Monitoring the structural behaviour and integrity of a bridge during complex rehabilitation process

Designed in 1922 and open to the traffic four years later, in 1926, the Hercílio Luz bridge served as the first road link between the Island of Florianópolis, part of the state of Santa Catarina, and the continent, in Brazil. Installed in a maritime area, for over 90 years it has suffered from enemies not often seen, including the corrosive action of the sea, the natural wear of the structure and the load increase as well as the number of vehicles, which reached values much higher than those at the time in which the bridge was designed.

Plans and actions for permanent preventive maintenance of the bridge were detailed, which allowed it to maintain its original structure even after almost 90 years. However, this did not prevent it from being completely closed since 1991 due to the precarious conditions in which the bridge was. In addition to being closed to the traffic, the asphalt floor of the central span was removed from the bridge to reduce permanent loads, a relief of 400 tons on the structure.

Problem

Since its inauguration, the Hercílio Luz Bridge faces different enemies. The corrosive action of the sea, the natural wear of the structure and, finally, the increase of the load and the number of vehicles, much higher than the one designed for the bridge in 1926. Even so, with permanent preventive maintenance services, the bridge has, after almost 90 years, maintained its original structure. Now, more than ever, there is the need to recover it with the added challenge of maintaining its originality, from the geometry to the details in its connecting parts.

Solution

In a project that could take almost 3 years, the company Teixeira Duarte was hired for the complete rehabilitation of the Hercílio Luz Bridge, including replacement of bearing supports in the main towers, reinforcement and recovery of foundations, rehabilitation of the metallic structure, replacement of eye-bars, saddles and main cables, including cargo transfer work. Application of new decks for car, pedestrian and motorcycle circulation. For this project, the company Teixeira Duarte decided to use HBM technology, including sensors and optical interrogators with Fiber Bragg Grating (FBG) technology to monitor strain at critical points in the structure, electrical sensors for reading of various signals including inclination, temperature, wind and sea current. All this combined with engineering services for sensor installation, software script creation for monitoring, data recording and alarm generation, as well as remote support in case of any failure.

Results

The most critical part of the project involving the transfer of load from the central span of the bridge to auxiliary structures has been successfully executed and there is a possibility that the structure will be monitored continuously after the restoration is completed.

General diagram with the main points where the sensors were installed for monitoring

Typical monitoring sections, with representation of the strain gauges in blue. — A) eyebars (O's);
B) lower auxiliary supports (AI's);
C) auxiliary towers (TA's);
D) lower strings (IC's);
E) upper ropes (CS's);
F) reinforcement diagonals (DR's)

The system architecture

For the instrumentation of the bridge structure, a hybrid architecture was defined, one that uses both optical sensors based on Fiber Bragg Gratings (FBG) and conventional electrical sensors for various quantities. The measurement of deformations, temperature, wind speed and sea currents are part of the global monitoring system. In total, 284 optical sensors were acquired by 3 HBM FiberSensing interrogators. Taking advantage of the optical network necessarily installed for the transmission of signals from the optical sensors, the communication of the HBM's PMX was integrated to read the electric sensors in the two 24-way multifiber cables, deployed along the 800m of the bridge.

At strategic points for sensor connection, the multifiber cable has been bled to individually connect each fiber to the sensors. All connections were protected by dedicated boxes along the structure.

For ease and speed of the installation process, an essential requirement for the project execution, the optical sensors were delivered as pre-assembled sensor chains. Two different methods for sensor installation were chosen depending on the structural parts where they would be placed. Since the bridge is a metal structure, the most obvious choice was that of spot welding sensors. For the central trusses and auxiliary supports - parts that remain in the structure during the whole project - this was the chosen form of installation. For the eye-bars - the parts to be replaced - glued sensors were selected. Sensor chains included strain and temperature sensors. The combination of the two is needed to compensate for temperature effects on strain measurements.

The more technically complex activity is the load transfer procedure which consists of the tension relief on the eye-bars allowing their sequential replacement. The execution of this procedure implied the instrumentation of several points of the original and provisional metallic structure with strain sensors and the application of other electric sensors as inclinometers, and climatic sensors making the total of 312 sensors. HBM presented the best offer of supply and installation, achieved through the optimized use of combined technologies - Optical technology for strain measurements and data transmission to the control center and electrical for other measurements - and a timely installation methodology that was required by the physical schedule of the work. Ricardo Martins, Project Engineer, Teixeira Duarte

Main control room and rack with data acquisition equipment

Data Acquisition System

Acquisition equipment is all connected to the control room. Both optical and electrical platforms communicate via ethernet and their connection is performed by optical Ethernet switches, as usual for networks of this type.

