Experimental investigation of lightweight aggregate concrete-filled cold-formed built-up box section (CFBBS) stub columns under axial compression

This paper presents an experimental investigation of cold-formed built-up box section (CFBBS) stub columns infilled with lightweight aggregate concrete under concentric loading. The experimental programme included a series of twenty-four lightweight aggregate concrete-filled built-up box sections comprising by two nested cold-formed thin-walled C-sections using breakstem rivets at flange and eight reference hollow sections for comparison purposes. These built-up sections of four different cross-section sizes were infilled with three different lightweight concrete compression strengths, i.e., 20, 30 and 40 MPa. The failure modes, ultimate compressive load and load-end shortening relationship of the test specimens were compared and discussed. A set of performance indices was calculated from the test results to evaluate the structural performance from several aspects. In addition, the confinement effect of the specimens was investigated extensively based on the strain development. Finally, the experimental results are used to evaluate the applicability of the design equations specified in the existing codified provisions, including Eurocode (EC4), Australia/New Zealand Standard (AS/NZS 2327) and America Specification (AISC 360-16). It was found that EC4 with consideration of effective area of the outer CFBBS in the design provided the most accurate and less scattered predictions. Predictions from AS/NZS 2327 were generally found to be slightly unsafe while those from AISC 360-16 were conservative.