5 Conceptual Options 5.1 Assumed Fundamental Constraints 5.2 Measures to Mitigate Load Changes 5.3 Functional Constraints 5.4 Particular Measures 5.5 Catalogue of Functional Options for the Main Bridge 5.6 Main Bridge – Changes in Dead Load 5.7 Approach Spans – Changes in Dead Load 5.8 Summary – Changes in Dead Load
5 Conceptual Options
5.1 Assumed Fundamental Constraints
It is assumed that:
a) No significant structural modifications will be made to the main bridge girders, hangers, suspension cables or tower supports. Modification to the cross girders and deck system is acceptable as these do not form part of the primary bridge system.
b) The aerodynamic properties of the bridge deck shall remain essentially undisturbed. It is preferable that the air vents be maintained at a similar permeability to the present design. However, some encroachment may be permissible. It will be necessary to check that aerodynamic stability criteria are met. (Which could be done by subsequent wind tunnel testing).
c) The articulation of the bridge will be retained.
d) The design life of elements of the bridge, in particular the suspension cables, are currently under review. The exact details of this review are not precisely known to us. It is a prime assumption, therefore, that any modification should attempt to not significantly increase the static load effects beyond those which are currently applicable.
5.2 Measures to Mitigate Load Changes
5.2.1 Static Loads (Permanently Applied Loads and Live Load)
Load increases can be mitigated in several ways:
a) Controlling the traffic on the bridge.
b) Removal of existing structural components and replacing with lightweight elements.
c) Removing / replacing existing live load facilities with alternative facilities
5.2.2 Environmental Loads (Wind and Temperature Loads)
It is presumed that the wind climate will not have changed significantly.
The temperature effects will not have changed significantly from those for which the bridge was designed.
It is, therefore, assumed that the response of the bridge under environmental loads will be similar both before and after any modifications.
5.2.3 Deformations
The most critical deformation is likely to be the rotation (in plan and elevation) of the movement joints at the main towers and abutments - particularly for railway usage.
It may be possible to alter the deck girders in the support regions to make them continuous for moment to reduce rotational displacements at the main joints. A system of piston type shock absorbers could be arranged to permit slow acting thermal movements but restrain short duration live load movements.
For the purpose of this study it is assumed that the articulation should remain as it currently is.
5.3 Functional Constraints
5.3.1 Foot and Cycle Use
There are no known constraints. The existing footways appear to be adequate for the expected pedestrian volumes.
5.3.2 Road Use
Elements of the bridge are known to be deteriorating at a greater rate than originally projected (i.e. when first constructed). Ultimately, for the structure to remain viable, the dehumidification will need to arrest this decline and leave it at an acceptable level. It is possible that in order to increase its useful life, the road traffic usage will have to be reduced - particularly heavy goods vehicle types.
Options which eliminate heavy goods vehicles from the crossing are likely to achieve an improvement in design capacity. In the absence of direct traffic control, say by toll booths, it is difficult to police a sophisticated sifting of the traffic flow. One simple measure, which can be policed, is to limit access only to vehicles which have two axles. This criteria is, therefore, employed in estimating the intensity of load applicable to any change in use relating to banning heavy goods vehicles.
The minimum desirable carriageway width, between barriers, for a single lane of traffic flow is 6.0 m. This width is sufficient to allow passage of vehicles in the event of a breakdown, or other blockage, of the running lane. Provision of a carriageway width less than this would represent a departure from current standards.
5.3.3 Rail Use
The particular issues associated with the design of rail systems on this bridge are as follows:
(a) Geometric:
a) Can one or two tracks be accommodated? A range of different configurations have been considered in the study.
b) Can maintenance / emergency walkways be provided? A maintenance / emergency access can be provided for the configurations considered, but clearly there are a number of ways in which this can be achieved.
c) Can OLE systems be accommodated? This will depend upon the type of power supply system required but some configurations have been illustrated.
(b) Design:
a) Rotation of the girders? Particular attention has been given to determining the level of rotations at the tower movement joints to confirm that they are within acceptable limits. Discontinuity of the deflection line occurs at the movement joints. Can rail joints be designed to accommodate these movements?
b) Fatigue demand from rail traffic? This is a consideration for rail structures. This has not been reviewed in this study, but it is not envisaged to be a critical issue. This will have to be confirmed by further studies.
c) Can the central, or outer, roadway vents be reduced or eliminated? The configurations considered in this report maintain the vents, albeit with some incursions, but the impact of covering them over, totally or partially, is worthy of consideration in further studies.
5.4 Particular Measures
5.4.1 Possible reworking of the edge parapet detail
The current edge parapet detail occupies approximately 700 mm of width plus around 150 mm clear to the face of the tower. There is a small opportunity to gain space by modifying this detail to accommodate a modern vehicle parapet system. Approximately 200 mm can be added to each carriageway with this modification. The slight gain in width is unlikely to be justified by the cost of making this modification.
5.4.2 Possible use of the footways to carry railways
The present layout provides for two approximately 4.65 m wide combined footpath and cycle tracks located on lightweight structures outside of the main trusses and outside of the legs of the main towers.
