Bird Lane South

Bird Lane South Overbridge is one of five weathering steel composite highway bridges on the A34 Chieveley/M4 Junction 13 improvement project. The scheme will reduce congestion at this major north/south, east/west interchange. Work was carried out under a design and build project.

Concept

The three-span, continuous integral bridge at 30 degrees skew is made with weathering steel at a standard minimum headroom of 5.3 m. The outline design was an insitu reinforced concrete bridge on the greenfield site. The weathering steel ladder deck solution was selected to minimise both the initial cost and future maintenance.

Key benefits of using steel

The key benefits of steel in this project included:

• Minimum capital cost.
• Minimum whole life cost (part of tender-evaluation process).
• Maximum off-site work. The contractor was concerned with the availability of specialist tradespeople (for example, carpenters and steel fixers) on-site.

Design details

The two main girders are 1.4 m deep and 10 m apart. The transverse girders are 750 mm deep and at 3.2 m centres. Integral abutments and the use of weathering steel minimises future maintenance requirements. Stability of the bottom flanges at the internal piers is provided by inverted U frames.

Construction details

The plate girders were fabricated on a “T & I machine” and transported to site singly (maximum length 35 m). The steelwork (span and cantilever girders) was erected using cranes. Precast “omnia” planks span longitudinally between transverse girders. A standard 1.5 m cantilever formwork system was used for all five bridges.

Summary