Continuous structural monitoring of the Tremor viaduct

Context

The Tremor Viaduct is located on the A6 highway in León (Spain) over the Tremor River. It has a total length of 470 m, which is divided into 11 isostatic spans of between 39 and 45 m. The deck is formed by two 3 m high box section girders, one for each roadway, with both girders resting on a single pier at either end of each span. Therefore, the bridge is in fact two separate viaducts. The spans were built using voussoirs, each span having an independent external post-tensioning system (Figure 1) that runs inside the box section and is anchored to the diaphragms located on the piers at the ends of each span.

Figure 1. Interior of the box section girder of the Tremor viaduct.

The special inspection, safety level assessment and rehabilitation works of the Tremor viaduct advocated by the Spanish Ministry of Transport and Sustainable Mobility includes the implementation of a continuous monitoring system aimed at assessing the structural safety of the viaduct, both in the repair and service phases. This system focuses on the behaviour of the external post-tensioning system and the continuous monitoring of the movements of the diaphragms, as well as estimating the deflection and stiffness of several spans. The term “continuous” refers to the need to continuously evaluate the structural condition at very short assessment intervals as this allows structural anomalies to be anticipated and/or helps guide decisions regarding repair work.

As part of the tender process for the execution of the Special inspection works and evaluation of the safety level of the Tremor Viaduct, the company LRA INFRASTRUCTURES CONSULTING, S.L. was appointed as the main contractor and assigned responsibility for evaluating the safety of the viaduct. Several systems were proposed at different levels. These included the continuous monitoring of the post-tensioning system and of some struts located between the lower part of the deck and the diaphragms, with the aim of controlling the relative movements of the diaphragms, as well as the continuous measurement of some spans.

The Structural Engineering Group (GIE) of the ETSI de Caminos, Canales y Puertos of the Universidad Politécnica de Madrid together with the company LRA designed and implemented the two aforementioned systems.

The work team was led by professors Jaime García Palacios and Iván Muñoz Díaz and included researchers Luis Chillitupa Palomino, Belén Vecino Muñoz, Javier Naranjo Pérez, Carlos Martín de la Concha Renedo and Christian Barrera Vargas, on behalf of the GIE, and by Tomás Ripa Alonso and Mario Martin Aguilera, on behalf of LRA.

The project requirements stated that the monitoring should integrate an accelerometer system oriented to the external post-tensioning system and a system of strain gauges to estimate the load on the metal struts located inside the box sections (Figure 2). The large number of measurement points (88 struts), the distance between them (approximately 500 m between the farthest ones) and their location inside two independent concrete box sections made a classical solution based on strain gauges and traditional wired, synchronised dataloggers difficult to adopt.

Furthermore, the system had to have a sufficiently high sampling rate to effectively measure the thermal effects and determine whether the estimated load variability is a result of these effects or due to other types of structural anomalies. Using an overall temperature value was therefore not sufficient, and it was necessary to take local measurements taking into account the large distances between the struts and the significantly different degree of environmental exposure of each span. The project also required real-time alarms that activate when pre-established load levels on the struts are exceeded.

Figure 2. Arrangement of struts in the viaduct.

Solution

Given the requirements of the project, a decision was taken to adopt a solution that uses low-power wireless Worldsensing equipment and LoraWAN communication that can interact over long distances. The system was implemented using 24 LS-G6-VW-5 wireless nodes for vibrating wire and thermistor, with 5 simultaneous channels and 2 gateways (one of which was configured as a secondary gateway). In addition, the Connectivity Management Tool (CMT) Edge was integrated in one of the gateways.

This allows the system to measure the deformation of the struts using vibrating wire gauges and continuously translate the measurements to load values, together with the thermal variations. The instantaneous measurements of the loads on all struts are taken with a sampling frequency that allows any unusual variations, i.e. those not due to temperature, to be detected and reported. Data readings are taken every two minutes together with the local temperature of each strut.

The sensors used to measure the deformations are vibrating wire strain gauges (Figure 3), which have high long-term stability. These allow micro-deformations due to thermal effects to be measured, as well as deformations close to the elastic limit of the struts. In the adopted configuration, each node digitises 4 measurements from the 4 struts located in the area of each pier or abutment (Figure 4) on both sides of the diaphragms. The data from all the nodes pass to the gateway and are sent remotely to a database via an MQTT protocol, where the deformations are transformed into loads and the corresponding alarm signals are emitted where necessary.

Figure 3. Vibrating wire strain gauges installed on the struts.

Figure 4. (A) Example of a GW with antenna on the viaduct. (B) Example of a node and location of the vibrating wire strain gauges on the viaduct.

Prior to installation, the client was supplied with two nodes and a gateway, with technical support from Worldsensing, for familiarisation and training purposes and to allow testing in the laboratory (Figure 5).

Figure 5. Pre-testing of the devices in the laboratory.

Installation of the strut monitoring system was completed in December 2023. The system has allowed the loads on the struts to be monitored during this period as well as the identification of any diaphragms experiencing major deformations. Periodic reports are issued every month and annual trends are analysed.

During this time, occasional events involving sudden variations in strut loads caused by maintenance and repair work on the viaduct have been detected and reported. The system has been found to function satisfactorily, as it has adequately identified these changes.

In general, the strut load measurements are stable over the long term and correlate with temperature. Figure 6 shows an example of the evolution of strut load and temperature values obtained during the month of December 2024. It is also possible to identify the correlation between temperature and load, highlighting the case of struts SP093 and SP094 of pier 9 in span 10, which are very sensitive to thermal changes.

Figure 6. Evolution of strut load and temperature in span 10, piers 9 and 10 during the month of December.

Similarly, Figure 7 shows the evolution of strut load and its correlation with temperature in span 7 during almost one year of measurement, from February 2024 to January 2025, i.e. the entire period in which the system has been operating.

Figure 7. Evolution of strut load and temperature in span 7, piers 6 and 7 during 2024.

“The continuous monitoring system allows anomalous situations that could compromise the structural safety of the bridge to be detected and maintenance and repair work to be planned more precisely.”

Iván Muñoz Díaz, Jaime García Palacios, Luis Chillitupa Palomino, Belén Vecino Muñoz, Javier Naranjo Pérez, Carlos Martín de la Concha Renedo and Christian Barrera Vargas, researchers in the Structural Engineering Group at the Universidad Politécnica de Madrid,Tomás Ripa Alonso, Mario Martin Aguilera, LRA Infrastructures Consulting.

Benefits

The continuous monitoring system has made it possible to track strut load and thus verify the behaviour of the diaphragms in the viaduct. This, added to the dynamic monitoring of the external post-tensioning system by means of accelerometers, offers the capacity to detect anomalous situations that could compromise the structural safety of the bridge and has allowed more precise planning of maintenance and repair work.

In general, it can be stated that continuous monitoring enables anomalies to be detected immediately, preventing them from evolving into major problems (the most serious outcome of which could be the collapse of the viaduct), and the detection of degenerative trends that require decisions regarding maintenance actions, which optimises resources and allows the adoption of preventive maintenance plans (as opposed to corrective ones), thus leading to long-term economic savings and increasing the useful life of the infrastructure.