Model-Free Methodologies for Data-Interpretation during Continuous Monitoring of Structures

Most civil engineering infrastructures, especially bridges worldwide, are approaching the end of their designed lifespan. They are continuously deteriorating due largely to material aging, excessive loads and changing environments. Therefore, it is crucial to evaluate the performance of existing structures to prevent catastrophic events. Structural Health Monitoring (SHM) has the potential to provide a proper assessment of structural performance and to further reduce cost through early damage detection and thus replacement avoidance. SHM integrates technologies to monitor structural behaviour and with current advances in sensor technology and measurement systems, the number of bridges that are continuously monitored is increasing. The bottleneck in SHM is data interpretation and this task is even more challenging in the presence of environmental variations. The main stream of data interpretation in SHM involves physics-based modelling and validation. However, building a model can be expensive, time consuming and difficult due to the structural complexity and uncertain environments. This research focuses on model-free methodologies for continuous monitoring of civil structures under environmental variations. The work involves important aspects of SHM such as damage detection, measurement system configuration and environmental variations. For damage detection, a novel model-free methodology that combines Moving Principal Component Analysis (MPCA) and regression analyses is proposed. Such approach aims to exploit the advantages of both MPCA and regression-analysis methods. The methodology has been tested on numerical and experimental studies including a full-scale bridge application. Results of a comparative study with other model-free methodologies demonstrate the superior performance of the proposed methodology in terms of damage detectability and time to detection. This research work also compares the performance of model-free methodologies for predicting natural frequencies of a continuously monitored bridge under environmental variations. Relative importance of environmental factors and traffic loading is also evaluated. Results of the case study reveal that traffic loading and temperature variations are the most influential parameters. A structural identification strategy that takes advantage of thermal variations is developed. The strategy utilizes thermal variations as load cases to evaluate structural performance. Results exhibit the promising potential of the strategy for enhancing structural identification tasks. As for measurement system configuration, a systematic approach that involves multi-objective optimization and Multi Criteria Decision Making (MCDM) methodologies is proposed. The approach is able to accommodate model-free methodologies for damage detection and provide support for selecting the best compromise configuration. It is also applicable for situations where several model-free methods are used for data interpretation.

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

Designing measurement systems for continuous monitoring of structures

Nizar Bel Hadj Ali, Irwanda Laory, Ian Smith, Ngoc Thanh Trinh

A good measurement system improves capability to identify the behavior of civil engineering structures. This paper presents a measurement-system design approach for continuous monitoring of structures. This approach performs two tasks: (1) determining the appropriate number of sensors to be employed and (2) placing the sensors in the most informative locations. Measurement systems are designed using three criteria: minimize the number of non-detectable scenarios, minimize the average time to detection and maximize damage detectability. To select the best compromise solution in the solution set obtained from multi-objective optimization, two multi-criteria decision-making methods, Pareto-Edgeworth-Grierson multi-criteria decision-making (PEG-MCDM) and Preference Ranking Organization METhod for Enrichment Evaluation (PROMETHEE), are employed. A railway truss bridge in Zangenberg (Germany) is used as a case study where measurement data are interpreted using a model-free (non-physics based) method - Moving Principal Component Analysis (MPCA). Results demonstrate that the proposed approach is able to provide engineers with measurement-system-design support for continuous monitoring of structures.

2011

Thermophysical and Thermomechanical Behavior of Cold-Curing Structural Adhesives in Bridge Construction

Omar Moussa

The use of the structural adhesive bonding technique is well established in the aircraft and automotive industries where, in most cases, joints can be fabricated indoors under controlled conditions. In the civil engineering domain, however, load-bearing structures are normally erected on-site in outdoor conditions, i.e. joint fabrication is exposed to varying outdoor temperature and humidity profiles and particularly in winter, when long periods of very low temperatures are frequent, the on-site joining of structural components must remain possible. Consequently, and due to the generally large scale of the structural components, cold-curing adhesives are used, unlike in other fields where hot-curing adhesives are used. Although it offers many potential advantages in bridge construction, structural adhesive bonding is not yet widely used in civil engineering structures. This can be attributed, amongst other things, to the lack of knowledge regarding adhesive behavior during exposure to varying environmental conditions. Despite significant research efforts concerning the characterization of structural adhesives, the question of the changes that occur in the thermophysical and thermomechanical properties of cold-curing structural adhesives during their service life, and particularly at early age and low temperatures, is yet to be addressed. The objective of this work was therefore, based on experimental and analytical investigations, to understand the thermophysical and thermomechanical behavior of cold-curing structural adhesives from the time of the mixing of the different components and throughout the long-term service life. Investigations of curing at early age and low temperatures (during winter) showed that curing already takes place at temperatures slightly above 0 °C. The process is very slow however and several days of curing are required to attain significant curing degrees. The glass transition temperature, Tg, develops even more slowly due to early initiation of vitrification. A new and practical experimental method was developed to establish the relationship between glass transition temperature and curing degree for different types of cold-curing adhesives. Furthermore, cure kinetics (developed for hot-curing adhesives) proved its applicability for cold-curing adhesives at low temperatures. In contrast to thermophysical properties, mechanical properties start developing significantly only after the onset of material vitrification. Lower curing temperatures also significantly decelerate the development process. An empirical model to predict strength and stiffness as a function of simple or complex curing temperature profiles particularly for curing at low temperatures was developed, taking the temperature-dependent vitrification into account. During summer, the temperature at specific locations of certain joints (e.g. joints below the asphalt) may exceed Tg, which may lead to a significant drop in mechanical properties. The investigations showed that when cooled to temperatures below Tg, the mechanical properties fully recovered and even a significant increase in properties is achieved due to post-curing. An existing model to predict temperature-dependent mechanical properties was extended to predict the change in stiffness and strength resulting from the exceeding of Tg and subsequently after recovery. Over the long term and during ambient curing, a significant increase in mechanical and thermophysical properties occurs over the years and decades due to the completion of curing. A model based on the similar long-term increase of concrete properties was revised to also predict the long-term strength and stiffness of cold-curing structural adhesives. Finally several case studies demonstrate how the outcome of this research can be applied, particularly to predict mechanical properties at early age and low temperatures. Such results can be used to plan construction stages or estimate the required waiting periods prior to a bridge being brought into service.