Reduced Beam Section Spring Constants

A moment connection that includes a wide flange beam with trimmed flanges is commonly known as a Reduced Beam Section (RBS) connection. Accurate analysis of frames incorporating RBS beams requires knowledge of the elastic stiffness matrix of RBS beams. In lieu of using this stiffness matrix, an RBS beam can be modeled as an Euler-Bernoulli frame element with rotational springs at each end, which can be easily implemented in structural analysis software. This paper presents the derivation of the formula for the spring constants of an RBS beam and validates it. From a study of the spring constants for a plethora of RBS beams, it was found that a strong linear relationship exists between the minimum plastic section modulus of RBS beams and their spring constants. This paper has direct applicability to the practical and accurate determination of the elastic drift of a moment frame with RBS connections.