Samuel Vurpillot

Structural monitoring by curvature analysis using interferometric fiber optic sensors

All structures undergo deformations under the effects of loads or degradation of the constituent materials. The deformations of any structure (bridges, dams, frames, shells, tunnels, towers, wings, trusses,) contain a lot of information about its health state. By measuring these deformations it is possible to analyse the loading and aging behavior of the structure. The presented method analyses a structure by subdividing it into sections and cells. The deformation of each of these macro-elements is first analysed separately to obtain local information about the materials, and then combined to provide insight on the global behavior. Examples of these techniques applied to civil engineering structures fitted with long-gage-length fiber optic sensors show the variety of information that can be obtained using this powerful analysis technique.

Mathematical model for the determination of the vertical displacement from internal horizontal measurements of a bridge

The evolution of the vertical displacements is considered to be a good criterion to assess the condition of a bridge. Most embedded deformation sensors (e.g. inductive sensors, optical fiber sensor...) give the relative displacement between two points in the same structure. Therefore, the vertical displacements of a bridge in an earth-bound coordinate system are not directly supplied by the deformation sensors. This paper presents an algorithm to determine the vertical displacement from internal deformation measurements in the bridge. It is shown that 6n deformation sensors are necessary to draw the exact vertical displacement of a bridge uniformly loaded on n plus 1 supports. A load test has been carried out on a 6 m long timber beam on 3 supports, instrumented with 12 fiber optic deformation sensors. The vertical displacements calculated by the mathematical model on the base of the horizontal measurements are found to be in good agrement with the ones obtained by external dial gages.

Vertical Deflection of a Pre-Stressed Concrete Bridge Obtained Using Deformation Sensors and Inclinometer Measurements

The serviceability of a bridge is generally analyzed by a comparison between the vertical deflections expected by the engineer and those measured during a load test or in the long term. The existing methods do not allow the determination of the vertical displacements from the measurements carried out by a network of deformation sensors placed inside the bridge. The mathematical model presented allows the determination of the displacement field from internal horizontal deformation measurements and helps in the design of the required sensor network. This model was tested on an experimental model and on the Lutrive Highway Bridge in Switzerland by comparing the changes in vertical displacements under daily temperature variations obtained with the proposed method, with those measured directly using an absolute hydrostatic leveling system. Fiber optic deformation sensors and electrical inclinometers were used to carry out the measurements. With this deformation monitoring system, featuring a precision of 10 micrometers on 1 m long deformation sensors, it is possible to retrieve the vertical displacement field of a beam with a global error less than 8%.

Bridge monitoring by fiber optic deformation sensors: design, emplacement, and results

In 1995, our laboratory fitted a highway bridge near Lausanne (Switzerland) with low- coherence fiber optic deformation sensors. The engineers, who had designed the steel-concrete composite bridge, were interested in the strain distribution inside the concrete slab and in the effects induced by the concrete shrinkage. More than 30 fiber optic deformation sensors, a few vibrating string deformation sensors, thermoelectric couples as well as resistive strain gages were installed in the concrete deck and on the steel girders of this bridge. Three phases of the bridge life were monitored: concreting and thermal shrinkage, load test with heavy trucks and long term deformations. This contribution presents the fiber optic sensor design, the installation technique and the preliminary results obtained on this bridge.

Development and field test of deformation sensors for concrete embedding

Our laboratories have developed a measurement system called SOFO, based on low-coherence interferometry in singlemode optical fibers and allowing the measurement of deformations of the order of 1/100 mm. This system is especially useful for the long-term monitoring of civil structures such as bridges, tunnels, dams and geostructures. The SOFO system requires the installation of two fibers in the structure to be monitored. The first fiber should be in mechanical contact with the structure in its active region and follow the structure deformation in both elongation and shortening. The second fiber has to be installed freely in a pipe near the first one. This fiber acts as a reference and compensates for the temperature dependence of the index of refraction in the measurement fiber. This contribution presents the design process as well as the lab and field tests of a sensor responding to these requirements and adapted to the installation in concrete structures. The active region can be between 25 cm and 8 m in length, while the passive region can reach at least 20 m. While the reference is free, the measurement fiber (installed in the same pipe) is pre-stressed between two glue-points at each end of the active region. The glue was chosen in order to avoid any creeping problems even at temperatures up to 160 degree(s)C and elongation up to 2%. The sensor was tested in laboratory and field conditions. The lab tests included survival to concreting, high temperatures, freezing, thermal cycling, vibrations, cracking and corrosion; response to elongation and compression, measurement range and creeping of the glue points at high temperatures and high tensions. The field tests included installation of a number of these sensors in a bridge deck and in a tunnel vault. In these applications we tested the ease of use, the rapidity of installation and the survival rate.

