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To apply the acceleration load at the water level, you would need to convert the acceleration time history record to a displacement time history record. Once you have the displacement record, define a Load pattern in which you apply a unit value of joint displacements in the direction of acceleration at the affected joint. For this to affect the structure, you also need to restrain these joints in the direction of loading only. Then create a time history function that will describe a variation of these displacements in time. Finally, define a time history load case that will be specified using both the displacement and the time function.
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