General

Box girders comprise two webs that are joined top and bottom by a common flange creating a closed cell that offers very good torsional stiffness, which may be required on highly curved bridges. In beam and slab bridges, box girders are an alternative to plate girders at the upper end of the span range, where they offer a lower steel weight, although this has to be balanced against increased fabrication costs. Such composite box girder decks may take the form of multiple closed steel boxes, with the deck slab over the top, or an open top trapezoidal box, closed by the deck slab. 

Longer spans of 100 to 200m typically use either a single box or a pair of boxes with crossbeams. For such long spans and for bridges such as lifting bridges, where minimising structural weight is very important, an all-steel orthotropic deck may be used instead of a reinforced concrete slab. Above about 200m, box girders are likely to be part of a cable-stayed bridge or a suspension bridge, where they are specially shaped for optimum aerodynamic performance.

Product Information

SMC always use high-quality structure steel that is readily available worldwide in certified grades, in products of various shapes and sizes. Prefabrication of steelwork in controlled factory conditions leads to high quality work at minimum cost. The excellent quality control is achieved through a thorough testing regime at the steel mills and during the fabrication processes of cutting and drilling, assembly, welding, and protective treatment. The quality assurance that is attained should give confidence to all clients and engineers who specify steel for their bridge project.

SMC will use Corten Steel un this type of bridge, Corten steel is a high strength low alloy steel that in suitable environments forms an adherent protective rust ‘patina’, to inhibit further corrosion. The corrosion rate is so low that bridges fabricated from unpainted weathering steel can achieve a 120 year design life with only nominal maintenance.

Construction

A wide range of construction methods and sequences are available for steel bridges. They can be lifted, piece-by-piece, by cranes or strand jacks; they can be launched by sliding or rolling from the abutment; or they can be slid or transported into position. In some instances a combination of erection methods are needed; these are called ‘Hybrid schemes’. Steel offers flexibility in terms of erection sequence and certainty in terms of the programme, once erected, the steel girders provide platforms for subsequent deck construction operations. That can reduce the construction time. 

Offsite prefabrication of steel components means that construction time on site often in hostile environments. The speed of bridge construction made possible by steel allows disruption to road and rail users to be kept to a minimum. The relatively low weight of structural steel components permits the erection of large sections; in some circumstances complete bridges can be moved into position overnight. Speed of steel bridge construction also benefits main contractors even on ‘green field’ sites, as it allows them to establish and maintain access for haul roads.