Civil Engineering Association

Using Non-Linear Vibration Techniques to Detect Damage in Concrete Bridges

There has been much work published in recent years on the use of vibration char-
acteristics to detect damage in bridges. Almost all of this work has been based on the
assumption that the vibration is linear, i.e. the natural frequencies are not dependent on
the amplitude of oscillation. The aim of the work presented here was to investigate the
possibility of using changes in the non-linear vibration characteristics to detect damage
in reinforced concrete bridges.
These changes in the non-linear vibration characteristics were studied by conducting
impact excitation vibration tests on reinforced concrete beams. The non-linearities
were detected by examining the changes in fundamental frequency over time (and hence
over amplitude of vibration). Several time-frequency distribution estimation tools are
discussed including the discrete Fourier transform moving window, the auto-regressive
model moving window, harmonic wavelets and examples of the Cohen class of bilinear
time-frequency distributions. A detailed investigation into these various distribution
predictors was conducted to assess which is most suitable for analysing the vibration
signals to detect changes in frequency with time.
To understand the non-linearities in the vibration characteristics, a time-stepping
model was described. The model is capable of including damage in the form of a moment-
rotation relationship over the cracked region. It was validated for linear vibrations
against theoretical values and the representation of a non-linear mechanism using the
model was compared with experimental data.
Static load tests were also conducted on the beams at various damage levels. They
involved the use of vibrating wire strain gauges to investigate the moment-rotation be-
haviour over the cracked region. Several possible non-linear crack mechanisms are dis-
cussed and two of them are assessed using the vibration and the static load tests. Future
experimental work is proposed to study the possible non-linear mechanisms further.
The beam tests demonstrated that there is a change in non-linear vibration behaviour
with damage. The change is greatest at low levels of damage and after the beam has
been loaded to 30% of the failure load in three-point loading there is a reversal in the
trend and a slight reduction in non-linearity with further damage.