Stiffening behavior can result from interaction between a structure (base system) and its surrounding environment as in the MSSS bridge soil-structure interaction. In this study, three MSSS bridge cases are parametrically analyzed and their dynamic characteristics and behavior are presented and discussed. Later, for investigating stiffening behavior, a simplified stiffening model is introduced and an extensive parametric study with more than 367,000 analyzed cases is performed. In the parametric study, different stiffening parameters (i.e., stiffness, strength, gap size, and mass), 41 strong motion records, and several damage criteria are considered and effects of the stiffening parameters variations on the dynamic response of simple stiffening systems are presented. It is shown that on the average the displacement response is lower for stiffening systems, consistent with pushover analysis based on the seismic codes. However, considering many other damage indices like dissipated hysteretic energy and low-cycle fatigue damage index, it is quite likely that a stiffening system would sustain more damage than an elastic-plastic system. At the end, suitable design response spectra for stiffening systems are developed and their practical implications for MSSS bridges are demonstrated.