Here is a picture of the structure. It is a tandem bicycle frame that has a constant downward force on each of the seat joints and then an oscillating force for each of the pedals. To simplify I think it is appropriate to assume that the two oscillating forces do have the same frequency and that the phase is 0. (This allows us to model the worst case scenario where the two riders exert the maximum downward force on the pedal on same side and at the same time) 
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