Experimental study on anchorage performance and mechanism of hooked bars with a pre-yield anchorage failure

In RC exterior beam-column joints, reliable anchorage of beam bars using hooked bars is a common approach to ensure sufficient joint rotation capability. The anchorage failure criterion and the influence of axial load and side cover on anchorage strength are unclear with the limitings of the existing test setup and test approach. Twenty-two simulated beam-column joint tests with hooked bars anchorage were therefore conducted. A specially designed test setup was proposed to ensure the simultaneous failure of hooked bars within a specimen, while a novel approach was employed to separate the anchorage force of the hook portion into bearing and bond forces based on stress distribution measurement. The experimental results show that the anchorage strength of hooked bars and hook portion bearing strength increase as side cover thickness and axial load increase, which may potentially alleviate joint congestion. Additionally, the study identifies that the hook portion bearing strength dominates the anchorage failure mechanism of hooked bars, prompting the proposal of an anchorage failure criterion. Based on this criterion, models for anchorage strength and anchorage force-slip relationship were proposed, which are applicable to hooked bars with varying inner bend diameters in actual structures. To examine the safety and validity of the proposed equation for anchorage strength, it was compared against a hooked bar test database with 363 anchorage specimens, as well as several existing models. The database analysis showed potential safety risks with the GB50010-2010 model. Notably, the proposed model demonstrated the highest accuracy in predicting the anchorage strength of hooked bars.