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The efficiency of a collar in reducing the scour depth around circular and rectangular piers is studied at different flow intensities (ratio of upstream shear stress to sediment critical shear stress). Rectangular Piers aligned with the flow as well as skewed at 5º, 10º, 20º were examined. Previous studies had shown that with collar the equilibrium time of scouring increases considerably. To reduce the time of experiments low density sediment was used as the bed materials. Comparison between test results and available results with natural sediment showed that, though the relative equilibrium depths were approximately similar, the time to reach equilibrium condition diminished to less than 10 hours with low density sediment. Experimental results for circular and aligned rectangular pier showed that at u*/u*c=0.95 to 0.75 the collar could reduce the maximum scour hole from about 20% to 60% respectively. In rectangular pier, by increasing the skew angle and/or the flow intensity, the efficiency of collar decreased.

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In this article, general procedures for vulnerability assessment and retrofitting of a generic seismically designed bridge are outlined and the bridge’s damage criteria for blast resistance are explained. The generic concrete bridge is modeled and analyzed with the finite element technique implemented in ANSYS LS-DYNA environment and explosion threats are categorized into three main levels. Uncoupled dynamic technique is adopted to apply the blast loads on the bridge structure, damage and performance levels are resulted based on quantitatively verified damage mechanisms for the bridge members. The results show that, amongst different loading scenarios, the explosions that happen under deck are more critical comparing to blasts initiating from over deck sources. Furthermore, two retrofitting methods 1) concrete filled steel tube (CFST) and 2) concrete jacket are applied on the bridge columns. The program AUTODYN is used with coupled dynamic analysis of a column to compare the effectiveness of each method. Afterward, more efficient method for a column is applied to the whole bridge and its efficiency is revaluated. It is shown that CFST can decrease concrete spall, scabbing, rotation, displacements and shear forces more than the concrete jacket. Considering the proposed damage and performance levels, the bridge retrofitted with CFST reacts with lower damage level and higher performance level to blast loads.