Highway Structures Design

This article outlines the design process for Highway Structures in the United Kingdom. Similar processes apply in the Republic of Ireland.

Introduction

Highway structures design is dealt with in Volumes 1, 2 and 3 of the Design Manual for Roads and Bridges (DMRB). Highway structures include:

• Bridges
• Sign Gantries
• Retaining Walls
• Culverts
• Earth retaining structures - either walls, or reinforced earth
• Cantilever mast for traffic signal and/or speed camera.
• Lighting column.
• High masts of more than 20m in height
• Mast for camera, radio and telecommunication transmission equipment.
• Catenary lighting support system.
• Environmental barrier.
• Proprietary manufactured structure.
• Highway signs on posts of more than 7m in height,
• Multi-level public car parks proposed for adoption by Roads Service (in Northern Ireland only).
• Mass gabion steep slope/retaining structure
• Reinforced/strengthened soil/fill structure where hard facings are not provided and the face inclination exceeds 45 degrees (in Wales only).

Note that definitions and design standards vary slightly amongst the constituent parts of the United Kingdom.

Highway structures are defined in one of 4 categories, ranging from Categories 0 to 3 inclusive, and they each have different approval, design and checking requirements.

Category Definitions

Categories are defined in BD2/05 Section 3.4. Typical examples of these structures are included below. Some highway authorities will class structures in a higher category than that listed in DMRB and advice should be sought from the highway authority from the outset.

Category 0 Structures

These are generally the smallest and simplest of structures, which conform to DMRB with no Departures from Standard and comply with Category 0 listing in BD2/05. Examples include:

• Small culverts with more than 1m cover to the top.
• Most environmental barriers
• Most lighting columns and CCTV masts
• Most sign installations
• Most traffic signal mast arms
• Small simply supported structures, less than 5m in span.
• Low retaining structures

Category 1 Structures

Structures, other than those in Category 0, which have no departures from standard, provided they also appear in the Category 1 listing, including:

• Simply supported span structures less than 20m in length and less than 20 degrees skew
• Retaining structures up to 7m in height
• Other lighting columns, CCTV masts, larger mast arms etc.
• Culverts up to 8m span

Category 2 Structures

Structures not in categories 0, 1 or 3.

Category 3 Structures

Any structure requiring sophisticated analysis techniques, or which comprise any of the following:

• Span greater than 50m
• Skew greater than 45 degrees
• Complex foundations
• Grouted duct post-tensioned concrete structures (e.g. Hammersmith Flyover)
• Bridges with suspended decks
• Steel decked bridges
• Moveable bridges
• Retaining Walls greater than 14m in height

Complex structures requiring sophisticated analysis

Approval In Principle (AIP)

Following the collapse of a number of structures in the early 1970s, the Ministry of Transport introduced the requirement that for all but the simplest of structures, the designer should first of all obtain Approval In Principle for the type and method of design and checking process to be adopted for the proposed structure. A formal AIP submission is required for any structure which falls into Categories 1, 2 and 3.

The full AIP process is set down in BD2/05, but a typical submission would include a description of the proposed structure; the category of the structure; details of the road it is on or adjacent to; the proposed loading criteria; proposed method of analysis of the structure; a schedule of applicable design standards; requirements for road restraint systems (parapets and safety fences); headroom requirements; details of other structural forms considered; details of any proposed Departures from Standard and any other information required by the Technical Approval Authority to determine whether the proposed design and checking regime is robust and acceptable.

In theory, prior to commencing design, the AIP must be signed by the Technical Approval Authority. In practice, programme constraints dictate that some design is carried out prior to obtaining a signed AIP, though this is at the designer / client's risk as the Technical Approval authority could require a change to the design process, resulting in the need to revisit the design.

Design Process

Designs are carried out in accordance with the process agreed within the AIP and the standards outlined in Volumes 1, 2 and 3 of DMRB. A completed design is certified by the Design organisation, stating that the design has been prepared in accordance with the AIP and applicable standards.

Checking Process

All structural designs must be checked by another engineer prior to construction, and a check certificate produced to demonstrate that this has been done.

Category 0 and 1 structures may be checked by another engineer from within the design team who has not worked on the design. Category 2 structures may be checked and certified by another engineer from within the design organisation, but they must be from a different design team, i.e. independent of the originating team. Category 3 structures must be checked and certified by an engineer from a different organisation from the design team.