Research On Design Method Of Micro Steel Fiber Reinforced Concrete Gradient Composite Beam

With the in-depth development and technology update of highway bridge,civil infrastructure and other engineering construction,higher requirements are put forward for concrete materials and structural performance.Steel fiber reinforced concrete can solve the brittle fracture problem of ordinary concrete by adding steel fiber into ordinary concrete matrix,which is an important research direction of concrete reinforcement and toughening.The efficient and economic layout form of micro steel fiber reinforced concrete can make up for the short slab of reinforced concrete beam easy to crack,In order to quantify the influence of high content of micro steel fiber on the bending and crack resistance of reinforced concrete beam,and form the design method of gradient composite beam based on active crack control.Based on the concept of functional gradient,this paper analyzes the calculation method and experimental study of rectangular section steel fiber reinforced concrete gradient composite beam,which can provide an important theoretical basis for the design of high volume fraction micro steel fiber reinforced concrete gradient composite beam.Firstly,combined with the micro element method and the constitutive model functions of steel fiber reinforced concrete and ordinary concrete,according to the mechanical structure characteristics of composite beams,based on the force balance and moment balance conditions,the calculation formulas of cracking load and ultimate load of reinforced steel fiber reinforced concrete gradient composite beams are derived;and according to the characteristics of over reinforcement failure and less reinforcement failure of composite beams,the calculation formulas of concrete cracking load in the top region are obtained.According to the concept of reinforcement design,the calculation method of balanced reinforcement ratio of beams with suitable reinforcement is obtained.Secondly,based on the effective moment of inertia method and the calculation theory of crack width,the effective simplified conversion of reinforcement and micro steel fiber is carried out,the full section moment of inertia before and after the crack of the beam are calculated respectively from the perspective of structural design,and the stiffness calculation model of the beam in normal use stage is derived by interpolation method.By introducing the influence coefficient of micro steel fiber crack width,considering the effect of micro steel fiber on the composite beam structure,the calculation methods of average crack spacing,average crack width and maximum crack width are derived.Then,three-point loading tests are carried out on ordinary reinforced concrete beams and steel fiber reinforced concrete gradient composite beams.According to the test results,the calculation models of cracking load,ultimate bending capacity,stiffness and mid span deflection,average crack spacing,average crack width and maximum crack width in the normal use stage of the beam are compared and analyzed,and the calculation error is kept within 6%.Combined with the test results,the influence coefficient of crack width of micro steel fiber is corrected and rechecked.Finally,in order to make up for the shortage of the actual test samples and verify the general applicability of the calculation model,the discussion and analysis of variable factors are added combined with the finite element simulation.Based on the plastic damage model of concrete,the extended simulation analysis of micro steel fiber reinforced concrete gradient composite beam is carried out,and the simulation results are basically consistent with the results of the calculation model.The comparison of the calculation model with the experimental data and the results of finite element analysis shows that the calculation model in this paper is feasible,accurate and safe.