The current technology manifests a great demand for high precision and high positioning accuracy in many engineering applications that range from robot manipulators and high-speed flexible mechanisms to supercritical rotors and the space deployable structures. A reliable dynamic model is crucial to the performance evaluation and precision control of such elastic mechanical systems. The modeling problem becomes even more difficult when higher frequencies of such large scale systems are excited. A pieced-interval modal analysis scheme is introduced to alleviate the problem of handling a system of widespread eignspectrum. A set of energy indices is defined to monitor the energy content in higher modes. The numerical scheme invokes a modal transformation with changeable basis that accounts for higher modes only when they are excited, and picked up more proportional values of the systems total energy. Numerical examples are presented to demonstrate the applicability of the developed method and to display its potential in performing efficient modeling of the dynamic behavior of such systems.