In this paper, the lateral–distortional buckling (LDB) of hollow tubular flange plate girders (HTFPGs) with slender unstiffened webs is investigated. Firstly, shell finite element analyses are performed in order (i) to study the elastic buckling behaviour of simply supported HTFPGs under uniform bending and (ii) to compare it with the behaviour of I-section plate girders (IPGs) with unstiffened and stiffened webs. The results show that HTFPGs (without stiffeners) have much higher critical moments than IPGs (with stiffeners). Secondly, the LDB behaviour of HTFPGs is investigated using finite strip analyses and the results show the girder length range in which LDB is relevant. After that, an analytical model to estimate the critical LDB moment is proposed and validated by comparison between analytical and numerical values. Thirdly, the investigation is extended to the inelastic domain and non linear shell finite element analyses are performed to evaluate the ultimate strengths of the HTFPGs. Several remarks regarding the flexural strength of HTFPGs are presented. Finally, the predictions given by Eurocode 3 are compared with the numerical results and it is found that the predictions given by the design equations are unconservative. Therefore, it is recommended to use the European specifications by adopting the proposed expression to estimate the critical LDB moment.
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