Behaviour of bolted connection system in pultruded GFRP structures

Behaviour of bolted connection system in pultruded GFRP structures

The pultruded GFRP hollow sections, in particular, have received growing interest from the engineering community due to better torsional rigidity, effective resistance of out-of-plane forces, high load transfer and improved strength and stiffness of the minor axis. However, one of the significant issues that hinders the widespread use of pultruded GFRP hollow sections is the inadequacy or unpredictability of its connection system. In this study, the behaviour of pultruded GFRP truss structure using through-bolt connection system was investigated based on the current industrial practice in Australia. The through-bolt connection system is incorporated with an FRP mechanical insert as a filled-type connection element and currently, there is no scientific research focusing on joint behaviour of pultruded GFRP hollow profiles with mechanical inserts. This has been the key motivation for this research whereby the suitability and joint strength adequacy of bolted connection with insert for pultruded FRP, in particular, tubular profiles were examined. Therefore, the study ultimately investigated this particular jointing technique on pultruded GFRP trusses and aimed to understand how the loads are resisted, transferred, and distributed to each FRP component. The experimental data and theoretical predictions developed in this study are critical to produce a safe, reliable and adequate connection system for pultruded GFRP hollow sections.

This thesis is presented as a compilation of technical papers. In the first paper, effects of threaded bolt with varying end distance to bolt diameter, laminate thickness, clamping pressure and laminate orientations (longitudinal and transverse) on the joint strength behaviour, joint efficiency and mode of failure were evaluated using a double lap joint test set-up configuration. The test results obtained from the effects of using threaded bolt were compared to that of plain bolt in order to assess the differences in joint behaviour and possible reduction in joint capacity. In this experiment, the joint was designed to promote bearing failure as it is preferable in composite joint due to its progressive nature of failure. From this study, approximately 30-40% reduction in joint strength was observed for specimens with longitudinal laminate orientation caused by laminate tearing of the bolt. In addition, under scanning electron microscope (SEM) imaging, this damaging effect was further observed to better understand its mechanism and how it affects the resulting mode of failures.

In the second paper, the joint behaviour of pultruded GFRP hollow sections with a single all-threaded bolt and mechanical insert connection system was investigated under elevated in-service temperature. A comparison of different bolted joint configurations of pultruded GFRP hollow sections, namely joint without mechanical insert (N), joint with mechanical insert with tight-fit attachment (I) and joint with mechanical insert bonded with epoxy adhesive (G) was conducted and the effects on the joint strength and failure mechanism were evaluated. The results of this experimental work have demonstrated that the bolted joint with adhesively bonded mechanical insert sustained the highest load-carrying capacity across the elevated temperatures compared to other configurations. Also, the proposed joint strength prediction equation, which incorporates the strength reduction and modification factors based on different joint configurations involving mechanical insert, produced reasonable outcomes against experimental failure load. These results suggest that, the use of mechanical inserts to strengthen bolted connections system can be adopted in pultruded GFRP hollow sections and the joint performance of this configuration at a structural level was discussed in the next paper.

In the third paper, the joint behaviour of through-bolt connection with mechanical insert under eccentric loading was investigated. The joint configuration was adopted in pultruded GFRP T-joints using both single and double bottom chords, with the former imbalanced configuration intended to impart load eccentricity. This eccentric condition can be found in composite truss bridges. The experimental results showed that, the presence of mechanical inserts in both single and double bottom chords of the T-joints had improved the joint strength and fixture stiffness when compared to their insert-less counterparts. It was found that the mechanical insert has prevented bolt flexure and contributed to the improvement in bending resistance when subjected to a couple moment developed due to eccentricity.

In the last paper, the structural behaviour of double-chorded pultruded GFRP trusses connected using through-bolt with mechanical inserts under different load cases were investigated. The structural performance of the trusses was described in terms of load-midspan deflection response, force distribution of internal members and mode of failure. The results of this study indicate that the adopted through-bolt with mechanical insert connection system possess high joint load-carrying capacity and demonstrated effective transmissitheoretical strength limits of pultruded GFRP truss members in tension, compression and flexural according to ASCE pre-standard were in close agreement with the experimental results. Meanwhile, the prediction equation proposed in the second paper was used here to evaluate the joint load-carrying capacity of the pultruded GFRP trusses. A two-dimensional numerical model to simulate the behaviour of the pultruded GFRP trusses was constructed using Strand7 finite element analysis software. Satisfactory comparisons against experimental results were achieved and this demonstrates the validity of the Strand7 simplified numerical model.

From this overall research program, it can be concluded that the combination of through-bolt and mechanical insert is a promising connection system for pultruded GFRP in truss application. The proposed factors and theoretical joint strength equations developed in this research can be important tools for practitioners to perform strength analysis of through-bolt with mechanical insert connection system, encouraging its acceptance and utilisation, especially in truss application.on of internal forces to other truss members. The