Advanced design of box girders

1. INTRODUCTION

Although steel or steel-concrete composite box girders are usually more expensive per tonne than plate girders because they require more fabrication time, they can lead to a more economic solution overall.

For bridges, box girders have several advantages over plate girders which make their use attractive:

• a very high torsional stiffness: In closed box sections, torque is resisted mainly by Saint Venant shear stresses and the torsional stiffness is normally much greater than that of open sections.
• closed steel boxes provide torsional stiffness during their erection: Thus, they avoid the need for the temporary bracing which is required with plate girders and which also interferes with the construction of the concrete slab. For highly curved spans, torsional stiffness is almost always essential during their construction.
• wide flanges can be used: This allows large span to depth ratios without resource to very thick material.
• box girders have a neat appearance: The stiffening can remain unseen inside the box.
• the facility to choose a good aerodynamic shape: This is particularly important for large suspension or cable-stayed bridges.
• box girders use fewer bearings: Usually, torsional restraint needs to be provided at only one position along a continuous box; single bearings can be used at all other supports. Further, with a highly curved box girder, single bearings at all supports is often sufficient.

Box girders are sometimes used in building structures, but this is not common. This lecture deals mainly with box girders as used in bridges - both all-steel construction and composite construction with a reinforced concrete deck slab. Most of the general remarks are applicable to box girders used in buildings.