Monitoring suspension bridges? Give us a ring!
As the world’s cable-stayed / suspension bridges get wider, with some surpassing the 2000m ‘span barrier’, the need for monitoring them during construction extends too. A new paper at Nature explains how the experience gained building the Tajo Bridge using load cells to measure direct stress has helped with the monitoring of elements only used during construction, such as temporary stay-cable towers.
Wires and cables
In a cable-stayed or suspension type bridge, the load cells are usually positioned to measure on e of the seven high tensile steel wires that form a single bridge stay cable. What’s more, those monitoring needed to “Establish a correlation coefficient between the deformation experienced by the wire and the deformation experienced by the strand.” (You can find the maths at Nature).
The Tajo Bridge
We’ve covered the use of load cells during the construction of the Tajo Bridge before, an impressive rail bridge in southern Spain. This time, load cells were used to determine the current levels of tension in the bridge stays during inspection, and structural repair work.
The Sensor System for the bridge has 114 sensors:
- 30 load cells in the suspension cables
- 8 load cells in the anchorages of the stay-cable towers
- 40 unidirectional strain gauges in the reinforcement of the half-arches
- 24 temperature probes
- 16 clinometers
- 3 anemometers
- 3 accelerometers
Load cells in a ring
The team devised a new load cell design that would monitor both the axial force transmitted by the bridge stays and the force transferred to prestressed structures. The clever solution is a metal ring that the bridge stay or prestressing unit passes through. Load cells placed around the ring measure the mean axial deformation of the ring. (As an image shows the result is a load cell that looks remarkably like a large cooking pot!) As a result of installing a prototype in the Tajo Bridge itself, he team aim to “determine the minimum edge of the device (h ≥ ∅/2) to achieve more adequate accuracy in stress measurement.”
The team also devised a “synchronised multi-strain gauge load cell network for the monitoring of short prestressing units”, including monitoring Macalloy bar stresses. (Macalloy bars basically anchor stay towers to the deck of the bridge.)
The Baltimore Harbour Bridge
Sadly, no amount of load cells could have saved the Francis Scott Key Bridge in Baltimore Harbour from the devastating impact of the container ship MS Dali striking one of its supporting columns. However, we’re sure load cells came into play in the complex and dangerous operation to lift and clear an estimated 45,000 tonnes of debris that had fallen into the Patapsco shipping channel.
According to Colonel Estee Pinchasin, the Army Corps of Engineers Baltimore district commander in charge of clearing the debris said:
“Everything (was) mangled up on top and around. You had these big, large spans that were just laying in the water. You see four-inch steel that’s been bent… how can you even start to think about the force?”
Robyn Bianchi, the assistant salvage master for Donjon Marine, explained the challenges for the divers who assessed the wreckage and ways to raise it.
“I kind of compared it to a metal jungle gym underwater. You turn off all the lights in the room and try tell me all the pieces of where they connect to that metal jungle gym. Oh, and by the way, the jungle gym is completely twisted and looks nothing like it did when it was built to perfection.”
Load cells for any size project
Whatever your load cell requirements, from major build to small scale testing, off the shelf to bespoke or specialist, we can help. Contact us to discuss your requirements, or browse our online shop for quick delivery across the UK and beyond.
