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GPS-only solution by 31%. It has been demonstrated that when GPS is augmented with
PLs similar positioning precision (sub cm) can be obtained for both the horizontal and
vertical components. The low-cost single-frequency CMC Allstar receiver provided good
quality GPS measurement data and tracked the pseudolite signals for the entire trial
without difficulty. These GPS receivers therefore have the potential to be used in a low-
cost system for deformation monitoring. Also, it was necessary to resolve carrier phase
ambiguities by appropriate software, which was developed.
2.3. Modal analysis of bridge vibrations
The structural modal parameters: resonant frequencies, mode shapes and modal dam-
ping are determined from dynamic measurements of structures like buildings, towers and
bridges. The modal parameters can be used for a variety of purposes: active control, if the
design requirements have been respected, and to update numerical models for better
understanding the dynamic behaviour of the structure under the dynamic loads - operative
or exceptional loads, produced by earthquakes or winds. Moreover, periodic monitoring
of the modal properties can be used to determine the structural integrity of investigated
construre. Structural vibrations have to be measure at different ambient excitations, such
as traffic, wind, earthquake and their combination. In this case, only response data of
ambient vibrations are measurable and the detailed loading conditions are unknown. For
most of the structures under natural operating conditions the loads are nonstationary,
cannot be easily measured, and the ambient excitation is taken as unobserved white noise.
Structure dynamic testing is considered as a procedure for determining the resonance
(natural) frequencies of a structure. The identified vibration mode shape for each natural
frequency corresponds to the deflected shape when the structure is vibrating at that
frequency. Each vibration mode is associated with a damping value which is a measure of
energy dissipation. The natural frequency, vibration shape and damping value are referred
to as the modal parameters of the particular mode. Dynamic tests involving identification
of modal parameters are also known as modal tests or modal surveys.
Full-scale dynamic testing of structures can provide valuable information on the ser-
vice behaviour and performance of structures. With the growing interest in the structural
state of highway bridges, dynamic testing can be used as a tool to assess the integrity of
bridges. From the measured dynamic response, induced by ambient or forced excitation,
modal parameters (natural frequency, mode shape and modal damping value) and system
parameters (stiffness, mass and damping matrices) can be obtained. These identified
parameters can then be used to characterize and monitor the performance of the structure.
Analytical models of the structure can also be validated using these parameters.
Two different techniques can be used to extract the modal properties of a structure
such a bridge under ambient vibration: eigensystem realization algorithm (ERA), from
early 1980s, and a new method based on wavelet transform (WME). Both techniques
need free-decay signals to extract the modal parameters. At this aim a random decrement
technique (RDT) has been used to process the environment-excited signal. These
techniques were applied to determine modal properties of the Nottingham Wilford sus-
pension bridge, (Fig. 7), under ambient vibration /9/. This footbridge exhibits large def-
lections (decimeter range) under normal environmental loading. Its span is held up by two
sets of suspension cables restrained by two massive masonry anchorages. The span
sidewalk is 3.65 m wide and 68.58 m long is composed of a steel deck and wooden floor.