Question
How do I define lanes when applying lane loads?
Answer
When defining lanes, you can use the [Load] tab > [Moving Load Analysis Data] group > [Traffic Line Lanes] feature to input the eccentricity and wheel spacing for lane elements and select the reference element when defining lanes.
Depending on whether you select Lane Element or Cross Beam in Vehicle Load Distribution, the way the vehicle load is loaded will be different.
When selecting Lane Element, both the vertical loads and the moments due to eccentricity are applied to the reference element defined for the lane. Even if the lane is placed over a cross beam, the load distribution due to the cross beam's effects will not be considered. If you assume that a vehicle load P is acting at the lane's position, the distribution of the vehicle load based on the Lane Element will be as follows.
When selecting Cross Beam, the eccentricity is only used to determine the distance from the reference element to the lane. The vehicle load is distributed to the cross beam using the defined girder elements. If you are modeling a bridge with multiple girders, it is more suitable to define lanes using the Cross Beam option. If you assume that a vehicle load P is acting at the lane's position, the distribution of the vehicle load based on the Cross Beam will be as follows.
Tip.1 Considerations when selecting Cross Beam for lane definition
If one axis of the vehicle load exceeds the Cross Beam, you will encounter an error message as shown below. This occurs when the input eccentricity and wheel spacing result in one side of the vehicle load exceeding the Cross Beam.
Tip.2 The purpose of applying the Impact Factor
Actual vehicle loads are dynamic loads that vary over time. However, it is not efficient to design using dynamic loads. Instead, vehicle loads are treated as static loads, and an impact factor is applied to consider the dynamic effects.
The impact factor is a safety factor applied to account for dynamic effects, ensuring that displacements are much larger than those occurring in actual dynamic conditions.
The impact factor for the superstructure is determined by the following equation and cannot exceed 0.3.
Where L is the length (m) of the span section where the live load is applied, considering the design member where maximum stress occurs when the live load is equally distributed.
(Refer to Table 2.1.5 in Road Bridge Design Criteria '2.1.4 Impact')
Tip.3 Considerations when defining lanes on curved bridges
When defining lanes on curved bridges, if you use the commonly used 2Points method for defining lanes, you may encounter an error message, and the lanes may not be defined.
To avoid this issue when defining lanes on curved bridges, use the Picking method to directly select the reference element for lane definition.