Composite action of FRP bridge decks adhesively bonded to steel main girders

Up until today reinforced concrete has been used in most cases for the manufacturing of bridge decks. Depending on the quality of the work carried out, defects can already occur after only a few years. These defects mostly appear in the form of corrosion of the steel reinforcement due to concrete's sensitivity to de-icing salts and water. To reduce maintenance costs, which are mainly caused by corrosion of the steel reinforcement, attempts were made to eliminate the steel reinforcement in the bridge deck. This was achieved for example by replacing the whole concrete bridge deck with an FRP1 bridge deck. FRP bridge decks, besides the advantage of the absence of steel reinforcement, exhibit the advantage of a low dead load (approx. 20% of a comparable concrete deck) combined with high strength. These properties resulted in the fact that today more than 200 bridges with FRP decks are in service worldwide. Most of them need steel or concrete main girders to bridge the required span. Despite the many bridges already in service, assessment of their load-bearing capacity or deflections still remains difficult. Some of the reasons for this this are as follows: Geometry and material properties vary considerably between different FRP bridge-deck types. The problem of the connection between main girders and bridge decks has only been partially solved. No design method exists which allows determination of the stresses and deflections of composite girders, and takes the degree of composite action of the bridge deck into account. This thesis contributes to solve these problems. Experiments with two different bridge decks were carried out in order to determine the necessary system properties (in-plane compression and shear resistance and in-plane compression and shear stiffness) for the calculation of the load-bearing behavior of steel/FRP composite girders. The method developed to determine the system properties can also be applied to other FRP bridge decks (e.g. sandwich decks). In a second step, four composite girders (two with each bridge deck) were manufactured by bonding the bridge decks onto conventional steel girders. Local failure of the bridge deck, as occurs in girders with stud or bolt connections, is therefore prevented and a clear load transfer in the joint is assured. One of the two girders, with each bridge deck system, was tested statically and the other statically and under fatigue loads. The results of the girder experiments showed that adhesive bonding is a reliable connection technique, since failure always occurred first in the bridge deck and then in the adhesive layer. The stiffness and failure load of the composite girders could be increased considerably in comparison with the pure steel girder. The determined system properties concerning in-plane shear and compression stiffness were confirmed with the girder experiments. The results of the experiments with the composite girders were compared with results of an analytical design method for concrete/wood girders adapted for steel/FRP composite girders. It was shown that the load-bearing behavior of composite girders consisting of steel main girders and adhesively- bonded FRP bridge decks can be determined with good accuracy in the linear-elastic region. Furthermore a design method was developed which allows the loadbearing capacity of the steel/FRP composite girders investigated in this thesis to be determined with very good accuracy. Subsequently a parameter study was carried out in order to verify the assumption of full composite action in the adhesively- bonded joint. This is one of the requirements for application of the developed design methods. The study showed that the assumption is applicable for different adhesives and even for thicknesses up to 50 mm. ---------------------------------------- 1 Fiber Reinforced Polymer

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

Modèle d'évolution de l'état des ponts-routes en béton

Guido Roelfstra

The management of existing road infrastructures is a multidisciplinary activity that involves structural engineering, material science, management, economics and ecology. The objective is to achieve maximum availability of road links at minimum societal costs. Recently, tools (Bridge Management Systems, BMSs) have been developed to help decision makers to determine the optimal management strategies within available resources. The condition development model is a key element of the BMSs. Currently this model is using empirical approach making use of data collected during inspections and is formulated with Markov chains. The assessment units are structural elements and these are classified into condition classes according to their visual appearance. The condition assessment is therefore rather subjective since no measurements are involved in it. Consequently, the condition development based on visual inspections will reflect this subjectivity. In order to improve the objectivity of condition forecasts physical and chemical phenomena governing deterioration has to be incorporated in the condition development model. Observations made on a bridge sample representative of the Swiss highway bridges showed that chloride-induced corrosion is the main deterioration mechanism. For this reason this research was focused upon modeling the chloride-induced corrosion. In this research, the condition development model is established, based physical and chemical phenomena involved in chloride induced corrosion. This analytical model is able to yield quantitative results on condition development. Accuracy and availability of input data govern the level of accuracy of such an analytical model. The existing knowledge on chloride induced corrosion and inspection techniques available on site are examined in order to develop a simple and reliable condition development model. Firstly, non-destructive testing methods are used in addition to visual observations, to determine quantitative values (permeability and thickness of the concrete cover) of parameters governing the segment deterioration. The structural elements are divided into areas i.e. segments where the parameters governing chloride-induced corrosion are assumed to be constant. It is proposed to divide structural elements into segments based on three concrete cover permeability classes, concrete cover thickness and three types of surface exposures to chloride contaminated water. Secondly, a chloride transport model is developed to simulate the real exposure of reinforcement to chloride contaminated water, taking into consideration the transport mechanisms of water and chloride ions. Simulations are performed for the various segment parameters that have been identified in the segmental approach. Thirdly, a relation is established between the free chloride ion content and the corrosion initiation probability. Using the results of the chloride transport model, the development of the corrosion initiation is determined for road bridges segments. An analysis is made to predict the corrosion initiation time of existing bridges as a function of their segment parameters (exposure and concrete cover permeability and thickness), but also to establish recommendations for the construction of new bridges. Fourthly, the corrosion rate i.e. loss of steel cross-section is then determined according to the concrete cover permeability and the exposure to chloride contaminated water. Fifthly, the results of the presented mode1 are mapped to conditions States as defined in the BMSs. Three degradation matrices (Markov chains) are determined, corresponding to three condition developments: favorable, normal and unfavorable. The three condition developments can be estimated for given quantitative values of the concrete cover permeability, thickness and surface exposure to chloride-contaminated water. In this way, the segment parameters may be used in BMSs to forecasts future condition of segments, elements and the whole structure. Finally the structural behaviour of the concrete road-bridges until failure is studied. The determinant failure scenarios are identified for each bridge type and a case study is performed on the sample representative for the Swiss highway bridges. For this purpose the determinant segments for the structural safety are identified. Additionally the required concrete cover depth for a service life of 50 and 100 years is expressed as a function of the permeability and the exposure to chloride contaminated water. This research outlines the need to measure the concrete cover permeability and thickness with non-destructive test methods as well as to determine the exposure to chloride contaminated water. These values are essential to predict the condition development of concrete road bridges.

EPFL2001

Prediction of fatigue life of reinforced concrete bridges using Fracture Mechanics

Eugen Brühwiler, Marina Rocha Pinto Portela Nunes

With the occurrence of higher and more frequent axle loads, bridges are more solicited by fatigue loading. Bridge elements like deck slabs are subjected to a high number of stress cycles at relatively small stress magnitudes. The application of Fracture Mechanics as a useful tool for the analysis of fatigue crack growth in steel elements was demonstrated by Paris et al. in the early 1960s. With respect to reinforced concrete, the fatigue strength of the steel reinforcement is determinant. The fatigue behaviour of the steel reinforcement is similar to that of structural steel. The fatigue relevant parameters are the stress ranges, the number of fatigue cycles and stress concentrations. This paper presents a study to predict the fatigue life of steel reinforcement based on the Paris law. The method will be validated by a case study.