Analysis of foundations vibrating under harmonic loads during emergency shutdown due to additional pulse loading

This study uses an elastic-perfectly plastic single degree of freedom (SDOF) system to investigate reaction of machinery-supporting foundations to harmonic and pulse loads. An exponentially decaying pulse is used and applied to an already operating machine foundation which may experience an emergency shutdown due to extreme loading. Based on response at end of positive phase of pulse, two cases are analyzed. The governing equations are derived and solved numerically. Random response of SDOF system, before vibrations die out, can be observed due to the interaction of the two different loadings. Overestimation of the response for zero decay coefficient, b representing a triangular pulse, is noted. The absolute maximum displacement, influenced by the negative phase, changes by approximately 10-11% as the mass increases from 180 tons to 220 tons. Stiffness of the system and damping ratio affected the response to maximum extent. For various input parameters considered, displacement reduces by an order of 73% with increase in stiffness and by about 65% when damping ratio increased. The results indicate that negative phase of pulse loading has significant impact, particularly on less stiff and less damped systems.