During the incremental launching of steel and composite bridges high support reactions have to be introduced into the slender steel webs of the bridge girder. In the past, the specific characteristics of bridge launching such as long loading lengths and longitudinal stiffeners were not covered appropriately by existing design formulae for patch loading. In the frame of the research project “Competitive Steel and Composite Bridges by Improved Steel Plated Structures (COMBRI)” a clear improvement of the patch loading resistances for I-girders and box sections could be derived leading to more economic solutions for this type of loading. The paper introduces the basic concepts of the developed models for both unstiffened girders and girders with open-section and closed-section longitudinal stiffeners as well as a software tool for the advanced determination of elastic critical buckling loads and modes. A comparison of the main area of application and the resulting economic advantages concludes the survey.

Incremental launching of steel bridges is a demanding undertaking, on the erection site as well as on the designers desk. Not seldom, the structure itself is during the launching subjected to high concentrated forces on the lower flange when passing over the launching shoe or a intermediate support (e.g. column). These concentrated forces, commonly referred to as patch loads, may be of such magnitude that it governs the thickness of the web in the bridge girder and a small increase in web thickness leads to a substantial gain of steel weight of the bridge. Hence also a higher material cost. One solution to this problem is to reduce the buckling length of the web with the use of a longitudinal stiffener of open (a plate) or closed type (closed profile of e.g. V-shape). The improved patch load resistance is in the Eurocode nowadays determined with the help of the yield resistance for the web and contributing parts of the loaded flange reduced with a factor dependent of the slenderness of the web and the influence of the longitudinal stiffener. Parts in the expressions for the yield resistance and the reduction factor have been somewhat questioned and over the years a substantial amount of tests and FE simulations of longitudinally stiffened webs has been carried out. This research work has produced a large amount of test data which has been used herein to further improve the prediction of the patch load resistance of longitudinally stiffened steel girder webs. Based on the use of the gathered test data from the literature and previously done research, a calibrated patch load resistance function was developed for both open and closed longitudinal stiffeners. Furthermore, a partial safety factor for the proposal was determined according to the guidelines in the Eurocode. In all, the proposal was proven to have an improved level of prediction than other comparable proposals (e.g. Eurocode). Another questioned part in the commonly used design codes is the reduction function concerning local buckling under uniformly in-plane compression. The nowadays used function (the Winter function) has been developed during the 30-ies based on tests on cold formed specimens and has been criticized as being too optimistic regarding plates with large welds. At LTU a series of tests on welded specimens made of high strength steel with large welds was conducted to investigate the aforementioned concerns. Along with test data found in a literature survey, the tests showed that the Winter function was shown to be too optimistic and a new reduction function, based on the test data, was proposed also for this application.

New structural steels, such as (liquid) quenched and tempered (QT) steels, with extra high strength, good toughness and weldability have been developed in Europe in recent years and are included in Eurocode 3 Part 1-12 for strengths up to S 700. However, extra high strength steels (EHSS) with a yield strength of 1100 N/mm2 so far have not been taken into account in the present version of Eurocode 3 due to insufficient knowledge on their buckling behaviour and fatigue strength and lack of associated experimental data. In this article results of experimental and numerical investigations concerning the local buckling behaviour of cold-formed and welded hollow sections made of EHSS are summarised. The investigations have been carried out within the scope of an European research project. Results have been used to check the applicability of existing design rules for local buckling to EHSS. As a consequence the scope of Eurocode 3, Part 1.12 could be expanded to cover stability rules also for such steels.

High strength steel provide designers with the possibility of creating more slender and weight efficient structures than would be possible if using steels with lower strength. To be able to do this, a structural designer needs updated and validated codes as aid in their work. This thesis addresses the behaviour of high strength steel with respect to local buckling and residual stresses. The thesis was aiming to determine if there exists any significant differences in the resistance to local plate buckling of high strength steel (fy > 460 MPa) compared to steels with lower strength. Furthermore, longitudinal residual stresses induced by welding were also considered on a basis of material strength. Experimental work considering these two issues was conducted concerning the three steel grades Domex 420, Weldox 700 and Weldox 1100. The investigation concerning the local buckling resistance comprises experiments on 48 welded box section specimens made of the three grades. Nominal plate slenderness values were altered between 0,7 and 1,5. Moreover, the experimental work was founded on plate theory with respect to local buckling and a survey of other conducted comparable experiments. The results from the tests and the literature survey were evaluated with respect to Eurocode 3. The gathered test results from literature and experiments showed that no significant difference between the local buckling resistance of different steel strengths could be concluded if compared to the Winter function. However, the Winter function was concluded to overestimate the resistance for more slender simply supported plates (plate slenderness > 0,9) with residual stresses (in as-welded condition). The residual stress state present in three box sectioned specimens made of the three grades was measured with the blind hole technique. Evaluation of the test results was made with respect to the steel strength and complemented with test results collected from a literature survey. The study showed that the tensile residual stresses induced by welding could not be directly correlated to the material strength. Results from measurements on high strength steel specimens showed that the longitudinal residual stresses was lower if made dimensionless with respect to the strength of the steel.