7 The Rail Movement Joint 7.1 The issue 7.2 Evidence of Other Existing Bridges 7.3 Conclusion

7 The Rail Movement Joint

7.1 The issue

The largest movement joint on the Forth Road Bridge (footway to main span) is described in the ICE report as providing for 6 ft (1830 mm of longitudinal movement). It is understood that this includes allowance for plan rotation as well as vertical rotation and axial movement components.

With the original design loading, the joint at the main towers was required to accept, in addition, around 0.044 rads (0.022 main + 0.022 side) in elevation and 0.033 rads (0.028 main + 0.005 side) in plan.

Additional study work has indicated that the rotations may be in a similar order for Light Rail operations.

Whilst the rotations are tolerable for road traffic, they require a special joint for railway use. There is around 8 m of space available to account for this change in slope.

7.2 Evidence of Other Existing Bridges

A useful summary table is contained in the East Corridor HCT report (ref 5 in the appendix), to which is here appended the predicted Forth Bridge values:

Movement	I-90 Bridge	Tagus Bridge	Sky Train Bridge	Forth Bridge
Longitudinal displacement	1250 mm	3050 mm	670 mm	2130 mm *
Horizontal rotation	+/- 0.019 rads	not known	not known	+/-0.033 rads**
Vertical rotation	+/- 0.038 rads	+/-0.060 rads	+/- 0.013 rads	+/-0.030 to 0.038 rads***

*Estimated. A figure of 6 ft (1830 mm) is quoted in the ICE report and is assumed to be the total range of movement at the main span side at the outer edge of the footway. An allowance for side span movement (mainly thermal plus deck rotation) has been added to this figure. It is understood that the ICE figure includes allowance for plan rotation as well as vertical rotation and axial movement components. The rail joints, which are located close to the centre of the bridge, are not as sensitive to the axial effect resulting from plan rotations.

** Based on ICE report

*** Lower figure for single track; higher figure for twin track rail loading

This comparison indicates that the order of magnitude of axial movement and rotation required can be achieved with a rail joint system. At the Tagus Bridge, approximately 8.5 m between joints on the main bridge and approach span was available. A similar space is available at the Forth bridge - so a solution seems entirely possible.

The Forth Joint and Tagus Joint are compared in the following diagrams:

Forth Joint: approximately 8.7 m space available

Tagus Joint: approximately 8.5 m space provided

7.3 Conclusion

Sufficient space is available to accommodate the joint. Existing precedent, particularly that of the Tagus bridge, would indicate that the technical issues associated with a rail movement joint can all be addressed. The joint would require development and testing in the normal way - but is entirely feasible.