When a newly built footbridge near Florida International University in Miami collapsed on March 15, 2018 killing several people driving beneath, it was a tragedy, but a rare one. Few pedestrian bridges collapse in this way. The loss of life is shocking because it is often assumed that bridges will stand the test of time.
But 54,500 bridges in the US alone are classed as “structurally deficient”, according to the National Bridge Inventory. There are no comparative figures available for the UK, but The RAC Foundation reports that 3,441 bridges are considered “substandard”. The Florida bridge was put in place just five days before it collapsed on March 15 and hadn’t yet been opened to the public, showing even new bridges can fail.
Yet cheap new technologies mean that it is increasingly easy to monitor the health of bridges and spot problems before they become fatal. If deploying these systems becomes more commonplace, future tragedies could be prevented.
Why bridges collapse
The cause of the Florida bridge collapse is not yet known, but structural forensics should eventually reveal the reasons. It is known that a crack was reported in the north end of the bridge. Now new information is starting to appear in the media which will shed more light on this catastrophic failure.
Previous experience shows that the majority of bridge failures are caused by design errors, being struck by vehicles, overload and natural disasters, such as flood. Design faults usually cause bridges to collapse either shortly after they’re built, or steel and concrete fatigue can cause them to fail while in service.
The Florida bridge was largely built in advance and then the main span lowered into place on supporting abutments, an increasingly common technique used to reduce construction time and risks to workers and the general public. Deflection monitoring systems are usually employed to help place a bridge accurately and allow the team to observe its deflections – the way a structure deforms when bearing a load – while it is moved and installed. Such a monitoring system is reported to have been used when the Florida footbridge was installed.
Long-term continuous monitoring of bridges aims to detect damage at an early stage to help prevent a catastrophic failure and aid regular bridge inspections. But there is no legal requirement for bridges to be continuously monitored. In the UK, thorough physical inspections take place every six years but these can be subjective and prone to human error. Plus, a lot can happen in six years.
If more bridges had continuous health monitoring systems in place, it could help detect damage, check bridge condition after natural disasters and prevent bridge failures. Developments in sensing technologies continue to lead to new viable solutions for measuring and collecting data. What’s more, modern technology means monitoring can be relatively simple and affordable.
We have the technology
Technological advances have led to robust, multifunctional and precise bridge monitoring systems. Here are the technologies most widely used today:
Fibre optic sensor system: Light signals transmitted through optical fibres are examined to determine changes in fibre properties when a structure comes under strain. These sensors can be embedded inside or attached to the bridge.
Micro-electromechanical systems: These are small electronic and mechanical systems (as small as a two pence coin), that can collect measurements and provide wireless data transmissions.
GPS: This can be used to measure bridge movements by tracking the location of GPS transmitters.
Non-contact measurement techniques: These capture structural changes without coming into physical contact with the bridge. An example is vision-based monitoring which employs cameras to capture images of a bridge, which are later analysed using image processing techniques. Installing sensors on bridges can be complicated due to health and safety concerns and the difficulties in gaining access. These challenges can be avoided when vision-based systems are used in bridge monitoring. Cameras can be set to collect images at certain frequencies. Even smartphones and GoPro-style cameras can be used to monitor the structural health of bridges over a period of time. Using high-resolution cameras, information extracted from images can be as accurate as that collected with sophisticated contact sensors.
Monitoring bridges doesn’t just mean assessing the structure. It can also involve spotting crossing vehicles that are too heavy for the bridge and long-term patterns of pedestrian movement. Video footage can also be used for security purposes and, in the case of a bridge failure, provide useful information on the nature of a collapse.
But bridge health monitoring is not just about collecting measurements. The greatest challenges are to make sense of the data and provide timely and reliable reports on bridge health.
In order to improve the safety of bridges and minimise the possibility of catastrophic failures, regular bridge inspections should be complimented with continuous monitoring systems. Such systems would provide real-time data of bridge conditions and help detect damage. If damage is dealt with quickly and properly, the lifespan of a bridge can be prolonged and made safer for all who use it.