The tensile stiffness of a novel anchored blind-bolt component for moment-resisting connections to concrete-filled hollow sections

The use of hollow section columns in steel construction is presently hindered by the lack of adequate connection technologies. Due to access constraints, standard bolting techniques are difficult to achieve, if not impossible without welding. As an alternative to welding, blind-bolting techniques were developed to provide desirable bolted configurations, allowing hollow column frames to be erected in the same way as open profile column frames. But the current blind-bolting techniques are restricted to the construction of simple connections because of their difficulties in achieving sufficient tensile stiffness. More recently, a novel anchored blind-bolt, labelled the Extended Hollo-bolt (EHB), has been developed at the University of Nottingham; as a modification of the standard Hollo-bolt. For the proposed connection technology, its potential in providing moment-resistance has been assessed successfully. However, the existing data related to the performance of this novel connector in tension is insufficient to permit its design. This work investigates the performance of the EHB blind-bolt under tension loading and focuses on determining, and modelling the stiffness of this novel technology in such a way to enable its application within the component method approach. An extensive experimental programme was devised to collect sufficient component characteristic data to enable the development of an EHB component model. This covered data deals with the overall response of the connector and the individual responses of its contributing elements. A total of 51 experimental pull-out tests and 20 pre-load tests have been performed. The force-displacement behaviour of the investigated joint component was determined under monotonic pull-out testing, where remote video gauge techniques have been adopted to capture the full non-linear response of the component, alongside traditional techniques to confirm the reliability of the data. The test matrix varies the grade and size of the component's internal bolt, the strength of concrete, and the depth of its mechanical anchorage. From the pull-out tests it was identified that the EHB component can ultimately develop the full tensile capacity of its internal bolt. This ultimate failure mode is confirmed for the range of parameters that was covered in this study. Increasing concrete strength had the most enhancing effect on the response of the component. A secondary programme was related to the measurement of pre-load that is induced in the internal bolt of the EHB component at its tightening stage; where pre-load was monitored over a five day period. The test matrix varies the grade and size of its internal bolt, and also considers various bolt batches. It was concluded that the relative level of component pre-load to ultimate strength increased only in the case where higher bolt grades were used. To model the tension behaviour of the EHB component, a mechanical model was developed that is based on an assembly of the component's different sources of deformation. The component model employs idealised springs with tetra-linear characteristics for the elongation of Its Internal bolt element, and springs with tri-linear characteristics for the slip of its expanding sleeves and mechanical anchorage elements. By comparing the predictions of the component model with relevant experimental data, the component model has been shown to be capable of describing the EHB component response with reasonable accuracy; capturing its tensile stiffness and its yielding trend. The accuracy of the component model has also been assessed in exclusion of pre-load effects. It was found that if the level of pre-load Is excluded from the assembly process, this can have highly undesirable effects on the predictions of the component's response. The findings of the supplementary pre-load testing programme assisted greatly in the accuracy of the component model by providing the necessary levels of pre-load. The proposed component model has demonstrated that the behaviour of the EHB component can be modelled by the component method approach; by employing Idealised models for the behaviour of its contributing elements. The validated component model is considered to simulate the tension behaviour of the novel anchored blind-bolt with sufficient fidelity that it can be considered as a benchmark for further studies