Research On The Shear Behavior Of The UHPC Beams With Small Shear Span To Depth Ratios

UHPC has excellent mechanical properties and has been gradually used in bridge engineering structures.However,there are still two challenges for the UHPC beams with small shear span to depth ratios,that is,the predictions of the shear capacity at the ultimate state were relatively conservative based the current codes and methods,and the serviceability behavior of the UHPC beams was unclear,especially for the crack width.To this issue,the elastic to plastic strut and tie model and modified compression field theory were adopted to find out t he design and analysis model for UHPC beams with small shear span to depth ratios at the ultimate state and serviceability state.This study was carried out based on the following aspects:(1)45 reinforced UHPC specimens were designed and tested,carried out,and the tensile performance of the reinforced UHPC showed approximately bilinear changes.In the uncracked stage of UHPC,the stress-strain curve showed linear elasticity;when UHPC cracked,its stiffness will decrease,and the stress-strain will generally change linearly at this time.Therefore,the tensile stress-strain curve of reinforced UHPC was established based on the single-fiber drawing model.(2)12 UHPC beams with small shear span to depth ratios were designed and tested.The experimental study showed that,for the same details of the stirrups,the cracking load and load-bearing capacity of UHPC beams decreased with shear span to depth ratios;when as for the same shear span to ratio,the load-bearing capacity did not change much,that was,the contribution of the stirrups to the shear capacity was very small for UHPC beams.(3)Based on the theory of elastic to plastic strut and tie model,an analysis model of the UHPC beams was constructed,and the principle of residual energy was used to decouple the arch mechanism and truss mechanism in the model,thereby the evolution law of the force mechanism was revealed.The ratio of the arch mechanism can be divided into three stages,the uncracked stage,the development stage of flexure cracks and the development stage of diagonal cracks.In the uncracked stage,the ratio of the arch mechanism was a certain value,and was linearly related to the shear span ratio of the UHPC beam;As for the development stage of the flexure cracks,the ratio of the arch mechanism decreased rapidly with the increase of the load;after the diagonal crack occurred,the ratio increased linearly with the increase of the load.(4)Following elastic to plastic strut and tie model,the method for prediction of the shear capacity was established and compared with the calculation methods of current design codes.The results showed that the calculation method in this paper was in good agreement with the experimental values,the mean ratio was 0.89 and the variance was 0.002,while the calculated value of the French code will underestimate the shear bearing capacity of UHPC beams.Moreover,according to the elastic to plastic strut and tie model,the responses of the UHPC beams during the entire loading process were analyzed and verified by the test results.(5)According to the modified compression field theory,and taking into account of the shear contribution of UHPC in the shear compression zone,an analysis method of the shear capacity of the UHPC beams with small shear span to depth ratios under the combined force of bending and shear was established,therefore,the calculation method of the shear capacity and the crack width of the UHPC beam s with small shear span to depth ratios were obtained,and based on the truss model theory and the maximum stiffness criterion,a calculation method for the angle of the diagonal crack was proposed.The average value of the ratio of the calculated value of the shear capacity to the test value was 0.93,and the variance was 0.004.