Performance of Pedestrian-Load Models through Experimental Studies on Lightweight Aluminum Bridges

Aluminum structures provide a high strength-to-weight ratio, are corrosion resistant, and are esthetically pleasing. Hence, their use in bridge construction has recently begun to increase, especially for pedestrian bridges. Because of their relative light weight as compared with other structures, they often exhibit fundamental vertical frequencies outside the range of normal pedestrian walking frequencies. Despite this, they tend to be lively structures, easily excited by pedestrians, resulting in large-amplitude accelerations. An experimental program was undertaken by the authors to study the vibration characteristics and performance of three full-scale aluminum pedestrian bridges, two in the laboratory and one in the field. Although the lowest fundamental frequencies for all three bridges fall outside the normal range of pedestrian walking frequencies, there is the possibility of higher harmonics of walking loads near resonance or in resonance with the fundamental vertical mode. The aluminum bridges studied were instrumented and subjected to a range of pedestrian-load tests to understand the performance of existing periodic models in predicting the dynamic response. The performance of these models (assuming pedestrian forces to be a summation of harmonics) has thus far only been assessed for cases in which the fundamental frequency is resonant with the first harmonic of walking, often considered the most severe case for design. This is the first time, to the knowledge of the authors, that these models have been studied for the case of bridges that are nonresonant with the fundamental mode. This article notes important observations regarding the performance of these models in predicting the serviceability of bridges made out of lightweight materials, such as aluminum, for cases in which the higher harmonics of pedestrian walking are either nonresonant or near resonance with the fundamental vertical flexure mode.

Size effects in the fatigue behaviour of tubular bridge joints

Luís Costa Borges

A critical aspect in the design of tubular bridges is the fatigue performance of the structural joints. Economic viability depends on it. Lower fatigue strength for joints with thicker failing members was observed in welded joints typical to the bridge application. Different approaches to this phenomenon, called size effect, have been suggested, all based on the thickness correction for welded plate joints first proposed by Gurney. For the welded tubular joints, few studies on the size effects have been carried out; most of the existing investigations refer to geometries typical to petroleum industry offshore structures. In contrast to offshore structures, bridge structures have different absolute sizes and different member proportions (in particular lower chord radius to thickness ratios, γ). Tubular joints are far more complex than welded plate joints, multiple parameters are needed to describe the geometry (α, β, γ, τ, ζ) and there are several load scenarios. For these reasons, the fatigue behaviour analysis of such joints is a complex task. Current design recommendations combine the use of the structural (hot-spot) stress at the weld toe with a correction factor to take into account the wall thickness of the failing member. This approach oversimplifies the problem and can be very penalising, in particular for joints composed of thicker tubes, as is commonly the case for bridges. Furthermore, the truss member sizes that result from static design are likely to fall out of the validity range of current recommendations. This thesis focuses on a case of commonly used tubular joints: welded steel K-joint made out of circular hollow section (CHS). The main goals of this research are to understand the fatigue behaviour of as-welded CHS K-joints and to clarify the influences/effects of the different geometric parameters on their fatigue strength. In order to carry out a thorough study on the geometric size effects in CHS K-joints for bridges, fatigue tests were conducted for large-scale specimens with crack depth measurements and an advanced 3-D crack propagation model was developed. The first chapters of this thesis provide an introduction and a brief review of the main concepts in tubular joint fatigue and size effects on fatigue behaviour. The experimental tests of two tubular trusses under fatigue loading are then outlined. Crack growth in selected truss joints is monitored using the Alternating Current Potential Drop (ACPD) system. An advanced 3-D modelling of welded K-joint with surface crack is implemented using the boundary element method (BEM). A crack propagation model, based on Linear Elastic Fracture Mechanics (LEFM), is then developed using a step-wise incremental crack growth strategy. This model allows for fatigue strength and life estimations. Furthermore, it considers the influence of all geometric parameters that define CHS K-joints in a realistic way. The validation of the crack propagation model is made by comparisons with experimental data at different levels (i.e. member and joint strains and stresses, ACPD crack growth data). A parametric study is then carried out on joint geometries typical for a bridge application (low chord radius to thickness ratio) considering three basic load cases. Examples of results are shown and analysed on a "geometry cause"/"effect over the stress intensity factor and fatigue strength" basis. Parametric study results are then analysed, highlighting the case where the joint is proportionally scaled. The geometry correction factor, Y, is introduced as a function of the relative crack depth that is common to homothetic joints. The influence of the absolute size of the joint, also known as thickness effect, is determined for the three basic load cases. Parametric results are finally explored bringing to light the effect of non-proportional scaling. It is shown that size correction factors for fatigue strength can be expressed as a function of the non-dimensional geometrical parameters β, γ and τ, chord thickness, T, and different load cases. A new fatigue design method is proposed for welded (CHS) K-joints, based on LEFM and accounting for geometric size effects.

EPFL2008

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

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

Irwanda Laory

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.

EPFL2013