News & Events

March 2022

Sign of the Times: Use and Application of CD 354 – Design of minor structures

Accepted as the cornerstone of the delivery and management of motorway and all-purpose trunk roads across the UK, and a respected document worldwide, the Design Manual for Roads and Bridges (DMRB) was first published in 1992. The suite of over 350 documents and associated Interim Advice Notes (IANs) was added to and updated on a relatively ad-hoc and disjointed basis up until April 2017 when technical experts and specialists at National Highways (formerly Highways England) began the unenviable task of updating the complete suite of documents.

This process was scheduled for completion in March 2020 and its focus was to make the DMRB clearer, more consistent, and easier to understand and implement. The update places design requirements at its core, intending to make it easier to incorporate developments in design best practices and achieve greater efficiency and innovation throughout the lifecycle of the assets.

Whilst some of the documents and requirements were readily and easily adopted and implemented by the industry, some parts have taken longer to be completely understood and integrated into the design, construction, operation and maintenance of highway authority assets and infrastructure.

One such example of this is the use and application of “CD 354 Design of minor structures”, specifically the design and specification of foundations for road traffic signs and street lighting columns. This is partially due to a ‘we have always done it this way’ attitude of designers, contractors/installers and manufacturers but also because National Highways (NH) has been developing its own understanding and experience of the design standard, and therefore NH, to date, has not been as stringent with enforcing design criteria and requirements. Furthermore, supplementary design advice and guidance, for example the Institute of Highways Engineers Sign Structures Guide 2020, has been revised to account for the 2019 update of EN 12767 (passive safe standard) and proving of ground bearing pressures in accordance with Eurocodes 7.

On several recent NH schemes, the design of these minor structures has been coming under greater scrutiny by NH and the design review service providers. This has required a more comprehensive and rigorous assessment and a package of supporting information and calculations to be produced as part of the design process.

In response to this, the Vectos Infrastructure team has undertaken a detailed review of CD 354 and how it should be used and applied. As part of this process, we have created several semi-automated and intuitive spreadsheets so that the assessment and design of the foundations for road traffic signs and lighting columns can be undertaken, assessed and evaluated quickly and efficiently, and so potential solutions and issues can be identified and resolved effectively. These spreadsheets have also been integral in providing information to geotechnical specialists for the production of geotechnical design reports and gaining approval of proposals from NH in a timely fashion.

Not only do the CD 354 assessment spreadsheets enable us to complete the assessments accurately, quickly and efficiently on behalf of our clients; they also further benefit clients and contractors as the designs produced are optimised to maximise capacity and therefore minimise foundation depth and size saving on material and construction time. They also benefit the overseeing organisations as they can check and verify the proposed designs more quickly and easily, as they have red, green and amber highlighted results.