Multi-beam

Multi-beam steel composite decks are very competitive for highway bridges in the span range of 15 – 100m. They comprise a reinforced concrete deck slab on top of several girders. The steel girders are connected to the concrete slab by shear connectors and the two act compositely together.

For simple spans up to 25m and continuous spans up to 33m, rolled universal beams are economic and are available up to 1000mm deep. Very little fabrication is required, usually only the fitting of stiffeners for bearings and  the attachment of bracing, and sections may be bent to provide precamber and horizontal curvature.

For longer span bridges, deeper girders are fabricated from plates. This gives scope to vary the web and flange sizes and the steel grades for an efficient design. Variable depth girders are often used to give greater economy, and also to enhance aesthetics. Such plate girders can be readily fabricated with curvature in plan & elevation.

Examples

The Thelwall viaduct (shown right) carries  the M6 over the Manchester Ship Canal. This 26-span, 1380m long structure is the longest, continuously welded, structure of its kind in the UK.

The A69 Haltwhistle Viaduct (shown right) is an example of a twin-girder ‘ladder-deck’. The bridge follows a tight 540m radius horizontal curve, and has 3.2m deep truly curved plate girders. A special feature of the design was the use of half-depth precast deck units to speed up construction over the railway.