Pultruded GFRP Spacer Tube — Material Identification & Long-Term Creep

A pultruded GFRP tube acting as a structural spacer between two concrete elements, carrying a sustained load as a short cantilever. Rather than sizing the tube on generic material datasheet values, the actual as-manufactured stiffness and failure behaviour of each candidate tube were identified directly from physical test data, then carried through a wide parametric FEM sweep to find a geometry that could meet a demanding civil-grade safety margin.

The result was a clear, evidence-based design recommendation: none of the tested tube geometries were adequate, and a specific larger-diameter direction was identified to close the gap.

Rather than trusting nominal GFRP material datasheet values, the actual stiffness of each candidate tube was identified through an inverse method: a three-point bending FEM model, built with the same section geometry, support arrangement, and boundary conditions as the physical test rig, was run and its simulated force-displacement response fitted against the recorded physical test curves. Matching the simulated slope to the experimental one back-calculated the tube's true longitudinal modulus, and the same correlation approach was used to identify each tube's failure strain from its measured failure load. This mattered because the identified stiffness values came out well below typical nominal GFRP figures — a discrepancy that would have gone unnoticed had the design simply used datasheet properties, and one consistent with variable fibre compaction quality across the pultruded sections tested.

With material behaviour identified from real test data rather than assumed, the same modelling approach was applied to the actual spacer tube geometry under its true service condition: a 400 kg sustained cantilever load over a fixed free span, encastred at the embedded end. A mesh sensitivity study first confirmed that a coarser, faster-solving mesh gave essentially the same displacement result as a much finer one, at a fraction of the run time — the practical enabler for the large-scale sweep that followed. A parametric study spanning outer diameter, wall thickness, and identified modulus was then run across more than 150 FEM configurations, tracking deflection, peak strain, and resulting safety factor for every combination, and revealing that tube outer diameter, not material stiffness, was the dominant driver of structural performance.

None of the three tube geometries physically characterised met the deflection and safety-factor targets simultaneously under short-term loading, and the best-performing candidate's margin collapsed further once long-term behaviour was considered: applying a standard design-code knock-down factor to account for decades of sustained load in a moisture-exposed insulated environment roughly doubled predicted deflection and halved the safety factor for every tube tested. That combined short-term and long-term assessment gave a clear, quantified answer to what would otherwise have been a judgement call: every tested geometry falls short of the civil-grade safety margin required, and a larger-diameter tube — increasing the section's resistance to bending rather than relying on material stiffness alone — is the necessary next step, with a specific target diameter range and minimum identified modulus defined to guide the next physical test campaign.