Research On Shear Capacity Of UHPC Beams Based On Truss-Arch Model

Ultra-high performance concrete(UHPC)is a new type of cement-based composite material that has a wide range of applications in bridge engineering.The shear failure of concrete beams is brittle damage,UHPC strength is high,and high strength may bring high "brittle hardness",making the shear failure brittleness of UHPC beams more serious.Steel fibers are incorporated into UHPC,but the contribution of steel fibers to the shear bearing capacity of UHPC beams has not been quantified,so it is necessary to quantitatively study the contribution of steel fibers to the shear bearing capacity of UHPC beams and propose a more economical and reasonable calculation formula for the shear bearing capacity of UHPC beams.This article mainly has the following work and conclusions:(1)By numerical analysis,the shear failure pattern of UHPC beam was studied,and the effects of longitudinal reinforcement rate,longitudinal rib strength,steel fiber doping and shear-span ratio on the load-displacement curve of UHPC beam were analyzed,and the shear resistance of ordinary concrete beams was compared.The results show that with the increase of the reinforcement rate of the longitudinal rib,the shear bearing capacity and ductility of the UHPC beam increase;With the increase of the strength of the longitudinal rib,the shear bearing capacity and ductility of the UHPC beam increase;With the increase of the amount of steel fiber,the shear bearing capacity and ductility of the UHPC beam increase;With the increase of the shear span ratio,the shear bearing capacity of the UHPC beam decreases and the ductility increases;The plastic damage of the UHPC beam is low,the initial elastic modulus is large,and the shear bearing capacity is high.(2)The similarities and differences between the shear calculation formula in the ordinary concrete specification and the UHPC specification were compared and analyzed,147 UHPC rectangular beam test data were collected,the shear test database was established,and the applicability of the shear calculation formula in the ordinary concrete specification and UHPC specification was verified;based on the shear test database,the effect of shear span ratio,longitudinal rib reinforcement rate,stirrup reinforcement rate and steel fiber characteristics on the shear bearing capacity of UHPC beam was statistically analyzed.The results show that the calculation method of the ordinary concrete specification cannot be directly applied to UHPC sheared members;In the existing UHPC specification,the calculation of the anti-shear bearing capacity is conservative;The shear bearing capacity of the UHPC beam decreases with the increase of the shear-span ratio,Increases with the increase of the longitudinal rib reinforcement rate,decreases with the increase of the reinforcement rate of the stirrup,increases with the increase of the length-to-diameter ratio,and increases with the increase of the amount of steel fiber;The optimal range of the reinforcement rate of the longitudinal rib is4%~8%,and the optimal range of the length-to-diameter ratio is 60~ 80,the optimal range of steel fiber doping is 1% to 3%.(3)Based on the truss-arch model,considering the influence of the reinforcing effect of steel fiber on the shear bearing capacity of UHPC beam,the force analysis of a single steel fiber is carried out,the bonding stress between steel fiber and UHPC is analyzed by using the principle of thick-walled cylinder,and the tensile force of steel fiber in UHPC beam is proposed by considering the fiber effect coefficient,and then the steel fiber is quantized as a truss lever for shear calculation,and the calculation formula of shear bearing capacity of UHPC beam is proposed,and the range of the oblique pressure abdominal rod inclination angle and softening coefficient and shear-to-span ratio in the formula is given.Based on the shear test database collected in this paper,the calculation value of the model and the test value in this paper are compared,the accuracy and applicability of the shear calculation formula derived in this paper are verified,and the contribution value of steel fiber in shear bearing capacity is discussed.The results show that the shear calculation formula derived in this paper has certain accuracy and is suitable for UHPC beams under different beam heights,and the steel fibers have obvious contributions to the shear bearing capacity of UHPC beams and cannot be ignored.(4)Based on the classical bonding-slip theory,the calculation formula of shear-slant crack width for UHPC beam is proposed,and the values and calculation formulas of relevant coefficients such as oblique crack load and comprehensive difference coefficient in the formula are given.