Comportement dynamique des dalles de roulement des ponts en béton sollicités par le trafic routier

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.

Berücksichtigung von dynamischen Verkehrslasten beim Tragsicherheitsnachweis von Strassenbrücken

Hannes Ludescher

It has been known for more than 150 years that action effects in bridges due to traffic action are higher than it has to be expected for purely static loads. In the design of road bridges, this difference is considered by multiplying static traffic loads with a "dynamic amplification factor". The amplification factors defined in codes are based on dynamic load tests on existing bridges. Despite of hundreds of tests in several countries, experimental investigation has not given satisfactory explanation of the observed phenomena, which has resulted in marked differences between amplification factors defined in different codes. This is due to the fact that the core of the matter – the dynamic interaction between vehicles and bridges– is a complex mechanical problem. Based on a detailed analysis it is shown in the introduction, that it can also be attributed to the fact, that the experimental investigation is more part of the problem than its solution. This thesis aims at getting a solid and systematic grounding in the problem using theoretical analysis. The centre of attention is the question, which importance dynamic phenomena have in those scenarios which are effectively relevant for the structural safety of a bridge. All scenarios are considered that justify an amplification factor, and not only dynamic vehicle – bridge interaction. The structural safety evaluation of a bridge includes the verification of the ultimate and the fatigue limit state. Accordingly, this thesis distinguishes between the interaction at ultimate limit state, for which inelastic bridge behaviour is assumed, and the interaction at service limit state with linear elastic bridge behaviour. The structural analysis of a bridge shows in addition, that the elements of the bridge deck differ considerably from the main girders: For the elements of the deck – i.e. primarily for the deck slab – dynamic interaction is of little importance, and amplification of action effects is essentially due to amplification of traffic action. In the case of the main girders, action effects are additionally amplified due to the oscillations of the structure. In order to analyse interaction at service limit state in detail, very sophisticated models are required, which do not only cover all relevant eigenmodes of the bridge but also the non-linear, dynamic behaviour of heavy vehicles and the precise road surface profile. Design and analysis of such models are mostly conferred to specialists in numeric analysis and structural dynamics. In the contrary, this thesis aims at capturing the fundamental connections by simple models, which facilitates the identification of the key parameters and the interpretation of their influence. The most important result of the analysis of vehicle – bridge interaction at service limit state is that the amplification factor is most influenced by the weight and the number of vehicles on a bridge. Whereas the amplification is negligible for high vehicle loads, tests with relatively lightweight vehicles on long bridges lead to a significant over-estimation of amplification factors. Furthermore it is shown that neither the span nor the natural frequency of a bridge is appropriate for fixing the amplification factor for a particular bridge and safety verification, respectively. It has been observed in dynamic load tests that deflection measurements consistently result in higher amplification factors than strain measurements. This phenomenon has been known for more than fifty years, but no explanation has been given so far. In this thesis an explanation is proposed and it is shown that deflection measurements result in an over-estimation of amplification factors. Similar considerations lead to a proposal for a more suitable application of amplification factors in the verification of shear force. A completely new approach is chosen for the analysis of vehicle – bridge interaction at ultimate limit state. The effective behaviour at rupture is taken into account, which necessitates first to deal with the influence of loading velocity on material strength. It is shown that only for impact loading of deck slabs due to dynamic tyre forces a minor increase in concrete strength can be expected. An important prerequisite for the understanding of dynamic behaviour at ultimate limit state is the "gravity effect", which is shown to cause massive reduction in the dissipation capacity of a structure. The determinant criterion with inelastic behaviour is deformability and not stiffness. Simple models are used to study the influence of deformability and gravity effect in the most important cases of dynamically amplified traffic action. The results show, under which conditions the dynamic amplification of action effects can be compensated by plastic deformation of the structure without causing its failure. If the steel yield stress is already attained due to the static part of traffic action, compensation of the dynamic part is only assured if the rupture behaviour is characterised by strain hardening. A simple condition of equilibrium shows that dynamic amplification due to centrifugal forces cannot be absorbed by deformations of the structure. However, rupture behaviour characterised by significant deformation causes a delay in the failure of the structure, which can be sufficient to prevent the definitive rupture anyway, depending on the scenario. In addition to these reflections, it is attempted to determine the importance of shear failures with respect to flexural failures, in order to estimate the probability of this comparatively brittle failure mechanism. In view of the application of the findings, the relevant results are synthesized and a concept for the safety verification accounting for dynamic traffic action is developed. The concept is based on the distinction between verifications at ultimate and service limit state on the one hand, and the separate treatment of elements of the deck and main girders on the other hand. This differentiation allows integrating risk based considerations using explicit hazard scenarios. An important point in the application of the findings is the recommendation to emphasize the benefit of good road surface evenness in the maintenance of structures. A necessary complement in establishing the recommended amplification factors is the detailed analysis of the reaction of vehicles to road surface irregularities. The dynamic tyre forces for different vehicle and axle types, respectively, are analysed, since the findings indicate that the amplification of tyre forces is much more important in fixing amplification factors than the dynamic behaviour of bridges. The investigations clearly show that higher axle loads imply lower amplification factors, and that the maximum amplification of axle forces in axle groups never occurs simultaneously for all axles. The thesis is finished by an annexe including introductions to the dynamic behaviour of vehicles and bridges as well as to the modelling of traffic loads and road surface irregularities. In addition to an extensive review of the state of the art, these introductions constitute an important basis of the work and facilitate understanding of the calculations in the main part.

EPFL2003

Fatigue Life Assessment of Existing Motorway Bridge

André-Gilles Dumont, Marc-Antoine Fénart, Alain Nussbaumer

The aim of this paper is to provide a fatigue-life assessment of an existing composite steel-concrete bridge within Switzerland. The Venoge bridge is a 219 meters length, four spans, four girder composite bridge, situ-ated on the Geneva-Lausanne Swiss motorway. Stress response influence lines at fatigue critical details are characterized with a refined three-dimensional (3-D) finite element (FE) model that takes into account the ef-fective transverse load repartition and the cracking of the concrete slab; the model is further validated using strain gauge measurements. Real traffic simulations are run to get the effective traffic histograms on the bridge. Weigh in Motion (WIM) data from the Denges station (1km from the bridge) are used as input for the simulations. The effective slow lane position over the bridge section is taken in account and future traffic in-crease is considered. The fatigue failure probability of the bridge at a critical location under the long-term ef-fect of highway traffic is calculated by defining a limit state function for fatigue reliability analysis. The re-sults show that fatigue is not a concern for the Venoge bridge, even though particular attention has to be paid to future change in traffic lane configurations which could require re-consideration of the fatigue critical de-tails.

CRC Press/Balkema2013

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

Herbert Gürtler

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