Some remarks on the influence of temperature-variations, non-linearities, repeatability and ageing on modal-analysis for structural health monitoring of real bridges

Abstract

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 stepby-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.

Comparison of different excitation‐ and data sampling‐methods in structural health monitoring

“…At −5°C, f 1 is around 5 Hz while it is around 4 Hz at 25°C, that is, more than 20% decrease in this operating temperature range. This variation is more important than a change due to severe damage …”

“…FRFs of Useldange Bridge in Luxembourg excited with five different swept‐sine force amplitudes between 900 N and 5400 N leading to a variation of 2.4% for the first eigenfrequency and 1.9% for the second …”

“…But it is important to note that even severe damage on real bridges changes the eigenfrequencies typically by less than 15% . In case of high sensitive bridges, the same range of variation may be caused by temperature effects without damage. Other sources of variation were as well reported in Reference , for example, boundary conditions or types of excitation.…”

“…Modal features like eigenfrequencies are rather dependent on the temperature distribution T ( x,y,z,t ) of a bridge (also variations up to 15%), which has to be measured in the healthy reference state, thus enabling later temperature compensation . This correction of measured data is mandatory and can be done in a physical or statistical manner, for example, based on principal component analysis (PCA) . While many works removed temperature effects from eigenfrequencies, modeshapes were studied for the localization in Reference only from numerical examples.…”

Static load testing with temperature compensation for structural health monitoring of bridges

“…Unfortunately, this effect is unavoidable, but may be limited by choosing cloudy days for the measurements without high and direct solar irradiation, which additionally leads to local temperature differences [17]. A temperature compensation algorithm is proposed based on the slope of the deflection-temperature curve.…”