By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 22 2024A recent article in Scientific Reports provided an overview of innovative construction techniques that improve the strength and precision of stress management in active anchorage and short prestressing units for long-span bridges, focusing specifically on potential risks.
Extensometers in the passive reinforcement of the Tajo Bridge arch: (a) extensometer in the reinforcement of the stay piers; (b) extensometer in the reinforcement of the half-arches. Image Credit: https://www.nature.com/articles/s41598-024-61873-y
Background
Cable stays and suspensions are widely used in the design of long-span bridges, where their longevity is influenced by fatigue and corrosion due to dynamic loads like traffic and wind. To assess the impact of these factors and the subsequent damage to the cables, axial stress monitoring is typically conducted.
Various techniques and devices have been developed to measure the stress in bridge cables. Direct measurement devices include load cells, fiber optic Bragg grating sensors, and elasto-magnetic strain sensors. For quicker and indirect assessments, vibrating wire methods are commonly used.
Additionally, auxiliary structural elements such as temporary stay-cable towers used during bridge construction also experience significant instantaneous prestressing losses. Monitoring the prestressing stress and its variation over time is critical to ensure that these elements perform as intended.
Methods
In this study, researchers conducted a comprehensive review of the stress monitoring systems utilized during the construction phase of the Tajo Bridge, a distinctive high-speed infrastructure project in Spain designed and built from 2012 to 2016.
The Tajo Bridge was meticulously engineered to adhere to high speed, efficiency, and safety standards while also providing a modern aesthetic. To assess the structural behavior of the bridge's central arch span, a sophisticated Structural Health Monitoring System (SHMS) was developed, integrating multiple devices and systems.
The SHMS included a Management and Unification System with the Project (M&USP), which housed databases provided by the bridge’s design and construction teams. Additionally, the Sensor System (SS) featured 114 sensors strategically placed across the bridge, such as load cells on the suspension cables and anchorages of the stay-cable towers and strain gauges on the half-arch reinforcements.
A Data Acquisition and Processing System (DA&PS) was incorporated to manage the inputs from various sensors. Moreover, a Data Management and Processing System (DM&PS) was established for data transmission, visualization, storage, and the development of early warning systems.
The Structural Safety and Assessment System (SS&AS) also played a crucial role. It included all parties involved in the bridge's construction, from technical to management teams. This subsystem was essential for monitoring the collected data and comparing it against theoretical projections. The outcomes of these comparisons updated the M&USP databases and enhanced the overall functionality of the SHMS.
The deployed SHMS effectively monitored various parameters, including the deformation of the half-arches, the stability of stay piers and towers, the acceleration of the northern half-arch, thermal gradients across different structural sections, and the wind conditions impacting the structure.
Results and Discussion
The Tajo Bridge project provided an opportunity to critically assess and refine the newly implemented monitoring systems for stress control. The load cells designed for active anchors demonstrated their capability to precisely measure the total axial force transmitted by bridge stays or prestressing units, offering a resilient solution for environments subject to extreme conditions, shocks, and impacts.
These load cells, uniquely designed as metal rings, enable the bridge stay or prestressing unit to pass through. This facilitates installation between the anchor and distribution plates on the structure without requiring signal integrators for direct measurement, enhancing the accuracy and reliability of the stress measurements.
During the construction, three types of devices were installed to monitor the bridge stays: load cells on active anchors, unidirectional strain gauges on a strand within the stays, and piezoelectric accelerometers on the stays. This setup proved instrumental in detecting a variety of structural phenomena, such as stress variations in the bridge stays during the concreting of successive segments, the analysis of force variation due to the stressing of different cables, and the detection of force variations from load readjustment operations in the suspension stay cables.
Additionally, a novel synchronized multi-strain gauge load cell network was proposed for each stay tower to monitor short prestressing units effectively. This network was crucial in ensuring proper prestressing and accurately quantifying the losses experienced by the prestressed connections, thereby confirming the effectiveness of the monitoring system in managing and optimizing the structural integrity of the bridge during its construction.
Conclusion
This study concentrated on refining stress management for bridge stays, suspension cables, and short prestressing units, with a focused emphasis on stress as a unified parameter. Advanced load cells were specifically designed and implemented in active anchorages to ensure robust and precise control of stress. Additionally, the introduction of a novel synchronized multi-strain gauge load cell network for monitoring short prestressing units was pivotal in scenarios where prestressing losses were substantial.
To substantiate these technological advancements, the researchers provided practical insights and results from their application during the construction of the Tajo Bridge, which utilized the cable-stayed cantilever technique. These methods proved essential in quantifying prestressing losses, which, in the case of the Tajo Bridge, exceeded 10 %. This data is crucial for planning new stressing operations and ensuring the structural integrity of such critical infrastructure.
Through these innovative techniques, the study demonstrates significant advancements in the field of structural health monitoring, offering a model for future projects requiring meticulous stress management.
Journal Reference
Gaute-Alonso, A., Garcia-Sanchez, D., Ramos-Gutierrez, Ó. R., & Ntertimanis, V. (2024). Enhancing stress measurements accuracy control in the construction of long-span bridges. Scientific Reports, 14(1), 10961. https://doi.org/10.1038/s41598-024-61873-y, https://www.nature.com/articles/s41598-024-61873-y
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