Experimental study on modified blind bolts anchored in concrete-filled steel tubular columns

Although concrete-filled tubular columns are becoming popular in multi-storey building frames, connections between the columns and beams using normal blind bolts are limited in the level of moment-carrying capacity and stiffness they can achieve. This is because of the extensive localised yielding of the thin tube wall caused by the blind bolts bearing on the inside of the tube wall. This paper reports the results of an innovative modified version of blind bolt that aims to develop economical semi-rigid to rigid connections that are simple to construct. It uses a high tensile threaded rod cut to the required length with two special nuts, one acting as a bearing head on the inner tube wall in association with ONESIDE split washer and the other as an anchoring device in the concrete core. The results from full-scale pullout tests on bolts connected to concrete-filled circular and square hollow sections are presented. The objective of this work was to investigate the anchorage performance of the special blind bolts, including failure modes, strength and stiffness. The parameters selected were tube wall thickness, blind bolt diameter, and concrete strength. The results show that the full tensile capacity of the threaded rod can be achieved in the circular columns with higher strength concrete, whereas the performance is compromised in the square column with lower strength of concrete. These test results provide valuable information for developing a novel concept of blind-bolted moment connections to ensure sufficient strength and stiffness when they are used in momentresisting frames in low seismic regions such as Australia.