2

where (KL/r)0 is the slenderness ratio of the built-up member acting as a unit, (KL/r)m is the modified slenderness ratio of the built-up member, a/r; is the largest slenderness ratio of the individual components, a/rib is the slenderness ratio of the individual components relative to its centroidal axis parallel to the axis of buckling, a is the distance between connectors, r; is the minimum radius of gyration of individual components, rib is the radius of gyration of individual components relative to its centroidal axis parallel to the member axis of buckling, a is the separation ratio = h/2rib, and h is the distance between centroids of individual components perpendicular to the member axis of buckling

Equation 31.115 is the same as that used in the current Italian code as well as in other European specifications, based on test results (Zandonini 1985). In the equation, the bending effect is considered in the first term in square root, and shear force effect is taken into account in the second term. Equation 31.116 was derived from elastic stability theory and was verified by test data (Aslani and Goel 1991). In both cases the end connectors must be welded or slip-critical bolted (Aslani and Goel 1991).

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