Truss end links repair
A major phased project to repair and replace the 'truss end links' on the Forth Road Bridge has been under way since a fracture was discovered at the north east main span during a routine inspection in December 2015.
The resulting closure of the bridge and subsequent HGV restrictions were described in Parliament as an event of national significance.
Against the backdrop of technical, operational, safety, media and political challenges the project to repair the truss end links has demonstrated exemplar innovation and creativity in engineering, management and communication. Completed during challenging weather conditions, and under extreme scrutiny, the project successfully delivered immense social value by sustaining one of Scotland’s arterial and iconic transport networks.
The expertise and dedication shown enabled completion ahead of programme, alleviating prolonged economic, social and environmental impacts associated with diverting over 70,000 vehicles 33 miles each day, whilst ensuring public safety and the structural integrity of the bridge.
New technology was harnessed including structural health monitoring, drone inspections and social media.
Amey’s strong culture of health and safety throughout the project resulted in zero accidents despite over 400 people on site.
Within six days, a welded stiffening plate and jacking solution was designed and checked. Within 21 days this design was fabricated and installed to all eight truss end link locations, including temporary scaffolding 50m above the River Forth.
Concurrently a Structural Health Monitoring (SHM) system was developed to provide live stresses, strains, displacements and temperatures at all ‘link’ locations. Over 125 progressive and controlled HGV load tests were undertaken to observe real-time structural behaviour. These tests allowed re-introduction of 91% of traffic to the bridge, but showed that lower pin joints of the links were partially seized, inducing high stresses, precluding the unrestricted re-introduction of HGVs.
By mid-January design and checks were completed on a second phase of repairs with new steel brackets attached to the main towers, from which the deck would be jacked and supported via steel cables. Similar solutions were implemented on all four main span links, allowing the bridge to fully reopen to HGVs on 20 February 2016.
The phased reintroduction of HGVs was trialled for safety during overnight load tests, with lorries merged into general traffic using a temporary ramp-metering system and the structural impact monitored in real-time using the SHM system.
The truss end link at the north east main span, where the original fracture occurred, has now been replaced with a world first permanent sliding bearing arrangement to be fixed to the tower beneath the existing end post. The new 5T bearing is supported on a new tower bracket below the bridge deck. These works were completed in spring 2017, with lessons learnt applied to the seven other links to be replaced by the spring 2018.
Thanks to the support provided by the temporary brackets already installed, this final phase can be carried out below deck level with minimal disruption to road users.
Reinforcing steel has to be assembled in confined spaces within the main tower, so a full size concrete replica was constructed to trial the buildability of the works and minimise construction and safety risks.
Structural health monitoring is being incorporated to complement that installed during the initial emergency works.
The works’ weather susceptibility at such exposed locations was managed by keeping personnel on standby, maintaining mobile welfare facilities on site and monitoring live wind speeds, temperature and rainfall in order to seize every opportunity safely. Despite significant disruption from storms, efficiencies in project management allowed the majority of steelwork fabrication to be completed indoors, minimising delays, maximising safety and permitting rapid fit-up on site.
Daily 'whiteboard' meetings during phases one and two, and close collaboration with Transport Scotland and Ministers, allowed all staff disciplines to contribute proactively. This allowed planned maintenance activities to be brought forward to take advantage of the bridge closure and minimise future disruption.
The dedication of the team enabled reopening to 91% of traffic within 21 days of the fracture being discovered, alleviating the closure’s economic impact estimated at £1 million/day and the environmental impact of traffic using the diversion route. Communities drew together and provided over 100,000 positive and supportive comments for the staff working tirelessly to reopen the bridge.
Broadcast, print and digital media were harnessed to promote public understanding and highlight the essential and positive role of engineers within society. Seven press conferences were held and over 1,860 articles generated. Live updates and engaging content were developed for social media, generating 13.5 million Tweet impressions, 4.5 million Facebook impressions and 1.1 million website views.
Feedback was overwhelmingly positive, meaning stakeholders had confidence in the team and the site workforce in particular became ‘heroes’ of the situation.