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The amount of forces exerted is mainly depends on the self-weight of the structure. They induce both vertical and horizontal forces during earthquake. When the bridge is to be built in seismic zone or earthquake zone, earthquake loads must be considered. To resist this, additional steel reinforcement perpendicular to main reinforcement should be provided. These stresses are tensile in nature so, concrete cannot withstand against this and cracks are formed. When the temperature is very high or very low they induce stresses in the bridge elements especially at bearings and deck joints. Thermal stresses are caused due to temperature. K = constant (value depending upon shape of pier) The forces caused by water currents are maximum at the top of water level and zero at the bottom water level or at the bed level. The water current induces horizontal forces on submerged portion. When the bridge is to be constructed across a river, some part of the substructure is under submergence of water. Is the depth of submergence is less it can be negligible. Hence, in this case design should be done for centrifugal forces also.Ĭentrifugal force can be calculated by C (kN/m) = (WV 2)/(12.7R)īuoyancy effect is considered for substructures of large bridges submerged under deep water bodies. If bridge is to be built on horizontal curves, then the movement of vehicle along curves will cause centrifugal force on to the super structure. So, IRC recommends 20% of live load should be considered as longitudinal force on the bridges. When the vehicle stops suddenly or accelerates suddenly it induces longitudinal forces on the bridge structure especially on the substructure. The longitudinal forces are caused by braking or accelerating of vehicle on the bridge. For long span bridges, wind load is considered in the design of super structure. But for medium span bridges, wind load should be considered for substructure design. For short span bridges, wind load can be negligible. Wind load also an important factor in the bridge design. For substructure greater than 3 m depth I f = 0.For substructure up to the depth of 3 meters I f = 0.5 to 0.If the length exceeds in any of the above limits, the impact factor should be considered from the graph given by IRC which is shown below.įor IRC class A and class B loadings Impact factor I f = A/(B+L)Īpart from the super structure impact factor is also considered for substructures The impact factors for different IRC loadings are given below.įor IRC Class AA Loading and 70R Loading SpanĢ5% up to 5m and linearly reducing to 10% from 5 m to 9 m. Impact factor is a multiplying factor which depends upon many factors such as weight of vehicle, span of bridge, velocity of vehicle etc. To consider impact loads on bridges, an impact factor is used. When the wheel is in movement, the live load will change periodically from one wheel to another which results the impact load on bridge. The Impact load on bridge is due to sudden loads which are caused when the vehicle is moving on the bridge. Both IRC class A and Class B are shown in below figure. This type of loading is used to design temporary bridges like Timber Bridge etc. When we design a bridge using class AA type loading, then it must be checked for class A loading also. It is considered as standard live load of bridge. This type of loading is used in the design of all permanent bridges. In class AA loading generally two types of vehicles considered, and they are This type of loading is considered for the design of new bridge especially heavy loading bridges like bridges on highways, in cities, industrial areas etc. The vehicle loadings are categorized in to three types and they are So, IRC recommended some imaginary vehicles as live loads which will give safe results against the any type of vehicle moving on the bridge. but it is difficult to select one vehicle or a group of vehicles to design a safe bridge. The moving loads are vehicles, Pedestrians etc. The live load on the bridge, is moving load on the bridge throughout its length. It is the first design load to be calculated in the design of bridge. The different elements of bridge are deck slab, wearing coat, railings, parapet, stiffeners and other utilities. The dead load is nothing but a self-weight of the bridge elements. Various design loads to be considered in the design of bridges are: Types of Loads for Design of Bridge Structures For IRC Class AA Loading and 70R Loading.
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Types of Loads for Design of Bridge Structures.