Bridge structures are designed with a variety of loads in mind. The safety of the bridge construction throughout its use in all conditions is determined by these loads and their combinations. For a bridge to be perfectly designed, the design loads must be appropriately taken into account. The following describes the various design loads that are applied to bridges.
Types of Loads in Bridge Design Structures
Various design loads to be considered in the design of bridges are:
- Dead load
- Live load
- Wind load
- Earthquake Load
- Impact load
- Longitudinal Load
- Centrifugal Load
- Settlement Loads
- Thermal Load
- Buoyancy effect
- Effect of water current
1. Dead Load on Bridge Structure
Simply put, the dead load is the bridge elements’ self-weight. A bridge’s deck slab, wearing coat, railings, parapet, stiffeners, and other utilities are its various components. In the design of a bridge, it is the first load to be estimated.
2. Live Load or Real-time loading
The bridge’s moving load along its whole length is known as the “Live Load.” Vehicles and pedestrians are among the moving loads, but choosing one or a group of vehicles to build a safe bridge might be challenging. In order to provide safe results against any kind of vehicle moving on the bridge, IRC suggested a few hypothetical cars as live loads. Three types of vehicle loadings are distinguished, and they are
- IRC class AA loading
- IRC class A loading
- IRC class B loading
3. Wind Load on Bridge Structures
The most commonly regarded load type, aside from dead and live loads, is wind load. Wind loads are imposed to structures. Wind pressure on the structure increases as the structure’s height increases.
Since the notional loads become essential at lower wind pressures, wind effect on smaller buildings is typically ignored. However, wind pressure becomes crucial as the building gets taller.
4. Loads from Earthquakes
Earthquakes are caused by plate movement. There are places where earthquakes are more likely to occur. The impact of the earthquake decreases with increasing distance from the plate borders.
In contrast to other kinds of loads, seismic loads are rarely applied to a structure.
The design takes ground acceleration into account based on the seismically active area. The design guidelines give designers acceleration coefficients for both horizontal and vertical accelerations. These figures are predicated on the earthquake’s likely magnitude.
An earthquake typically lasts between 30 and 40 seconds. On some occasions, the length is significantly longer.
One of the most used structural design standards for estimating the impact of earthquakes on structures is UBC.
5. Impact Loads on Bridge Structures
Unexpected loads created when a vehicle is driving across the bridge are the cause of the impact load on the bridge. The impact load on the bridge is caused by the live load fluctuating frequently from wheel to wheel while the wheel is in motion. An impact factor is used to account for impact loads on bridges. The impact factor is a multiplying factor that is dependent on a number of variables, including the vehicle’s weight, bridge span, and velocity, among others.
6. Longitudinal Load
The vehicle’s acceleration or braking on the bridge generates the longitudinal forces. The bridge structure experiences longitudinal loads, particularly on the substructure, when the vehicle abruptly stops or accelerates. Therefore, 20% of the live load should be taken into account as longitudinal force on the bridges, according to IRC recommendations.
7. Centrifugal Load
The movement of the vehicle over curves will exert centrifugal force on the superstructure if the bridge is to be constructed on horizontal curves. Therefore, in this instance, centrifugal forces should also be considered when designing. The formula for centrifugal force is C (kN/m) = (WV2)/(12.7R). W = live load (kN) in this case V = Design velocity in kmph R is the curve’s radius (m).
8. Settlement Loads
Additional loads on structures are encountered throughout the structural design process as a result of structural settlement. Different settlements could result from variations in the ground’s state, adding to the structural elements’ stress.
As an illustration, think of building a raft foundation on a surface where one portion of the foundation rests on rock and another on soil. Different settlement is unavoidable if there is no movable joint when the ground conditions change.
The portion of the foundation built on the rock does not move downward, whereas the portion built on the earth may sink. After consulting with the geotechnical engineer, this information will be carefully taken into account in the structural design. The subgrade reaction could be taken into account in the structural modeling during foundation design in order to depict the varying ground stiffness. To accurately depict the site’s real condition, the structural model must incorporate pertinent stiffness.
9. Thermal Loads
The structures are impacted by changes in the external atmosphere’s temperature. The internal stresses of structural elements rise as a result of the temperature structures being raised.
The concrete construction is not greatly impacted by temperature variations in the surroundings, which could result in structural breakdowns. It has an impact on the structures’ longevity, though. This subject goes into great length about the durability requirements in reinforced concrete design as well as the factors that affect concrete durability.
10. Buoyancy Effect Load
The buoyancy effect is taken into account for huge bridge substructures that are submerged in deep water. If the submersion depth is smaller, it may not be noticeable.
11. Forces by Water Current
A portion of the foundation is submerged in water as the bridge is being built across a river. The submerged section experiences horizontal stresses from the water current. At the top of the water level, the forces produced by water currents are at their greatest, whereas at the bottom or bed level, they are at zero. P = KW [V2/2g] is the pressure caused by the water current. P stands for pressure (kN/m2). K = constant (value based on pier shape) W is the water’s unit weight. V is the velocity of the water current (m/s). G = gravitational acceleration (m/s2)