اثر میانقاب های بنایی در خرابی پیشرونده در سازه‌های قاب خمشی فولادی

When one or more vertical elements of a structure fail due to defects in construction stages or over loading or etc., load distribution path of the structure changes and local failure arises in the damaged area. This kind of damage is not considered by engineers and can cause local collapse. The local collapse can spread vertically or horizontally to the other areas of the building if no alternate path exists to redistribute the loads. Therefore, limiting the local collapse in the damaged area is major idea to mitigate progressive collapse in the buildings.
Nowadays, analyzing the structures which are designed based on the current standards, against progressive collapse and offering ways to improve and strengthen them is leading to part of the designing stages of the special buildings. Thus, some standards and codes in this field are being produced or updated. The most common method to analyze the structure against progressive collapse is the alternate path method. In this direct design method, the critical columns be removed immediately and stability of the remaining structure is investigated. But there is no references talk about the effect of lateral resistant of the infill panels. This is one of the simplifier assumptions which are used in numerical studies of progressive collapse phenomenon in structures indicate inconsistency between the numerical and experimental full-scale results. Unlike numerical studies, experimental studies showed that the structure remain stable even if more than one column removed.
As a case study, in this research, a steel structure with 8 stories with moment resistant frame is analyzes and designed considering effect of unreinforced masonry infill panels (URM). URM infill panels in full contact with the frame elements on all four sides shall be considered as primary elements of a lateral force-resisting system. Recognizing this behavior, the stiffness contribution of the infill is represented with an equivalent compression strut connecting windward upper and leeward lower corners of the infilled frames. So, analytical macro-model based on the equivalent strut approach is used to simulate the effective infill panels. Potential of progressive collapse of the one of the peripheral frames is evaluated with the Opensees program based on the nonlinear dynamic analysis. Researchers found that linear static analysis might result in non-conservative results since it cannot reflect the dynamic effect caused by sudden removal of columns. So, time-history analysis should be applied to seek dynamical response of the structure.
Results indicate that considering effect of the infill panels increase axial force of the columns and decrease bending moment of the beams and nodes displacements. So results are closer to the experimental studies and prove stability of the structure after column removal and increase resistant of the building against progressive collapse.
As it distinct, modeling the infill panels in the analysis is complex and time-consuming, so in this research, the coefficients are proposed to apply to the load combinations instead of modeling the infill panels in order to closer the results together. The proposed coefficients are larger than one for columns forces and smaller than one for the beams forces.