The distribution of moments in a steel frame is directly correlated, beside other parameters, by the rigidity of the connection between the constituent elements. Depending on the type of beam-column connection, i.e. depending on its initial rotational stiffness, a diagram of internal static quantities is also generated. In design practice, the traditional way of treating a beam-column connection is carried out by assuming one (or their mutual combination) of the following ideal cases:
1. Absolute rigid connection
2. Absolute pinned connection
This of course, misinterprets the actual behavior of the structure and contributes to unnecessary increased material consumption The real rigidity of a connection is always between these two extremes cases. Eurocode 3, Part 1-8 provides rules and procedures for quantifying the initial rotational stiffness of a bare steel connection, but does not specify how to determine the moment in the connection joint from the influence of given external loads. The purpose of this paper is to describe the distribution of moments in a non-sway steel frame with semi-rigid connections under the influence of a uniformly distributed vertical loads and to answer the following question: Is it more convenient to analyze the beam as an isolated element with semi-rigid connections at the ends or is it necessary to include the rigidity of the vertical elements (columns)? This goal has been achieved by proposing a two-parameter analytical model that includes both the rigidity of the columns and the rotational rigidity of end beam connections, all together included in one model. The proposed model is compared to the Liu and Chen model [3] which gives the dependence between the external load and the negative moment in the semi-rigid connection, taking into account only the initial rigidity of the beam-column connection. According to this model for determining the distribution of moments, the rigidity of the vertical construction – columns is neglected. This way of neglecting the columns in analysis of a steel frame is also adopted in Eurocode 3, Part 1-8. This of course contributes to the slight overestimation of the negative moment in the beam-column relationship. Determination of the moment in the sagging and hogging region (connection node) from a given external uniformly distributed vertical load is directly dependent on the rotational rigidity of the connection (secant stiffness, Sj ) and the rigidity of the vertical structure ( kc ).