General interaction relations for hollow structural square sections subject to generalrncombinations of normal force, twisting moment, biaxial bending moments, and biaxial shearing forces arerndeveloped. The lower bound theorem of plasticity is employed to obtain the fully plastic resistance of thernsection as determined by the maximum distortional energy density criterion. Previously establishedrninteraction relations for hollow structural sections subject to bending moments, axial force and twistingrnmoments are recovered as a special case of the general solution. The developments are expressed asrnuniversal non-dimensional relations suitable for limit states design. A stress resultant transformationrnscheme is adopted in order to reduce the number of interaction relations from 20 cases to only threernfundamental cases. The applicability of the interaction relations in the design of hollow structural sectionsrnsubject to combined stress resultants is illustrated through an example.
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