7. Assessment of Options for Approach Spans
7.1 Three Corridor Deck Options
An orthotropic deck has not been considered for the approaches due to the high cost of the steelwork which would be required. Whilst the additional expense is justified for the cable stayed bridge where significant savings can be made in the stay cables and tower due to the lighter deck weight, the same savings are not achievable in the approaches.
There is no argument related to visual continuity of the main spans that could suggest that an orthotropic deck be adopted in the approaches despite the higher cost since the composite deck option for the approaches is aesthetically the same as an orthotropic deck option.
7.1.2 Composite Box Girder
The natural choice for the approach spans is to continue with the composite deck section right through to the abutments. This has clear advantages for the visual continuity between main bridge and approach. It also offers the potential of structural continuity between the two elements. Composite construction is generally a competitive economic solution for the 80 - 90 m spans proposed here. However, due to the large width and use of stiffened steel plate soffit, it is not clear if it maintains this economy for this particular configuration. Composite construction offers the option of push launching from the abutments. The approach bridge for this option is illustrated in Drawing FRC/C/076/S/301
7.1.3 Concrete Box Girder
Although the composite deck approach spans is undoubtedly aesthetically favourable since it provides perfect visual continuity with the main span deck, this may result in increased cost compared to a concrete deck approach bridge. A concrete option has therefore been developed to allow comparative cost estimates to be carried out and allow the cost-benefit of the composite option to be assessed.
The prestressed concrete option has been designed to provide a reasonable balance of aesthetics and cost. The key driver for aesthetics is of course visual continuity with the main spans and whilst this cannot be achieved as perfectly as for the composite box girder option certain measures can be adopted in the design which provide for a reasonable synergy between the cable stayed bridge and the approaches:
- Continuity of visually striking edge detail
- Constant structural depth (and structural depth the same between approaches and main spans)
The approach bridge for this option is illustrated in Drawing FRC/C/076/S/321
7.2 Double Level Deck Options
For the double level options, the only aesthetically viable solution is for the approaches to be a truss of the same form as that adopted for the cable stayed bridge.
For the Warren Truss options, whether Two Plane or Four Plane, the superstructure is well suited to being continued into the approach spans with no discernible variation in structural form. Spans of 144 m are considered to be the most economic solution, albeit with shorter end spans.
The Vierendeel Truss is less well suited to the approach spans on account of its relatively poor capability to carry large global shears arising from self weight effects. As a result the end bays of the truss are provided with diagonal web members. These are orientated to act in tension and as such can be relatively slender. Nevertheless, the appearance of the structure is unavoidably modified.
The approach bridge for these option are illustrated in Drawings FRC/C/076/D/341 and FRC/C/076/D/342
7.3 Pier Forms
The pier is a very important feature in providing visual continuity between the cable stayed bridge and the approach spans. The factors which have been addressed are:
- Relationship between pier form and tower form
- Cable stayed bridge side span piers read as a continuation of the approach bridge piers
- Any pier widening necessary to accommodate movement joints to be carefully considered with a view to the movement joint pier being read as the same as the typical pier
- Rhythm of the pier spacing
- Accommodation of the large variation in pier height when the approach bridge is over land.
7.3.1 Three Corridor Option
Approach Pier Form
Both the Needle and the Inverted Y towers have common features of curved faces, inclined legs and a connection between the legs at water level. These features are adopted in the approach piers. The small variation in pier height over water due to the vertical profile of the bridge is planned to be accommodated by cropping the top of the pier so that the shape of the pier remains constant. The large variation in pier height over land is achieved by varying the inclination of the legs which results in a small number of unique pier shapes.
7.3.2 Double Level Option
The approach span piers for the Warren and Vierendeel Truss options consist of simple rectangular sections tapered in both elevations. In transverse elevation the inclination of the tapered edge matches the inclination of the main tower legs. The pier cross section will be of hollow reinforced concrete construction.
Approach Pier Form