The Approval In Principle (AIP) document outlines the concept for the design of the structure.
This will be used for most highway structures and incorporates the Technical Approval Schedule (TAS) which lists all the current British Standards and documents that are relevant to the design of highway structures.
According to BD2/12 (Document: Design Manual For Roads And Bridges (DMRB) ):
Summary of what an AIP is
An AIP is a standard documentation required for any structure (e.g. Bridge, retaining wall, gantry, etc.) constructed in the civil engineering industry.
This document will 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
The 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
Conceptual drawings (if applicable)
Details of any references from Standards and any other information required by the Technical Approval Authority to determine whether the proposed design and checking regime are robust and acceptable.
In theory, prior to commencing design, the AIP must be signed by the Technical Approval Authority. In practice, program 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.
The History of the AiP
In the early 1970s, failures at Yarra (Australia), Milford Haven (Pembrokeshire, Wales), Koblenz (Germany), and over the Danube (Austria) occurred during erection.
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Resulting from these failures and the subsequent Report of the Merrison Committee, the following important changes were made by the then Ministry of Transport:
(i) The Department would continue to examine design criteria and methods but not computations.
(ii) The requirements by the Department for a certificate of independent check of the design and computations.
(iii) The application of Approval in Principle (AIP) stage to all but minor structures, which would cover the selection of bridge type, the materials for its construction, and methods of analysis and design to be adopted.
Technical Approval
(BD2/12 - 2.26) The Designer must provide sufficient information to enable the TAA to carry out the following aspects, where applicable:
(i) Appraise the proposed design or assessment criteria, principles and methods.
(ii) Agree on the required working life for the structure and its main components.
(iii) Agree on the Category of the Proposals.
(iv) Ensure consideration has been given to any special studies concerning safety and risk assessment and management that have a bearing on the final design or assessment or the construction process.
(v) Be satisfied that adequate consideration has been given to safety, sustainability, buildability, traffic management, environmental impact, aesthetics, structure robustness, durability, maintainability, access and inspection, upgradeability, whole life costs, demolition and compliance with the Overseeing Organisation’s requirements.
(vi) Agree on the list of documents included in the TAS and Departures.
(vii) Appraise the geotechnical conditions and other relevant investigations.
(viii) Appraise the adequacy of existing records and investigation data and the need for further investigations or studies that have a significant bearing on the preliminary or final design, assessment, execution, operation, maintenance or demolition processes.
(ix) Review the adequacy of consultation with other stakeholders and the incorporation of agreed requirements.
(x) Agree proposed Category 3 Checker based on their relevant experience and competence.
(xi) Resolve any point(s) of difference between the Designer or Assessor and the Checker.
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Checking Procedure
(BD2/12 - 2.32) Assessments, designs, and drawings, together with bar bending schedules, must be checked as follows:
Structures, which conform in all aspects of design, assessment, and execution to DMRB and MCHW Standards and contain no Departures, provided they also conform to one of the following:
Category 0 and 1 Structures
(a) Categories 0 and 1 require an independent check by another engineer who may be from the Design/Assessment Team.
3.4.1 Category 0:
(a) Single span simply supported structures with a span of less than 5m.
(b) Buried concrete boxes, buried rigid pipes, and corrugated steel buried structures of less than 3m clear span/diameter and having more than 1m cover.
(c) Multi-cell buried structures, where the cumulative span is less than 5m, and having more than 1m cover.
(d) Earth retaining structures with an effective retained height of greater than 1.5m (1.0m or greater in Northern Ireland) but less than 2m.
(e) Minor structures within the scope of BD 94 (DMRB 2.2.1) and not situated at a very exposed site as defined in BD 94.
(f) High masts ≤25m and not situated at a very exposed site as defined in BD 94.
3.4.2 Category 1:
(a) Structures with a single simply supported span of 5m or greater but less than 20m and having less than 25° skew.
(b) Buried concrete boxes, buried rigid pipes, and corrugated steel buried structures with a clear span/diameter of 8m or less.
(c) Earth retaining structures with an effective retained height of 2m or greater but less than 7m.
(d) Minor structures outside the scope of BD 94 (DMRB 2.2.1) or situated at a very exposed site as defined in BD 94.
(e) High masts >25m or situated at a very exposed site as defined in BD 94.
(f) Environmental barriers 3m or more in height or with overhangs.
(g) Portal and cantilever sign and/or signal gantries with a span of less than 20m.
Category 2 Structures
(b) Category 2 requires a check by a Check Team, which may be from the same organisation but must be independent of the Design/Assessment Team.
Structures, not within the parameters of Categories 0, 1 or 3.
Category 3 Structures
(c) Category 3 requires a check to be carried out by a Check Team from a separate organisation proposed by the Designer or Assessor and agreed by the TAA.
Complex structures, which require sophisticated analysis or with any one of the following features:
(a) High structural redundancy.
(b) Unconventional, novel or esoteric design aspects.
(c) Any span exceeding 50m.
(d) Skew exceeding 45o.
(e) Difficult foundation problems.
(f) Moveable bridges.
(g) Moveable inspection access gantries, gantry rail and gantry support systems.
(h) Bridges with suspension systems.
(i) Steel orthotropic decks.
(j) Internal grouted duct form of post-tensioned concrete structures.
(k) Earth retaining structures with an effective retained height of 14m or greater.
(l) Rock anchorages (Wales only).
Sources: BD2/12, www.sabre-roads.org.uk