| The steel-concrete composite beam combines the mechanical advantages of steel and concrete,and its larger stiffness,higher span capacity and larger span ratio make it have a good application prospect.However,under the effect of negative bending moment,steel beam compression and concrete tension make the structural system form a weak section in the negative bending moment region.The cracking of concrete in negative moment region is an urgent problem to be solved.Ultra-High Performance Concrete,also known as Reactive Powder Concrete,is a super high tensile/compressive strength,toughness and strong durability of cement base composite material.Researches have shown that UHPC under 0.05 mm width crack still has the tensile strength,and the permeability resistance does not fall,not more than 0.05 mm width of crack negligible influence on the durability of structure.UHPC is widely recognized as a new structural material with great development and application prospects in the future.Its ultra-high tensile strength and toughness make it more and more applied in the section of steel-composite structure with negative bending moment.However,the difference(including: cracking strength,strain hardening effect and ultimate crack strength)between the properties of ultrahigh performance concrete and normal strength concrete makes the design and calculation method in negative moment region of steel-normal strength concrete composite beams not suitable for the steel-UHPC composite beams.The research on the mechanical properties difference of steel-UHPC composite beams is the key to the popularization and use of this new type of composite beams(Including: failure form,crack resistance,bending capacity and deformation capacity).Therefore,in order to compare and analyze the mechanical characteristics of steel-UHPC composite beams in the negative moment region,including failure form,crack resistance,flexural bearing capacity and deformation capacity,the negative moment loading test of steel-UHPC composite beams was designed,and the theoretical calculation method of the flexural characteristic value of steel-UHPC composite beams was derived on the basis of the test.Finally,the finite element analysis of steel-UHPC composite beam was carried out by Abaqus finite element software.The research work and conclusions are as follows:(1)There are 4 types of steel-UHPC composite beams,including stud connector,high-strength bolt connector,rectangular-seam and T-seam.At the same time,a steel-normal strength concrete composite beam is designed for comparison.The failure morphology,cracking characteristics,interface slip characteristics,deformation capacity,stress-strain curve and ultimate bearing capacity of steel-UHPC composite beams were measured by single point loading of negative bending moment,and the corresponding comparative analysis was carried out.(2)The test results show that: Ⅰ.The final failure pattern of steel-UHPC composite beam and steel-normal strength concrete composite beam is similar,which is manifested by several cracks on the top of UHPC(NSC)wing plate,steel beam web and lower flange plate buckling.Ⅱ.Compared with the normal strength concrete,UHPC greatly improves the crack resistance of composite beams in the negative moment region.The initial cracking load of UHPC is3.4 times higher than that of normal strength concrete.Compared with the traditional stud connector,the high-strength bolt connector further improves the crack resistance of steel-UHPC composite beam.The existence of seam weakens the crack resistance of steel-UHPC composite beams,but its crack resistance is still greatly improved compared with that of steel-normal strength concrete composite beams.Ⅲ.The frictional slip caused by the highstrength bolt connectors maximizes the relative slip value in the composite beam interface.The larger slip leads to the generation of a neutral axis in the section of the UHPC and steel beams of the composite beam respectively.The second is the steel-normal strength concrete composite beam,the larger crack width has a greater impact on the interface slip.The slips of T-seam and rectangular seam steel-UHPC composite beams are relatively close to each other.The steel-UHPC composite beam with stud connector has the smallest slip value due to its strong stud welding,crack resistance and good UHPC integrity.Ⅳ.Compared with normal strength concrete,the high tensile strength of UHPC reduces the ductility and rotational capacity of steel-UHPC composite beams,but the rotational stiffness of the composite beams is improved.At the same time,the ductility and rotation ability of the composite beam can be effectively improved by using the high-strength bolt connector relative to the stud connector,and the traditional stud connector can be replaced by the high-strength bolt connector to reduce the reduction of the deformation capacity of the composite beam generated by UHPC.Ⅴ.Compared with stud connector,the frictional slip of high-strength bolt connector reduces the restrained tensile stress of UHPC and the internal steel bar strain,increases the strain of steel beams,and slightly reduces the bearing capacity of composite beams.Compared with normal stregnth concrete,the high tensile strength of UHPC greatly improves the bearing capacity of composite beams.At the same time,the weak section of seam interface leads to a slight reduction in the bearing capacity of steel-UHPC composite beam,but the bearing capacity of the composite beam is still greatly improved compared with that of the normal stregth concrete composite beam.(3)According to the stress characteristics of negative moment region of steel-UHPC composite beam,simplified formulas for calculating the cracking load of 0.05 mm of steel-UHPC composite beam(SU-S and SU-B),the initial sliding load of high-strength bolt connector and the flexural capacity of steelUHPC composite beam are proposed.The calculated results of the formula and the experimental results are compared and verified,the theoretical calculated values are in good agreement with the experimental values,and the bending capacity calculation method has a good safety factor.(4)In combination with the modeling techniques of previous scholars,the basic parameters of materials obtained in the experiment,including: steel bar,steel beam and UHPC,were used to conduct finite element modeling of the test model through the large-scale universal finite element analysis software Abaqus,and on the basis of this modeling,the finite element parameters of the test beam were analyzed,such as bolt shear connection degree,opening aperture of upper flange of steel beam,preload of highstrength bolt,thickness of UHPC wing plate and diameter of steel reinforcement. |
PDF Full Text Request