Warping influence on the resistance of uprights in steel storage pallet racks

Cold-formed thin-walled members are used worldwide to realize semi-continuous steel frames to store goods and products (rack systems). Usually, monosymmetric cross-section members are employed for rack columns but their design is carried out by assuming very simplified approaches, which consider the centroid coincident with the shear center of the cross-section. In routine design, non-uniform (warping) torsion influence is sometimes neglected in structural analysis: internal forces and bending moments are hence incorrect and the coupling between flexure and torsion is never considered also in buckling analysis. As a consequence, an unsafe design could be developed, owing to the use of non-appropriate tools for analysis and verification checks. A numerical study is currently in progress in Italy to define suitable design rules accounting for the monosymmetry of the cross-section members forming rack systems. This paper summarizes the main results related to the warping effects on the resistance checks. A numerical study on typical medium-rise rack frames is presented. In particular, two racks have been selected, differing for interstorey height and total height. Both configurations of unbraced and braced frames have been considered. Four different load conditions have been identified as relevant for rack design and, for each of them, a parametric analysis has been carried out by varying the degree of beam-to-column joint stiffness. Furthermore, warping influence on the distribution on the longitudinal stresses on the cross-section has been investigated, with the scope to quantify the errors when the sole uniform torsion is taken into account in structural analysis, i.e. when the bi-moment is neglected. Design results are then critically analyzed, singling out the importance of the warping on the cross-section resistance checks. Finally, suitable safety factors are proposed to be used when design is developed neglecting warping but an adequate safety level has therefore to be guaranteed.