For the global system, two computers were used: one focused on the monitoring and recording of the continuous data 24/7 and another working on the post processing of the data already recorded. catman software was used for data acquisition, recording and analysis. Several scripts were created to automate certain tasks such as the periodic generation of reports and alarms for both on-site operators and remote engineers, including HBM that was scheduled to remotely access whenever needed.

The importance of strain measurements during bridge rehabilitation

During the rehabilitation of the Hercílio Luz bridge, some of the abovementioned steps were very critical. For examplethe load transfers at the beginning and at the end of the woks. These load transfers are the moments where the hydraulic jacks are actuated to raise the deck of the bridge, in order to transfer the load supported by the eye-bars to the lower temporary structure, which are the four "Support Towers" or "TAs", built for this project. In a structure of this size, and with severe damage as in this case, a load transfer of this type can cause a structural failure in the bridge or temporary structure, and it is at this point that the instrumentation becomes fundamental. To ensure that the entire process was carried out with maximum safety, the multiple strain signals in the temporary structure were measured to assess whether the load was being homogeneously transferred by the structure and the structure of the bridge to evaluate if the load relief was occurring as expected.

The load transfer process was carried out in stages, and after each step the results were compared with numerical simulations by the engineers responsible for the project, in order to confirm that the behaviour of the structure was within the expected.

Tilt measurements were also performed on the towers of the temporary structure and on the bridge itself.

Another important point are the loads generated by the wind and the tide. Being a bridge in a coastal region, located at a narrowing point between the island and the continent, the winds can reach high speeds, compromising the restoration works. For this reason, sea current and wind measurements were taken along with all the strain and temperature sensors installed along the bridge.

Replacement of the saddle at the top of the main tower

Replacement of main tower support devices

Above: Application of rivets Below: Strain gauges installed on eyebar (left) and upper rope (right) — Above: Application of rivets
Below: Strain gauges installed on eyebar (left) and upper rope (right)

The choice of optical strain measurement technology

When using electrical strain gauges in large structures where a large number of sensors is required, the amount of electrical cables becomes a considerable problem, since instrumentation cables are expensive and bulky. In addition, measurements with this type of sensor over distances of hundreds of meters are not viable, due to the influence of electromagnetic noise in the long cables of the installations. In these cases, it is necessary to spread the data acquisition systems along the structure, which is a disadvantage due to the risk of damage to the equipment, not counting the connection issues between these equipment and the data storage system.

Fiber optic sensors have enormous advantages, such as high limits of fatigue, multiplexing and immunity to electromagnetic interference. HBM FiberSensing's range of strain gauges and other optical sensors based on FBG are the perfect choice for demanding test and measurement applications. Know the full range of optical sensors offered by HBM here.

Principle of operation of optical sensors based on Bragg networks. Only certain wavelengths are reflected and the variation of the peak position of the reflected wave can be converted into values of the magnitude to be measured.

HBM demonstrated high competence during the installation of the entire monitoring system and was able to meet the deadline for system completion despite the adverse weather conditions that occurred during most of the installation period. After the end of the installation, HBM provided direct support in the first months of operation, followed by a technical assistance agreement that will remain in place until the transfer of cargo has been completed. The maintenance of the system is carried out by Teixeira Duarte's technician, who has been trained by HBM who is permanently on site and from HBM's remote support team. This relationship of cooperation has allowed to keep the monitoring system in proper operation, says Ricardo Martins.

Teixeira Duarte

Having started its activity in 1921, Teixeira Duarte is now leader of one of the largest Portuguese Economic Groups.

Based on its structuring values: Ingenuity, Truth and Commitment, Teixeira Duarte has fulfilled its Mission: Doing, contributing to the construction of a better world.

Teixeira Duarte Group currently has around 10,500 employees in 18 countries on four continents and is active in six different sectors such as Construction, Concessions and Services, Real Estate, Hospitality, Distribution and Automotive.

Today, Teixeira Duarte assumes responsibility for the image they created of a company with great value and pioneering in construction, where its teams and structures interconnect to a final result of recognized capacity in the conception, innovation, construction and management of large projects and enterprises.

From large infrastructure such as bridges, dams, hospitals, roads and other public works, to the great buildings that mark our history, talking about Teixeira Duarte is synonym of an excellent know-how and a constant presence in our day-to-day.

HBM FiberSensing

HBM FiberSensing, SA belongs to the German HBM Group, the world leader in Testing and Measurement, and is dedicated to the development and production of optical systems based on Bragg fiber optic (FBG) network technology for advanced monitoring applications.

Backed by expertise ranging from optical fiber technology to optoelectronics, digital electronics and instrumentation, the company markets a broad portfolio of optical sensors, interrogators and software applications as well as complete monitoring solutions.

HBM FiberSensing is located in Portugal, operating internationally through the global network of the HBM Group present in more than 80 countries.