4. Assessment of Functional Cross Section Options 4.1 General considerations 4.2 Assessment of options 4.3 Three Corridor Option Connectivity 4.4 Double Level Option Connectivity 4.5 Comparison
4. Assessment of Functional Cross Section Options
4.1 General considerations
The bridge design must allow for the multi-modal corridor to be used for trams or LRT systems in the future. However, as there are currently no details for the installation of such a system the bridge design must also provide for the corridor to be used in the interim.
Initial concept development assumed that the interim use of the corridor could be either as High Occupancy Vehicle (HOV) or bus lanes. It is now proposed that only HOV use of the multi-modal corridor should be considered. The hardshoulders are widened to 3.3m compared to the 2.75m required for a standard D2UM cross section in order to allow buses to use the hardshoulders during peak hours.
Whilst for bus usage an unsegregated highway cross section on the multi-modal corridor was assumed this is not considered adequately safe for HOV usage. Therefore the HOV lanes will be separated by a central reserve incorporating a VRS.
Locating the footway / cycleway adjacent to and on the same level as the main carriageways was seen to be advantageous since it allows the footway to be used as a refuge for passengers of broken down vehicles as well as providing greater security for users of the footway since they are clearly visible to motorists and the risk of assault is reduced.
4.2 Assessment of options
At the concept design stage the functional cross sections were assessed against a range of criteria comprising:
- Road connectivity
- Multi-modal (Bus/HOV) connectivity
- Multi-modal (Tram/LRT) connectivity
- Operational access to multi-modal (Tram/LRT)
- Traffic crossovers
- Tower aesthetics
- Foundation costs
Both of the selected functional cross sections provide good operational access to the multi-modal corridor with both tracks of the Tram/LRT located adjacent to each other and with sufficient space for maintenance walkways adjacent to the tracks.
The Double Level option has the ability to provide traffic crossovers (contra-flows) between the main carriageways on the bridge which is an advantage but this is not considered a particularly important criterion since the length of the structure is not excessive (crossovers could be arranged beyond the end of the bridge) and wide hard shoulders are proposed which could allow resurfacing without the need for crossovers to allow contra flows.
At the concept stage, tower aesthetics and foundation costs were expected to be slightly better for the Three Corridor option than the Double Level option. This remains true and is further discussed in Sections 6 and 8 of this report.
The key issues for further assessment of the functional cross sections is connectivity, meaning how the motorway corridors and multi-modal corridors connect to the network at either end of the bridge.
4.3 Three Corridor Option Connectivity
Preliminary studies have been carried out regarding the connectivity for the three corridor option. As noted in Section 4.1 above the studies assume that the multi-modal corridor will be used for HOV lanes initially.
4.3.1 Southern Approach
At the southern end of the bridge the main highway alignment is D3M as it approaches the bridge. The central reservation is widened to allow future operation of a tram or LRT system within the centre. For the initial case of HOV usage of the multi-modal corridor, HOV’s will have been streamed into the off side lane on approach to the bridge. On the southern approach viaduct of the Main Crossing the HOV lanes transition from the off side lane of the motorway into the multi-modal corridor reducing the main cross section to D2M by the time it reaches the cable stayed bridge.
If a tram or LRT system is installed in the future then it will run between the carriageways. The approach cross section will still be D3M and will taper to D2M over the length of the southern approach viaduct. The LRT will cross either above or below the southbound carriageway of the motorway with all grade separations occurring off the main crossing.
A schematic alignment of the southern approach multi-modal arrangements is included in Appendix E.
4.3.2 Northern Approach
At the northern end of the bridge the basic concept is similar although the details are slightly more complex due to the interchange immediately behind the northern abutment.
The approach viaducts to the north of St Maragaret’s Hill will provide a D3M cross section on separated structures with sufficient space between the structures to construct a future viaduct to carry the tram or LRT. The gradient of the future viaduct would differ from the road in order to bring the LRT either over or below the northbound carriageway of the motorway.
