Behaviour of blind-bolted connections in concrete filled steel tubular columns

Abstract

The concrete filled steel tubular (CFST) members have been widely adopted in construction industry in the past few decades owing to its superior strength, ductility, and aesthetics. Its performance in better fire resistance and resisting higher loads in post-yield capacity have been the main reasons for its growing popularity. The performance of CFST members is dependent on composite action of the surrounding steel tube and the infill concrete. As in a realistic building the load is transferred from the floors to the column via the beam, it is therefore important to study the connection behaviour in a steel-beam and CFST column joint. The connections of the open-section steel beams with closed-section hollow or CFST columns are generally observed to be welded. But, the process of welding develops heat affected zones, and the fabrication is usually cumbersome requiring skilled manpower and thus expensive. The bolted connections have gained several advantages over the welded connection due to easier and faster fabrication. For fabrication of open-beam to closed-section column connection, the standard bolts are not feasible as one cannot access the other end of the bolt, and therefore blind-bolts are used. The blind-bolts can be inserted and tightened from outside of the steel tube without accessing inside of the tube. But the usage of bolts has been limited due to its severe slippage and column surface deformation. Also, the bolted connections are usually referred to as pinned connections due to its insufficient moment-resisting capacity. Thus, developing a semi-rigid or rigid moment-resisting bolted connection has been a challenge for the structural engineers. Therefore, researchers recently modified the blind-bolts to enhance the connection performance and is observed to have potential for developing moment-resisting frames. The blind-bolts like Ajax Australia bolt and the Lindapter hollo-bolt have been modified by researchers in Australia and the United Kingdom with an extended shank and headed circular nut. The purpose of the extended shank with headed nut is to provide anchorage into the concrete core of the CFST column. The previous findings with hollo-bolted CFST column connections identified two primary failure modes: the bolt failure and column-face failure. But in an actual construction site condition, a combined failure mode would be developed, rather than a bolt failure or column face failure. Therefore, this current research has delved into an extensive investigation to explore the combined failure modes for both single and group anchored blind-bolted CFST connections under direct tensile loading. From preliminary studies, it is observed that concrete cone failure is the predominant mode of failure, therefore the interaction between anchored blind-bolts when in group needs was assessed with due consideration to the bolt pitch and gauge distances. The experimental programme is supported by material tests for all the connection components. Numerical simulations are also carried out to supplement the experimental data. It is observed that with an appropriate strength combination, the combined failure mode can be achieved, and displayed higher ductility. It was also seen that the connection stiffness is influenced by the width to tube thickness ratio, concrete strength, bolt diameter and bolt anchorage length. A component model based on non-linear behaviour of individual connection element is also presented that can predict the global connection behaviour. For the group of anchored bolted connection, it is observed that total connection strength cannot be determined by summation of individual bolt capacity. Also, a bolt anchorage length of 3.75 times bolt diameter may be adopted to achieve 60% connection capacity of the group bolt, and a minimum of 50% of the total applied loading was borne by the concrete, displaying enhanced composite behaviour. As the bolts in a connection will be subjected to combined forces of tension and shear, and thus to understand this combination, the performance of extended hollo-bolts in pure shear was investigated. The influential benefit of extended hollo-bolt under shear loading is realised, and the existing design codes are assessed. With different bolt embedment lengths, the failure mode was consistent having fracture of bolt shank and sleeve along the shear plane, and full ultimate bolt capacity can be achieved. The shear load was significantly transmitted to the concrete core by bearing, and tube wall yielding was delayed. The findings from this research will be able to provide directions towards developing moment-resisting bolted connections. This research is also expected to provide some insights towards load-introduction mechanism in bolted connections.