Various types of loads are considered for design of bridge structures. These loads and their combinations decides the safety of the bridge construction during its use under all circumstances. The design loads should be considered properly for perfect design of bridge. Different design loads acting on bridges are explained below.
Types of Loads for Design of Bridge Structures
Various design loads to be considered in the design of bridges are:
- Dead load
- Live load
- Impact load
- Wind load
- Longitudinal forces
- Centrifugal forces
- Buoyancy effect
- Effect of water current
- Thermal effects
- Deformation and horizontal effects
- Erection stresses
- Seismic loads
1. Dead Load
The dead load is nothing but a self-weight of the bridge elements. The different elements of bridge are deck slab, wearing coat, railings, parapet, stiffeners and other utilities. It is the first design load to be calculated in the design of bridge.
2. Live Load
The live load on the bridge, is moving load on the bridge throughout its length. The moving loads are vehicles, Pedestrians etc. but it is difficult to select one vehicle or a group of vehicles to design a safe bridge.
So, IRC recommended some imaginary vehicles as live loads which will give safe results against the any type of vehicle moving on the bridge. The vehicle loadings are categorized in to three types and they are
- IRC class AA loading
- IRC class A loading
- IRC class B loading
IRC Class AA Loading
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. In class AA loading generally two types of vehicles considered, and they are
- Tracked type
- Wheeled type
IRC Class A Loading
This type of loading is used in the design of all permanent bridges. It is considered as standard live load of bridge. When we design a bridge using class AA type loading, then it must be checked for class A loading also.
IRC Class B Loading
This type of loading is used to design temporary bridges like Timber Bridge etc. It is considered as light loading. Both IRC class A and Class B are shown in below figure.
3. Impact Loads
The Impact load on bridge is due to sudden loads which are caused when the vehicle is moving on the bridge. When the wheel is in movement, the live load will change periodically from one wheel to another which results the impact load on bridge.
To consider impact loads on bridges, an impact factor is used. Impact factor is a multiplying factor which depends upon many factors such as weight of vehicle, span of bridge, velocity of vehicle etc. The impact factors for different IRC loadings are given below.
For IRC Class AA Loading and 70R Loading
|Span||Vehicle type||Impact factor|
|Less than 9 meters||Tracked vehicle||25% up to 5m and linearly reducing to 10% from 5 m to 9 m.|
|Wheeled vehicle||25% up to 9 m|
|Greater than 9 meters||Tracked vehicle (RCC bridge)||10% up to 40 m|
|Wheeled vehicle (RCC bridge)||25% up to 12m|
|Tracked vehicle (steel bridge)||10% for all spans|
|Wheeled vehicle (steel bridge)||25% up to 23 m|
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.
For IRC class A and class B loadings
Impact factor If = A/(B+L)
Where L = span in meters
A and B are constants
Apart from the super structure impact factor is also considered for substructures
- For bed blocks, If = 0.5
- For substructure up to the depth of 3 meters If = 0.5 to 0
- For substructure greater than 3 m depth If = 0
4. Wind Loads
Wind load also an important factor in the bridge design. For short span bridges, wind load can be negligible. But for medium span bridges, wind load should be considered for substructure design. For long span bridges, wind load is considered in the design of super structure.
5. Longitudinal Forces
The longitudinal forces are caused by braking or accelerating of vehicle on the bridge. When the vehicle stops suddenly or accelerates suddenly it induces longitudinal forces on the bridge structure especially on the substructure. So, IRC recommends 20% of live load should be considered as longitudinal force on the bridges.
6. Centrifugal Forces
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. Hence, in this case design should be done for centrifugal forces also.
Centrifugal force can be calculated by C (kN/m) = (WV2)/(12.7R)
W = live load (kN)
V = Design speed (kmph)
R = Radius of curve (m)
7. Buoyancy Effect
Buoyancy effect is considered for substructures of large bridges submerged under deep water bodies. Is the depth of submergence is less it can be negligible.
8. Forces by Water Current
When the bridge is to be constructed across a river, some part of the substructure is under submergence of water. The water current induces horizontal forces on submerged portion. 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 pressure by water current is P = KW [V2/2g]
Where P = pressure (kN/m2)
K = constant (value depending upon shape of pier)
W = unit weight of water
V = water current velocity (m/s)
G = acceleration due to gravity (m/s2)
9. Thermal Stresses
Thermal stresses are caused due to temperature. When the temperature is very high or very low they induce stresses in the bridge elements especially at bearings and deck joints. These stresses are tensile in nature so, concrete cannot withstand against this and cracks are formed.
To resist this, additional steel reinforcement perpendicular to main reinforcement should be provided. Expansion joints are also provided.
10. Seismic Loads
When the bridge is to be built in seismic zone or earthquake zone, earthquake loads must be considered. They induce both vertical and horizontal forces during earthquake. The amount of forces exerted is mainly depends on the self-weight of the structure. If weight of structure is more, larger forces will be exerted.
11. Deformation and Horizontal Effects
Deformation stresses are occurred due to change is material properties either internally or externally. The change may be creep, shrinkage of concrete etc. similarly horizontal forces will develop due to temperature changes, braking of vehicles, earthquakes etc. Hence, these are also be considered as design loads in bridge design.
12. Erection Stresses
Erection stress are induced by the construction equipment during the bridge construction. These can be resisted by providing suitable supports for the members.