Types of Loads for Design of Bridge Structures
Here, the different types of load impacted in the bridge are as follows.
- Dead load
- Superimposed Dead Load
- Live load
- Impact load
- Wind load
- Channel Forces
- Longitudinal forces
- Centrifugal forces
- Differential settlement
- Shrinkage and creep
- Earth pressure
- Buoyancy effect
- Effect of water current
- Thermal stresses
- Seismic loads
- Deformation and horizontal effects.
- Erection stresses
1. Dead Load
2. Superimposed Dead Load
Other permanent objects such as parapets and road pavement, as well as other non-structural and architectural attachments to the bridge, are called superimposed dead loads. These things are long-term, though they may be replaced during the structure’s lifetime.
3. Live Load
The moving load on the bridge corresponds to the live load the entire length of the bridge. Vehicles, pedestrians, and other moving loads are examples. Selecting one vehicle or a group of vehicles to construct a safe bridge is difficult.
In anticipation of any form of vehicle moving on the bridge, IRC proposed some fictional cars as live loads, which will provide secure outcomes.
The following are the several types of vehicle loadings:
- IRC class AA loading.
- IRC class A loading.
- IRC class B loading.
3.1 IRC Class Aa Loading
This form of loading can be used to design new bridges, especially those with heavy loads, such as highway bridges, city bridges, and industrial bridges. Normally, the following sorts of vehicles are taken into account when loading a class AA vehicle.
- Tracked type
- Wheeled type
3.2 IRC Class A Loading
All permanent bridges must be designed with this type of loading in mind. It is handled as a conventional bridge live load. When designing a bridge for class AA loading, it should also be checked for class A loading.
3.3 IRC Class B Loading
This sort of loading is used to create temporary bridges such as Timber Bridges. It’s considered a light load.
4. Impact Loads
These sudden loads generated as the vehicle is travelling on the bridge induce the impact load on the bridge. Whenever the wheel is in movement, the live weight shifts from one wheel to the next, causing the impact load on the bridge.
An impact factor is used to calculate the impact loads on bridges. The impact factor is a multiplication factor that’s also depending on numerous elements such as vehicle weight, bridge span, vehicle velocity, and so on.
5. Wind Loads
The wind load is also a significant consideration in bridge design. Wind load on short-span bridges can be insignificant. For medium-span bridges, wind loads should be taken into account while designing the substructure.
Wind load is taken into account when designing superstructures for long span bridges. The surface areas of all elements (including superstructure and substructure) as seen in elevation are considered the exposed zone of the bridge (i.e., perpendicular to the longitudinal axis).
AASHTO specifies the loading on a bridge due to wind forces based on a wind velocity of (160 km/h). This translates to an intensity of (2.40 kN/m2 ) for standard girder/beam bridges, with a minimum total force of (4.38 KN/m).
5.1 Wind Load on Structure
- Transverse Loading = (2.40 kN/m2 ).
- Longitudinal Loading = (0.58 kN/m2 ).
5.2 Wind Load on Live Load
- Transverse Loading = (1.46 kN/m) .
- Longitudinal Loading = (0.58 kN/m).
6. Channel Forces
The loads exerted on a structure by water course-related features are known as channel forces. Stream movement, floating ice, and buoyancy are only a few examples of these forces. Substructure elements are predominantly affected by channel forces, which are equivalent to seismic forces.
7. Longitudinal Forces
Braking or accelerating a vehicle on the bridge causes longitudinal forces. When a vehicle comes to an abrupt stop or accelerates, longitudinal forces are applied to the bridge structure, particularly the substructure. As a result, the IRC advises that 20 percent of the live load be treated as longitudinal force on bridges.
The longitudinal force is exerted above the deck surface (1.8 m). The assumption is that all travel lanes are travelling in the same direction. The influence of longitudinal forces (breaking force) on the superstructure is minor.
8. Centrifugal Forces
The movement of vehicles over curves will create centrifugal force on the superstructure if the bridge is built on horizontal curves. As a result, in this scenario, centrifugal forces should be considered as well. Centrifugal force can be calculated by
C = ( W xV 2 ) / (12.7 R )
- W = live load ( kN )
- V = Design speed ( kmph )
- R = Radius of curve (m).
Decks attached to the primary members of the superstructure (for example, a composite concrete deck integrated with steel girders using shear connectors) transmit centrifugal forces to substructure elements via secondary components and bearings at piers and abutments.
9. Differential Settlement
10. Shrinkage and Creep
Concrete beams that were pre-stressed showed signs of shrinkage and creep. This distortion may have an impact on bridge design. Furthermore, when considering the composite action of bridges, cast in-situ concrete set on a specific beam subjected to shrinkage comes into play.
It adds to the stress level. It will be necessary to detail reinforcements in order to avoid this consequence.
11. Earth Pressure
The typical method of applying earth pressure to the bridge can be used. Surcharged loads must be considered for the design based on the location. Coulomb’s method or any other applicable approach could be used to compute active, at rest, and passive pressure coefficients.
12. Buoyancy Effect
The buoyancy impact is taken into account for huge bridge substructures submerged in deep water. If the depth of submersion is low, the effect may be insignificant. Bridges with submerged underwater components (e.g., piers) are susceptible to the effects of buoyancy. This is usually just an issue with very big hollow constructions.
On pier footings and piles, buoyancy may provide an uplifting force.
13. Effect of Water Current
When constructing a bridge across a river, a portion of the foundation will be submerged in water. Horizontal forces are induced by the water movement on the submerged section. Water current forces are greatest at the top of the water level and zero at the bottom of the water level, or at the bed level.
The pressure by water current ( P ) = KW [V2/2g]
- 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)
14. Thermal Stresses
The temperature causes thermal stresses. Extremely high or low temperatures cause stresses in bridge elements, particularly around bearings and deck joints. Because these strains are tensile in nature, concrete is unable to withstand them and cracks emerge.
15. Seismic Loads
When constructing a bridge in a seismic zone or earthquake zone, earthquake loads must be taken into account. During an earthquake, they produce both vertical and horizontal stresses. The amount of force applied is mostly determined by the structure’s self-weight.
Larger forces will be exerted if the structure’s weight is greater. Seismic forces are a significant loading factor that frequently influences bridge design in seismically active areas. The acceleration coefficient at the bridge site is used to determine the degree of seismic activity.
Structures with only an acceleration coefficient higher than 0.19 are thought to be in a seismically active location. These coefficient, as well as whether or not its bridge is categorised as essential, are being used to provide a seismic performance category to the bridge (SPC).
16. Deformation and Horizontal Effects
17. Erection Stresses
While prestress is more of a means of resisting load than a loading, it is often more convenient to treat it as such for analytic purposes. The approach of equivalent loads can be used to deal with prestress, just as it can with temperature.