Fatigue Behaviour of the Web-to-Flange Junction in a Glass Fibre-Reinforced Polymer Web-Core Sandwich Panel Bridge Deck Subjected to Bending

Abstract

Many steel bridges in the Netherlands, built in the 1950s and 1960s, are nearing the end of their service life, with steel bridge decks suffering from fatigue damage due to high traffic loads. Replacing these decks with Glass Fibre Reinforced Polymer (GFRP) Web-Core Sandwich Panel (WCSP) decks is a potential solution due to their superior strength-to-weight ratio and better in-plane fatigue performance.

A critical aspect of using these bridge decks safely is verifying the fatigue life of the Web-to-Flange Junctions (WFJs), which connect the webs to the facing. Fatigue damage is known to occur in such components with changing cross-sections, leading to stress concentrations. Current design codes lack verification equations or S-N curves for this component, necessitating further research.

This research investigates the static an fatigue performance of the WFJ by performing tests and identifying key parameters influencing this response. Through testing, the static bending moment resistance and the dominant failure mode are determined. Using the safe life approach, an S-N curve is generated by measuring the number of cycles until crack initiation occurred during cyclic loading.

Static tests revealed a constant rotational stiffness followed by a significant reduction due to delamination. Fatigue tests showed progressive stiffness degradation and crack propagation, with some crack retardation indicating a stabilisation phase before ultimate failure. Finite Element Modelling (FEM) accurately predicted initial stiffness but overestimated post-crack rotational stiffness, suggesting the need to incorporate additional parameters like material stiffness degradation or cohesive zone modelling.

This research identified important parameters such as waviness, web thickness, and radius affecting the response of the WFJ. However, testing did not confirm the predicted linear relationships between web thickness, radius and moment resistance as suggested by equations given by Lekhnitskii. Additionally, no direct correlation was found between waviness and moment resistance. It was observed that specimens with greater web thickness often also had higher waviness, and it is hypothesised that these parameters influence each other which would explain the non-linear relationship found from testing. Future research is needed to verify this hypothesis and include methods for quantifying the 'waviness' parameter.

This research enhances the understanding of the static and fatigue behaviour of WFJs in GFRP Web-Core Sandwich Panel bridge decks. The findings reveal that the dominant failure mode of the WFJ subjected to bending is delamination, due to out-of-plane stresses, as predicted by equations given by Lekhnitskii. Therefore, it is suggested that future design codes for FRPs incorporate these equations and delamination S-N curves to verify the fatigue safety of this component. Additionally, the WFJs were observed to be damage-tolerant, suggesting the potential for alternative design concepts to the fatigue life approach. Static tests revealed that although increased web thickness enhances the strength of the WFJs, it is also correlated with increased waviness, which is found from previous research to reduce strength. Future research could explore acceptable levels of crack growth and rotational stiffness degradation for safe bridge design, contributing to the development of design guidelines for GFRP WCSP, making it more viable to use these bridge decks in bridge renovations.