Biaxial properties of structural steel under nonproportional loading

Most research studies on inelastic material behavior of steel structures under earthquake ground motions are concentrated on general uniaxial cyclic loading conditions. Very limited attention has been given to the responses of structures subjected to multiaxial and nonproportional loading conditions. This study is concerned with the observations of experimental behaviors of structural steel subjected to biaxial nonproportional loading conditions. Six annealed A-36 steel tubular specimens were tested under constant strain rate and room temperature using an MTS axial-torsional testing system under prescribed strain paths. Results of three of the six tests are reported in this paper (the other three are primarily duplicates). Four important observations on the material characteristics are described and discussed. They are the initial plastic plateaus of structural steel under biaxial loading, the hardening behavior under multiaxial cyclic and nonproportional loading, the material memorial characteristics, and the effect of nonproportional loading.