Many commercial software are unable to provide an overview of displacement, i.e. internal static quantities, when it comes to composite steel-concrete frames. The problem arises precisely because of the variation of the stiffness of composite beam in the region of positive and negative moments along its length. It is well known that the problem can be solved numerically by modeling the composite beam using a finite element mesh, however dividing the beam into finite elements requires knowing in advance where the bending stiffness change for each beam, which basically explains the idea of ​​this paper.

In this study, the equivalent bending stiffness of composite beam as part of a composite frame is analyzed.

Determination of the region of negative moments is calculated depending on the phase of loading, according to the principle proposed by Wong [8].

Depending on the loading phase, geometrical and mechanical property of the beam as well as the stiffness of connection, different lengths of these regions are generated. Determining the exact values ​​of these regions is in principle a long and complicated procedure and that is why, for example, considering Eurocode 4, at each end, 15% of the composite beam length is suggested as one of the negative moment segments, and the reminder of the span is defined as the positive moment segment,

In this study, to investigate the length of these regions depending on the phase of loadings, five composite frames subjected to the same level of loads, but with different levels of rotational stiffness are considered. Moreover, with the help of this method, a comparative analysis was made with the proposals from EN 1994-1-1.