Elizabeth Line Gantry Rigorous Assessments

South East England, UK

Project details
Client

Network Rail

Duration

2012 – 2019

Services provided by Buro Happold

Bridge engineering and civil structures

Europe’s largest transport scheme, Crossrail is delivering a new railway for London and the south-east – the Elizabeth line – that will carry an estimated 200 million passengers every year. Conceived to ameliorate crowding, ensure faster travel and provide an expanded range of direct journey options, this ambitious infrastructure project will engender positive social and economic effects felt throughout the country.

Crossrail encompasses 27 miles of existing Great Eastern and Great Western track for overground sections between Stratford and Maidenhead. Network Rail is responsible for the installation of new overhead line equipment (OLE) and Traction Power Supply (TPS) to serve this route. Various types and configurations of gantries are already in place to carry TPS/OLE elements; in the case of Great Eastern, many of these supports have been standing since the UK railways were first electrified in the 1940s. As loading on all gantries will change as a result of the works, an assessment was necessary to determine how many of the structures would be fit for purpose.

Challenge

Based on a traditional codified approach, previous structural assessments indicated that most of the 570 existing gantries – approximately 430 – were unsuitable for carrying new loading from the upgraded equipment and so would need to be replaced. However, Network Rail considered these initial findings to be unduly conservative in some cases. Consequently, Buro Happold’s structural experts were contracted to carry out a critical review of the assessment work. The aim was to ascertain, as realistically as possible, the structural utilisations and determine the number of viable gantries more precisely.

View of Great Eastern and Great Western railway track next to a cityscape
A lattice portal gantry at Stratford Station; one of 570 similar structures assessed by our experts. Image: Buro Happold

Solution

In assessing all 570 gantries to a demanding timescale, Buro Happold developed a procedure incorporating detailed non-linear structural analysis to eliminate unknowns and remove conservatism while maintaining a suitable level of safety. This involved deriving equipment weights, wind, ice and wire tension loading as accurately as practicable. To check condition and document unrecorded modifications, all structures were inspected via walk-through and high-level surveys. Material samples were collected for laboratory testing to verify the strength of historic steelwork.

Our approach yielded a significant improvement in the number of gantries that were demonstrably capable of sustaining the new TPS/OLE loading compared to the linear analysis. Overall, this number increased from 40% of the gantries considered to 75%. The use of rigorous non-linear analysis for the slender structures proved particularly beneficial when considering susceptibility to lateral torsional buckling.

Several of the remaining structures failed evaluation due to condition. A frequent defect was corrosion at the mast base. Where this was discovered – or mast base capacity was inadequate – the proposed solution was a steel strengthening collar filled with a small amount of concrete batched on site. This uncomplicated measure was the most common reinforcement to be deployed.

Before Buro Happold’s engagement, the client had entertained the prospect of replacing over 400 gantries. Ultimately, all but six were safely retained with minimal intervention.

Engineer working on a railway gantry
Material samples were gathered to allow testing to inform the assessment process. Image: Buro Happold

Value

Giving renewed purpose to historic infrastructure increases the resilience of rail transportation while preserving part of the UK’s engineering heritage. In direct sustainability terms – assuming like-for-like replacement of gantry types – our work drove down the amount of new steel required by an estimated 3,054 tonnes. For production alone, that equates to 4,191 tonnes of embodied carbon. Demolition and waste processing of the existing gantries, together with transportation and installation of the replacements, would see that figure rise.

As a result of our help, Network Rail made substantial savings in terms of the time, labour and money – direct project cost and risk allowance along with the associated programme – involved in replacing these structures. The concurrent OLE upgrade – which had been restricted by dependency on gantry replacements – was also greatly simplified.

Reducing the overall labour requirement was more than a financial win, as construction in the railway environment is a highly constrained and hazardous activity. Our use of advanced analysis techniques, combined with a pragmatic approach to mitigative strengthening, avoided the need for significant construction altogether.

Moreover, the client has gained an improved understanding of their assets. Over the course of the project, a set of structural datasheets were produced that incorporate inspection observations, site measurements, material testing results and assessment utilisations. Previously reliant on partial record information, Network Rail now has a detailed and easily accessible inventory for maintenance. This project unequivocally demonstrates the benefits of bringing innovative structural engineering to a traditionalist industry sector. The procedures developed are applicable to other areas of the rail network and can deliver similar value. To streamline such comparable projects, a parametric approach to generating analysis models and deriving loading has been created.

Close up of a railway gantry
An innovative steel collar design was used to strengthen the base of masts. Image: Buro Happold

Awards

2021

IStructE Award for Minimal Structural Intervention

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