1. What is moving load?
Let's explore the moving loads acting on civil structures.
Moving loads refer to loads that travel a certain distance and are a type of Live Load.
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The common moving loads in civil structures include vehicle loads, train loads, pedestrian loads, and special loads.
- Vehicle Load: This refers to loads on roads such as standard truck loads and lane loads.
- Train Load: This refers to the load from trains.
- Pedestrian Load: This includes loads from pedestrians or bicycles.
- Special Load: This refers to loads generated by acceleration and braking as vehicles move.
These moving loads can be considered in two major forms when acting on structures:as concentrated loads and distributed loads.
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Taking vehicle loads as an example, it is reasonable to consider them as concentrated loads when vehicles are moving freely, and as distributed loads when vehicles are stationary.
2. Various types of loads to be aware of for moving load design
Next, let's look more closely at the loads to consider when designing for moving loads.
1) Dynamic Force
Dynamic Force primarily refers to the force generated by moving vehicles.
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When vehicles are stationary, they are classified as a Static Load, which applies a constant and unchanging force to the structure.
On the other hand, moving vehicles apply a force that varies over time and position, considered a Dynamic Force. Bridges vibrate in response to the load (Dynamic Force) of moving vehicles.
The response of the bridge can vary depending on the speed and weight of the vehicle and the structural characteristics of the bridge. It is crucial to consider for moving load design that if the vibration caused by vehicles crossing the bridge matches or is close to the bridge’s natural frequency, resonance can occur, significantly increasing the bridge’s vibration.
2) Vehicle Load Distribution Across the Width
Considering the lateral distribution of vehicle loads is crucial for the design and safety assessment of structures like bridges and roads.
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Especially when vehicles are concentrated in the middle or skewed to one side of the bridge, the structural response and stress distribution can vary, necessitating consideration of various distribution scenarios. For instance, when vehicles are skewed to one side, the part of the bridge under that area receives a heavier load, which can cause asymmetric bending or fatigue damage to the bridge.
3) Horizontal Load Due to Centrifugal Force on the Upper Deck
When vehicles travel on curved roads, a centrifugal force acts outward from the center of the road curve.
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The magnitude of the centrifugal force is proportional to the square of the vehicle's speed and inversely proportional to the radius.
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This centrifugal force impacts both the superstructure and substructure. The farther a girder is from the center of impact, the greater the member force, and the closer a girder is, the smaller the member force, resulting in an asymmetrical structural response. The impact on the substructure is transmitted through the superstructure as a horizontal load.
4) Horizontal Load on the Substructure Due to Braking Force
Braking force refers to the force generated by the friction between the wheels and the road when a vehicle slows down or stops. This force primarily depends on the vertical force the vehicle's wheels apply to the road surface and the friction coefficient between the tires and the road. Braking force plays a crucial role when a vehicle rapidly decelerates, especially during emergency braking situations.
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The friction force generated during braking affects both the superstructure and the substructure of a bridge. While the braking force of vehicles acts in the axial direction on the bridge deck, it acts as a horizontal force on the substructure.
Therefore, the substructure should be given more careful consideration during the design process.
Topics
Moving Load
Moving Loads
Eurocode
Dynamic Force
Vehicle Load
Centrifugal Force
Braking Force
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Hello. My name is Sungjin Jo. I majored in Civil Engineering and currently oversee operations at MOTIVE site.
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