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seismic acceleration time history input
Posted 12 sept. 2018, 23:37 UTC−4 Structural & Acoustics, Geomechanics, Structural Mechanics Version 5.3a 1 Reply
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I am modeling structure fixed on the ground，the acceleration time history need to be apllied to the boundary .So I have to set prescribed displacement/rotation and prescribed acceleration to the boundary to the same region simultaneously.But they cannot work together, the former is overriden by the latter. I want to know if the comsol can realize my idea,how. If it could not do this in this in my idea, some other mehod is also OK.
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Hi,
For physical reasons, it is not possible to prescribe displacement and acceleration simultaneously for the same degree of freedom, since one is the second time derivative of the other.
If, however, you have a known displacement in one direction and a known acceleration in an orthogonal direction, it would be admissible from a physical point of view. Then you must use some kind of trick to circumvent the override rules. Some suggestions:
a) Integrate the acceleration twice, and enter both as prescribed displacements.
b) Compute the second time derivative of the displacement, and enter both as accelerations. This is less numerically stable, and also computationally more expensive that a)
c) You can also use a more raw approach, where you add a Pointwise Constraint in which you set the prescribed displacement. A Pointwise Constraint does not override other boundary conditions, so there are no checks against conflicts with a prescribed acceleration.
Regards,
Henrik
Henrik Sönnerlind
COMSOL