A large-scale analytical and experimental program was conducted on the compressive instability failure modes of multicellular thin-walled box members. The specimens were made from high strength low alloy (HSLA) steel with a minimum yield strength of 550 MPa. Altogether, nine specimens were fabricated and tested to failure under axial and lateral distributed loads. The test results showed that coupled local-flexural instability was an important determinant of strength, especially in members having slender component plates. Nonlinear finite element analyses generally predicted the ultimate strengths of the specimens to within -6percent to +16percent of the experimental values. An alternative and simpler beam-column model, based on numerical integration, was also developed, and was able to predict the strengths to about +-11percent. An analytical stub-column model was also proposed and gave good predictions of locally buckled box column capacity based on the tangent modulus method, compared to several other methods.
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