Identifying hidden defects in bridges
We've compiled a range of case studies, looking at our application of various, specialist methodologies in the detection of hidden defects in bridges across numerous structural types.
Metallic bridge applications
M6 Bescot and M5 Oldbury Viaducts (CS4)
Hidden defect: Internal box sections
Problem: Concerns were raised regarding the condition of welded box sections on the bridge structure and their susceptibility to fatigue damage.
Solution: James Fisher Testing Services designed and installed unattended dynamic monitoring systems to measure the strain under live traffic loading over a one month period at each site. Each location was fitted with 150 gauges and the data collected was used to calculate the fatigue life consumption at both locations.
Trowse Bridge and Caerphilly Station
Hidden defect: Deck plates, jack arches
Problem: The degree of load transfer from the deck plates to longitudinal beams was unknown, causing assessment calculations to call for a weight limit to be imposed on the bridge.
Solution: James Fisher Testing Services completed controlled static load testing. The use of known weights combined with the measurement of displacement and strain enabled more accurate assessments to take place at the site.
Forth Road Bridge
Hidden defect: Fatigue cracking
Problem: During a routine inspection, damage to the bridge was noted. This included the complete fracture of one element of the truss end link on the mains span section of the eastern leg of the north tower of the bridge, and the partial fracture of its counterpart. The bridge was closed whilst repairs were carried out.
Solution: James Fisher Testing Services deployed a comprehensive health monitoring system to the bridge, including more than 200 strain gauges, temperature and tilt sensors to measure stress in the bridge members. This monitoring enabled the identification of the root cause of the link failure; a seized pin, and a phased repair was implemented as a result of the findings.
Concrete bridge applications
A4 Hammersmith Flyover
Hidden defect: Pre-stressing wires / strands / anchorages
Problem: Breakages of post-tensioning tendons meant that residual strength testing was required to monitor the effect of failures on the structure.
Solution: James Fisher Testing Services undertook programmes of in-situ stress measurement in the post-tensioned concrete structure. This was part of the assessment of the structure’s residual strength following well publicised breakages of post-tensioning tendons, as detected by acoustic emission monitoring. James Fisher Testing Services also set up instrumentation for monitoring the structure during re-stressing operations.
M8 Kingston Bridge complex
Hidden defect: Half joints
Problem: The renewal of approach viaducts required strength testing to be carried out on existing half joints.
Solution: As part of a renewal of approach viaducts, a test to destruction of a half joint was undertaken, with monitoring by James Fisher Testing Services. This was used to provide information about any hidden strength and to give confidence about the likely condition of the dozens of other similar joints within the junction complex.
M56 Bowden View
Hidden defect: Missing / inadequate rebar / pre-stressing
Problem: Risk of sudden bridge collapse.
Solution: James Fisher Testing Services ascertained the dead load condition within a segmental post-tensioned concrete bridge deck. A monitoring system was fitted to the bridge, including acoustic emission sensors to detect pre-stressing wire breaks. Strain and displacement sensors were employed to monitor potential crack development.
Masonry arch bridge applications
Isle of Man
Hidden defect: Arch barrel
Problem: Deformations of the arch barrel.
Solution: James Fisher Testing Services implemented a camera-based system to monitor the deformation of the arch barrel under load. The resulting information gave an indication of load paths and locations of high strain, pinpointing areas where cracks could develop.
Bell Busk Viaduct
Hidden defect: Spandrels
Problem: Longitudinal cracks had appeared beneath the spandrel and arch barrel interface.
Solution: James Fisher Testing Services undertook long term monitoring for deterioration and short term testing using special 3D dispal cement sensor arrays. These tests were used to identify whether new freight wagon arrangements were causing more distress to the structure due to a more concentrated load application.
Bearing and expansion joint applications
M5 / M6 Midlands links
Hidden defect: Un-inspectable details
Problem: Data relating to bearing behaviour was required to plan maintenance work on multiple motorway links.
Solution: James Fisher Testing Services set up monitoring at four separate locations on the Midlands links in order to gather information about the actual behaviour of bearings and supporting structures, facilitating maintenance planning.
Hidden defect: Bearing seizure
Problem: Data was required to measure long term structural articulation.
Solution: James Fisher Testing Services instrumented a ten span section of the bridge, enabling long term monitoring. The piers were slender and there was no evidence of movement in the steel bearings. In order to record any movement between the piers, monitoring included displacement between them so that any overall sway could be detected.
Warrington Bank Quay
Hidden defect: Elastomeric degradation
Problem: Measurement of deformations in elastomeric bearings was required.
Solution: James Fisher Testing Services instrumented elastomeric bearings in order to measure the deformations in three dimensions under known traffic loading. This information was then compared with the design parameters.
Moveable bridge applications
North Bridge, Hull
Hidden defect: Fatigue in track girders
Problem: Longitudinal cracks were detected during a repainting contract. These were in angle sections connecting flanges to webs of the deep box section track girders, beneath the rolling part of the bridge.
Solution: Strainstall devised a monitoring system comprising strain gauges and displacement sensors to track further deterioration and to detect whether any residual global deformations were developing. The bridge was maintained open for traffic in this condition.
Somerleyton / Reedham
Hidden defect: Articulation and load bearing issues
Problem: General structural performance.
Solution: Monitoring was implemented in order to identify how the bridge was operating and where the load paths were under trafficking.
Hidden defect: Settlement
Problem: Weetwood Bridge was suffering from leaning side walls.
Solution: The method to halt and arrest the movement involved excavation of fill from between walls and above the arch barrels; this was replaced with polystyrene fill and transverse ties were fitted. James Fisher Testing Services provided a movement and settlement monitoring system to measure any heave resulting from the removal of significant dead load.
Hidden defect: Scour
Problem: The bridge is located across an upland river which is subject to flash flooding and fast flowing water. Scour of the bridge foundations was a concern.
Solution: The risk of scour undermining the foundations had been mitigated by the construction of a concrete apron to channel the water away from the structure. However, this was being undermined by scouring action, indicating that there remained a risk of bridge foundation scour occurring. Sensors were installed in conjunction with a solar powered data logger to monitor inclination of the concrete apron and to provide a real time alert of possible scouring.