Study On Bending Strength Of Partially Filled Narrow Steel Box-UHPC Composite Beam

The concrete flanges in the support zone of steel-concrete continuous composite beams are susceptible to cracking due to the negative bending moment,which in turn has an impact on the later use of the structure.Ultra-high performance concrete(UHPC)has good resistance to cracking and high strength;the narrow steel box composite structure benefits from a smaller base plate width to thickness ratio,which ensures structural stability while reducing the arrangement of stiffening ribs and facilitating design and construction.Based on the above advantages,a new structural form,the partially filled narrow steel box-UHPC composite beam,is proposed.The following work has been carried out to investigate the flexural performance and bending damage mechanism of this new composite beam under negative bending moments:(1)Model test: Nine partially filled narrow steel box-UHPC combination beams of 3m long and 0.6m wide with different parameter variables were subjected to mid-span reverse single-point static loading tests using counterforce frames and hydraulic jacks.It was found that the test beams entered the plastic deformation stage after reaching the ultimate load and eventually broke down due to excessive deformation of the web and bottom plate of the steel box beam,showing good ductility;the cracking load of the UHPC flange with 3% steel fibre doping was63.1%,29.8% and 7.7% greater than that of the UHPC flange with 0,1% and 2%steel fibre doping respectively;the reinforcement ratio of the flange was increased from 1% The yield loads were increased by 13.4% and 16.1%,respectively,by increasing the reinforcement rate from 1% to 2% and 3%;the cracking resistance was increased by 56.2% and the ultimate load was increased by 7% by replacing 1/2thickness of plain concrete in the flange with UHPC;the yield loads were increased by 38.3% and 43.1% and the flexural load capacity was increased by 50% and 60.9%for half-filled and full-filled compared to 0-filled core concrete.The above results show that the addition of steel fibres can control the development of cracks and increase the plasticity of the wing slabs,which can effectively improve the brittleness phenomenon during damage,and that filling the core concrete can significantly increase the structural load carrying capacity.(2)Finite element simulation analysis: ABAQUS was used to build finite element models of seven test beams with different flange sizes.The applicability of the built models was first verified by comparing them with the load-deflection curves of the test beams.Afterwards,the effects of structural parameters such as flange width,flange thickness and UHPC flange length of the composite beam on its flexural performance were analysed.The results show that the flange thickness has a more significant effect on the cracking load,yield load and ultimate load than the flange width,up to 14.2%,40% and 50%,and the cross-sectional dimensions are optimised with reference to the results obtained.(3)Calculation of flexural load capacity:Based on the test and finite element analysis results,the calculation models of elastic and plastic flexural load capacity of the composite beam were established,and the formulae for calculating the flexural load capacity of the section under negative bending moment of the composite beam were derived.The application range of the two calculation methods is discussed,and the average accuracy of the calculation results is between 0.9 and0.95,which verifies the accuracy of the two ultimate flexural load bearing capacity calculation methods.