Keywords: saving steel, saving fabrication expenses, thin-walled construction, effect of initial imperfections, stability, repeated loading, cumulative damage, fatigue cracks, "erosion" of post-buckled behaviour, user-friendly design.

Two ways to increasing the competitiveness of steel structures are discussed:. (i) to save steel (using thin-walled construction); and (ii) to reduce fabrication expenses (using economic fabrication structures), the optimum solution being understandably achieved by combining both of them.

With respect of point (ii), the effect of the initial imperfections of the webs of thin-walled girders are analysed by means of the elasto-plastic large-deflection theory, and it is concluded that, for the products of highly accredited steel fabricators, no straightening of webs is needed.

The design of thin-walled structures is mainly governed by stability considerations, the beneficial influence of the post-buckled behaviour of their plate elements being of course taken into account. However a great part of such structures are subjected to many times repeated loads, the impact of cumulative damage generated thereby is investigated by means of numerous experiments. The test girders used in them are fabricated by a highly experienced Prague steel fabricator, where ordinary steel bridges are produced; and, compatibly with the economic-fabrication strategy, no heat straightening is applied to the webs of the test girders. Two kinds of loading are used in the experiments: (i) combined shear and bending, with shear predominating, and (ii) partial edge loads. The phenomenon of the many times repeated buckling (so-called breathing) of the slender webs of the test girders is studied, and the process of the initiation and propagation of fatigue cracks generated in them is investigated. Special attention is paid to the impact of these fatigue cracks on the failure mechanism of the whole girders and their limit state; and in so doing the difference in the fatigue performance of webs subjected to predominantly shear on the one side and webs under the action of partial edge loading is highlighted.

Of course, the main objective of the authors' investigation is to contribute, hand in hand with other researchers, to an improvement in the economical and safe design of steel thin-walled girders. Within the framework of these activities the authors consider only the results of the experiments carried out on their large-size girders, so that the test girders used are similar, in both the fabrication process and the size of girders, to the characteristics of ordinary steel girders. Based on these results, simple and user-friendly formulae are established such as to make it possible completely to disregard the detrimental effect of web breathing for any number of loading cycles.

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