The Problem
Built in 1932, Abercorn Bridge spans the River Strule and provides access to Newtownstewart, County Tyrone.

Over the years since 1994, a considerable number of condition surveys and assessments were undertaken, which ultimately determined that the viaduct section of the bridge was considered beyond repair. As a result, temporary traffic management was installed at the bridge and a 3.5 tonne weight restriction imposed whilst assessment works were completed. Transport Northern Ireland issued a tender for the rehabilitation of the structure in 2013, which included the demolition and reconstruction of the span 4 viaduct section.

The Solution
In conjunction with Graham Structural Solutions, McFarland Associates Ltd proposed an alternative scope of rehabilitation works, which included the retention of the span 4 viaduct section. McFarland Associates Ltd undertook a Detailed Survey and a series of Non-Destructive Tests to determine its current condition. A Finite Element Analysis (FEA) model of the bridge was developed to determine the structural behaviour. McFarland Associates Ltd engineers temporarily installed a series of fibre optic sensors at critical locations on the structure and a series of Load Tests were performed in order to confirm and calibrate assumptions made during the FEA assessment including: load dispersal through the structure; material strengths; arching action; and support conditions. Concrete repair works included the installation of a Cathodic Protection system and associated monitoring equipment. A new 100mm overslab was install on the bridge, which consisted of lightweight concrete and the use of inert basalt reinforcement.

The Result
The alternative rehabilitation approach proposed by McFarland Associates Ltd and implemented by GRAHAM Structural Solutions delivered considerable cost and time savings for the Client. Our sustainable approach in retaining the span 4 viaduct section as opposed to demolition, negated the need for waste material ending up in landfill and losing the embodied energy within the structure. Our load tests, following the completion of the repairs, confirmed that the structure was capable of sustaining 18t GVW, which was vital for the local farming community.

Awards

McFarland Associates were subsequently jointly awarded a Certificate of Excellence in the Restoration Category by the Concrete Society in 2014 for the works to the structure.