FBG sensors for seismic control and detection in extradosed bridges

Structural health monitoring (SHM) has become increasingly valuable in recent years and is starting to be widely used in the field of structural engineering applications (Rodrigues et al., 2010). However, the necessity to have the precision resolution in real-time way in monitoring applications requires the manufacturer to look for new technologies. The novel technology found is the fiber Bragg grating (Sun et al., 2007; Vorathin et al., 2019). The implementation of such technology at design stage of the civil engineering structures is a good practice to the Master of the work which provides detailed knowledge of the behavior of these structures dice their exploitation in real-time way.

Actually, the fiber Bragg grating (FBG) technology has enormous progress to meet the requirements of industrial applications thanks to their advantages like great resolution with precision, high sensitivity, immunity from electrical and magnetic interference (Shen and Shen, 2008) and multiplexing (Kang et al., 2005). It mixes the optical sensing with the optical communication (Li et al., 2011). The FBG sensors can easily measure with proper installation: temperature, strain (Kerrouche et al., 2009), pressure, frequency, chemical, biological, biomedical, rate, flows, etc. But temperature and strain still the base of any FBG sensor. However, the design of the FBG uniform does not remain stable over time or at elevated stress. That is why we can have false results. To beat this difficulty, we proposed and developed a new design of FBG rather than the uniform type (Lu et al., 2008; Palaniappan et al., 2007). It is a special chirped FBG.

In the first part of this work, we present the new design of the FBG sensor which is validated. It gives a linear response to the strain and/or to the temperature with acceptable sensitivity in compared with all recent publications in this field. This element is the base element of the robust FBG sensors network that is applied, in a demonstrative example, in civil engineering structure in the second part of this work as SHMS. The test structure is one of the two towers of the bridge body of Rades-La Goulette in Tunisia. This work is the first of its style in Tunisia. The developed network will be used as little system for SHM applications for real-time monitoring. And so, the ultimate goal of this research is to improve the role of the conventional monitoring system that is already installed in this structural engineering (Kang et al., 2005; Kerrouche et al., 2009; Li et al., 2011; Lu et al., 2008; Palaniappan et al., 2007; Piot, 2009; Rodrigues et al., 2010; Shen and Shen, 2008; Sun et al., 2007; Vorathin et al., 2019), and to strengthening detections more selective and precise. The system proposed has been tested under particular conditions, where the tower is stressed.

This particular condition is the seismic problem, and for the first time their effect is calculated in civil engineering structure body using the technology FBG specially using our proposed SHMS since our location, the Mediterranean, in one of the locations that is subject to earthquake effects and their happening risks increase from one year to another in coastal areas. Therefore, the need for such a precise structural health monitoring system (SHMS) in all engineering structures like a bridge is mandatory. This little SHMS allows us to appreciate, continuous, the ability of the service of the tower when the bridge is completely installed, during his life, where we quantify the normal and exceptional vertical or horizontal tilt sustained by the structure body by an earthquake. Also this proposed SHMS able to indicate the potential and the degree of such effect. On the other hand, the first Bridge that used the FBG technology for its structural monitoring is the Beddington Trail in Canada (Raikar et al., 2011) where the manufacturers used 20 FBG sensors to measure the temperatures and strains.

Therefore, our purpose is to validate an SHMS based on FBG technology in engineering structures like bridges, stadiums, and buildings, and the test procedure and results obtained are discussed in detail. The error range found by this technology does not exceed 10 to 12. In Addition, our little SHMS allows us, absolutely, to reduce the cost of the monitoring and maintenance. On the other hand, in such structure engineering applications, the exact study of utilized systems and FBG sensors locations has an essential role to have the desired efficiency and efficiency monitoring.

