2 Description of the Existing Bridge 2.1 General Arrangement 2.2 Original Functionality 2.3 Main Bridge Deck System and Articulation 2.4 Towers

2 Description of the Existing Bridge

2.1 General Arrangement

The General Arrangement of the existing bridge is, in general, taken from the Jacobs / Faber Maunsell drawing (see Ref 7 in Appendix B). There are some minor variations between this drawing and the account given in the ICE report (see Ref 1 in Appendix B). Extracts from this drawing, supplemented by data from the ICE report, follows with the key dimensions assumed in this study.


Key dimensions in elevation:


Main Span:

1006 m

Side span:

408 m


Height of tower above base:

150.0 m (taken from ICE report p.339)


Elevation on Tower Cable IP:

156.100 m

Elevation on Side Span Cable IP:

47.900 m

Elevation on Mid-Span Cable IP:

64.700 m

Height of cable in main span:

91.4 m (given as 90.5 in ICE report p.379)

Height of cable in side span:

108.2 m

Sag of Cable in main span:

91.4 m (given as 90.5 in ICE report p.379)

Sag of Cable in side span:

20.3 m (taken from ICE report p.379)


Length of centre hanger in main span:

2.44 m (taken from ICE report p.379)


Elevation on centre line of truss at tower:

53.4 m (approx)

2.2 Original Functionality

The existing layout comprises two carriageways of 24'0" (7.3 m) and two footways / cycle tracks of 15'3" (4.65 m) width across the entire bridge.

On the main bridge, there is a 10'0" (3.05 m) median and the carriageway and footway are separated by 14'9" (4.5 m) to accommodate the towers and suspension cable system.

An air gap separates the footways from the carriageways. The carriageways are also separated by an air gap below the median, which is covered by an open grillage structure for safety.


On the approach viaducts the median remains at 10'0" (3.05 m) but the verge between the carriageway and footway is reduced from 14'9" (4.5 m), at the side span towers, to 5'0" (1.525 m) beyond. This transition is believed to take place within one span of the approach viaducts.


2.3 Main Bridge Deck System and Articulation

The deck system of the main bridge is separated into two distinct parts: the primary support and the deck systems.

2.3.1 Primary System

The primary support system consists of two parallel continuous warren type trusses with a nominal panel length of 60'0" (18.124 m - figures in brackets taken from reference 1). The truss is supported at the ends of each panel by cable hangers clamped to the main suspension cables. The truss is 27'6" (8.382 m) in depth on the centres of the top and bottom chords. The trusses, and their suspension system, are 78'0" (23.774 m) centre to centre in plan. Diagonal bracing in plan is provided to both top and bottom planes. Additional lateral bracing members are provided to the top plane coincident with the deck cross frame elements. The rectangular truss forms a box system designed to carry bending and shear in both vertical and horizontal planes as well as torsion about its longitudinal axis.

2.3.2 Deck System

Cross girders, and their associated frames, are at nominal centres of approximately 30'0" (9.062 m). The cross girders span transversely onto the vertical posts of the truss. These frames, in turn, support the road way and footway panels, which span longitudinally between cross frames. The deck system is of the non participating type. i.e. It is designed to carry only its own weight and local loads applied to it - it does not participate in the global behaviour of the main truss. The road way panels span longitudinally between the cross frames with joints every second frame offset from the frame by approximately 5'0" (1.524 m). The footway panels, which also span longitudinally, are jointed at every frame with the joints lying above the frame.

The deck panels are stiffened plates (trough type stiffeners) in the main span and reinforced concrete slabs in the side spans. The footway panels are stiffened plates (flat type stiffeners) throughout. In all cases longitudinal steel stringers transfer the loads to the cross beams.

The road panels were originally surfaced with 1½ " (38 mm) of asphaltic type surfacing. The footway panels were protected by a thin rubber bitumen coating. It is assumed that the current surfacing components are similar and of the same weight.

The deck features three longitudinal air gaps The first separates the two carriageways along the median. The other two separate each footway from the carriageway. The ICE paper mentions that these slots are required for aerodynamic stability and it is assumed, therefore, that they should be maintained.



2.3.3 Articulation

The road decks are provided with local movement joints at approximately 60 feet intervals. The footways are jointed every 30 feet, approximately. In this way the local expansion/ contraction of the deck is effectively isolated from that of the main truss.

The main truss has movement joints at the side span piers (referred to as 'towers' in the ICE report) and at both sides of both main span towers. The joints at the main towers permit longitudinal movement along with rotation in elevation and plan. Those at the side span piers allow only rotation in both elevation and plan.


The vertical restraint to the truss at the main towers is provided by pendulum links which connect the lower chords of the main truss to brackets attached to the side faces of the towers. These links are designed to carry both tension and compression forces and thus cater for normal reactions, uplift and torsion.

Lateral restraint at the main towers is provided via a bearing connected to the last cross frame of the girders and, via diagonal bracings, to the cross frame at the tower. This bearing also permits longitudinal movement between the girders and the towers. At the side span piers, a similar centralised pin exists - but in this case longitudinal movement is restrained.

(a) Carriageway Movement Joints to the Main Towers

The main truss frames terminate either side of the main towers. The end cross frames of each of the side and main spans are at approximately 28'8" (8.74 m) centre to centre. A transom beam connects the two legs of the towers transversely. This beam supports a frame system in plan which provides lateral restraint to the main girders. A space of approximately 8.1 m longitudinally by 2.1 m deep is available below the existing carriageways to accommodate the 'Demag' joint system. This is illustrated in the following sketch extracted from Fig 2.25 of the ICE paper. It is believed that these joints are due for replacement following over 40 years of service.


(b) Carriageway Movement Joint at the Side Piers

More conventional carriageway 'metal comb type' movement joints are provided at the end of the side spans to allow for the small movements at deck level that result from rotation of the girders.


2.4 Towers

The tower section at deck level is 2.895 m (9' 6") wide on the main box section, which is symmetrical about the cable plane, and 3.657 m (12' 0") overall on the stiffening rib. Thus, from the cable plane, the tower extends laterally 1.45 m on the outer side and 2.21 m on the inner (traffic face) side.