Equivalent Mass

kashoo

As we have soem specific formulas for finding Stiffness for Masses connected in series or in parallel when considering system as Multiple degree of freedom Systems in Vibrational Analysis(Eigen Value Calculation)....What are formulas used for Equivalent Mass for Masses connected in Series or in Parallel.....

Like i have one signle mass on top then in middle three masses in parallel then again a single mass at bottom ....i m taking equivalent stiffness of middle part as (3k...formula for stiffnesses in parallel) ...and mass as 3m...is it right approach to take mass as 3m???
coz i m getting complex result for this three degreee of system result....