Load-bearing capacity of masonry arch bridges using a plastic model

Determination of the load-bearing capacity of natural-stone masonry arches is an important aspect in the field of conservation of architectural heritage. Structural verification of bridges of this type requires thorough methods in order to make efficient use of the considerable reserve commonly found in their structural capacity. The failure of these structural elements often involves successive formation of hinges until failure. The present study aims to establish a novel analytical model to evaluate the load-bearing capacity of stone masonry arch bridges. The model takes into account both the non-linear behaviour of masonry and the high ductility of such arches. Furthermore, the analysis considers the effect of typical deficiencies of masonry arches on their load-bearing capacity. The application of the developed model is illustrated in a case study.

Capacité portante de ponts en arc en maçonnerie de pierre naturelle

Alix Grandjean

Historical masonry arch bridges are an integral part of our cultural heritage and deserve the attention and the efforts required to safeguard their patrimonial value. There is currently a diverse range of methods available to engineers for evaluating the load-bearing capacity of masonry arches. The major drawbacks are that these methods are either too conservative or computationally intensive. The study presented here proposes a model based on the theory of plasticity. The result is a transparent model that is both rapid and easy to use. Moreover, its application does not involve heavy computation tools. The model is validated by comparison with available experimental results and with the results of a widely-recognised analysis software. In order to take into account the material and structural behaviour of an arch in an optimal way, the present thesis provides a new integral model divided into three evaluation levels. At the micro-level, the response of masonry under pure compression is used to characterise the material of an arch. At this level, numerical models are used to describe the behaviour of natural-stone masonry. These models distinguish between the constitutive units and mortar as well as the unit – mortar interface. The shape and arrangement of the units have the most influential role in defining the failure mode in comparison to the properties of each constitutive material. At the meso-level, the material behaviour from the preceding level is used to define the limit for the local plastification of an arch section under combined compression and flexure. This phenomenon is described as the formation of a hinge. The resistance of an arch section can be expressed using the bending moment – normal force interaction diagram based on the stress-strain curve of the masonry material. Each point along the interaction diagram refers to a state of stress causing the local plastification of an arch section due to the formation of a hinge. The macro-level focuses on the structural analysis of a masonry arch – a structure with four degrees of indeterminacy. The arch failure corresponds to the successive formation of four hinges defined by the yielding condition determined at the meso-level. The analytical model developed in this study considers an arch at the state of being a statically determinate structure – after three of the four hinges are developed. Using the principle of equilibrium, the model calculates the load-bearing capacity of the arch at the ultimate limit state. A large number of existing natural-stone masonry arches include deficiencies accumulated throughout years of service without any regular maintenance. Three common types of deficiencies on these bridges are considered. The model is extended to take into account their effects on the load-bearing capacity of arches. Finally, the model developed in this study is incorporated into a global methodology for the examination of masonry arch bridges. This methodology implies two complementary approaches, which have to be applied parallel to one another. The first uses the developed model and gives a quantitative evaluation of structural security. The second includes a qualitative appreciation of the risk, based on the condition survey of the structure. This methodology allows an objective examination of the structure and recommendations for intervention measures.

EPFL2010

A monitoring system for determination of real deck slab behaviour in prestressed box girder bridges

Eugen Brühwiler, Mark Anthony Treacy

Due to ever increasing traffic demands the fatigue safety and service life of the bridges of a railway line in Brazil need to be examined. Conventional assessment methods using load models and approaches as suggested in codes lead to conservative results resulting in significant strengthening interventions. Due to the important direct and indirect costs of the intervention, more detailed examination methods based on data as obtained from monitoring is suggested. This paper reports on an ongoing study to examine the fatigue safety of one standard bridge type, i.e. riveted steel truss structure. First resultsDeck slabs of road bridges are particularly exposed to high numbers of fatigue inducing stresses. It can be difficult to accurately model their service behaviour as secondary elements such as parapets, kerbs and surfacing layers reduce the stress levels in the steel reinforcement bars. Changes in material properties over time and thermal variations further complicate the process. For these reasons a monitoring regime is favoured for the accurate determination of real ‘action effects’ within the bridge elements. This paper presents a 1960’s prestressed concrete motorway bridge in Switzerland which is instrumented with a remotely accessible structural health monitoring (SHM) system. The key focus of the study is on the direct measurement of strains in reinforcement bars to provide a better understanding of the real structural behaviour of such bridges. The system is used to show the signature of an extreme vehicle crossing event as strain and vibrational response in the structural elements. show that all bridge members are safe under the various limit states. The level of stress ranges found in the truss members due to fatigue loading are low such that only two members experience fatigue damage. The bridge structure has thus significant reserves in capacity which makes a future increase in axle loads feasible.

CRC Press/Balkema2012

Reinforced concrete bridges under increased railway traffic loads

Andrin Herwig

The aim of this thesis is to provide rational engineering methods for the fatigue safety examination of existing railway bridges. Increasing railway traffic loading and enhanced structural analysis allow for higher exploitation of the capacity of bridge elements. This implies that increasing action effects due to higher service loads may become fatigue relevant in the future. The application of the results allows for more precise fatigue examinations. As beneficial consequence, most railway bridges may be operated safely in the future without costly strengthening measures. Calculated stress ranges in the reinforcement that governs the fatigue resistance of structural elements are too high when using conventional resistance models. The load carrying contribution of non-structural elements and a not entirely developed crack pattern lead to fatigue relevant reduction of the stress range in the rebars. A theoretical study shows that reductions of 5 – 15 % may be obtained by considering the composite effect of the continuous rail grid. Measurements of the dynamic traffic effects were conducted on a single-track bridge. The effects of passenger and freight trains at different velocities on the structural response were measured. The results are interpreted based on the observed vertical shape of the railway track. The observed dynamic behaviour could be correctly represented by simulations using simple dynamic models. The effect of other velocities and in-creased carriage loading degrees are investigated in a parametric study. The findings of previous investigations for road bridges and the investigations of this research work for railway bridges show that the Dynamic Amplification Factor (DAF) decreases for increasing loadings. Thus, smaller DAFs are applied for fatigue relevant causing heavy vehicles. Fatigue stress ranges may be reduced by supplementary 3 – 6 %. A method based on Linear Elastic Fracture Mechanics theory is presented for fatigue life determination. Fatigue life typically is very sensitive to different calculation parameters, rendering the calculation of a reliable fatigue life difficult. The refined analysis of fatigue tests reported in literature allows better understanding of the fatigue behaviour of bridge elements. Thus, a fatigue safety concept is established that compensates the inconvenience of the high sensitivity of fatigue life calculations. This fatigue safety concept is based on the observed load carrying behaviour of inspected bridge elements. The specific fatigue behaviour with visible signs allows for the examination of fatigue safety in a pragmatic way. Specific inspection of bridge elements becomes pertinent as soon as general examinations exhibit an insufficient fatigue resistance. Fatigue behaviour of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) is characterised by increasing deformations during cyclic tensile loading. Under a maximum tensile stress level of 4 MPa the deformation of the UHPFRC layer becomes stable during cyclic loading. For slightly higher stresses, the deformation increases and the UHPFRC layer fails due to steel fibre pull out. Application of the UHPFRC layer in the flexural tension zone considerably reduces the fatigue stresses in the steel reinforcement. As the UHPFRC layer deforms, the stress range in the reinforcement is effectively and permanently reduced and may remain below the nominal fatigue limit. For design of members with a layer of UHPFRC for waterproofing and fatigue strengthening, the maximum allowed strain for stable behaviour under tensile fatigue loading should be verified.

EPFL2008