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