Investigation of orthotropic steel deck design using fem

There are many orthotropic steel highway bridges in the world including damages, which are induced by either qualitative or quantitative reasons. Qualitative reasons of damages can be inadequate design, workmanship or material, whereas quantitative reasons are in general increased traffic loads and dimensions of load bearing structures. Most of these bridges especially built in Germany in 1960s show damages as a result of constantly increasing traffic density in heavy traffic lanes. In this thesis the quantitative reasons of damages in steel highway bridges will be assessed under conservative static wheel loads by changing one chosen unique dimension, while keeping other costants. So as to make this investigation possible, all dimensions and spans of orthotropic steel bridge are defined as variables in well known commercial FE- method programme ANSYS by means of APDL (Ansys Parametric Design Language). In the first introductory chapter the history of orthotropic steel deck is summarized and famous bridges possessing orthotropic steel deck are given. Afterwards, the general design of orthotropic steel deck together with its load carrying components are introduced. Finally the methods used to analyse deck structure to obtain the internal forces and stresses are briefly explained. In the second chapter, the ductility of steel ever known as non- linear material property, which causes stress relief with plastic hinge formation and increases the load carrying capacity of structures made of steel, and its contribution to stress calculations are investigated by means of Finite Element Method (FE- method). The ductility of steel is considered in stress calculations either as a simplification in calculations or directly incorporated into calculations as non- linear material property. For instance the stress calculation of truss structures is reduced in calculation of only normal forces aligned with members' direction, due to plastic hinge formations, which indeed is consequence of steel ductility. In case the form of structure composed of steel load- bearing components is complex because of geometry, not like truss members, the ductility of steel shall be taken into account by incorporating material non- linearity into stress calculations. The phenomenon, incorporating the complex geometry of steel components and material non- linearity in stress calculations can be achieved by means of FE- method. Steel orthotropic highway bridges, which are composed of deck- plate, longitudinal stiffeners going through cross – beams, cross- beams with cut- outs, main girders, bridge extensions for pedestrians, are extremely complex in geometry and are made of steel, which is ductile in nature. As a result, stress calculation of orthotropic steel bridge using FE- method is a perfect example for the investigation of ductility induced stress relief. The traditional constructional details and their dimensions of steel orthotropic highway bridges are given in Eurocode 3 Part 2. In the scope of this thesis, based on dimensions recommended in Eurocode 3 Part 2 and kinematic hardening material property of steel material, a FE- model of steel highway bridge is prepared. Results of calculations with and without material non- linearity are presented and compared with each other in order to demonstrate the decrease in stress because of steel ductility. In the scope of the third chapter wearing coarse on deck plate is considered as uniformly distributed dead load. The dimensions of steel orthotropic bridge are chosen depending on Eurocode 3 Part 2 and a FE- model of orthotropic steel highway bridge will be prepared to analyze the effect of deck plate thicknesses of 12, 14 and 16 mm. Results of calculations will be presented and compared with each other in order to demonstrate the effect of deck plate on strain amplitudes, which are indicators of damages developed in structural parts of orthotropic steel deck. In the fourth chapter, effects of cross beam web thickness and cross beam spacing on orthotropic steel bridge are assessed by means of a parameter study. Consequently, dimensional and constructional recommendations in association with cross beam thickness and spacing are presented. In the fifth chapter, fatigue lives of four fatigue sensitive structural parts of orthotropic steel bridge are calculated. These are critical section in web of cross girder due to cut- outs, weld connecting deck plate to trapezoidal rib, continuous longitudinal stringer and deck plate. Finally, required thicknesses and spacings of these structural parts depending on their fatigue lives and design categories are given. In the sixth chapter, moment of inertia of longitudinal stiffeners depending on cross- beam spacing is chosen as recommended in Eurocode 3 Part 2. The results of FE- analyses performed in this thesis state a numerical proof and validity for recommendations of Eurocode 3 Part 2 regarding longitudinal stiffener and cross beam spacing. In the seventh chapter, it is focused on trapezoidal ribs, since they are dominantly used in industry. Three different slopes of trapezoidal rib web are assessed using FE- method, while rib width, height, spacing, span and thickness are kept constant. Results show that stresses especially in cross- beam and deflections of deck plate change depending on slope of trapezoidal stiffener webs. In the eight chapter, effect of Elastic moduli, Poisson ratio and thickness of wearing surface on the stresses emerged in steel deck and wearing surface itself is investigated,while wearing surface is modeled by 3D finite elements, instead of considering as dead load. In the ninth chapter, rib width to height and rib spacing to deck plate thickness ratios are assessed by means of the stresses developed under different ratios of these parameters. Afterwards necessary FE-analyses are performed to reveal the stresses developed under different rib width to height and rib spacing to deck plate thickness ratios. Based on the results obtained in this thesis, recommendations regarding these ratios are provided for orthotropic steel deck occupying trapezoidal ribs. In the last section results and discussions obtained so far are restated and a summary of all chapters is given. Key words: FEM, orthotropic steel deck, highway bridges, kinematic hardening, ductility, cross- beam, deck plate, fatigue, logitudinal stiffener,stress analysis, wearing surface, bonding layer, Eurocode 3.