THE LINK BETWEEN SUPERSTRUCTURE OF BRIDGES AND IT'S FOUNDATION (SUB-STRUCTURE) SHOULD BE DETERMINED BY LEVEL OF EROSION AND THE LIMIT OF SCOUR.

For the last some times bridges are facing infrastructure failure, more than required and it is necessary to verify the facts and reasons behind this, even though the sub -structure below the bearings and above foundation is a typical arrangement know as sub -structure, where in the intermediate support units over superstructure rests are known as piers and the end supports of a superstructure are known as abutments. The top of the pier and abutments are generally provided with coping or bed blocks and the bottom part of the pier knows as footing is to transfer the load from pier to the foundation. In bridge over water way the height of bridge pier is determined by the level of erosion. The presence of bridge piers and abutments causes obstruction in water way and reduces the effect ive cross sectional area of the stream. This results increase in the velocity of the flow and raises water level. The presence of the obstruction due to piers sets up eddy currents around piers cross section. The effect of eddy currents and increase in the velocity is to cause scour. If the scour depth exceeds the foundation depth there is possibility of the bridge failure. In order to avoid bridge failures due to erosion, the depth of pier should be more than the lower limit of scour, as there is no reliable analysis to determine the depth of erosion. A general rule is to take depth of erosion, as four times the height through which the water level rises at the pier due to obstruction. The bottom dimensions of the pier are determined from the considerations that no tensile stresses are produced in the section due to all the forces acting on it. The piers are generally provided with batter between 1 in 50 to 1 in 12.The pier footing is generally rectangular in shape and projects about 30 cm beyond the edge of the pier at the base and may rest directly on the firm soil or other suitable foundation. As for as design load for piers is concerned, it is relatively simple and does not present any problems, however the loads on piers may be properly computed and the design verified for the worst combination of loads and the following loads should be taken into consideration while designing the bridge piers:-(1) Dead load of the superstructure (2) Dead load of the piers less than any hydrostatic uplift caused by surrounding water. (3) Live load on the superstructure, the effect of the load eccentricities due to only some spans loaded should be taken into consideration. (4) Lateral forces perpendicular to the centerline of the superstructure. These forces include effect of wind on superstructure and moving loads, wind pressure on the portion of pier above water level pressure due to water currents wave action and ice and the centrifugal force in case of rail road bridges on the curves. (5) Longitudinal forces acting parallel to the direction of the bridge. These forces are caused by tractive effort of vehicles, braking of vehicles, unequal frictional resistance of bearing and unbalanced thrust in case of the arch bridges. (6) Seismic forces, considered for computation of straight forward in case of the dead loads. The design of bed block is simple but it is necessary to follow effects of the combination of link between superstructure of the bridges and its foundation and appropriate design requirements for structure below bearings, known girder bearings, rests over the bed block and coping must have suitable strong hammer head piers design layout plan for the typical RCC trestle , so required to avoid any losses at this place of the load bearings, failing which it may be distressed if not designed as per the parameters required by the design load of a pier. :- Er Fateh chand Guleria, RTI welfare Association registered number HPCD, 3552 Bilaspur Himachal pradesh phone number, 9459334377.