Research On The Mechanical Performance And Bearing Capacity Calculation Method Of The Castellated Composite Beam In The Negative Bending Moment Zone

The castellated steel beam is combined with the concrete slab in the steel frame to form a steel-concrete castellated composite beam,which gives full play to the good performance of the castellated steel beam and concrete,effectively reduces the structural weight,facilitates the insertion of pipelines,saves the net height,and significantly increases the stiffness of the member.In the frame structure,there is a negative bending moment area at the beam-column node and the secondary beam support.The negative bending moment causes the composite beam concrete slab to be tensioned,and its force characteristics are quite different from those under compression.Existing researches mostly focus on the static performance of castellated composite beams.There is not much research on the performance of castellated composite beams in the negative bending moment zone.There is no complete calculation method for the bearing capacity of the negative bending moment zone.This hinders the promotion and promotion of castellated composite beams to a certain extent application.Therefore,studying the bearing capacity and force characteristics of the negative bending moment zone of castellated composite beams has important theoretical significance and practical value.This paper takes the regular hexagonal hole castellated composite beam as the research object,and adopts the method of combining model test,theoretical analysis and numerical simulation to develop the calculation method of the mechanical performance and bearing capacity of the tensile zone(negative moment zone)of the concrete slab analysis.Through the test of 2 castellated beams and 4 castellated composite beams,the load-bearing capacity,failure mode and stress distribution law of the hole circumference of the castellated composite beams in the negative bending moment area are compared and analyzed.According to the test results,a reliable numerical model was established to further analyze the elastoplastic development law of the negative bending moment zone of the castellated composite beam,and the influence of parameters such as concrete slab thickness,slab top and bottom reinforcement ratio,steel beam flange width-to-thickness ratio,steel beam web height-to-thickness ratio and opening ratio on the flexural and shear performance of castellated composite beams is studied.Based on the results of experiments and numerical simulations,the factors affecting the bearing capacity in the negative moment zone of the castellated composite beam are analyzed,and the bearing capacity calculation formula is established.The research results show that the bearing capacity of the castellated composite beam in the negative moment zone is affected by the ratio of width to thickness of the flange of the concrete slab,steel beam flange,the ratio of web height to thickness and the opening ratio.The stress value of the top steel bar of the concrete slab under load is larger than that of the bottom steel bar.It is the main stress position and has a significant impact on the bearing capacity.With the increase of the thickness of the concrete slab,the contribution to the bearing capacity gradually increases;at the same time,The bearing capacity decreases with the increase of the width-thickness ratio of the steel beam flange,the height-thickness ratio of the steel beam web and the opening ratio.The failure mode of the negative moment zone of the castellated composite beam changes from shear failure of the piers between holes to compression failure of the flange after the flange width-to-thickness ratio increases to 25,after the web height-tothickness ratio increases to 47.2,it changes from the flange compression failure to the shear failure of the piers between the holes,and after the opening rate increases to 64%,it becomes hole corner failure.The negative bending moment zone of the castellated composite beam is symmetrically distributed along the vertical direction under the condition of pure bending.The hole angle near the concrete slab is under tension and the hole angle near the compression flange is compressed.When the ultimate bearing capacity is reached,the compression hole angle yields but the tension hole angle does not yield;the stress around the hole shows the same tension and compression state of the oblique hole angle under the combined action of bending and shear,and both the compression and the tension hole angle stress can reach the yield strength under load.The calculation formula for the flexural bearing capacity obtained by considering factors such as the height of the composite beam,the thickness of the concrete slab,the reinforcement of the composite beam,the size of the steel beam flange and web,and the opening ratio can accurately reflect the flexural bearing capacity of the castellated composite beam in the negative moment area,and has good engineering application value.