Project Overview and Monitoring Objectives
The core objective of a bridge health monitoring project is to achieve real-time perception of structural conditions and anomaly alerting. The system typically covers the following monitoring dimensions: main girder deflection, pier settlement, cable force/stay force variation, bearing deformation, environmental corrosive media, temperature field distribution, and so on.
Monitoring during the construction phase must focus on: temporary support deformation, form traveler travel posture, and structural response during the lifting of large components. These data serve both as real-time assurance of construction safety and as part of the baseline database for later operation and maintenance.
Key Points of Safety Management
Personnel safety is the first priority for on-site implementation, with specific requirements including:
- All personnel entering the site must receive safety technical disclosure and be familiar with the special plans for work at height and over water
- Installation of monitoring equipment constitutes work at height: safety ropes and fall arresters must be provided, and a harness must be worn above 2 m
- For work over water, life jackets must be worn, and warning signs and interception facilities must be placed downstream of the work zone
- Electricians must hold valid certifications; verify reliable grounding before energizing equipment
Regarding equipment safety, sensors and optical fiber are delicate. Before construction, coordinate work faces with the civil contractor to avoid cross-operations damaging already-installed sensors. Once a sensor is in place, fit a temporary protective cover and hang warning signs nearby.
Quality Control of FBG Sensor Installation
Fiber Bragg grating (FBG) sensors are precision optical components, and installation quality directly determines the reliability of monitoring data. Key control points are as follows:
| Control Item | Quality Requirement | Inspection Method |
|---|---|---|
| Fiber fusion splicing loss | ≤0.05 dB/point | OTDR testing |
| Total channel attenuation | ≤12 dB (recommended value) | Bidirectional measurement with optical power meter |
| Sensor bonding/welding | Firm, no voids, positional deviation ≤5 mm | Visual inspection + hand-pull test |
| Sensor numbering and labeling | Clear numbering, consistent with drawings | Cross-check against drawings |
| Fiber routing markers | Marked throughout, securely fixed | Patrol inspection |
Temperature compensation: strain sensors are temperature-sensitive. Deploy temperature sensors simultaneously for thermal-drift correction; keeping the two within 50 cm of each other yields good results.
Lightning protection and grounding: bridge sites are typically open environments. The optical fiber and sensor metal housings must be reliably grounded, with grounding resistance not exceeding 10 Ω, to prevent lightning damage to the interrogator.
Data Quality and System Acceptance
After system installation, data quality verification is the core of acceptance testing. Common acceptance items include:
- Data completeness: verify that every channel reports data continuously and that packet loss stays within the allowed range
- Data plausibility: cross-check sensor readings against site conditions (e.g., deflection changes under load tests, correlation between temperature and strain)
- Alert function verification: inject simulated signals to confirm that platform alerts trigger correctly and are pushed to designated personnel
- System stability: continuous operation for 72 hours or more with no disconnections and no data dropping to zero or spiking abnormally
Common Issues and Remedies
Issue 1: Fiber channel down or excessive loss
Remedy procedure: first locate the break or high-loss point with an OTDR → check whether connectors inside the splice closure are loose → confirm whether the cable is being squeezed or bent excessively → re-splice and retest if necessary.
Issue 2: Data remains flat over long periods
Possible causes: sensor debonding/desoldering, broken signal cable, or interrogator channel failure. Check physical connections first, then perform on-site calibration comparison of the sensor.
Issue 3: Frequent false alarms
A common cause is a fixed threshold set too low. It is recommended to switch to dynamic thresholds plus trend-based pre-judgment, and to set differentiated thresholds for night and day to reduce ineffective alerts.
Relevant Standards for Reference
- JTG/T 2350-2021 Technical Specification for Structural Monitoring of Highway Bridges
- DB37/T 5245-2022 Shandong Province Technical Standard for Smart Bridge Health Monitoring
- GB/T 34065-2017 Technical Specification for Structural Monitoring of Buildings and Bridges