An important problem of modern engineering is to ensure the stability of buildings to a progressive collapse. One of the options for making calculations is to perform a static calculation with the use of the dynamic coefficient. The dynamic factors of damage of the structure depend on the failure time of the element, i.e. time during which the resistance of the damaged element is reduced to zero.To determine the time of exclusion, a series of experimental studies of stretched and compressed steel samples was carried out. The work of elements was studied after they had exhausted their carrying capacity (strength or stability). The results of a numerical study of a truss with damaged elements are presented. It is established that a significant part of the load is redistributed to the intact elements of the truss during deformations of the excluded element are smaller than the experimentally established deformations of the rods.
An important role in the robustness analysis of damaged structures, is played by time during which local damage of the elements of the system occurs. Numerical studies have shown that the shorter the failure time of a particular farm element, the greater the dynamic forces arising in the structure. The time from the beginning of destruction to the complete failure of an element, which consists either in destruction or in loss of stability, is called the time of exclusion. The article describes the results of a series of numerical calculations of steel trusses as part of the building frame with local damage of individual truss elements. The influence of the time of exclusion of the element and the location of its damage on the value of dynamic coefficient was studied. The frame of the building is considered as a space frame, including not only trusses with local damage and the columns on which it relies, but also neighboring trusses, columns, bracing and purlins. This approach allowed to study the effect of local damage on the behavior of intact bearing structures. Based on the calculations, dynamic factors were established for the load acting on the truss with local damage and for the load on adjacent structures of the space frame. Recommendations are offered on constructive measures to increase the robustness of a building with steel trusses. The most unfavorable options for local damage of the trusses have been established. Recommendations are given on the assignment of dynamic coefficient for conducting a static calculation that takes into account the dynamic effects in the building frame with local damage of the truss.
Introduction. To ensure the safety of buildings and structures in case of emergency, structures and individual structural elements must maintain their bearing capacity in the event of local damage. The purpose of the article is to study the effect of rigidity of a steel truss on the dynamic coefficient in the process of analyzing resistance to progressive collapse. A solution to this problem is a stage in the development of a practical method for analyzing steel trusses in cases of local failures of elements. Materials and methods. Within the framework of the study, several numerical and analytical computations of steel trusses, having spans of 24, 48, 72, 108 and 144 m, were performed. Steel trusses were subjected to different local failures of elements in top and bottom chords. Results. The effect of rigidity of trusses on the dynamic coefficient is studied for cases of withdrawal of individual elements from the analytical model. Values of dynamic coefficients, obtained by means of analytical and numerical calculations, demonstrated good convergence (the discrepancy did not exceed 15 %). The dynamic numerical calculation of the 3D roofing skeleton with a standard pattern of horizontal ties was performed, taking into account the local failure of a truss element. The distribution of the dynamic coefficient within a damaged truss and the nearest trusses was obtained. Conclusions. Analytical and numerical studies showed that an increase in the rigidity of a truss caused a reduction in the value of the dynamic coefficient. Analytical dependences can be used as the bearing capacity reserve when making analytical calculations. Numerical analysis showed that if a damaged truss resisted loading, being part of the 3D framework of an industrial building, the load, acting on the damaged truss, was redistributed to neighboring trusses, and dynamic forces in it were smaller than those in an independent load-bearing truss. The damage of the compressed chord was a worse case; the value of the dynamic coefficients exceeded the values for the case of stretched chords by an average of 26 %, and the truss resisted loads nearly independently from neighboring trusses, behaving as a flat element.
To ensure the safety of buildings and structures, they must maintain their bearing capacity in case of local damage. The article is devoted to the study of the robustness of damaged steel trusses, as well as the time during which local destruction of the trusses occurs. Finding a solution to this problem is one of the key stages in the development of a practical methodology for calculating steel trusses’ resistance to the local destruction of elements. The time from the beginning of destruction to the complete failure of the element is proposed to be called the exclusion time. Based on the performed theoretical and numerical studies, for the quasi-static calculation it is recommended to use the values of the dynamic coefficient for the considered steel trusses. In the numerical formulation, the work of steel trusses as part of the building frame was investigated along with local destruction of individual elements of the truss. Numerical studies have shown that the shorter the failure time of the truss element, the greater the dynamic forces arising in the structure. The frame of the building is considered as a spatial system. A numerical dynamic analysis of the spatial coverage is carried out, taking into account the local failure of one of the truss elements. The distribution of the dynamic coefficient over the steel trusses is obtained. This made it possible to study the effect of local failure on the intact load-bearing structures. The article presents the results of a series of experimental studies of flat trusses aimed at determining the time of failure of a steel truss element. Based on the results obtained, the failure time of the damaged rod and the redistribution of efforts to the neighboring elements of the truss were calculated. In accordance with the calculated failure time, recommendations were formulated to reduce the value of the dynamic coefficient used in the static calculation, depending on the types of damage to the truss.
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