A6 Toome Bypass - Bridge
Two competing factors influenced the selection of the type of bridge being built:
- the need to provide 3.5 metre navigation air draft above high river level; and
- the need to keep approach embankments as low as possible to reduce visual and environmental intrusion in the environmentally sensitive flat floodplain.
A 'Bowstring' or tied arch design shaped like a bow with the string downward allows a shallow deck to be used which reduces the height of the approach embankments. Bridges of this type are aesthetically pleasing, and give the motorist a feeling of openness and an unobstructed view of the river.
A strong girder ties both ends of the supporting arch together, thus the name 'tied arch'. The girder connects horizontally between the arch springing points, balancing the large horizontal thrusts in the arch. Foundation design is much simpler for a tied arch than it is for an ordinary arch because the horizontal thrust is balanced internally in the girder tie. Hangers attached to the arch provide intermediate support for the tie girder.
Pre-construction sketches of proposed bridge
Bridge erection sequence
Temporary steel ‘dolphin’ supports were installed in the river to allow segmental construction of the bridge over the river. After completion of the bridge, the temporary supports were cut off 600mm below the river bed and removed.
Temporary ‘dolphin’ supports in river
The steel superstructure was fabricated by Fairfield Mabey and delivered to site in segments. Assembly of the superstructure was carried out in the five stages as illustrated below.
Five stages in erection of steel deck
As built bridge