(a) Design Capacity
The original design was made for 4 kN/m2 on the 1.85 m footway and 2 kN/m2 on the 2.75 cycle way, that is: 12.9 kN/m run for the loaded lengths which would apply to the footway or its support brackets. The LR loading will be 20 kN/m with a KEL of, 100 kN plus Dynamic Amplification Factor (DAF) of, say 20%. (based on DLR/Tramway type loading).
The support brackets are at 9.1 m (30 ft) centres. The brackets would attract, say, around 130 kN load each. This gives around 370 kN per bracket - approximately 300 % increase in live load.
By inspection, therefore, the supporting structures of these footways cannot carry rail loading should it be considered a functional option. They would require significant strengthening.
(b) Alignment
The alignment of the footpaths is affected by the transition between main bridge and approach viaduct, as seen in the following aerial photograph:
The transition length appears to be inappropriate for light railway use. Extensive modifications would, therefore, be required to the viaduct structures.
(c) Conclusion
This option is not considered further. The layout is presented as drg 071 in Appendix A for reference purposes only.
5.4.3 Possibility to Remove the Outer Footway
It is possible to consider that the footways are entirely removed and relocated either along the current carriageway or diverted onto the Replacement Crossing. The advantages associated with this otherwise loss of benefit, are as follows:
- Reduce the overall loading on the bridge in its current configuration - both dead and live.
- Change the aerodynamic properties of the bridge, which may be favourable.
This option is not pursed further at this stage as it would require major modification to the existing structure.
5.4.4 Possible Single Track Railway Working
The study has considered both twin and single track arrangements.
The possibility of single track working should be considered at this stage, particularly - especially since it is the only configuration which permits the use of a minimum (according to current standards) 6.0 m wide single carriageway with only minimal modification to the existing bridge deck structure. Cross-overs could be provided outside of the abutments to the crossing, which are approximately 2.5 km apart - about 2 - 3 minutes at 50 - 80 kph. This option has, therefore, been reviewed (see below) and presented in drg 021 in Appendix A.
5.5 Catalogue of Functional Options for the Main Bridge
The following options are presented (refer to the Appendix for the relevant drawings):
Option |
Carriageway |
Footway / Cycleway |
Rail track |
OLE |
Footway in rail corridor |
Drg Ref |
|---|---|---|---|---|---|---|
0* |
2 x 7.30 |
2 x 4.65 |
0 |
n/a |
n/a |
|
1 |
n/a |
2 x 4.65 |
2 |
yes |
yes |
|
2 |
2 x 6.00 |
2 x 4.65 |
1 |
yes |
yes |
|
3 |
2 x 5.18 |
2 x 4.65 |
2 |
no |
no |
|
4 |
2 x 4.81 |
2 x 4.65 |
2 |
yes |
no |
|
5 |
2 x 4.53 |
2 x 4.65 |
2 |
yes |
yes |
|
6** |
2 x 6.00 |
2 x 4.65 |
2 |
yes |
yes |
* This option represents the 'do nothing' base case and is used for comparison with the proposed options listed.
** This option does not strictly conform to the restriction on closing off parts of the roadway vents; the 6.00 m carriageway is reduced to 4.73 m at the towers.
5.6 Main Bridge – Changes in Dead Load
(Refer to section 3 for a breakdown of the loading applied).
5.6.1 Main Bridge – Option 0
No change in Dead Load.
5.6.2 Main Bridge – Option 1 (Twin Track only)
Remove |
Add |
||
14.63m of road surfacing |
-14.6 |
2 track slabs |
+13.4 |
-14.6 kN/m |
+13.4 kN/m |
||
Net Change
+1.2 kN/m (-0.8% increase in main span)
5.6.3 Main Bridge – Option 2 (Single Track/ 2 x 6.00m carriageways)
Remove |
Add |
||
2 vehicle barriers |
-1.4 |
1 rail tracks all inclusive |
+8.0 |
1 grillage |
-1.4 |
2 road barriers |
+2.0 |
1.03m of road decking |
-2.2 |
||
1.63m of road surfacing |
-1.6 |
||
-6.6 kN/m |
+10.0 kN/m |
||
Net Change
+3.4 kN/m (+2.2% increase in main span)
5.6.4 Main Bridge – Options 3 (Twin Track/ 2 x 5.18m carriageways)
Remove |
Add |
||
2 vehicle barriers |
-1.4 |
2 rail tracks all inclusive |
+16.0 |
1 grillage |
-1.4 |
2 road barriers |
+2.0 |
3.25m of road decking |
-6.9 |
||
3.25m of road surfacing |
-3.3 |
||
-13.0 kN/m |
+18.0 kN/m |
||
Net Change
+5.0 kN/m (+3.3% increase in main span)
5.6.5 Main Bridge – Options 4 (Twin Track/ 2 x 4.81m carriageways)
Remove |
Add |
||
2 vehicle barriers |
-1.4 |
2 rail tracks all inclusive |
+16.0 |
1 grillage |
-1.4 |
2 road barriers |
+2.0 |
3.99m of road decking |
-8.5 |
||
3.99m of road surfacing |
-4.0 |
||
-15.3 kN/m |
+18.0 kN/m |
||
Net Change
+2.7 kN/m (+1.8% increase in main span)
5.6.6 Main Bridge – Options 5 (Twin Track/ 2 x 4.53m carriageways)
Remove |
Add |
||
2 vehicle barriers |
-1.4 |
2 rail tracks all inclusive |
+16.0 |
1 grillage |
-1.4 |
2 road barriers |
+2.0 |
4.55m of road decking |
-9.7 |
||
4.55m of road surfacing |
-4.6 |
||
-17.1 kN/m |
+18.0 kN/m |
||
Net Change
+0.9 kN/m (+0.6% increase in main span)
5.6.7 Main Bridge – Option 6 (Twin Track/ 2 x 6.00m carriageways)
Remove |
Add |
||
4 vehicle barriers |
-2.8 |
2 rail tracks all inclusive |
+16.0 |
1 grillage |
-1.4 |
4 road barriers |
+4.0 |
4.55m of road decking |
-9.7 |
3.93 of road decking |
+8.4 |
4.55m of road surfacing |
-4.6 |
3.93 of road surfacing |
+3.9 |
-18.5 kN/m |
+32.3 kN/m |
||
Net Change
+13.8 kN/m (+9.0% increase in main span)
5.7 Approach Spans – Changes in Dead Load
Removal of existing median = 0.15 x 3.05 x 24 = -11 kN/m
Removal of existing verge = 0.15 x 1.0 x 24 = -3.6kN/m
Track slabs = allow +6.7 kN/m / slab including rails etc excluding waterproofing.
