Why Do Bridges Increasingly Need Long-Term Safety Monitoring?

After a bridge enters operation, it is continuously affected by vehicle loads, temperature changes, wind, rainfall, flood scouring, material aging, environmental erosion and other factors.

Many structural changes do not form obvious defects within a short time, but manifest as:

▪ Slow changes in girder alignment;

▪ Abnormal displacement of bearings or expansion devices;

▪ Settlement or tilt of piers and abutments;

▪ Load states of cables and hangers have changed;

▪ Structural vibration characteristics gradually change;

▪ Local cracks keep propagating.

Therefore, one inspection can show the bridge's state at a certain time point, but can hardly fully answer when the structural change started, whether it continues to develop, and what the bridge experienced during special events.

According to data published by the Ministry of Transport, by the end of 2023 China had about 1.08 million highway bridges, including 10,239 extra-large bridges and 177,700 large bridges. As bridges serve longer and environmental corrosion and long-term load effects accumulate, bridge maintenance management places higher demands on continuous monitoring and data application.

What Is the Difference Between Bridge Inspection and Bridge Monitoring?

Bridge inspection usually includes frequent inspection, periodic inspection and special inspection, mainly to discover structural appearance defects, component damage and functional anomalies.

Bridge structural monitoring, through long-term deployed measuring devices, continuously or periodically collects environment, load, structural response and structural change data.

The two are not mutually substitutable.

Inspection can confirm cracks, corrosion, damage and component anomalies; monitoring provides the change process before and after anomalies occur. Only by combining the two can one further judge:

▪ Whether the defect is still developing;

▪ Whether special events changed the structural state;

▪ Whether the current change is normal fluctuation or an abnormal trend;

▪ Whether inspection frequency needs to be increased;

▪ Whether repair, reinforcement, speed limitation or traffic control is needed.

JTG/T 5122-2021 Technical Specification for Maintenance of Highway Cable-Structure Bridges states that the design of bridge structural health monitoring systems and the selection of monitoring items should be based on the actual bridge, so as to grasp the dynamic and static structural state and operational safety state in real time and effectively respond to specific emergencies.

Which Indicators Does Bridge Safety Monitoring Usually Monitor?

1. Displacement and deformation

Including girder deflection, pier displacement, tower top offset, bearing displacement, expansion joint changes and structural alignment.

Displacement data directly reflects the geometric state of the structure and is an important basis for identifying long-term settlement, local offsets and abnormal deformation.

2. Strain and stress

Strain data can be used to understand the load response of key components under vehicle, temperature and environmental actions, and to help judge whether local loading is abnormal.

3. Vibration and acceleration

Bridges vibrate under vehicle, wind and earthquake actions. Through parameters such as acceleration, frequency and damping, the bridge's dynamic response and dynamic characteristic changes can be analyzed.

4. Cable force

The cables, hangers or tie bars of cable-stayed bridges, suspension bridges and tied-arch bridges are important load-bearing components. Abnormal cable force, uneven cable force change or continuous drift needs further analysis combined with the structural state.

5. Crack and component condition

For key cracks, attention should be paid to crack width, length and development trend. For cables, hangers and connecting components, visual inspection, non-destructive testing and other sensing methods should be combined for comprehensive judgment.

6. Environment and load

Information such as temperature, wind speed, wind direction, humidity, vehicle loads and water level is an important basis for interpreting bridge structural responses.

For example, girder displacement changing periodically with temperature may be a normal response; but if the displacement baseline keeps changing under similar temperature conditions, further investigation is needed.

Why Bridge Monitoring Cannot Set Just One Alarm Value?

Bridge structural response is affected by many factors together. Simply setting a fixed threshold easily leads to two problems:

One type: short-term over-limit caused by normal temperature changes or traffic loads, leading to frequent false alarms; the other type: data has not reached the fixed threshold, but the change rate, correlations or long-term baseline have become abnormal, so risks are not detected in time.

