Flexural Bearing Capacity Research Of Steel Truss And Concrete Composite Beams

Steel truss-concrete composite girder is a new type of composite beam structurewith superior space-using performance and significant project economic benefits.As apromising structural system of application, the research of steel truss-concretecomposite girder in China is still at the starting stage, related theoretical andexperimental research is still insufficient, especially the lack of research on bearingperformance and the design method suitable for needs.Because of the complex mechanical performance of steel truss-concretecomposite girder and the extensive experimental study is not realistic.A relativelysimple circular steel tube composite truss girder has been used as the research objectin this paper.The theoretical analysis, finite element numerical simulation andexperiments are adopted to study the properties and the influence factors of bendingbearing capaticy of the steel truss-concrete composite girder.Firstly, this thesis uses the experimental method to study the steel truss-concretecomposite beam flexural capacity.The CB1, CB2a, CB2b and CB3specimen aretaken for example.The test results show that: the whole section strain of the steel-trussconcrete composite girder of obeys the plain section assumption.Therefore, thetheoretical calculation with the plane section assumption is reasonable. It is alsofound that the concrete composite beams shear compression flange exist obvioushysteresis effect, therefore, the shear lag effect should be considered during thedesigns.The results show that: the bearing capacity and deformation performance ofcomposite truss beam is good.On the basis of experimental researches, the elastic and plastic stress analysis ofthe solid-web steel-concrete composite beams is put out, the circular steel tube trusstype steel concrete composite beams are analyzed and the stress and the effectivesection width is discussed, the formulas of the ultimate bending bearing capacity ofthe composite beams are derived. Theoretical analysis shows that: the compressiveconcrete flange effective width is an important factor which affects the bending bearing capacity.The accurate effective width should be calculated in design.For thesteel truss-concrete composite girder, there are only two basic failure modes offlexural failure, namely the lower chord tension yield failure or upper concrete plateis pressed.Designers should choose the failure mode and calculate the bearingcapacity of the corresponding limit. Analysis shows that:the4specimens of finiteelement calculation of elastic stiffness are larger than the test of elastic stiffness; frombearing capacity, bearing capacity and the test results are close to the ultimate finiteelement, the deformation performance is consistent with the test results. This providesa basis for the further study of parameter analysis.In order to facilitate the design application,a simplified calculation formula forthe effective width of steel-truss concrete composite girder is put out,which isconvenient for engineering application. In order to understand furtherly of the factorswhich have influence on the bearing capacity of the steel-truss concrete compositegirder, this paper also analyses the ventral rod thickness, width of concrete slab,girder truss chord plate width and spacing of studs and other parameters.On this basis,this paper presents a method to design a steel-truss concrete composite girderpreliminarily.In order to further understand the influence factors of the bending bearingcapacity of composite beams, the ventral bar section, concrete slab thickness andwidth, upper plate truss beam width and stud shear connection degree and otherparameters are analyzed.The characteristics of these design parameters haveimportmant influences on the flexural performance of composite truss girder.Research results show that: when the other parameters are fixed, composite beam webrod diameter thickness ratio increases, the initial stiffness of the composite beam, postyield stiffness and ultimate bearing capacity had no obvious change. So the designappropriate to consider the use of concrete wing plate is thick and broad, not only canimprove the bearing capacity of composite beams and can improve the deformationproperties of composite beams. In addition, the degree of shear connection upperplate width and studs are combined to improve the bearing capacity of beam in acertain range.The theoretical analysis is put out in order to understand better of the theory of flexural performance of composite truss girder. Comparison of solid web steel-concrete composite beam from the two levels of elasticity and plasticity, and the fullshear connection composite beams with partial shear connection composite beamplastic analysis results were put out. Research shows that: for non full shear trusscomposite beam (CB1), finite element calculation and theoretical values are almostthe same, while the test is smaller than the theoretical value of5%. For the completeshear truss composite beams (CB2a, CB2b and CB3), average7%greater than thetheoretical value of the test value, finite element calculation value is about19%higher than theoretical value.Therefore, the theoretical calculation formulas of theultimate bending bearing capacity of the composite beams are suitable for use,and itcan ensure enough strength and ductility reserve.At present, the traditional stud connectors are widely used in most steel-concretecomposite structures in China, the connection pieces root is relatively weak, the shearability is poor, the connection is easily failure.These disadvantages cause great wastein steel-concrete composite structures in China.Aiming at this problem, this paperproposes a novel double screw form head stud shear connection, and put out thepush-out test and finite element simulation analysis. On this basis, the main factorsinfluencing the new stud shear connectors carrying capacity are discussed and somedesign suggestions are put forward. Finally, the bending bearing capacity of asteel-truss concrete composite girder with the new style double-nut stud shearconnectors is discussed. The results show that: when the other parameters areconstant, the higher bearing capacity and better deformation ability should beobtained in the composite beams with new style double-nut stud shearconnectors.And the new stud shear connector has a good application prospects in thesteel-concrete composite structures.