Experimental study of residual stresses in thick steel plates

Residual stresses must be taken into account when evaluating the stability of a steel bridge girder. Residual stresses are unavoidable, as manufacturing processes for steel plates introduce thermal residual stresses in the material. For welded built-up sections, such as those commonly used in large steel plate girders, these thermal residual stresses are due to 1) cooling after rolling, 2) flame cutting of the flanges, and 3) welding between the web and the flange. Residual stresses can influence the lateral torsional buckling resistance of structural steel members and need to be considered in design. This paper experimentally investigates residual stresses due to flame cutting using the sectioning method. In this study, residual stresses are measured after flame cutting and welding 60 mm thick steel (S355N) flanges, a typical size used in the fabrication of bridge girders. Results from flame cutting confirm the findings of previous studies which reveal that residual stress distributions have a high tensile component at the flame cut edge. Procedures and testing of welding residual stresses are also presented. Residual stress measurements from this study could be used to develop a longitudinal and through-thickness residual stress model for thick flame-cut plates and welded plates used in bridge I-girder construction. Such a model could serve as an initial state in non-linear finite element analyses to predict the residual stress influence on the LTB of steel bridge girders.

Résistance au déversement des poutres métalliques de pont

Raphaël Thiébaud

Steel and steel-concrete composite bridges made of plate girders represent an economical structural form and are largely used for short and medium spans. Two parallel plate girders connected by cross-beams define a twin-girder bridge. This type of bridge transfers the loads from the deck to the supports essentially through bending and shear of I-girders. Associated with the two resistance modes are the following instability modes: lateral torsional buckling (LTB) and local buckling. Design experience of steel bridges made of plate girders shows that LTB determines the dimension of the upper flange of an I-girder at mid-span during construction phase, and the dimension of the bottom flange next to the intermediate supports during construction by launching or in service. In practice, LTB verification is carried out using reduction curves (i.e. buckling curves) which decrease the resistance of a beam with respect to the sensitivity to LTB; thus, the more sensitive the beam, more severe the reduction. A comparison of different standards shows reduction curves available for steel bridge girders have non-negligible differences which can reach 30 % of the resistance. A more refined study of the buckling curves shows that it is semi-empirical in nature, where the empirical part is mainly based on numerical and experimental research for building sections. These findings justify the goal of this thesis which is to propose a verification method for LTB that better takes into account the special features of bridge girders. To reach this goal, study of parameters that influence LTB, like re-sidual stresses, geometric imperfections, cross-beams, loading and beam geometry, is required. In this research, the experimental studies carried out on the residual stresses propose a new model which takes into account flame-cutting and welding of thick plates used in the fabrication of steel bridge girders. Experimental measurements of geometric imperfections performed on real bridge girders show the difference between fabrication tolerances and imperfections measured. These results are then used as input data for numerical simulations of LTB by two approaches: beam and bridge. The beam approach, which considers a simple span I-girder with fork support and a constant bending moment, reveals that the influence on the resistance to LTB can reach 10 % for the residual stresses and 12 % for the geometric imperfections. The bridge approach, which considers a twin-girder bridge with cross-beams and a constant load on the upper flange, shows that the influence reaches 17 % for cross beam spacing and 27 % for the type of cross-beams (frame, truss, diaphragm). Based on the experimental and numerical results, a design method for LTB is proposed. It recommends the use of reduction curve c for SIA263:2013 or reduction curve c of Eurocode EN1993-2:2006.

EPFL2014

Experimental and numerical characterisation of heat flow during flame cutting of thick steel plates

Jean-Paul Lebet, Raphaël Thiébaud

This paper concentrates on the characterization of the heat flow present during flame cutting of 60 mm thick steel (S355N) flanges, a typical size used in the fabrication of bridge girders. Experimental results are associated with a finite element analysis of the process. Temperature measurements around the flame and extensions of the fusion and heat affected zones are used to determine the input parameters of a 3-D thermal model of flame cutting. For the sake of simplicity, the volumetric heat input within the plate is modelled using only two parameters. The heat loss associated with the air gap that forms behind the flame is adjusted to reproduce the measured cooling curves. Such a thermal model is the first step in a better understanding of thermally induced stresses during flame cutting.

2012

Strain-based approach to fatigue crack initiation on high-strength-steel welded joints under multiaxial loading

Martin Antonio René Garcia, Alain Nussbaumer, Heikki Tapani Remes

A recently novel strain-based approach recently proposed by Remes (Remes 2013) for the fatigue strength modelling of welded steel joints is extended here to consider multiaxial loading conditions and three dimensions finite element models. Only the first step of the approach to initiate a crack is presented in this paper. The multiaxial fatigue number of cycles to initiate a crack of full-scale tube-to-plate specimens in S690QL steel is assessed and compared to experimental results. As fatigue cracks are initiating at the weld toe, the statistical distributions of the main geometrical parameters of the weld are defined by the mean of 3D laser scans and used to perform parametrical analyses. In addition, variations in the material characteristics due to the welding process, i.e. in the heat affected zone and the base material, are considered. Effects of the residual stresses are also considered. Results of the parametrical analyses and comparisons with the experiments are presented and discussed.

CRC Press (Taylor & Francis)2019

Résistance au déversement des poutres métalliques de pont

Raphaël Thiébaud

EPFL2014