The existing Forth Road Bridge (FRB) forms a key link in Scotland’s transport network. The crossing currently carries some 66,000 vehicles per day which includes over 70 percent of travellers across the three Forth bridges (Kincardine, Forth Road Bridge and Forth Rail Bridge).
In 2007, the Scottish Ministers announced that a Replacement Forth Crossing would be promoted by the Scottish Government. Previous work undertaken by Transport Scotland included consideration of alternative corridors and structures for the Replacement Forth Crossing and on 19 December 2007, the Scottish Ministers announced that the Replacement Forth Crossing would cross the Firth of Forth immediately upstream of the existing Forth Road Bridge and would be a cable-stayed bridge.
The Jacobs-Arup commission is for the management and delivery of the Replacement Forth Crossing Project inclusive of all roads and other infrastructure associated with such a structure.
Recent investigations of the cables of the existing bridge have indicated that the working life of the bridge, given the current level of traffic demand, may be longer than projected at the time of the commission. A 'twin bridge strategy' could, therefore, be advantageous.
A 'twin bridge strategy' envisages that the existing Forth Road Bridge has a useful ongoing potential and may, therefore, be included in the functional strategy for the construction of a new Forth Bridge. The existing bridge may potentially be considered to carry any of the following functions:
- A reduced volume of unrestricted road traffic
- Light road traffic, excluding heavy vehicles
- Pedestrian / cycle traffic.
- Light rail / tram traffic.
This report reviews the structural implications of this proposal.
The report has been prepared by Arup on behalf of the Jacobs Arup Joint Venture with support from Flint & Neill, and draws upon information that is within the public domain. In draft form, it was audited by Faber Maunsell. This process is recorded in the separate report "Forth Road Bridge - Audit of Feasibility of Future Multi-Modal Use – Summary Report", by Faber Maunsell, issued as Revision 4, third issue on 27/11/2008. The two reports should therefore be read in conjunction with each other.
1.2 Scope of this report
The feasibility of possible structural options available to accommodate various reconfiguration strategies is considered. The discussion is generally limited to the physical arrangement of the deck system and consideration of the bridge articulation. The analysis of the main girders, suspension system, towers and foundations are excluded at present from the review - except by simple comparison with estimated actions for the current status quo. The premise of this report is, therefore, 'are any of the proposals likely to make the present state worse?'.
1.3.1 On General Loading
It is known that the existing bridge has been calculated to have a reduced design capacity in the main cables. Intervention in the form of cable dehumidification is presently under way. This report has been prepared on the basis that the dehumidification halts the deterioration in the cable such that the global factors of safety remain acceptable. It is assumed, therefore, that the effects resulting from the general level of static loading on the bridge should not be increased, and in some areas (such as main cable tension) a decrease would be desirable.
1.3.2 On Permanent Applied Loading
As far as possible, it is preferable that the current geometric state of the bridge under permanent loading be maintained. Thus refurbishment schemes should, as far as possible, attempt to maintain equality with the current level of permanently applied dead and superimposed dead loading.
1.3.3 On Aerodynamic Loading
The bridge, as designed, features three vent areas in the deck surface. In order to maintain the present aerodynamic performance these vent areas should be maintained. The aerodynamic performance is also sensitive to detailed cross section changes; this will have to be kept in mind when considering any changes to parapets or barriers.
1.4 Functional Definitions
1.4.1 Unrestricted Vehicular Road Traffic (four or two lanes)
Previous studies (see Ref 3 in Appendix B) have been carried out based on the actual traffic moving across the bridge and a Bridge Specific Assessment Live Loading (BSALL) developed. This BSALL is based on four lanes of unrestricted road traffic on the bridge.
As part of this study, traffic load models have been derived to give an indication of the intensity of load if only 2 lanes of traffic (one in each direction) were permitted along the bridge.
Traffic models that are based on traffic flow without any restriction being placed on different vehicle types is referred to as 'all inclusive' in this report.
1.4.2 Restricted Vehicular Road Traffic (four or two lanes)
A number of strategic options may be considered in which, for instance, only high occupancy vehicles and/or buses were permitted to use the bridge. The intensity of loading associated with this type of restriction has been characterised as being associated with vehicles having only two axles. This is a reasonable characteristic to enforce as it may be tested by a simple visual inspection of vehicles.
Whilst some modern bus and coach types, may have twin back axles (i.e. 3 axle rigid configuration), the total loading per metre length of bridge associated with this type is not believed to be necessarily any larger than the loading associated with twin axle types.
Approximate traffic models have been derived to give an indication of the intensity for this type of load as applied to both 4 and 2 lane operation.
This data, which relates to 'restricted' or 'reduced' traffic flow, is also referred to as 'two axles' in this report.
1.4.3 Pedestrian and Cycle Traffic
The existing bridge currently carries unrestricted pedestrian and cycle traffic. It is assumed that this situation will be maintained. There does not appear to be any corroborative data for the intensity of pedestrian and cycle traffic currently using the bridge. However, in recent assessment work it is understood that a total load of 0.3 kN/m has been allowed (regardless of loaded length) in combination with the BSALL vehicular loading. This represents around 1000 people distributed along the bridge at one time. This study assumes that the same assumptions may be applied in comparison between various options.
1.4.4 Light Rail/Tramway Traffic (twin or single tracks)
The distinction between light railways and tramways is a little obscure. The Office of Rail Regulation (ORR) recommends that a tramway system which is completely isolated from pedestrian and vehicle access is better treated as a railway for the purposes of planning and safe operation.
In this study, the two types of traffic are considered interchangeable in terms of loading on the bridge. However, one important requirement for a tramway is that the power delivery system must be isolated from the public. This is traditionally achieved using an Overhead Live Electrification (OLE) system and associated catenary support masts. An isolated light railway, on the other hand, may also utilise systems such as a third rail which does not require a catenary support structure. Thus in the report, a distinction is made where space for an OLE system is included in the functional arrangement.
1.5 Analytical Studies
In order to carry out this initial feasibility work, a preliminary two dimensional analytical model for studying the global behaviour of the bridge has been set up. This model has been based on some basic survey data on the existing bridge but mainly upon data given in the original ICE report published just after construction (See Reference 1 in Appendix B). It is known that some strengthening work and modifications have been made since opening the bridge. The major change is the addition of a new prestressed strut in the centre of each tower leg - which has little effect on bending stiffness. It may be assumed that these modifications do not materially affect the global static behaviour within the scope of the objectives of this study.
The behaviour of the model in respect of deflections, rotations and moments in the girders under the original BS 153 loading have been compared with the results given in the ICE report. Good agreement with these values was achieved for the vertical loading conditions considered. Further analyses will be necessary (using the current construction information) to be able to confirm the effects of twisting and lateral loading. However, it is not anticipated that this will alter the conclusions of this report.