When modern structural codes are used for plastic analysis, it is important to define the class of the section. One of the main issues for the class classification is the rotation capacity of the section, that is, the inelastic rotation that the section can sustain after the plastic hinge is formed. EC3 requires class1 to perform plastic analysis, unless the rotation capacity is known. Sometimes, the election of class 1 is too restrictive and plastic analysis could be performed with class 2 provided that the section has sufficient inelastic rotation [
]. For this reason, it is important to know the inelastic rotation of a section, and it could be provided in property tables of sections.
There are two reliable ways to calculate the rotation capacity: experimental tests and simulation by finite element analysis. In general, both of them are expensive. This paper presents a new and reliable method to obtain the rotation capacity of square and rectangular hollow sections. The method expands the original work of Kecman [
], simulating the plastic hinge model by a complete multibody system. The method performs successive static equilibriums, as described [
], for each angle of rotation of the plastic hinge. The forces considered in the model are in agreement with the elasto-plastic theory of materials The work developed by Kecman only considers the post-critical behavior of plastic hinge, however the proposed method, provides the complete moment-rotation curve. This method allows performing a quick simulation to obtain the inelastic rotation instead of large FEM simulations or expensive execution of experiments.