Britain’s 2.2 km long Humber Bridge carries the A15 dual carriageway over the Humber estuary between Hessle in East Yorkshire and Barton in North Lincolnshire. Opened in 1981, its 1410 m suspended main span it made it the world’s longest single-span bridge until 1997.
The 4.5 m deep main-span deck box are supported at the suspension towers on four steel A-frames, which carry up to 1600 t each and allow longitudinal movement of the deck. Routine inspections showed the bearings on these A-frames were wearing prematurely and at risk of seizing.
Following design optioneering by employer Humber Bridge Board’s designer Arup, it was decided the main span’s A-frame rocker bearings should be replaced with an innovative system of vertical pendels and horizontal wind shoes – and that the bridge should remain open to traffic throughout the whole of the works. The challenging replacement project was let to contractor C Spencer Ltd under an NEC3 Engineering and Construction Contract option A (priced contract with activity schedule), with Arup retained as NEC project manager and supervisor.
The award-winning project, which included fitting temporary pendels to allow the A-frames to be removed, involved threading 160 t of specialist steelwork and bearings into a confined environment directly under live traffic and 30 m over water. Work started on site in July 2013 and was successfully completed exactly two years later.
According to project manager Michael Nichols, ‘NEC is generally the contract suite of choice now as it provides a clear allocation of risk and responsibilities, as well as a clearly defined process to follow for the agreement of change and contract administration'.
‘We chose ECC option A as the design provided to the tenderers was well defined. As a result of the complex material requirements and specification there was little opportunity for value engineering. The main savings would be method related – and the contractor has greatest control over working methods" says Nichols.
‘The scheme also had a clear set of activities, which meant identifying an activity schedule was simple to do. Tenderers were encouraged to provide a further breakdown under each task. To assist the chosen contractor with cash flow, it was agreed from the outset that payments would be made on completion of each sub-activity as the duration of each activity was many months' concludes Nichols.
Nichols says that despite a well-defined design being prepared prior to tender, the extremely tight constraints on site – including restricted space, the need to keep the bridge open throughout and a number of elements of the existing bridge not being as originally assumed – meant that a collaborative approach was required to resolve the issues encountered.
‘NEC facilitates a collaborative approach by bringing together the expertise of the contractor and the employer’s designer while maintaining clear lines of responsibility. The early warnings process meant outcomes could be managed to minimise costs, enabling a mutually cooperative approach to resolving issue' says Nichols.
‘Under NEC, changes to the contract were generally quick and simple to resolve as the contract clearly identifies where risk lies. Generally the process for agreeing compensation events works well too' adds Nichols.
In terms of administering the contract, Nichols says that while most construction practitioners are now familiar with NEC processes, an initial NEC refresher for all parties proved to be beneficial in setting expectations and aspirations for the project.
BENEFITS OF NEC3
- NEC enabled a complex bearing replacement project on the UK’s longest suspension bridge to be successfully completed under live traffic conditions
- The contract facilitated a collaborative approach by bringing together the expertise of the contractor and the employer’s designer while maintaining clear lines of responsibility
- For projects with a well-defined design, ECC option A encourages the contractor to achieve savings through method-related improvements
- Changes to the contract were generally quick and simple to resolve as NEC clearly identifies where risk lies
Contact: Michael Nichols, Project Manager, Arup
Tel: +44 113 2378 218
Web: www.arup.com / www.icevirtuallibrary.com/doi/full/10.1680/jcien.15.00066