Lateral-Distortional Buckling of Steel-Concrete Composite Beams: Kinematics, GBT Constrained Modes and Analytical Formulae

Steel-concrete composite beams are extensively used in the building construction industry - their well-known structural advantages stem from the joint work of the steel profile (often an I-section) and concrete slab. While the concrete slab provides full restraint against lateral-torsional buckling to the I-section beam regions under sagging bending moments, the same is not true for the regions under hogging bending moments. In the latter, the restraint provided by the concrete slab changes the buckling mode nature from lateral-torsional to lateral-distortional, involving lateral displacements of the unrestrained compressed flange and web transverse bending (single or double curvature).

Lateral-distortional buckling (LDB) of restrained I-section beams has been studied for the past four decades and several simplified approaches were proposed to calculate LDB moments. However, consensus about the mechanics underlying this phenomenon has not yet been achieved, since discrepancies are observed in the LDB moments yielded by the various approaches, which stem from the different kinematic assumptions adopted. Therefore, it is fair to argue that the kinematics and mechanics of LDB in elastically restrained I-section beams are not yet fully understood. The theoretical solutions available concern almost exclusively web transverse bending in single curvature and fully fixed connected flanges.

This work is based on Generalized Beam Theory (GBT) and sheds fresh light on the kinematics and mechanics of LDB (single and double curvature) in I-section steel beams elastically restrained by concrete slabs, namely by providing the cross-section properties characterizing this buckling phenomenon. GBT constrained deformation modes incorporating the displacement/rotation restraints due to the slab are employed to derive novel analytical formulae to calculate LDB moments. Several illustrative examples are presented to assess the performance of these formulae and compare them with those available.

Learning Objectives:
Understand the kinematics of lateral-distortional buckling in steel-concrete composite beams.

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