Auto Centrifugal Load Generation
Centrifugal load can be generated from two locations in the program.
First is the generation from Centrifugal load case dialog. Secondly it can be generated along with Live load generation. When centrifugal load is generated from its own load case, only one load case is generated by the program. When the centrifugal load is generated along with the live load generation, program generates same number of CF/CE cases as it generates live load cases. For this case, every CF/CE case is generated for same number of live load lanes loaded as in the live load case. For example, if program generates 5 live load cases, it will generate 5 CF/CE cases. If the first live load case generated is based on two lanes loaded simultaneously, then first CF/CE case will also have two lanes loaded.
As per AASHTO Standard Art. 3.10, the centrifugal load as a percent of live load is,
| C = (6.88 S2)/R |
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| C = (0.79 S2)/R |
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| C = 4/3 v2/gR |
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In this generation, all the axles of a truck are considered for generation even if the span is too short to accommodate the entire length of the truck.
The program also provides an option by which the total live load values for truck can be specified rather than selecting a truck from the library. When total truck load is specified, program uses the value input as total truck load per lane.
Once the percent (or factor) C is computed, program then determines the total centrifugal load and then computes the bearing loads. In computing bearing and cap loads it assumes that the total longitudinal load is applied at 6 ft above the deck in the direction of stationing. The load direction is as you specify.
For LRFD California, you can input the height above deck, the default value in this case is considered 0.
The curvature of the bridge alignment defines the direction of the centrifugal force on the bridge. If the pier view direction is upstation and the bridge curvature is such that the center of the curve is on the right side of the alignment looking upstation, you should specify to apply the load in - X direction. See the below figure a, b, c, and d (shown below) for four possible cases considering curvature and the pier view direction.
The total centrifugal load is distributed among all the bearings as bearing loads. The moment about the X axis is applied at cap load and the moment computed about Z axis is applied as a force couple in Y direction at exterior bearings. If there are two bearing lines, half of the force is distributed on bearings in each bearing line. For skewed bridges, Substructure resolves the load into X and Z resulting in bearing loads in both X and Z directions of the pier.
