Keywords: thin-walled steel continuous beams, thin-walled steel frames, buckling, post-buckling, design, direct strength method.

This paper presents and discusses the results of an ongoing numerical investigation aimed at (i) assessing the buckling, post-buckling, strength and collapse behaviour of thin-walled steel continuous beams and frames, and (ii) developing an efficient direct strength design method to estimate the ultimate strength of such structural systems. The results currently available concern two and three-span beams and simple frames subjected to various loadings causing non-uniform bending. These results are obtained through (i) generalised beam theory (GBT) buckling analyses and (ii) elastic and elastic-plastic shell finite element post-buckling analyses. The numerical ultimate strengths obtained are compared with the estimates provided by a specific application of the current direct strength method (DSM) design curves, which were developed and validated in the context of isolated members.

The main conclusions of this study are the following:

(i)

The continuous beam and frame buckling modes often exhibit a "mixed" nature, thus precluding their direct classification as local, distortional or global. Thus, it is necessary to resort to the "dominant buckling mode nature" concept in order to classify those buckling modes - the use of GBT-based buckling analyses makes the application of this concept fairly straightforward.

(ii)

The beam and frame ultimate strength may be heavily affected by the cross-section elastic-plastic strength reserve and moment redistribution - efficient design procedures must take into account the influence of these two effects.

(iii)

On the basis of the parametric study carried out in this work, it appears that the most rational design approach for continuous beams consists of developing and calibrating design curves based on (a) the elastic-plastic collapse load, for beams with low-to-moderate slenderness, and (b) the elastic buckling load, for very slender beams.

(iv)

Since the (modified) current DSM strength curves were developed and validated in the context of isolated columns or beams, it was expected that they would only provide satisfactory (safe and reasonably accurate) ultimate strength estimates in frames that buckle and fail in modes triggered by members subjected almost exclusively to pre-buckling axial compression (columns) or bending (beams) i.e., not beam-columns (members subjected to pre-buckling axial compression and bending).

(v)

The current DSM design curves are not able to predict accurately the ultimate strength of frames (a) containing members under bending and high compression ("clear beam-columns") or (b) that buckle and fail in modes involving relevant joint deformations. In such cases, new or modified DSM design curves must be developed and calibrated. Note that the existing DSM design curves do not at present cover isolated beam-columns.

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