Monitoring of concrete bridges with long-gage fiber optic sensors

In many bridges, the vertical displacements are the most relevant parameters to be monitored in both the short and long term. Current methods (such as triangulation, water levels or mechanical extensometers...) are often tedious to use and require the intervention of specialized operators. The resulting complexity and costs limit the temporal frequency of these traditional measurements. The spatial resolution obtained is in general low and only the presence of anomalies in the global structural behavior can be detected and warrant a deeper and more precise evaluation. To measure bridge vertical displacements at low cost and frequently in time, one solution consists of installing a network of fiber optic sensors during concrete pouring or installing them on the surface of the structure. By subdividing the whole structure into structural elements and those elements into cells that are analyzed by the sensors, it is possible to obtain information about the average cell deformation (e.g., mean curvature) that can then be combined to obtain the global structural displacement field. In 1996, a concrete highway bridge near Geneva (Switzerland) was instrumented with more than 100 low-coherence fiber optic deformation sensors. The Versoix Bridge is a classical concrete bridge consisting in two parallel pre-stressed concrete beams supporting a 30-cm concrete deck and two overhangs. To enlarge the bridge, the beams were widened and the overhang extended. In order to increase the knowledge on the interaction between the old and the new concrete, we choose low-coherence fiber optic sensors to measure the displacements of the fresh concrete during the setting phase and to monitor the long term deformations of the bridge. The aim is to retrieve the spatial displacements of the bridge in an earth-bound coordinate system by monitoring its internal deformations. The vertical and horizontal curvatures of the bridge are measured locally at multiple locations along the bridge span by installing sensors at different positions in the girder cross-section. By taking the double integral of the curvature and respecting the boundary conditions, it is then possible to retrieve the deformations of the bridge. This measurement methodology was also applied to the Lutrive Highway Bridge in Switzerland in order to measure the variation in vertical bridge displacements due to a static load test. The results obtained using the low coherence interferometric sensors of the SOFO system were then compared with the displacements obtained through an optic leveling system. In the case of this cantilever bridge of 60 meters half-span equipped with 30 fiber optic sensors, a discrepancy of less than 7% was obtained between the two measuring systems.

In-line coherence multiplexing of displacement sensors: a fiber optic extensometer

Civil smart structures often require displacement sensors with measurement bases between a few centimeters and a few meters with a precision of the order of 1/100 mm. Low-coherence interferometry offers these performances even for long-term measurements. Being a non- incremental setup, it does not require an uninterrupted monitoring. The main drawback of this technique resides in the fact that a separate sensor is required for each section to be measured. A typical civil structure such as a bridge requires up to 50 sensors for each span, so the complexity for this type of instrumentation and the number of connections often limit its large- scale application. It would be interesting to subdivide the fiber sensor in domains that can be measured separately but have a single lead-out connection. This contribution presents an in- line multiplexing scheme for displacement sensors based on low-coherence interferometry and partial reflectors installed in pairs along the sensing fibers. The multiplexing of up to ten displacement sensors along the same fiber line is demonstrated theoretically and experimentally. Different types of partial reflectors are also compared. The special case of structures that are constructed in sections is also analyzed.

Embedded and surface mounted fiber optic sensors for civil structural monitoring

Civil structural monitoring by optical fiber sensors, require the development of reliable sensors that can be embedded or surface mounted in concrete, mortars, steel, timber and other construction materials as well as in rocks, soils and road pavements. These sensors should be rapid and simple to install in order to avoid any interference with the building site schedule and not to require specialized operators to accomplish the task. The sensors have to be rugged enough to withstand the harsh conditions typically found in civil engineering including, dust, moisture, shocks, EM disturbances and unskilled workman. It is also desirable that the instrumentation survives for tens of years in order to allow a constant monitoring of the structure aging. This contribution presents the results of a four-year effort to develop, test and industrially produce a palette of sensors responding to the above requirements and adapted to different applications and host materials. These sensors include a small version (length up to 2 m) adapted for embedding in mortars, grout and glues, an intermediate version of length between 20 cm and 6 m adapted to direct concrete embedding or surface installation and a long version adapted to measure large deformations (up to 2%) over length up to 30 m and especially adapted for geostructures monitoring.

Bridge monitoring by interferometric deformation sensors

In many concrete bridges, the deformations are the most relevant parameter to be monitored in both short and long- terms. Strain monitoring gives only local information about the material behavior and too many such sensors would therefore be necessary to gain a complete understanding of the bridge behavior. We have found that fiber optic deformation sensors, with measurement bases of the order of one to a few meters, can give useful information both during the first days after concrete pouring and in the long term. In a first phase it is possible to monitor the thermal expansion due to the exothermic setting reaction and successively the thermal and drying shrinkages. Thanks to the long sensor basis, the detection of a crack traverse to the measurement region becomes probable and the evolution of cracks can therefore be followed with a reduced number of sensors. In the long-term it is possible to measure the geometric deformations and therefore the creeping of the bridge under static loads, especially under its own weight. In the past two years, our laboratory has installed hundreds of fiber optic deformation sensors in more than five concrete, composite steel-concrete, refurbished and enlarged bridges (road, highway and railway bridges). The measuring technique relies on low-coherence interferometry and offers a resolution down to a few microns even for long-term measurements. This contribution briefly discusses the measurement technique and then focuses on the development of a reliable sensor for direct concrete embedding and on the experimental results obtained on these bridges.

Monitoring of a concrete arch bridge during construction

The Siggenthal Bridge is a concrete arch bridge with an arch span of 117 m, being built over the Limmat River in Baden, Switzerland. This bridge has been instrumented with 58 long- gage SOFO fiber optic deformation sensors, 2 inclinometers and 8 temperature sensors to monitor its deformations, curvatures and displacements during construction and int eh long-term. The sensor have been built installed successfully and the arch was monitored during the removal of the formwork and supports. It was therefore possible to observe the deformations of the arch wen being loaded by its dead load and by the daily temperature fluctuations. The measurements have shown that the temperature changes produce deformations of the same order of magnitude as the dead loads. The out-of-plain displacements obtained by double- integration of the measured curvatures are in good agreement with the direct triangulation measurements. Monitoring was also carried out during the construction of the superstructure, with the associated change of the load distribution in the arch. This paper briefly introduces the functional principle of the long-gage sensors used in this application, illustrates their installation and discusses the measurement results obtained during the bridge construction.