Estimation of the Elastic Critical Resistance of the Braced Portal Frame from the Lateral Torsional Buckling Condition of the Beam

In this paper, a stability analysis was conducted of the lateral torsional buckling of the beam of a braced portal frame. The elastic critical resistance (ECR) of the frame was estimated for: (a) a volumetric 3D model (Abaqus/CAE 2017) including the interaction of the component members (beam, columns) in welded corner joints and (b) a simplified 1D bar model (LTBeamN, v 1.0.3) including the parameters of elastic restraint (at support nodes) for the so-called critical member extracted from the structure. In the case of the analysed portal frame, the critical member that determines the lowest elastic critical load of the frame was the transversely bent beam. The following parameters of the elastic restraint of the beam in the support members were taken into account: (1) restraint against warping, (2) restraint against lateral rotation, and (3) restraint against rotation in the bending plane My. The paper demonstrates that the use of a simplified model can enable an efficient and, from an engineering point of view, sufficiently accurate estimation of the ECR of the analysed braced portal frame. The underestimation of the calculated elastic critical resistance of the structure in the simplified 1D model (discrepancies of −10.3% to −18.8%) is compensated for by the lack of the need to prepare a relatively complex volumetric 3D model of the frame (Abaqus). Warping restraint, when neglected, reduces ECR by up to 25% for HEA300 columns.