Effect of Inertia Forces on Support Reactions of Beams subjected to Uniform Blast Loads

摘要

One commonly used method for designing elements to resist dynamic blast loads is single-degree-of-freedom (SDOF) analysis. This method uses an equivalent mass and stiffness and provides accurate displacement estimates for the center of the beam. This procedure allows for the consideration of different support conditions as well as various loading configurations. Although very practical for the calculation of displacements, the SDOF method does not provide member end reactions which are required for the design of the members' connections to the supporting structure. Simplified equations have been formulated to obtain these reactions based on force and moment equilibrium. These equations assume that the inertia forces always oppose the blast loads and that their distribution follows the first mode of the beam. This paper presents the results of analytical finite element (FE) analyses of a simply supported beam with a uniform blast load. These analyses show that the inertia forces do not always resist the blast load; in some instances they add load to the beam while the blast loads are still being applied. Additionally, it is shown that the inertia forces' distribution changes with time and that it is influenced by the first few modes of the beam. As a result of the behavior of the inertia forces, for beams undergoing high levels of ductility, the FE peak reactions can be significantly higher than reactions calculated with the simplified equations. The parameters that influence this response are presented.