| As one of the main load-bearing structures,steel beams play a vital role in steel structures.In the process of production and construction,the steel beams might have some defects,such as scratches,micro cracks and micro-holes.Compared with ordinary steel beams,the bearing capacity and stress distribution of the steel beams could be affected due to the microscopic defects to some extent,thus the mechanical properties of the steel structures could be changed.At present,the researchers remain centre on mechanical properties of steel beams under static loading.Some researchers have also carried out research on the dynamic performances of ordinary steel beams under dynamic load(explosive load,seismic load,etc.),and they have achieved some experimental and theoretical results.Research methods mainly include experimental research and numerical simulation studies.However,limited scholars have carried out the research on the dynamic properties of steel beams with micro-defects under blasting load.In view of the greater damage to the steel structure caused by the impact of the explosion,it has brought great economic losses to the society in recent years,and it has also caused great threats to life safety for social citizens,and even paid the price of life.The dynamic behaviors and reinforcement modes of steel beams with microscopic defects under blast loading are investigated in this paper to reasonably predict the mechanical properties of them under blast loading.The results could provide some theoretical help for the anti-explosion reinforcement design of steel structures.In this paper,the finite element software ANSYS/LS-DYNA is used as the main tool of simulation.Firstly,based on the theory of characteristics of explosion shock waves,numerical simulation method is used to study the burst trajectory of triple point of shock waves when detonating TNT explosive in the air above concrete ground.The influence of the blasting height,quantity of the explosive,and shape of the explosive on the heights of triple point is also discussed.Secondly,based on the reported experimental results of ordinary steel beam under blast loading,the rationality and validity of the numerical simulation method are verified to study the dynamic performance of steel beams.Thirdly,a series of numerical studies on ordinary steel beams,defective steel beams and strengthened steel beams with CFRP sheets and steel plates are carried out to investigate the failure modes and dynamic responses of the steel beams under different conditions.The coupling fields of temperature and explosion are considered in the study of defective steel beams with microscopic defects.As for the reinforcement research of steel beams,it mainly involves two modes: steel plate reinforcement and CFRP reinforcement.The following conclusions are obtained:(1)The blast shock wave propagated around the center of the explosive in the explosion field,and the height of the triple points increased gradually.Regardless of changing the quantity of the explosive charge or the blasting height,the growth rate of the triple point was slow in the midfield(4.0 ~7.0 m)of blasting and the growth rate in the far field(> 7.0 m)increased sharply.When detonating the same mass and the same blasting height of the explosive with the different shapes,the height of the triple points formed from detonating the cylindrical explosive in the midfield are slightly higher than that formed from cubic explosive;meanwhile the growth rate of it also is relatively slower.The heights of triple point substantially in the far field were almost equal and the growth rate was rising sharply and reaches a certain value,however.Compared with the influence of explosive shape,the blasting height and the quantity of the explosive had more influence on the height of triple point of the blast wave formed from detonating TNT explosive.(2)The transmission and reflection phenomenon are observed in steel beams exposed to explosive shock waves.When the blast waves acted on the steel beams,they firstly contacted the upper flange of the steel beams.The shock waves transmit and reflect simultaneously,and the overpressure peak on the steel beam increases significantly.The upper flange of the steel beam is the compressed area and the lower flange of it is the tensioned area.When the shock wave is gradually transmitted to the lower flange,the bending failure of steel beams occurred.(3)When the fields of temperature and explosion act together on the span of steel beam with some micro cracks,the maximum deflection in the mid-span also increases with the increasing of crack widths.After reaching a certain value,the steel beams will be completely broken.When the initial cracks with the widths from 1 mm to 10 mm are located at the half web of the steel beam,the maximum displacements of the beam span are changed substantially linearly.When the cracks are located at the half flange of the steel beam,the steel beams are more easily broken where the cracks lied.Moreover,and as the increases,the growth rate of the mid-span displacement increases with the crack width increasing.(4)When the blast loading is applied directly above the span of the steel beam under the coupling fields of temperature and explosion,the positions of the cracks have great influence on the anti-explosion performances of the steel beams,where the crack located on the web of the beam.The bearing capacity was increased significantly.When the cracks are located at the upper flange of the beam,the existence of the cracks has no significant effect on the bearing capacity of the steel beams,even if it is a certain distance from the span.That is,when the crack appears on the flange of the steel beam,the crack has a great influence on the anti-explosion performances of the steel beams.(5)When the crack is located at different positions of the steel beams under the coupling fields of temperature and explosion,different failure modes occurred on the beams with the same detonation point and the explosive mass.When the crack is located at the flange of the steel beam,the bending failure is occurred.Moreover,when the crack is located on the web of the steel beam,bending-shear failures is occurred on the steel beam.(6)When the cracks are lied on one side of the web of the steel beam under the coupling fields of temperature and explosion,the reinforcement effect of the cracked beams,such as the integrity,stiffness and anti explosion performance were more improved on the web of it strengthened with steel plates.Moreover,compared with the beam strengthened with steel plates,the anti-explosion performances of the cracked steel beam are not greatly improved under the same condition of explosion. |
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