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It is well-known that the responses of a structure are different when subjected to a static load or a sudden step load.The dynamic amplification factor (DAF),which is defined as the ratio of the amplitude of the vibratory response to the static response,is normally used to depict the dynamic effect.For a single-degree-of-freedom system (SDOF) subjected to a sudden dynamic load,the maximum value of DAF is 2.Many design guidelines therefore use 2 as an upper bound to consider the dynamic effect.For a civil engineering structure,which is normally a multiple-degrees-of-freedom (MDOF) system,the DAF may exceed 2 in certain circumstances.The adoption of 2 as the upper bond as suggested by the design guidelines therefore may lead to unsafe structural design.Very limited studies systematically investigate the DAF of a MDOF system.This study theoretically investigates the DAF of a MDOF system when it is subjected to a step load based on the fundamental theory of structural dynamics.The condition on which the DAF may exceed 2 is defined.Two numerical examples and one experimental study of a cable-stayed bridge subjected to sudden cable loss are presented to illustrate the problem.