As you load up your mini-van with the kids and the dog on your way back from a sporting event or family visit this fall, keep these sobering statistics in mind:
- 1 out of 3 U.S. bridges, nearly 224,000 – requires repair or replacement, including one-third of all Interstate Highway bridges
- 78,800 bridges should be replaced
- The cost of this work is estimated at 260 billion dollars, taking 30 years to complete
- Iconic structures, such as New York City’s Brooklyn Bridge, included
- These bridges have arrived at this state due to old age, overuse, and inadequate maintenance. Ongoing investment has not kept pace. (ARTBA 2022)
Bridges are critical infrastructure. Outdated and deteriorating bridges cause traffic slowdowns, road closures, and limited access for heavy trucks causing inconvenience, delay, inefficiency, and elevated costs.
This issue is well known. Last year, the government endeavored to address this problem with the passage of the massive one trillion-dollar Infrastructure Investment and Jobs Act (IIJA).
Maximum Return on Investment (ROI) demands that the IIJA funds allocated to bridges be invested wisely.
There is a myriad of best practices, building and construction methods and newer technology ensuring maximum ROI, including BIM, Digital Twins, 4D Planning and Scheduling, Accelerated Bridge Construction, Innovative and New Construction Materials, Pre-Fabrication, Artificial Intelligence, Virtual Reality, Drones, Robots, etc. These are all topics for another blog. Today, I will focus on Structural Health Monitoring (SHM).
The advantages of Structural Health Monitoring (SHM) ensure that it will become standard practice in the quest to manage the new and refurbished bridges’ lifecycle efficiently.
A typical bridge is designed to provide service for 100 years. External events (earthquakes, floods, acts of war) or poor maintenance will shorten this lifespan. Conversely, proper maintenance can extend the lifespan.
The goal is to extract 100% use of the structure over its designed lifespan. Structural Health Monitoring can play a significant role.
What is Structural Health Monitoring?
A human’s health is monitored by tracking height, weight, blood pressure and chemistry, pulse, etc. When the results fall outside the expected range, a doctor can perform enhanced testing such as an EKG (electrocardiogram) to clarify the issue and recommend an intervention (medication, surgery, lifestyle change) to mitigate the problem. Health monitoring can uncover defects early, allowing for a successful intervention before the issue becomes unmanageable.
Structural Health Monitoring provides the same benefit for bridges, tunnels, buildings, etc. Structural Health Monitoring is the process of Monitoring, over a period of time, variations in the geometric (bending, twisting) and material (strength, cracking) qualities of a structure which adversely affect the structure’s functionality.
Structural Health Monitoring involves using automated tools, sensors, and systems to improve inspection procedures and methods of maintenance and repair. Measurements are taken at key locations at set intervals utilizing diverse networked sensors, often in combination with traditional inspection techniques, to diagnose the structure’s condition at any given time.
Like human health monitoring, Structural Health Monitoring can uncover structural defects at an early stage, allowing the implementation of maintenance, repairs, or total replacement in an orderly manner, ensuring the overall safety and functionality of the structural system.
Why Structural Health Monitoring?
Structures can fail due to multiple causes, including design and construction error, geological instability, poor maintenance, deterioration of material, etc. Structural Health Monitoring aims to extract 100% functional use of the structure over its lifespan. Structural Health Monitoring offers advantages compared to legacy practices in public safety, quality of life, and economics. Some of the benefits are as follows:
Public Safety / Quality of Life
- Detects deteriorating structural conditions or risk of structural failure allowing mitigating measures to be applied (shut down the structure, decrease load capacity, repair, rebuild, replace, etc.), avoiding catastrophic failure, enhancing public safety
- Avoids unplanned shutdowns due to the need for an immediate / emergency repair
- Allows for planned shutdowns to perform maintenance and repairs in an orderly fashion
- Provides a rapid condition assessment after the occurrence of a natural or manufactured disaster (e.g., earthquake, collision)
- Extends the service life of the structure by identifying essential maintenance
- Avoids unnecessary maintenance of structures in good health
- Allows for detailed planning of maintenance and repair activities, avoids unplanned shutdowns by reducing the need for immediate / emergency repair.
- Improves design methodology by uncovering flaws in current designs and eliminating these flaws in future designs
- Reduces cost and increases accuracy related to an inspection using automation
How is Structural Health Monitoring Applied?
