دراسة عددية لشكل الدعامة الأمثل للحصول على جسور آمنة

Numerical Study of Optimum Pier Shape for Safe Bridge

     Local scouring around bridge piers has been identified as one of the main causes of bridges failure around the world, which is significantly affecting the total construction and maintenance costs. Research on local scour around the pier of bridges have shown the lack of understanding the effects of the main parameters such as a pier shape on local scour depth. Thus, the main goal of this study is to investigate the effects of bridge piers’ shape on local scour to determine the optimum pier shape that gives minimum scouring. In addition, an empirical equation has been developed to predict scour depth based on parameters obtained from dimensional analysis in terms of shape factor, flow intensity, flow depth, pier width and angle of attacks by employing gene expression programming (GEP) and statistic non-linear regression using Statistical Package for Social Sciences (SPSS).

       Various shapes of piers are used in this study including circular, square, rectangular, elliptic, oblong, octagonal, hexagonal, ogival and lenticular to decrease the influences of local scour around bridge piers. A computational fluid dynamic -based simulation to compute the depth of local scour around piers using Flow-3D model is used in this study. The effectiveness of the model evaluated and validated using experimentally obtained data from Melville 1975. 729 runs were performed for each pier shapes at three different values of flow intensity V/VC (0.55, 0.76, 1), fluid depth ratio y/b (0.2, 0.98, 2.95), pier width ratio b/B (0.11, 0.15, 0.2) and angle of attacks (0, 30°, 45°). All these runs were simulated in non-cohesive bed sediment under clear water scour conditions.

      The outcome of comparing numerical results of predicting scour depth around circular pier with laboratory experiment of Melville 1975 shows that the numerical model validates a good agreement with experimental model with the maximum percentage of error between the experimental and the numerical models of 10%.

      Models results revealed that maximum scour depth observed at rectangular shape while minimum depth of scour occurred at lenticular shape. The results obtained by the simulation on four pier shapes namely circle, square, elliptical and lenticular that have the same volume of concrete and surface area, also showed that minimum depth of scour was observed around the lenticular pier with about 70% lower than square pier shape. Based on the results, it can be said that lenticular pier provides optimum hydraulic design of pier. Additionally, the scour depth increases with increasing flow intensity, flow depth, pier width and angle of attacks.

     Based on the simulation results, the equation obtained using GEP model performed better predicting to scour depth than SPSS model with coefficient of determination (R²) and root mean squares error (RMSE) of 0.89 and 0.152, respectively. The empirical equation derived to predict the scour depth based on shape factor, flow intensity, flow depth, pier width and angle of attacks. Sensitivity analysis results suggests that the flow depth has a major influence in predicting the local scour in comparison to the other input parameters.