Automatic evaluation of plastic collapse conditions for planar frames with vertical irregularities

The plastic load and failure modes of vertically irregular planar frames are studied by means of an original software code developed in the agent-based programming environment NetLogo with a user-friendly interface. The proposed method lies in the limit analysis framework and is based on the generation of elementary collapse mechanisms and on their linear combination aimed at minimizing the collapse load factor. The considered irregularities consist in the absence of an arbitrary column in a regular grid of the frame and require considering additional elementary mechanisms, here presented for the first time, with respect to those associated to the corresponding regular frame. A further novelty of the method is the adoption, in the linear combination of elementary mechanisms, of negative coefficients, which, as better shown in the applicative section, is fundamental to grasp the actual collapse mechanism in irregular frames. The minimization procedure is efficiently performed by means of genetic algorithms, which allow computing both the collapse load factor and the correspondent failure mode with great accuracy and in a very short computing time. Many applications have been performed considering seismic load scenarios. Finally, by means of a parametric study, some general considerations on the weakest configurations of this typology of vertically irregular frames are provided.