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Smart Infrastructure | Forth Road Bridge

Installing strainguages on tel 1

The Scottish Parliament’s inquiry into the closure of the Forth Road Bridge in December 2015 following the discovery of a fractured truss-end link concluded that, “the failure had been unforeseen and unforeseeable”.

Published in March 2016, the findings also indicated that the decision taken to monitor the structure’s behaviour using instrumentation and analytics soon after the defect was found, was the right one – any worrying trends could then be picked up at the earliest opportunity, allowing action to be taken to prevent failure.

The fractured link had been discovered by an engineer taking a group of visitors on a tour of the bridge. That member was only subject to a six-monthly inspection, but others would only be inspected every two years. Within nine days of discovering the fractured link, structural health monitoring using strain gauges and displacement sensors was deployed on all eight links on the bridge. Specialist monitoring company Strainstall, which was already working on the neighbouring Queensferry Crossing, supplied the software and equipment.

Frb

Frb

All eight links were then strengthened using jacks and steel plate splints (New Civil Engineer, November 2016) to allow the bridge to reopen. A speedy repair was needed because the closure of this vital route across the Firth of Forth was costing the economy £1M a day. The defect was attributed to a pin, which allowed the truss-end to move. It had seized up, leading to a huge transfer of load to the truss-end, causing the link to fail. Instrumentation deployed on to the seven other links also showed that these pins were all seizing up to varying degrees and so would also require the same treatment.

After the bridge reopened HGVs were initially not allowed to use it as further work had to be carried out to install a high-level frame and strand jacks to support the truss-end at all eight locations. Load testing (using loaded trucks) and structural health monitoring data proved that the new supporting arrangement for these troublesome end members worked, allowing the bridge to open to all traffic just eight weeks later.

“We were able to say to the [Scottish Parliament] Ministers that we knew the bridge could safely carry the loads that reopening would subject it to, not because a computer said so, but because we loaded it 125 times and used the structural health monitoring to measure it and analyse the results,” recalls Ewan Angus, major bridges director for maintenance contractor Amey. The contractor is responsible for maintaining the Forth Road Bridge and the Queensferry Crossing when it opens on 30 August.

Forth road location

Forth road location

The long term solution for the truss-end links was to replace them with a new end-post supported by a new lower bracket. Full replacement of the fractured link using this method was completed in April this year and work to replace the others will be completed by April 2018.

Structural health monitoring of the bridge now incorporates nearly 500 sensors to understand the real-time behaviour of these connections (see table). Various aspects monitored include: strains in key members, global and local temperature, wind speed and direction, vehicle load, speed and axle weight, GPS position of bridge elements, bridge end displacements and rotations. The structural health monitoring system transfers data from sensors via fibre optic cables to a cloud server where it is sorted and stored and where analytics process it into more relevant information, such as bending stresses. Charts or speedometer-style dials then convey to engineers how the bridge is responding. With trigger levels embedded in the software, they are alerted to take action if readings approach the red “danger zone”.

Load testing

Load testing

Load testing following the truss end link repair

Structural health monitoring data can be reviewed easily from automatically generated reports for any period from the previous minute to an entire year. It is complemented by information held longer term on Amey’s in-house Inspection Defects & Repair Management Database (IDRMD) asset database. Work to integrate the structural health monitoring and IDRMD is underway. Until then, manual intervention to inform the forward programme of work is required. The idea is to create a more autonomous system to plan work via an interactive 3D model interface.

According to Angus, the Forth Road Bridge warrants such care and attention because, as a suspension bridge, its parts move, but many of these elements cannot be inspected to check they are behaving as designed. He says: “Suspension bridges, in particular, are at risk of unforeseen element failures, and visual inspection alone is not sufficient to evaluate the risk of such failures.

“When bridges are numerically assessed to establish their load carrying capacity, it is necessary that this work includes consideration of unexpected effects not designed for,” he says. These effects include articulating elements becoming stuck or less free to move, defects in hidden components, susceptibility to fatigue and unexpected material properties.”

Tiltmeter on trussend link pinjpg

Tiltmeter on trussend link pinjpg

Tiltmeter on truss end link

Only constant monitoring could have highlighted the risks on the truss end links, Angus believes. “Structural health monitoring of the bending stress in the link would, had it been in place, have shown that that risk of fracture was very high,” he notes. “Structural health monitoring of the truss-end links currently uses displacement sensors to measure how much they are moving and rotating and strain gauges to indicate stresses in the members. As each truss-end link is replaced with a new connection, structural health monitoring also indicates how the bridge is responding and provides proof that it is functioning at the capacity required.

“We could monitor more things, but the system already generates a lot of data which requires resourcing,” admits Angus.

Only the truss-ends are currently being closely monitored, but eventually structural health monitoring will be used on the towers as well. This already amounts to 1Gb of data daily.

8,000 elements

“We have 8,000 elements on the bridge which are currently being reviewed to decide how resilient they are and then we will identify the ones most at risk,” explains Angus. Temporary structural health monitoring on a member can help identify if long term monitoring, strengthening or replacement, is the best whole-life solution.

The Queensferry Crossing will have nearly 2,000 sensors installed as part of its structural health monitoring system, producing 8Gb of data daily. This is partly because sensors are easier to install during construction than to retrofit, but also because the different bridge-type – a three-tower cable-stayed structure – needs different elements to be monitored. It will also have an IDRMD and Amey intends to pull information from both bridges’ SHM and IDRMD to aid decision-making.

Maintenance resilience

Angus says: “We will then have the resilience to be able to carry out major maintenance and renewals on the Forth Road Bridge which has not been easy up until now due to the need to keep four lanes open to get 100,000 people a day across the Forth Estuary. Using all of our developing systems, we can identify how best Transport Scotland can spend its maintenance budget for the next 10 years, in theory, but also continually augment and update those plans based on actual ongoing conditions”.

Smart asset management is not a term that Angus uses to describe the work carried out to plan and manage the Forth Road Bridge’s maintenance and operation. Perhaps if something really is that smart, it needs no spelling out. N

 

Structural health monitoring (SHM) Sensor Type Number of sensors on Queensferry CrossingNumber of sensors on Forth Road Bridge

Accelerometers

102

-

Air Temperature Sensors

13

2

Anemometer

11

2

Asphalt Temperature Sensors

40

6

Barometers

2

1

Bearing Gauges

16

8

Bearing Pressure Sensors

-

8

Concrete Deck Temperature Sensors

70

-

Concrete Tower Temperature Sensors

46

-

Corrosion Sensors

360

-

Displacement Transducers

32

48

Dynamic Weigh-in-Motion Sensors (DWIMS)

96

64

GPS Location

21

10

Rainfall Gauges

2

1

Relative Humidity Sensors

12

34

Strain Gauges

887

128

Stay cable temperature sensors

56

-

Steel Surface Temperature Sensors

158

32

Main Suspension Cable Acoustic Monitoring

-

116

Tiltmeters

48

16

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