11. Preliminary Consideration of Assessment of Anticipated Departures From Standard 11.1 Functional Cross Section 11.2 Post-tensioned grouted ducts
11. Preliminary Consideration of Assessment of Anticipated Departures From Standard
The UK is in the process of adopting Eurocode as the basis of structural design. The two year transition phase to the full use of Eurocode in the UK runs from April 2008 to April 2010 during which time Approval in Principle (AIP) documents for bridges may be submitted either in accordance with the old bridge code, BS 5400, or in accordance with Eurocode as modified by the UK National Annexes (NAs). Some National Annex documents remain unpublished. The design of the Forth Replacement Crossing will be in accordance with Eurocode, and a project specific Design Memorandum will document the design rules adopted and include supplementary rules which will complement Eurocode.
Requirements for the main carriageways are based on a dual 2 lane urban motorway, but hard shoulders of 3.3m width, rather than the required 2.75m width will be incorporated.
The Multi-Modal Corridor when operated as HOV lanes does not correspond to a standard Road Type. It is proposed that the Multi-Modal corridor shall consist of 3.65m lanes separated by a central reserve incorporating a VRS. A total width between the faces of the central and nearside VRS of 6,000 mm minimum will be generally provided to allow additional width in the event of a vehicle breakdown.
A Departure From Standard is anticipated regarding the VRS set-back requirement at the central reserve from the value of 1200mm given in Table 4.11.13 of TD 27/05 for central reserves to a value of 500mm which is the setback required for the working width of the VRS. The justification for the departure is:
- Significant cost associated with provision of an additional 1.2m width of bridge deck for this long span cable supported structure
- The multi-modal corridor does not include an adjacent lane but does include an adjacent hardstrip. Furthermore the HOV usage will not include wide vehicles. Therefore vehicle positioning may be towards the nearside white line and the reduced offside VRS setback will not have a significant effect on driver behaviour and driver shyness.
A further Departure From Standard is anticipated regarding the VRS setback at the towers for the Three Corridor Option. The hardstrip is discontinuous at the towers and generally the minimum VRS setback would be 1200mm. A locally reduced value of 350mm is proposed. The justification for the departure is:
- Significant cost associated with provision of an additional 1.7m width of bridge deck for this long span cable supported structure
- HOV usage will not include wide vehicles
- The reduced value is localised and the general VRS setback will be compliant.
Internal post-tensioned grouted ducts are under consideration in two locations:
- transverse prestressing in the concrete slab of the Composite Deck
- longitudinal cantilever prestress in the deck of the Concrete Approach
A moratorium on the use of post-tensioned grouted ducts was lifted in 1996 but certain restrictions remain in place which are described in Interim Advice Note 47/02 which makes reference to the Concrete Society Technical Report TR 47 (2002). IAN 47/02 notes that the design of the post-tensioning system will be classed as an aspect not covered by standards, and subject to departure procedures.
A moratorium for precast concrete segmental construction using internal grouted tendon systems still remains in force. Although precast slabs are under consideration for the Composite Deck this moratorium is not relevant since the in-situ stitches allow effective splicing and continuity of the duct.
However, the moratorium is relevant to the longitudinal cantilever prestress in the Concrete Approach. Preliminary discussions with Transport Scotland indicate that a Departure From Standard to allow construction of the approach using internal tendons in precast segmental construction would not be approved. Therefore the design will be progressed assuming in-situ construction.