The determination of dynamic response of structures subjected to dynamic loads is an important step in the design of structures. A large number of structures can be Idealized as an assemb1age of discrete beam elements or beam and truss elements in a plane or space. The combination of such elements in a plane form planar structures like multistorey frames, trusses, bridges, arches and dams etc. The idealization of structure as an assemblage of beam/truss elements with the massel conoentrated at the connecting nodes is commonly termed as lumped mass model or mathematical model. The formulation of a mathematical model of a structure is an important step in the dynamic analysis and requires some judgement by the analyst.