Waterproofing, assume 25mm of asphalt = 22.5 x 0.025 = 0.56 kN/m_
5.7.1 Approach Spans – Option 0
No change in Dead Load.
5.7.2 Approach Spans – Option 1 (Twin Track)
Remove |
Add |
||
14.63m of road surfacing |
-14.3 |
1 track slab |
+13.4 |
-14.3 kN/m |
+11.3 kN/m |
||
Net Change
-1.2 kN/m (-0.8% increase in main span)
5.7.3 Approach Spans – Option 2 (Single Track/ 2 x 6.00m carriageways)
Remove |
Add |
||
1 median |
-11.0 |
1 track slab |
+6.7 |
2.63m of road surfacing |
-2.6 |
2 road barriers |
+2.0 |
4.68m of waterproofing |
+2.6 |
||
-13.6 kN/m |
+11.3 kN/m |
||
Net Change
-2.3 kN/m (-1.5% increase in main span)
5.7.4 Approach Spans – Options 3 (Twin Track/ 2 x 5.18m carriageways)
Remove |
Add |
||
1 median |
-11.0 |
2 track slabs |
+13.4 |
4.25m of road surfacing |
-4.3 |
2 road barriers |
+2.0 |
6.3m of waterproofing |
+3.5 |
||
-15.3 kN/m |
+18.9 kN/m |
||
Net Change
+3.6 kN/m (+2.3% increase in main span)
5.7.5 Approach Spans – Options 4 (Twin Track/ 2 x 4.81m carriageways)
Remove |
Add |
||
1 median |
-11.0 |
2 track slabs |
+13.4 |
5.0m of road surfacing |
-5.0 |
2 road barriers |
+2.0 |
7.0m of waterproofing |
+3.9 |
||
-16.0 kN/m |
+19.3 kN/m |
||
Net Change
+3.3 kN/m (+2.2% increase in main span)
5.7.6 Approach Spans – Options 5 (Twin Track/ 2 x 4.53m carriageways)
Remove |
Add |
||
1 median |
-11.0 |
2 track slabs |
+13.4 |
5.55m of road surfacing |
-5.6 |
2 road barriers |
+2.0 |
7.6m of waterproofing |
+4.3 |
||
-16.6 kN/m |
+19.7 kN/m |
||
Net Change
+2.7 kN/m (+1.8% increase in main span)
5.7.7 Approach Spans – Option 6 (Twin Track/ 2 x 6.00m carriageways)
Remove |
Add |
||
1 median |
-11.0 |
2 track slabs |
+13.4 |
2 road barriers |
-2.0 |
4 road barriers |
+4.0 |
5.6m of road surfacing |
-5.6 |
7.6m of waterproofing |
+4.3 |
1 verge |
-3.6 |
2.92m of road surfacing |
+2.9 |
1.46m of footway surfacing |
-1.5 |
||
-23.7kN/m |
+24.6 kN/m |
||
Net Change
+0.9 kN/m (+0.6% increase in main span)
5.8 Summary – Changes in Dead Load
Option |
Main Bridge |
Approach Spans |
|---|---|---|
0 |
- |
- |
1 |
-1.2 kN/m (-0.8%) |
-1.2 kN/m |
2 |
+3.4 kN/m (+2.2%) |
-2.3 kN/m |
3 |
+5.0 kN/m (+3.3%) |
+3.6 kN/m |
4 |
+2.7 kN/m (+1.8%) |
+3.3 kN/m |
5 |
+0.9 kN/m (+0.6%) |
+2.7 kN/m |
6 |
+13.8 kN/m (+9.0%) |
+0.9 kN/m |