The anchorage of deformed bars in reinforced concrete members subjected to bending

摘要

The conditions favorable for the development of uniform bond stress in a short anchorage length of a reinforcing bar in a pull-out test are rarely present in practical reinforced concrete (RC) members. The determination of a representative average ultimate bond stress for use when designing the anchorage of tensile bars in RC flexural members is complicated by fluctuating bond stress conditions arising from the presence of cracks crossing the anchorage length. This research is aimed at assessing typical bond failures at the anchorages of tensile bars in RC beams and slabs, and hence determining the representative value of average ultimate bond stress for designing the anchorages. An experimental study was conducted to assess the impacts of monotonically increasing loads on the anchorage requirements of small diameter deformed bars in tension, including the cases of end development and lapped splices.The study revealed that the value of the average ultimate bond stress that develops at failure in an anchorage is not only heavily dependent on the bar diameter, but is also dependent on the anchorage length. The measured value of average ultimate bond stress for shorter anchorage lengths is higher than that for longer anchorage lengths. Analytical modelling based on the structural mechanics of bond has been undertaken to reasonably represent the variable degree of plasticity in the concrete around an anchored bar, and to calculate a more reliable estimate of the average ultimate bond stress. The practical implications of the mean local bond stress-slip models for modelling longer anchorages have been assessed, and the bond-slip models have been recalibrated to account for the variations of average ultimate bond stress for different anchorage lengths and bar diameters. The recalibration of the bond-slip laws for selected RC anchorage specimens has been undertaken according to a viable analytical modelling procedure, and a modified model of the peak bond stress has been proposed for the splitting type bond failures typically observed in RC flexural members. Finite element modelling of the RC anchorage specimens has been undertaken using the recalibrated bond-slip laws and the numerical results are in good agreement with the laboratory measurements. The analytical and numerical modelling provides for rational simulations of the strength and failure modes of anchorages of tensile bars in RC flexural members.