As a definition, the Bragg grating is a spectral filter that allows a part of the incident signal to be reflected and etched in the transmitted signal, see Figure 1. The central wavelength of this reflected and etched part is (Kang et al., 2005; Kerrouche et al., 2009; Li et al., 2011; Lu et al., 2008; Palaniappan et al., 2007; Piot, 2009; Raikar et al., 2011; Rodrigues et al., 2010; Shen and Shen, 2008; Sun et al., 2007; Vorathin et al., 2019): ().(1)λB=2neffΛ(z)(1)with λ_B is the central wavelength of the spectral signal reflected and etched by the Bragg grating, n_eff is the effective refractive index of the fiber core, and Λ(z) is the grating period. The last is a constant in uniform case. In this work we have developed a new type which is a special chirped Bragg grating where the grating period is defined with: ()(2)Λ(z)={Λ0(1−cpz)for[0...z2]Λ0'(1+cpz)for[z2...z](2)with Λ₀ and Λ₀′ are the nominal grating periods and c_p is the chirp coefficient. It is defined in nm/cm. On the other hand, all the physical parameters of an FBG can be varied: profile of the effective refractive index, profile of the effective refractive index modulation, length, apodization, chirp coefficient, and whether the grating has a counterpropagating or copropagating coupling at a Bragg wavelength (Hill et al., 1997; Suresh and Tjin, 2005). For our proposed design of FBG, it watches a good performance. Figure 2 presents their reflected signal without the need for the apodized function. The reflected signal is calculated basis in matrix solution with the following parameters: Λ₀ = 0.53 µm, Λ₀′ = 0.5284 µm, c_p = 1.5 nm/cm, n_eff = 1.456, length of 4 mm, and average refractive index modulation equal to 2.5 × 10^-4. And so we have λ_B = 1.5424 µm. From Figure 2, we have the same shape of reflectivity if we compare it with reflectivity that is from the conventional shapes of FBG that are described in all research works (Kang et al., 2005; Li et al., 2011). Also, we have a reflectivity in the order 100%. In addition, our design presents robustness as the random fluctuations that are produced for different noises to the grating period, Λ(z), therefore automatically to the effective refractive index within a range ~ ±0.03%, which is a high value, of Λ(z) have not effect in any way the structure of the sensor design. In addition, if we exceed this value the fluctuations would destroy the period structure and divide the whole grating into lots of pieces, which results in interference among those pieces. So, we must be sure of the fluctuations values in the manufacturing process. However, our sensors design gives a large margin of security in the manufacturing process. On the other hand from the reflective spectrum of our FBG sensor, FWHM has a value of 0.226 nm which is very little value and can increase the multiplexing solution. So this proposed FBG is suitable for optical communication with grate long-term robustness.

Figure 1:

Figure 2:

On the other hand, we can characterize the robustness of our design by calculation the coupling coefficient, k. Basis in several references (Aubin, 2009; Hill et al., 1997), we can express this coefficient as: ()(3)K(z)=πδ¯neff(z)λB,zν(3)where λ_B,z is the central wavelength of each uniform Bragg section in matrix solution, υ is a term of coherence which is taken equal to 1 and δ¯neff(z) described the profile of the effective refractive index modulation, also for each uniform Bragg section.

In our case, we use an SMF doped-germanium optical fiber. The radius of the fiber core is equal to 4.6 µm, that of the optic cladding is of the order of 62.5 µm and that of the mechanical protective cladding is 125 µm. This type of fiber and the parameters mentioned above are used in the SHMS that will be presented in next sections of this paper.

In this part, we calculate the strain and temperature sensitivities in our FBG which acts as a sensor for monitoring applications. We remember that these sensitivities conduct to shift in Bragg wavelength, which is the basic function of an FBG sensor, as indicate the following expression (Kersey et al., 1997): ()(4)ΔλB=2neffΛz((1‒[neff22(p12‒v[p11+p12])])ε+[α+dneffdTneff]ΔT)(4)with Pij are the Pockel’s coefficients, υ is the Poisson’s ratio, ε is the applied strain, α is the fiber linear thermal coefficient, and ΔT is the temperature change. In addition, we can make the last expression simpler as (Vorathin et al., 2019): ()(5)ΔλBλB=(α+ξ)ΔT+(1‒ρ)ε(5)with α is the fiber linear thermal coefficient, ξ is the thermo-optic coefficient and ρ=neff22(P12−v[P11+P12]) is the effective photo-elastic coefficient (Kersey et al., 1997; Shen and Shen, 2008). This last has a numerical value; it is varied about 0.22 (Aubin, 2009) and 0.26 (Li et al., 2011) according to the material of the fiber. The sensitivities found are 1 pm/µε and 10 pm/°C, respectively, for strain and temperature as shown Figures 3 and 4.

