Bridge failure, and how to stop it before it happens.
The Sydney Harbour Bridge is 86 years old. It was only built to last 100.
The X-ray machine was invented in 1895 and MRI technology has been around since the 1970s, so it is a little surprising that most inspections for damage on built structures are still undertaken with a simple tech-free visual inspection. Is there an affordable and scalable alternative to a MRI to monitor a structure under stress?
The collapse of the Morandi bridge in Genoa in August turned that question from a casual curiosity to a racing urgency. The international and local spotlight has turned to structural health monitoring (SHM) of bridges. This is pertinent to Australia, where our most used (and most famous) bridges are approaching their originally planned retirement. The Sydney Harbour Bridge is 86 years old, and was originally intended to only have a 100-year life span.
Managed by the Roads and Maritime Services (RMS) NSW, the Sydney Harbour Bridge has undergone maintenance multiple times since construction was completed in 1932. This included major strengthening works in the mid-2000s which strengthened the steel arches and had new workman’s gantries fitted to the underside. Workers previously had to abseil off the bridge to inspect its underside.
SHM technology, developed by the CSIRO Data61, has been installed in 2,400 sensors across the bridge’s span since 2015. The sensors comprise of three accelerometers encased in a unit with a Linux-based computer processor. The units are set on the underside of each of the bridge’s arches, and linked via Ethernet to each other and Data61’s data centre in the inner West of Sydney. The accelerometers track fluctuations and changes in the bridge’s vibrations and changes in the bridge’s responses. A machine learning algorithm at the edge of the sensor provides successful damage identification.
“We measure the traffic load and the bridge’s responses to know whether it is performing according to the design assumptions – the system gives an alert if it is not performing properly,” says Queensland University of Technology (QUT) civil engineering Professor Tommy Chan, co-founder of Australian Network of Structural Health Monitoring (ANSHM).
The system, Chan describes, “uses accelerometers to identify the dynamic characteristics of bridges, such as frequencies and mode shapes.”
Robots have also been deployed to assess potential damage to the Harbour Bridge. A 2016 trial of a robot developed by University of Technology Sydney (UTS) and RMS could access areas dangerous for humans – both because of height or because of low-air flow in access tunnels.
Similar SHM sensor systems have been installed on the West Gate Bridge in Melbourne and the 76 year old Storey Bridge in Brisbane. The SHM for the latter, being a suspension bridge, also includes anemometers to monitor wind speed and direction, especially during tropical weather events. Other SHM systems track temperature changes, particularly within concrete pylons where thermal fluctuations cause the concrete to expand and crack.
The systems are most effective when they are installed up-front, though costly. It’s estimated installation of SHM during construction could be one to five percent of the total construction cost. Even so, structural health monitoring is becoming more commonplace in buildings, dams and towers. Visual diagnoses of structural damage are receiving an upgrade.