More reasonable monitoring analysis usually needs to consider simultaneously:

▪ Whether absolute values exceed limits;

▪ Whether the change rate is abnormal;

▪ How long the anomaly has lasted;

▪ Whether multiple measuring points change synchronously;

▪ Whether structural responses still maintain their original relationships with temperature, wind and vehicle loads;

▪ Whether the structural baseline changed before and after special events.

The Ministry of Transport's Guidelines for Spatiotemporal Big Data Application in Structural Monitoring of Long-Span Highway Bridges divide monitoring data applications into scenarios such as over-limit alarms, emergency response, maintenance evaluation and basic research, emphasizing moving from data collection to data analysis and use.

What Role Does Monitoring Data Play After Typhoons, Vortex Vibration, Earthquakes and Ship Collisions?

After a special event, manual on-site inspection often can only see the structural state after the event ended, while online monitoring retains the complete data before, during and after the event.

Relevant guidelines of the Ministry of Transport state that when a bridge experiences events such as vortex-induced vibration, typhoons, earthquakes, vehicle overloading, ship collision or abnormal vibration of cables or hangers, the monitoring data of the whole special event should be analyzed to support emergency response decisions and evaluate the structural health state.

These data can be used to answer:

▪ Maximum displacement and maximum vibration during events;

▪ How long the anomaly has lasted;

▪ Whether the structure has returned to its original stable range;

▪ Whether natural frequency, damping or structural response relationships have changed;

▪ Whether special inspection, load testing or expert judgment is needed.

▪ What is vision-based displacement monitoring suitable for on bridges?

Vision-based displacement monitoring uses cameras to continuously identify targets placed on the bridge structure, obtaining the position changes of the targets relative to the measurement reference.

Suitable for:

▪ Vertical deflection and transverse displacement monitoring of girders;

▪ Horizontal displacement and tilt trend monitoring of piers and towers;

▪ Relative displacement monitoring of bearings, expansion joints and connection parts;

▪ Coordinated deformation monitoring at different positions of multi-span bridges;

▪ Areas difficult for personnel to reach long-term;

▪ Review of structural displacement before and after special events.

For large-span or heavily obstructed bridges, multi-head, front-rear camera or daisy-chained arrangements can be adopted according to on-site sight distance and structure form, so that different measurement sections form a continuous monitoring network.

Vision-based displacement data can also be combined with temperature, wind speed, acceleration, strain and cable force data to judge the bridge condition from different measurement dimensions.

What Are the Features of the Haytham Bridge Safety Monitoring Solution?

Haytham online visual trackerUsing targets as the measurement object and continuously computing target position changes through vision algorithms, it can be used for non-contact displacement monitoring of key bridge parts.

For bridge site characteristics such as large span, scattered measuring points and obvious environmental changes, the system can be configured per project conditions:

▪ Multi-target recognition by a single device;

▪ Synchronized measurement with multiple measuring heads;

▪ Flexible networking of front and rear cameras;

▪ Continuous monitoring of long-distance measuring points;

▪ Comparing temperature data with displacement trends;

▪ Reference change identification and periodic calibration;

▪ Cloud data, trend curves and alarm push;

▪ API interfaces and monitoring platform data integration.

In project scheme design, one should not first determine the number of devices and then look for measuring points; instead, one should first clarify the risk objects, structure type, measurement direction, range, accuracy, sampling frequency and on-site sight distance, and then determine the system form.

How to Judge Whether a Bridge Monitoring System Is Truly Effective?

It can be judged from four questions:

▪ Is the data continuous and credible?
Whether device offline status, reference drift and environmental interference can be discovered in time.

▪ Can the anomaly be explained?
Whether the monitoring system synchronously records temperature, wind, working conditions or other influencing factors.

▪ Can the alarm be executed?
Whether different warning levels correspond to review, inspection, speed limitation or special inspection measures.

▪ Does the data enter maintenance decisions?
Whether monitoring results are used to adjust inspection frequency, determine key parts and formulate repair plans.

The value of bridge safety monitoring lies not in generating more curves, but in converting continuous data into executable inspection and maintenance bases.