Determine if a Structural Health Monitoring System is appropriate. Establish the objectives of the Structural Health Monitoring system; a cost-benefit analysis weighing public safety and quality of life against the economics is an essential first step
Consider Structural Health Monitoring in the following situations:
- New structures incorporating innovative design, construction, and materials such as Accelerated Bridge Construction and Ultra-High-Performance-Concrete
- New structures located in areas of known geotechnical risk, including weak bearing layers and seismic activity
- New structures located in corrosive environments commonly associated with de-icing chemicals and coastal locations (salt)
- New structures subject to inclement weather, including extreme temperature variations, wind, flooding, and snow
- New structures representative of a large number of similar structures. Analyze the performance of one typical bridge if you plan on building 100 more.
- Existing structures whose disruption will affect the critical transportation network, including high-capacity bridges
- Existing structures with known deficiencies
Analyze Risk, Identify the Response:
- List events and degradations that can adversely affect the structure, such as foundation settlement, corrosion, impact loads due to collisions, weather-related events (wind, flood), flawed design, defective construction, etc
- For each risk, identify the corresponding consequence and response:
- If corrosion is the risk, the consequence will be a chemical change resulting in the degradation of the strength and durability of the material
- If corrosion is the risk, corrosion sensors should be specified.
Establish a Budget:
- The cost of a complete Structural Health Monitoring system includes design, procurement, installation, calibration, testing and commissioning, training, system maintenance, and future upgrades, along with backup and redundant capability/capacity to act in a reserve function.
Design the Structural Health Monitoring System
A Structural Health Monitoring system is a problem-specific, custom-designed system, often utilizing off-the-shelf components.
Type of monitoring to consider:
- Continuous Monitoring – data collected 24/7
- Triggered Monitoring – data collecting initiated by a specific event – vibration monitoring only when a train passes by
- Periodic Monitoring – data collected at set intervals over a period of time – hourly for a year
Time strategies to consider:
- Short-term Monitoring
- State of the structure occurs during a specific event, such as nearby construction activity
- State of the structure occurs only at a specific point in time, such as during rush-hour traffic
- Implemented if the visual inspection uncovers damage to collect specific data
Carried out for multiple years, which may include the structure’s lifetime. Advantageous when changes are slow, such as the onset of corrosion damage.
Damage identification requirements to consider:
- Damage in general
- Location of the damage
- Severity of the damage
- Estimate of remaining service life of the structure
Select Components of the Structural Health Monitoring System
Sensors are available utilizing numerous technologies, including fiber optics, electro-mechanical, vibrating wire, etc. Land survey technologies such as Robotic Total Stations and Global Navigation Satellite Systems (GNSS) are also used. These sensors are designed to monitor numerous conditions, including:
- Stress and strain
- Cracks and deformations
- Settlement, displacement, inclination
- Moisture and chemical content
- Wind and wave speed, pressure
- Temperature, barometric pressure, humidity
- Event-Related (collisions, earthquakes, floods, hurricanes)
- Acoustic emissions (snapped cables)
- Visual Activity (time-referenced video)
Data Acquisition System (DAS)
The DAS, typically located at the structure, controls the overall execution of the SHM process, including the sensors, instruments, and devices collecting data. The system can be hard-wired, with a physical link between every sensor and the DAS, or wireless. Trade-offs should be analyzed for the specific situation. A wired system may not be practical for large structures with long cable runs; a wireless system may be slower and less secure. Backup sensors and equipment should be provided to ensure that Monitoring continues regardless of individual component breakdown.
The communication system provides a data link from the DAS to the point of processing. Data can be transported via existing telephone lines, Wi-Fi, or cellular networks. The system should ensure that information or data is not lost in communication. Data security, transmission speed, and cost may limit the choice.
Data is federated, formatted, and analyzed for simple, fast, and accurate interpretation. Unnecessary and redundant data and noise is cleaned up and removed.
Data Storage / Retrieval / Diagnostics
A system is required to enable data storage for rapid diagnostics and retrieval for future interpretation. Diagnostics convert data into useful information about the condition of the structure, empowering decision-makers to recommend or advise action:
- Build a new structure
- Strengthen the existing structure
- Restrict use of the existing structure – reduce traffic, speed, limit vehicle types.
- Recommend continuous Monitoring with an alarm system
- Structure does not meet desired safety requirements under extreme loads (flood, earthquake, etc.)
Structural Health Monitoring is vital to the safety and preservation of critical infrastructure. Digital Construction Works (DCW) has the resources, services, and solutions to help. Our in-house experts have deep experience in the design, planning, and execution of monitoring projects. We work with asset owners and responsible contractors to integrate project data to assist in making better-informed decisions. For more information visit www.dcwmonitoring.com or contact us today.