Influences Of Masonry Infill Walls On Steel Frames To Resist Against Progressive Collapse

A lack of effective load transfer path after failure of some local components due to accidental load(fire,explosion,impact,etc.)can lead to collapse of the entire structure or a large proportion of it,i.e.,causing progressive collapse.In view of the serious consequences associated with the progressive collapse,many efforts have been devoted to investigate the progressive collapse resistance of structures under various occasions.So far,quite a large number of experimental and numerical studies on the performance of building structures against progressive collapse have been carried out,based on which many countries have stipulated in their design codes the requirement for progressive collapse resistance of building structures.Owing to ease of experimental study,past researches have mainly focused on single-story substructures including beam-column and beam-slab substructures.These results are useful for understanding the local mechanisms of structure to resist the progressive collapse,but may be inadequate for global mechanisms.It is necessary to investigate the progressive collapse resistance of multi-storey structures.In addition,the research on the influence of masonry infill wall on the progressive collapse resistance of multi-storey steel frames is very limited.Therefore,this thesis aims to investigate the progressive collapse resistance of steel frames with masonry infills.The main work includes:1)the quasi-static vertical loading tests on four two-story steel frames with and without infill walls are carried out,and the vertical load-displacement curves,horizontal reaction force-displacement curves,deformation and internal force are obtained;2)the refined finite-element models of the tested infilled steel-frame specimens are established by using LS-DYNA and are validated against the experimental results,based on which further parametric studies are carried out;3)the simplified numerical models of infilled steel-frame specimens are established based on the component method and equivalent strut method and are validated,based on which further parametric studies are conducted.The main conclusions are drawn as follows:(1)The progressive collapsing process of both bare and infilled steel-frame specimens have demonstrated three stages:initial stage,bending stage and catenary stage.The masonry infill wall has shown significant contribution to the collapse resistance of the specimens in all the three stages.(2)Masonry infill walls have increased the initial stiffness by 7.7 times,1.5 times and 1.2 times for fully infilled specimens,and partially infilled specimens with window opening and door opening,respectively,as compared to that of the bare frame specimen.Similarly,the masonry infill walls have also increased the maximum peak load of the fully and partially infilled specimens by 170%,38%and 19%,respectively.It is worth noting that openings in the infill walls reduce these contributions.(3)Compressive reaction forces measured in the horizontal direction indicates that the equivalent strut mechanism forms in the masonry infill wall during the collapsing process to transfer part of the vertical load to the surrounding nodes,thereby enhancing the structural resistance to progressive collapse.Masonry infill walls without openings,and with window and door opening have increased the maximum horizontal compression force of the bare frame specimens by 3.4 times,2 times and 1.3 times,respectively.Moreover,the masonry infill wall will delay the specimens to enter the catenary stage.(4)The parametric studies based on refined numerical models show that the height of infill wall affects the collapse resistance of the specimens.The initial stiffness,maximum peak load and the dynamic bearing capacity are increased by 1.3-5.2times,61%to 190%and 62%to 226%,respectively,by full-height infill wall.The opening ratio also has an impact on the collapse resistance of the specimens.The initial stiffness of the infilled steel frame is 1.1 times and 1.2 times higher with the minimum opening ratio of 7%than other opening ratios,and maximum peak load and the dynamic bearing capacity are increased by 2%to 15%and 8%to17%,respectively,by opening ratio of 7%.Furthermore,the position of the door opening will affect the collapse resistance of the specimens.The results show that,when the door opening is shifted from the central of the span or the side of the remaining side column to the side of the removed middle column,the initial stiffness of the specimens are increased by 1.1 times and 1.4 times respectively,and the maximum peak load of the specimens are increased by 10%to 24%with the opening positioned at the central of the span as compared with those with other opening positions.(5)The parametric studies based on the simplified models show that the initial stiffness,peak load and dynamic bearing capacity of the specimens increase with the increase of compressive strength of the masonry infill wall.The initial stiffness,maximum peak load and the dynamic bearing capacity predicted by using the 2.0f_m simplified model are 1.1-1.8 times,1.1-1.6 times and 1.1-1.6 times those by the other masonry compressive strength models.Generally speaking,the improvement in the structural resistance is roughly proportional to the masonry compressive strength.