Experimental determination of critical loads in thin-walled bars with Z-section subjected to warping torsion

•A thin-walled bar with Z-section was examined experimentally.•Local buckling of the section was generated by warping torsion of the thin-walled bar.•The method of the experimental estimation of local critical bimoment was developed.•“Local ordered deflection interval” was determined for the sake of this method.•Two local critical bimoments occur in the thin-walled bar with Z-section.

Critical loads were determined experimentally from the condition of the local buckling of thin-walled bars with Z-section subjected to warping torsion. The experimental investigations were carried out using simply supported models, loaded with a concentrated torsional moment at the mid-span. A method of determining the so-called “local ordered deflection interval” was developed. In this interval, the modeled deflection of the component plates (walls) of a thin-walled bar with random wall geometrical imperfections is compliant with the local buckling mode. The “local ordered deflection interval” makes it possible to adjust the known experimental methods so that they could be used to determine critical torsional loads and local critical bimoments. Experimental investigations showed the occurrence of two local critical bimoments in bars with Z-section. The bimoments are different in their absolute values, depending on the sense (sign) of the torsional load. Experimentally determined critical loads were compared with theoretical results.