Figure 3:

Figure 4:

All the types of FBG as apodized, chirped, uniform, etc. are performing and make an accurate measurement, but our FBG is more robustness and their lifetime is more important even under continuous stress. And the obtained results confirm their good performances and accurate measurement even under random fluctuations. This new sensor is suitable for both optical communication and optical sensing in health monitoring applications due to the acceptable obtained sensitivities to strain and temperature, also due to the weak FWHM. On the other hand, in our SHMS installation, presented in the next section, our basic design is our FBG with all parameters defined above. In addition, we take FWHM as 0.3 nm instead of the real value of 0.226 nm for safety reasons when applied multiplexing.

As we have talked before, the first bridge that used the FBG technology for its structural monitoring is the Beddington Trail Bridge in Canada. In addition, this is the first extradosed concrete road bridge built in Canada. Also, it is defined as the first smart highway bridge in Canada since it uses a smart SHM system. It began service on November 5, 1993. It uses 20 sensors for measuring strain and temperature along the whole of its length. An image of this bridge which has an average length of 21.03 m is shown in Figure 5. In addition, Table 1 gives other examples of bridges that use the FBG technology as the basis of its SHM system. This table is considered in agreement with the work of Rodrigues et al. (2010).

Figure 5:

On the other hand, Table 2 gives some examples of bridges that use fiber optic technology, and not the FBG technology, as the basis of his system of SHM. This table is also considered in agreement with the work of Rodrigues et al. (2010). And it is very clear from the two tables that the number of sensors in bridges using FBG is much more important than other technologies. This indicates that the technology is booming and very effective. It is growing from one year to another. On the other hand, all the technologies presented in this section are based on optical fibers includes the FBG. Thus, the measurement parameters for these different technologies as the SHMS are based on the different properties of the light propagating in the fiber include phase, intensity, wavelength, and polarization. As an example, the interferometric sensors and the Low Coherence sensors detect the light phase, while our FBG sensor detects the wavelength shift in agreement with the external factor. In addition, the FBG technology is the most effective and can be applied in structural supervision, specifically in civil structures, i.e. bridges as we have seen before. And in the same line, in the following section, we describe the bridge Rades-La Goulette in Tunisia, which is the first of its style in Africa with its system of SHM.

Bridge Rades-La Goulette is the first of its style in Tunisia, and even in Africa. It belongs to the family of concrete extradossed bridges of stay-cable where the deck is suspended by cables from two pylons. This structure is opened to traffic on March 21, 2009. And the final construction cost is approximated by 100 million $. For the location, the bridge spanning the channel of Lake Tunis, where he connects the area of Rades to the area of La Goulette. The main bridge has a length of 260 meters, a width of 23.5 meters, and a height of 20 meters. It is located in a coastal aggressive environment, see Figure 6. The opening movement is delayed for two years after the initial date of completion. The reason is certainly the nature of the soil, which is very fragile and aggressive. A general picture of the bridge is shown in Figure 7.

Figure 6:

Figure 7:

The semi-maritime environmental feature of the bridge and its location geotechnical cause rigorous aggression on its construction. All these properties absolutely accelerate aging. That is why Tunisia has decided to equip it with a system of SHM. On the other hand, manufacturers taking into account the respect of the seismic conditions of work and block their lateral movement. And for the SHMS, manufacturers install throughout the main bridge 55 sensors and a dozen cameras. These devices are connected to a control room where they allow management measured to trigger alerts and alarms in case of the presence of anomaly data. And all the devices of the SHMS are installed on a structure or in different spans of the structure under construction. Table 3 summarizes the various sensors installed in the bridge. This table is similar to that presented in Piot (2009) where the total number of sensors installed is 55. And outside of the measured parameters, we can classify the sensors according to the location where we can find two types. The first is installed on the bridge body, and the second is installed inside the bridge body. The first type, it is intended to measure the phenomena that affect the bridge body, such as wind and ambient temperature. The second type gives the responses of the bridge body against certain parameters such as loads, accelerations, strains, inclinometer, etc. Similarly from the table, the sensors can be classified also according to the static or dynamic measure. For the first type, the data are kind of slow. We can tell they are made with periods that exceed 10 min as the temperature and inclinometer pylons. And for the second type, the data are kind of fast. We can tell they are made with less than 1 sec as the wind and acceleration periods.

