This paper is concerned with the initial phase of behavior of a long built-in beam when subject to a dynamic transverse load which is uniformly distributed over a central segment. The beam is assumed to be made of a rigid, ideally plastic material and the yield condition is assumed to depend only upon the bending moment. The effects of transverse shearing deformation, rotary inertia and strain rate are neglected. In the analysis, the beam is taken to be of infinite length. Four possible types of initial phase of motion are found. The conditions under which the results of this analysis are applicable to finite built-in beams are defined and the behavior of finite beams when these conditions are not satisfied is briefly discussed.