MHB Consultants were contracted by AMCO Giffen, on behalf of Network Rail, to undertake the strengthening and refurbishment of Orchy Viaduct to ensure the structure would be able to achieve a load rating of RA5 @ 45mph and RA7 @ 10mph and remain maintenance free for the next 25 years.  Orchy Viaduct is a 140m long, 7 span, half-through, metallic underbridge forming part of the Crianlarich to Oban line and carries a bi-direction track over the outflow of the Rive Orchy into Loch Awe. 

The works undertaken on the project by MHB Consultants included:

  • Initial Site Investigation including condition survey of metalwork and masonry.
  • Topographical survey.
  • Structural calculations of the bridge to determine strengthening requirements.
  • Development of a strengthening scheme.
  • Development of condition led repairs to metalwork and masonry.
  • Design of timber deck renewal.
  • Temporary works checks as required.

These tasks were completed by September 2018 with the project scheduled for completion on site in January 2019.  To date, a disruptive possession of the railway has taken place with successful installation of the new timber deck achieved.  

MHB Consultants offered all the services required to deliver the work from initial site investigations to final delivery of the design.  We were able to carry out all inspection works, topographical surveys, calculations and designs and work as a team to ensure all processes were carried out to the benefit of the project.  

During the initial reviews a computer model was developed of a single span only that cut down on the analysis and model development time which minimised the time taken for the assessment.  Additionally, MHB Consultants undertook a sophisticated non-linear buckling analysis on the top chord of the Main Girder Truss to better capture the effects of the U-frames, large non-standard intermediate stiffeners and cross girders on the top chord behaviour than the conservative approach in BS5400-3:2000.  This innovative approach ensured that the element capacity was maximised and led to minimisation of the strengthening scheme required.

MHB Consultants undertook further detailed analysis to compression and tension diagonals of the main girders to confirm the required strengthening extents by undertaking a buckling analysis.  This advanced analysis removed the requirement to strengthen 84 sets of flat plates saving significant sums of money.

MHB Consultants also increased the sustainability of the project by minimising the volume of timber required for the installation of the new timber deck.  This was achieved by refining the required extents of the timber without reducing the functionality of the bridge.  This change meant that 5% less timber (approx. 6 tonnes) of timber was saved.

Below photos are courtesy of our client AMCO Giffen.