ABSTRACT Recent experimental programs at Monash University on CHS KT-joints and CHS YT-joints have shown incremental collapse occurring at loads up to 18% below the static strength of the joint. Incremental collapse occurs when the joints are subjected to high amplitude, low frequency repeated loads. Little theoretical analysis has been undertaken thus far and current research aims at predicting shakedown loads for these joints. The Upper Bound Theorem of shakedown is used to assess the shakedown limit, and this requires an envelope of elastic response of the structure to all load cases, and a kinematically admissible strain rate. The kinematically admissible strain rate is basically a valid collapse mechanism and can be quantified using yield line analysis. One advantage in the use of the Upper Bound Theorem of Shakedown is that a simple elastic envelope of stresses is required with the only areas of interest being the yield lines. The elastic stress envelope can be determined using finite element analysis, and is the topic of this paper.
INTRODUCTION Cyclic load tests on CHS connections at Monash University have revealed incremental collapse of the connections occurring up to 18% below the ultimate static collapse load of the joint. Goh and Grundy (1994) applied cyclic loads to the diagonal of a YT-joint, from about 18% to 87% of the static strength, observing that incremental collapse occurred. Milani and Grundy (1996, 1997) followed on, with experimental work on profile cut KT-connections, and innovative KT-connections under variable repeated loads. The innovations "consisted of flattening the ends of the brace members of the connection. The findings of this research indicate for the conventional profile cut KT-joints that the shakedown limit was less than the static collapse limit by up to 17%, with a corresponding reduction of 20% in the case of the innovative KT-joints.