عنوان

بررسی اثرات فاصله پایه های پل بر شدت آبشستگی موضعی اطراف پایه ها

Text of Note

The scour of bridge piers is one of the most important factors that effects stability of constructed bridges over rivers. As a result of scour , a cavity is formed around the piers which causes destruction of the bridge in case of a big flood.To date, so many researches and studies have been done to estimate the depth and intensity of scour around the bridge piers. However, due to the complexity of the problem, no solution has yet been presented to enable the designers to estimate the intensity of scour . Each researcher has presented his own formula and theory which has its own specific applicational area.Researchers such as Laursen, Shen, Breusers, Elliott, Baker, Raudkivi and Ettema have all done valuable studies and have examined the effective factors on scour . The specifications of flow, geometric form of the waterway and pier, and the specification of sediment are among the important factors in this regard, which are studied by the mentioned researchers. The pier spacing is one of the effective factors which is studied the least. Elliott and Baker (1985) studying the effect of the above mentioned factor in fixed depth with various velocities, have surveyed the effect of this factor on clear water.In order to expand the studies in this field, the I5 bridge of California has been modelized using BRI-STARS software. The BRI-STARS model is a semi-two-dimensional model for water and sediment-routing along with an integrated graphical interface that is capable of calculating the erosion or alluvium accumulation through subcritical, supercritical or a combination of both forms of flow which include hydraulic jumps. This software is based on the theory of stream tubes. Using stream tubes enables us to have change in longitudinal and lateral hydraulic conditions along the canal length in coordination with the activity and movement of sediments in different cross sections existing in the study area. With the selection of several stream tubes, the model is made to be two-dimensional, so that the geometric changes of cross sections along with bed elevation can be studied in longitudinal and lateral directions. In the mentioned software, all the energy, momentum and sediment-routing equations are used, which by using the theory of total stream power minimization, the probability of progress of scour or aggradation in lateral or vertical directions is surveyed and finally the water surface profile is computed.After modeling the I5 bridge study area using the existing data, firstly, there was the possibility of a comprehensive study on its hydraulic, hydrologic, geometric and sedimentary parameters in the initial state. If these changes, which are the worst during a flood, are selected as a basis for comparisons, we can gain different results with modeling speculative pier(s) in different sections for bridge construction Continued: in various numbers, dimensions and x/b ratios. It should be mentioned that for the computation of the scour of the areas around the bridge piers, in accordance with FHWA recommendations, the formulas of Froehlich and CSU are more suitable than others.In the present study, at first, a comparisons was made between these two formulas and their related results and then studying the results and output data from different models analysis on the basis of Froehlich equation, in addition to the selection of the optimum section and bridge location with regard to the scour conditions, the changes in Froude number and flow type plus grade of local scour because of the position of the piers and also changes in depth of flow have been studied and with the processing of the output data, the best and suitable diagram along with the equation of increase of the depth of scour according to different x/b for selected cases have been foreseen. Finally, the effect of x and b parameters during the decreases of x/b, when we get closer to critical conditions, has been studied separately and therefore the amount of the influences of x and b parameters on x/b, and eventually changes made in bed elevation can be displayed. We should mention that, in all researches done on the spanning the piers and its effect on scour , x/b ratio have been considered. The computed figures of bed elevation in the end of last interval for similar models with Froehlich equation is lower In other words, Froehlich equation comparing with CSU is more conservative and computes scour more intensely using csu formula the increase of scour in models with ratio, x/b>7 can be ignored and changes are very low, and this result conforms to the studies of Elliott and Baker. For Froehlich formula the above mentioned conditions are provided during x/b>8 and we ignore the effect of the piers spacing on scour.The condition of one pier is more suitable because of its more space for water and also the decrease of the possibility of interference of horseshoe vortexes , and section 13 is not suitable for bridge construction because of river canal contraction at the entrance orifice which shows more scour.Section 11, which is selected at orifices of canal entrance, because of its location in the begining of the canal contraction has the most water speed and max. Changes of water depth and bed elevation this section includes a critical case in the worst conditions, so far all sections and cases it will be Fr? 1.Generally grade of local scour, along with decrease of the x/b ratio specially for figures less than 4, is increased. The depth of flow also is effective on scour in y/b<3.2, which conforms to the research of Raudkivy and Ettema (1983). With the decrease of piers distance, flow depth effect reaches y/b<2.75.With regard to the changes in the bed elevation, when x/b ratio is decreased, in different cases of modeling, we notice that for x/b<4.2 decreasing x or increasing b have the same effect on scour changes. For 4.26.7 ratios, increasing b will have more effect