Abstract
Structural Health Monitoring (SHM) intends to identify damage by changes of characteristics as for instance the modal parameters. The eigenfrequencies, mode-shapes and damping-values are either directly used as damage indicators or the changes of derived parameters are analysed, such as e.g. flexibilities or updated finite element models. One common way is a continuous monitoring under environmental excitation forces, such as wind or traffic, i.e. the so-called output-only modal analysis. Alternatively, a forced measured external excitation in distinct time-intervals may be used for input-output modal analysis. Both methods are limited by the precision or the repeatability under real-life conditions at site. The paper will summarize several field tests of artificially step- by-step damaged bridges prior to their final demolishment and it will show the changes of eigenfrequencies due to induced artificial damage. Additionally, some results of a monitoring campaign of a healthy bridge in Luxembourg are presented. Reinforced concrete shows non-linear behaviour in the sense that modal parameters depend on the excitation force amplitude, i.e. higher forces lead often to lower eigenfrequencies than smaller forces. Furthermore, the temperature of real bridges is neither constant in space nor in time, while for instance the stiffness of asphalt is strongly dependant on it. Finally, ageing as such can also change a bridge's stiffness and its modal parameters, e.g. because creep and shrinkage of concrete or ageing of elastomeric bearing pads influence their modulus of elasticity. These effects cannot be considered as damage, though they influence the measurement of modal parameters and hinder damage detection.
Highlights
Damage due to corrosion and fatigue of a bridge leads to reduction of its stiffness and in a smaller extend to changes in the damping behaviour
Structural Health Monitoring (SHM) intends to detect damage by measurement and analysis of changes of characteristics leading to the changes in the stiffness matrix
With increasing excitation force we find decreasing stiffness and decreasing resonance frequencies, which is a non-linearity and which has an impact of 2% or even 3% on the measured resonance frequency
Summary
Damage due to corrosion and fatigue of a bridge leads to reduction of its stiffness and in a smaller extend to changes in the damping behaviour. Structural Health Monitoring (SHM) intends to detect damage by measurement and analysis of changes of characteristics leading to the changes in the stiffness matrix. These changes can be identified in multiple ways, but very often the modal parameters are the starting point This monitoring and surveying of a bridge is either done continuously and automatically or in discrete timeintervals. It may be input and output or output-only measurements. Most methods try to reveal changes of modal parameters or deduced characteristics to compare them to a healthy reference state in order to reveal stiffness changes, i.e. damage. We will analyse output-only monitoring results of a new small and healthy two span composite bridge in Useldange Luxembourg that we follow-up for approximately 10 years
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