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Concept# Pont à poutres

Résumé

Un pont à poutres est un pont dont le tablier est porté par une ou plusieurs poutres en bois, en acier, en béton armé ou précontraint. Les ponts à poutres n’exercent qu’une réaction verticale sur leurs appuis intermédiaires ou d’extrémités et les efforts engendrés dans la structure sont principalement des efforts de flexion.
Deux critères permettent de différencier les poutres : la forme ou le matériau, le croisement des deux permettant de déterminer un grand nombre de poutres. Il existe quatre formes de poutres : les poutres à âmes pleines, les poutres caissons, les poutres treillis et les poutres bow-strings. Le matériau de constitution de la ou des poutres peut être le métal, le béton armé, le béton précontraint, le bois ou, plus récemment, des matériaux composites.
Histoire des ponts à poutres métalliques
thumb|right|Pont Rio-Niterói - Rio de Janeiro - Pont à poutres métalliques
Les poutres métalliques peuvent être positionnées sous la chaussée ou de part et d'autre de

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Pont en treillis

vignette|Pont Bailey sur la Meurthe, France. Pont provisoire en treillis, à mise en place très rapide.
Un pont en treillis, pont en poutre en treillis ou pont-treillis est un pont dont les poutres la

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Introduction à la conception et au dimensionnement des ponts routiers en construction mixte, en béton armé et précontraint. Ce cours porte sur le choix du type de pont, des principales dimensions des éléments structuraux, du matériau, du mode de construction et sur les exigences à satisfaire.

Ce cours traite les principaux aspects de la conception et du dimensionnement des ponts en béton armé et précontraint. L'accent est mis sur les ponts poutres. Etude des aspects suivants : optimisation du comportement structural, dimensionnement, dynamique des structures et technologie des matériaux.

Advanced topics in structural stability; elastic & inelastic column buckling; lateral-torsional buckling of bridge/plate girders; nonlinear geometric effects; frame stability; computational formulation of stability theory; Geometric stiffness
method; Plate buckling; Plastic collapse analysis

The management of a network of existing road bridges involves interventions in order to maintain safety and the priority of these interventions is often determined by safety criteria. During the evaluation of safety, the dynamic effect of traffic actions is considered using equivalent static loads determined by multiplying the effect calculated using traffic load models by a dynamic amplification factor. During the evaluation of a deck slab, the application of inappropriate dynamic amplification factors could have significant financial implications, all the more since the local dynamic effects of overloaded trucks are determinant for this type of structural element. Dynamic amplification factors defined in codes have usually been derived from the measurement of global traffic action effects in the main structural elements of bridges. Unfortunately, local dynamic effects in deck slabs have not been studied in detail until now. A better understanding of the dynamic behaviour of deck slabs will lead to the definition of more accurate dynamic amplification factors and avoid the use of values that are too conservative. The behaviour of deck slabs of six typical Swiss highway bridges has been simulated in order to study their dynamic response during the passage of trucks. The structural arrangement of the deck slab was different for each of the six bridges. A parametric study was based on the simulation of various scenarios involving the passage of trucks for various combinations of speed, path and road surface roughness. Deck slab response was obtained by numerical simulation based on models of the bridge, truck and road surface. This system was resolved using a prediction - correction algorithm that considers the dynamic interaction between a bridge and trucks. Dynamic amplification factors were subsequently calculated from strains and deflections obtained from independent static and dynamic simulations. The simulation of numerous scenarios enabled the evaluation of the influence of different parameters on the dynamic response of a deck slab: The road surface roughness was found to be one of the most important parameters, with an increase in roughness leading to an increase in dynamic amplification factors. An overloaded truck produces a lower dynamic amplification factor in a deck slab than an empty truck. The truck speed influences the dynamic interaction, but a clear relationship between speed and dynamic amplification factor could not be identified. The maximum dynamic amplification factor does not vary significantly from one point to another over a deck slab in a girder bridge. The structural arrangement of a deck slab in a girder bridge has little influence on dynamic amplification factors. Overall, the different deck slabs on the girder bridges studied were all equally sensitive to the dynamic effects of road traffic. In many cases considered for the framed slab bridge, the maximum dynamic amplification factor was found to occur over the supports. For such bridges, the sensitivity of the deck slab to the dynamic effects of road traffic is not uniform. Finally, two approaches to evaluating deck slabs of existing road bridges are proposed. The first approach is only applicable in certain situations and involves a simplified evaluation using an updated traffic load model and a dynamic amplification factor. In situations where a simplified evaluation is not applicable, the second approach is to evaluate a deck slab using numerical or experimental analyses.

