STATIC AND DYNAMIC TESTING OF BRIDGES: USE OF IBIS-FS FOR MEASURING DEFORMATION AND IDENTIFYING MODAL ANALYSIS PARAMETERS

Donato Fiore , Stefania Candela , Francesco Mittiga , Allison De Simone

GROUND-BASED RADAR INTERFEROMETRY (T-InSAR)

The technology of ground-based radar interferometry (T-InSAR) can be used to determination of deformations of structures with accuracy up to 0.001 mm in real time using a contactless technology. It is also possible to measure the main vibration frequency up to 200 Hz.

Deformations can be determined simultaneously in multiple point of the structure. Compared to conventional contact sensors, like accelerometers or optical target, the use of non-contact radar-based techniques overcomes some limitations and drawbacks, especially in the activities on structures in operation where time and space is very restricted. In addition, the system can be accurately reposition at different epochs, if a periodic control in a long period is need.

This instrumentation can be use both day and night. The Frequency Modulated Continuous Wave (FMCW) technique is use to resolve the scenario in the range direction, i.e. to detect the position in range of different targets placed along the radar’s line of sight. FMCW technique provide the operator with a plot called “Range Profile”. To compute the displacement of each target by comparing the phase information the back-scattered electromagnetic waves are collect at different times. Since two images acquired at different times exhibit phase differences, depending on the motion of the scatterers along the direction of wave propagation, the radial displacement dr (i.e. the displacement along the radar Line Of Sight – LOS) is simply compute from the phase shift Δρ as:

d= λ/(4π ) Δθ

where λ is the wavelength of the electromagnetic signal. The IBIS-FS sensors detect the LOS displacement of the target points illuminated by the antenna beam, which is essentially the projection of the real 3-D displacement of the measurement point along the LOS of the system; so, the measured displacement may include components of the vertical, longitudinal and/or transversal movement of the bridge element.

Full-scale dynamic testing of structures can provide valuable information on the service behaviour and performance of structures. With growing interest in the structural condition of bridges, dynamic testing can be used as a tool for assessing their integrity. From the measured dynamic response, induced by forced excitation or OMA test, modal parameters (natural frequencies, mode shapes and modal damping values) can be obtained. These identified parameters can then be use to characterize and monitor the performance of the structure.

Terrestrial Interferometric Radar IBIS-FS

The IBIS-S system is a microwave interferometer capable of remotely and simultaneously measuring the (static or dynamic) deflection of several points on a large structure with sub-millimetric accuracy. IBIS-FS radar technology is used for vibration and displacement measurement

Bridge Element Movement Along Two Or Three Directions

The IBIS-S system measures the displacement of a measuring point along the Line of Sight (LOS), so it measures a fraction of the real displacement, the extent of which depends on the angle between the real displacement direction and the LOS direction.

WHERE:

• α is the angle between the LOS and the longitudinal direction (grazing angle);
• R is the distance of the measurement point from IBIS-FS;
• H is the vertical distance of IBIS-FS from the bridge deck;

Live static load test

In static IBIS-FS is ideal for: Structure load test, displacement of the structure, risks of collapse, conservation of cultural heritage. The load static test has the objective of measuring the maximum and the residual deformation of the deck, the pillar or of any other significant element of the bridge under the load and after its removal. The instrumentation used continuously records the movements over the period considered necessary for the stabilization of the loads and unloads operation of the structure.

Dynamic Test

In dynamics IBIS-FS is the best solution for: Measurements of the resonant frequency of the structure, the relative accelerations of the structure and the analysis of the structural modal form. Dynamic configuration, allows to estimate the displacements of the span under examination in the presence of a moving load. Displacement data are processed to identify the deck resonance frequencies and modal shapes. These identified parameters can then be use to characterize and monitor the performance of the structure. From the analysis of the acquired data, the following were evaluate: Displacement and Vibration frequencies.

OMA (operational modalanalysis)

From the OMA procedure it was possible to evaluate the natural frequencies of the structure for different loading and unloading capacities. OMA with open traffic underneath the viaduct(Fig.1).OMA with unloaded bridge(Fig.2). OMA with loaded bridge(Fig.3).In the OMA configuration using environmental vibrations as input, the stress on the structure is evaluated. Compared to accelerometers, the same frequency values were detected in field tests.

GREAT WORKS APPLICATION: SAN GIORGIO BRIDGE

On the highway overpass bridge called San Giorgio Bridge in Genoa, were conducted a series of static load tests for the final test of the construction project. The check was perform with the aid of the interferometric technique for reading the intermediate deformation steps and for checking that the deck has stabilized under load in real time.

Dynamic Test

T-InSAR quickness, safety and accuracy of data

Experimental field tests by Terrestrial Radar Interferometry system (IBIS-FS by IDS, Italy) are performed for the static and dynamic characterization of bridges. In one only day of field activity, the deflection and the vibration of the structure under different loading scenarios are accurately detected allowing the determination of the actual structural capacity with significant money savings. The capability of the T-InSAR technology to remotely observe a structure and provide information on almost any point of interest, even if it is inaccessible to operators due to the presence of unsafe conditions or if it is not suitable for the installation of accelerometers, represents a key point for the exploitation of such technology in structural health monitoring. In addition, the quick setup of the instrument, the direct measurements of displacements along with the quite rapid data processing make the radar interferometer suitable for emergencies.

REFERENCES:

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