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