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

The construction principles of "Timber-Glass-Composite-Girders" demand practical and theoretical research to ensure reliable designs. Toward this objective the mathematical description of load-bearing and the non-rigid bond is the subject of this work, i.e. to define the contributing parameters for the design of these girders. The non-rigid-bond of the adhesive joint is of great importance because of its influence on the load-bearing of the system as a whole. The loading of girders is dominated by permanent forces thus leading to the obligation of knowing the long-term behaviour of the system and its components. The definition of the characteristic values, taking into account the long-term load-bearing, is an intrinsic part of this investigation. It can be shown that the load-bearing of the girders depends on the fracture of the glass panes. The properties of glass-fractures are a function of the residual internal prestresses due to the heat-strengthening of the panes. The load-bearing, particularly the post-cracked behaviour, changes with respect to the intensity of these internal prestresses. The timber is able to reinforce the cracked glass, leading to a ductile load-bearing behaviour as in the girders, with a dependency upon the size of the remaining fractures. A model based on the differential equations of the non-rigid bond is defined in order to calculate the loading-peaks of the adhesive joint and the material loading itself. The characteristic values of the non-rigid bond, such as the number and distances between cracks and the load introducing length, were evaluated in tests on composite slabs and small scale girders. The existing calculation models which take into account non-rigid bonding were modified to adapt the influence of the width and thickness of the adhesive joint. In order to calculate the stressing of the material the existing calculation models were adapted to calculate the post-cracked situation. The load-bearing behaviour of glass-panes bent with respect to its strong neutral axis needs other safety-considerations than panes, bent off their plane. It is shown that the existing safety considerations subject to the use of glass can not easily be adapted onto the composite girders. The bending with respect to the strong neutral axis and the reinforcement of the glass of the timber demands a different hypothesis to adapt the fractural mechanical analysis and to establish a safe limit conception. This is part of the performed research of this document. The following descriptions present briefly the obtained results: Depending on the quality of the glass (residual stresses due to heat-strengthening, e.g. annealed glass, heat-strengthened glass, fully tempered glass), the ductility and the mode of failure of the girders does change. As a result of its failure mode, annealed glass without internal prestresses offers the highest remaining load-carrying potential after the first crack has appeared. This ductility and thus the structural safety, diminishes with an increasing degree of internal prestressing due to thermal treatment. Heat-strengthened glass (with various degrees of prestressing, various residual stresses) shows a decrease in remaining load-carrying capacity with an increasing degree of prestressing until it fails in a brittle mode as fully toughened glass does. This has to be well considered in respect to safety considerations. Girders with panes made of glass having internal residual stresses due to heat strengthening below 50N/mm2 are considered to collapse ductily, residual stresses bigger than 50N/mm2 cause brittle failure. The effective stiffness of the girders decreases under permanent loads in function of the bondage. Both, timber and adhesive take part in this decrease; the influence of both of the involved materials has been defined. Since the design of conventional glass constructions which uses the concept of principle stresses cannot be adapted, unidirectional shear stresses had to be determined for the shear strength of glass, which is defined as 25N/mm2 for annealed glass. The characteristic values of the adhesive to define the non-rigid bonds have been determined, likewise the influence of the width and thickness of the adhesive joint. The load introducing length, the distances between cracks and the number of cracks are defined with tests on composite slabs. A model to describe the theoretical bond with the defined parameters based on the differential equations was developed. This model allows the calculation of the material stressing at the load introduction and on both sides of a crack. To calculate the stressing of glass and timber existing calculation models which take into account non-rigid bonding were modified to adapt the influence of the width and thickness of the adhesive joint. In order to calculate the stressing of the material the existing calculation models were adapted to calculate the post-cracked situation. The safety factors for the influences of the glass surface, environmental conditions, load duration and the parameters of fracture mechanical analysis as developed as they are known for semi-probabilistic safety concepts for glass constructions cannot easily be adopted. The bending with respect to the strong neutral axis demands a different hypothesis in order to adapt the fractural mechanical analysis and to develop the safety parameters. For the reliable design of timber-glass-composite-girders a contribution to the appropriate use of probabilistic, semi-probabilistic and deterministic safety concepts are given. A realised construction (Hotel "Palafitte" in Monruz (NE)) shows an example and the experience made in designing girders.