SIMULATIONS

Abstract eng:
Three-span slab bridge is a very common structural type, often used as overpass at traffic route crossings. This paper deals with seismic fragility of these structures. Infrastructure of the traffic network contains many of these bridges, which were constructed in different periods, and their design thus follows rules of different seismic code generations. The assessment of seismic fragility of existing bridges according to current codes is an important step towards increasing resilience of transport networks. To facilitate the assessment of these structures, this study presents results of seismic analysis on a generated population of bridges of this type. Further, simplified equations for seismic capacity estimate were derived using the basic structural properties. Characteristics of the bridge population were defined using 12 probabilistic variables that define the bridge geometry (length, height, skewness, pier dimensions, etc.), reinforcement amounts, and the soil/foundation. The reinforced concrete slab is supported by two rows of piers with monolithic connection; and elastomeric bearings are at both abutments. Properties of individual bridges were generated by Latin Hypercube Sampling of the probabilistic variables. The generated bridge population contains 20.000 samples in total. Geometric variables were modeled with a uniform distribution within the bounds that are commonly observed among existing structures. Since the reinforcement ratios vary considerably among existing structures, the longitudinal reinforcement ratio of the piers ranged from 0.4 % to 5 % in the whole bridge population. Similarly, amount of shear reinforcement varied considerably. Seismic analysis was performed on all 20.000 bridge samples using the response spectra method and singlemode pushover analysis. In here, the post-buckling behavior of longitudinal reinforcement in compression was considered using a simplified multilinear stress-strain diagram. The generated results enabled calculation of fragility curves for different sub-groups of the generated bridge population. Most critical behavior showed structures with low reinforcement ratios and small heights. The generated set of inputs (bridge properties) and outputs (seismic capacity) allowed creating simplified equations that define their relation. In here, the coefficients of the equations were fitted by regression techniques; and they provide estimate of seismic capacity and its variance.

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