On the other hand, according to Table 3, we can know the places of installation of all the sensors:

2 anemometer sensors: they are of ultrasound type. They are located on the deck to measure the direction and speed of the wind. These last two parameters can influence and biases the bridge body.

On the other hand, for each sensor suppliers combine two terminals between which the sensor can grow without triggering an alert. These two terminals are defined by the providers and can set from the results obtained in experiments testing with a learning period. They present the safety margin of a sensor. And so in the case of exceeding one of the two terminals, alerts are instantly displayed in the general interface of the control software. And then the control software can send alerts or reports to the responsible.

Our goal is to validate a proposed SHMS in civil engineering structures such as bridges, stadiums, and buildings for seismic effects. Test procedure and results obtained are discussed in detail. The studied structure is one of two towers of Rades-La Goulette Bridge, which is installed in aggressive environment and coastal area. In addition, this bridge is the first work of its style in Tunisia and in Africa. On the other hand, the manufacturers do not take into account the control and the detection of the seismic effect in the installed SHMS, however, the seismic risk exists. The proposed SHMS is based on FBG technology, especially a new FBG design. This design is efficient as a strain and temperature sensors. Also, it is expand to the field of civil engineering. So, this paper provides a perspective for the construction of new civil engineering structures based on FBG sensors technology proposed as an SHMS in an aggressive environment, where an improve in the role of the conventional monitoring system, as electric monitoring, and strengthening detections more selective and precise is warranty.

The little SHMS network proposed of three FBG sensors is installed into the tower. The tower is stressed with seismic effects. As we know their happening risks increase from one year to another in coastal areas. The generated effects are mainly: flexion, deviation, tilt, and crakes. These effects are variables depending on the nature of the land crossed by seismic waves, the distance from the epicenter, the depth of the tower under and above the sea level and also the vulnerability of buildings of tower. In addition, we can mention that the depth of the tower under the sea level affects the obtained results and not the efficiency of the sensors used. For us, we are interested mainly in the tower deviation from its vertical position. The obtained results are the results of an imitate model where the FBG sensors allow us to see the tilt effects of seismic waves on the tower in each degree. They show high efficiency and thus allow a real time monitoring to the structure. In addition, these results are compatible with those expected because our little SHMS is based on FBG sensors characterized by error range does not exceed the 10^-12, although imitate model do not take into account the exact topology between the seismic epicenter and the position of the tower where may be the seismic wave undergoes slight amplification. In addition, the bedrock of the tower is not defined exactly. Similarly, our results are confounded with the scale of seismic intensity used in Europe: EMS. Also, it is important to know that an earthquake is often followed by aftershocks. It should be expected. The aftershocks of magnitude are generally lower that the initial earthquake. In addition, in this part, we do not talk about the hydraulic system of a tower in semi-maritime zone. Otherwise, from observing the effects of an earthquake by FBG sensors network, we can characterize the severity of earthquake in surface.

The proposed SHMS network can be investigated too in all body of Rades-La Goulette Bridge but with a large number of FBG sensors to replace the maximum number of 55 conventional sensors which are installed as conventional SHMS and to ensure entire coverage of the bridge to monitoring: load, strains, temperature, cable-stayed extensometers, tilt, accelerometer, force, etc. But in this case, the result of tower tilts will be different because we have a new hydraulic system that is installed that respects the equilibrium and giving more rigidity to the tower. Also, we can apply this concept of FBG network to calculate the tsunami effects that attack the coastal area structures. Otherwise, the environment of our example is an earthquake zone nevertheless not subject to cyclical seismic vibrations being in a coastal area where the most from soils subjected to cyclic seismic vibrations. Thus, the manufactures will follow the rules of earthquake-resistant construction, solid foundation, chaining, since we never know exactly the date of the next earthquake.