Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 202

physical properties of the works will undergo. It is thought that soon, dynamic
testing will be used to not only to detect, locate and quantify damage, but also to
assess the likely productive life of all civil engineering works. This chapter briefly
describes the instrumentation used to perform dynamic testing. Both forced
vibration and ambient vibration tests, widely used for assessing civil engineering
structures, are presented. Some examples of real-life applications on large structures
- a cable-stayed suspension bridge, a pedestrian footbridge, a large gravity dam and
a large arch dam - are used to illustrate different test techniques. Tests used to detect
damage in a civil engineering structure are also explained, and the most recent
research in this area is described.
6.1. Instrumentation
The main consideration in any experimental investigation is the accuracy and
reliability of the instrumentation being used. In a dynamic test, all the instruments
can modify the measured signals. Such modifications must be taken into account
during data processing. To do this, it is necessary to consider all the components of
the instrument, from the measuring device to the analog-to-digital converter of the
acquisition device. Appropriate corrections can then be made at the data processing
stage. A review of the different instruments used for tests on large civil engineering
structures is presented and includes accelerometers, hydrophones, strain and
displacement gauges, and connection wires and data acquisition devices.
Force - balanced or piezoelectric accelerometers are commonly used for structure
testing. At the low frequencies typically associated with civil engineering structures,
the most commonly used devices are of the former type, generally seismic-
mass accelerometers whose frequencies range from 0 to 300 Hz, the 0 to 50 Hz
range being the most common. The advantage of using such accelerometers is that
load amplification is not required.
An accelerometer behaves as a low-pass filter that reduces the amplitude and
modifies the high-frequency phase of the measured signal. Figure 6.1 shows the
frequency response curve of an accelerometer with a cut-off frequency of 50 Hz (3
dB reduction, or about 30% of the amplitude of the measured signal).