The potential of glass fibre reinforced polymers (GFRP) as structural shear panels, i.e. slender elements loaded primarily in in-plane shear, is far from being exploited. Current design proposals only consider the resistance up to elastic panel buckling, leading to a relatively low economic competitiveness of such structural elements. This paper reports on a comprehensive research project on the structural behaviour of Vierendeel frames made of GFRP U-profiles, braced with riveted or bolted GFRP panels of variable thickness, particularly targeting their post-buckling (i.e. post-critical) shear resistance. The contribution discusses the results of full-scale tests and evaluates the influence of the different types of connections on ultimate loads and general bearing behaviour of the test specimens. It reports on findings from extensive non-linear FE analysis and compares them to the full-scale test results as well as to predictions based on existing proposals for the post-critical shear resistance of slender steel panels, i.e. tension field approaches. It further draws conclusions on the appropriate structural design method for determining the post-critical resistance of slender GFRP panels loaded in in-plane shear and identifies future research needs.