In order to consider the dynamic effects of a moving vehicle, the program allows to create a time history analysis load case based on a "Multi Step Bridge Live" load pattern type.
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Parameters correlation
The parameters on the "Multi Step Bridge Live Load Pattern Generation" (Load-Pattern) form would be typically correlated to the parameters on the "Time History Load Case Data" (Load-Case) form as follows:
(a) The Duration of Loading on the Load-Pattern form would match (Number of Output Steps)*(Output Time Step Size) on the Load-Case form.
(b) The Discretize Load every X seconds parameter (input time step) on the Load-Pattern form would match the Output Time Step Size parameter on the Load-Case form.
However, neither of the above conditions are absolutely necessary. If condition (a) is not met, the time history load case may represent the vehicle traveling over a shorter distance that defined in the Load-Pattern form, or the load case may continue after the vehicle has stopped moving. Both situations may be appropriate depending on your modeling needs. For example, extending the load case beyond the duration of the load pattern may be useful to observe residual vibrations.
For condition (b), note that the response is always calculated at the smaller of either the input time steps from the Load-Pattern form and the output time steps from the Load-Case form. Considerations:
- Using smaller input time steps improves the accuracy of the loading by creating a smaller spatial distribution of load points. This can be important to capture response near supports and other discontinuities in the model.
- Using smaller output time steps increases the accuracy of measuring high-frequency response, particularly for accelerations.
- In most cases keeping the input and output time steps the same size produces the most information
- You should consider starting with larger time steps at first to reduce analysis time while calibrating the model, then reducing the time steps until the response of interest does not significant change anymore.
