Abstract: In this paper,the shear of beams in elastic and elastic-plastic responses is investigated by analytical method and nonlinear finite element method,respectively.A shear dynamic factor,which is one of important parameters in the design of blast-resistant structures,is introduced.It is demonstrated that(1) Under the triangle load with zero rise time,the positive shear dynamic factor increases with the increase the ratio of duration of the load to the first natural period,t_d/T₁.The negative shear dynamic factor increases with the increase of t_d/T₁ first and then decreases slightly.Under the ramp load,the positive shear dynamic factor decreases with the increase of the ratio of load rise time to the first natural period,t_r/T₁.(2) The positive and negative shear dynamic factors increase with the increase of height to length ratio of beams.(3) When entering into the plastic response,the shears at the end of beams decrease comparing with the elastic response.The longer the duration or load rise time,the more decrease of the shears.