Lane transitions either streaming the HOV lanes or else tapering the cross section down to D2M will take place on the northern approach spans of the Main Crossing as well as on the adjacent viaducts. The detailing of the viaducts will need to allow for the future change in use but this is anticipated to be relatively straightforward.
A schematic alignment of the northern approach multi-modal arrangements is included in Appendix E.
4.4 Double Level Option Connectivity
4.4.1 Southern Approach
As for the Three Corridor Option, the highway alignment is D3M as it approaches the bridge with HOV’s streamed into the off side lane. This lane is separated from the main carriageway and then drops downwards within a structure between the carriageways in order to be at the lower deck level at the Main Crossing abutment. The width of the structure will have to include for sightlines on the curve.
Provision will be made within this structure to bring a future tram or LRT system beneath the southbound carriageway, into the structure and therefore onto the lower deck of the Main Crossing.
If this is implemented then the D3M section needs to be tapered down to D2M. This cannot be safely done in the tight radius curve behind the south abutment so must take place on the southern approach of the Main Crossing. Therefore the D3M section needs to be carried through to the southern abutment which will require the structure containing the trams / LRT to be contained within a box section rather than being in an open channel. It is anticipated that the roof of the box will be provided when the structure is first built. This means that the HOV lanes need to achieve the lower level alignment a short distance behind the southern abutment. The future conversion would involve construction of a wall closing the end of the box and then filling in of the open channel ramps leading up to the box.
The widening of the Main Crossing itself is achieved by providing structural cantilevers to widen the upper deck.
A schematic alignment of the southern approach multi-modal arrangements is included in Appendix E.
4.4.2 Northern Approach
At the northern end of the bridge the basic concept is similar although the details are more complex due to the interchange immediately behind the northern abutment.
The HOV lanes are brought to a low enough alignment sufficiently far behind the north abutment to allow the main carriageways to converge before reaching the cable stayed bridge.
Conversion of the viaduct structures will be required in the future in order to modify the functional use. The detailing of the viaducts will need to allow for this future conversion which may not be straightforward
A schematic alignment of the northern approach multi-modal arrangements is included in Appendix E.
4.5 Comparison
Connectivity of both of the Functional Cross Section options appears feasible. However the Three Corridor Option is more favourable than the Double Level Option for a number of reasons.
4.5.1 Functionality
For the Phase 1 usage of the multi-modal corridor as HOV lanes the connectivity of the Three Corridor Option is significantly better at the southern end of the bridge since the streaming of HOV’s off the main carriageway onto the multi-modal lanes is on a straight section rather than a curve and takes place approximately 800m further away from the adjacent interchange than for the Double Level Option. This allows safer lane transitions and better connectivity for both northbound and southbound traffic.
Furthermore, the grade separation required for the Double Level Option is likely to result in poor driver perception of the road layout since the lane transitions will involve curved ramp structures and minimum sight lines. In contrast the lane transitions for the Three Corridor Option will be on straight level sections of road with open sight lines.
For the Phase 2 usage with the multi-modal corridor for trams or LRT the connectivity between the two functional cross sections is almost identical. The Three Corridor option is slightly better at the southern end because the tapering of the section from D3M to D2M is further north of the tight radius bend beyond the south abutment.
4.5.2 Cost and disruption
The initial cost of the transition structures required for the Double Level Option will be higher due to the grade separation requirement which results in additional structures. Furthermore the structures for the northern approaches will have to be detailed for future conversion which is likely to require indirect load paths and greater materials.
The cost of construction works associated with converting to the Phase 2 usage as well as the disruption during the conversion is anticipated to be higher for the Double Level Option. In particular for the northern approaches the conversion works will require the demolition / removal of the viaducts which stream the HOV lanes into the multi-modal corridor and the reconstruction of a new viaduct in between the main carriageways.