Design of box girders
To describe the main features and advantages of box girders; to introduce the methods of global analysis used and the typical reinforcing details using stiffeners and diaphragms.
The advantages of box girders are compared to those of plate girders. Their structural behaviour is discussed in global and detailed terms, covering matters such as diaphragm and stiffener design, web buckling, and torsion. Recommendations regarding fabrication details are also given.
Box girders are used in building structures (see Figure 1) as well as in bridges (see Figure 2). In general, they are more expensive than plate girders because they require more time to make. They have, however, several advantages over plate girders which make their use attractive:
- very good torsional rigidity; for highly curved spans, box girders are almost essential
- very wide flanges allow large span to depth ratios
- a neater appearance (since the stiffening need not be visible) especially when the webs are inclined. In certain cases, for aesthetic reasons, it is the only solution which is preferable to concrete (see Figure 2a).
- a very good aerodynamic shape, which is important for large suspension or cable-stayed bridges where the resistance to lateral wind is the main issue (see Figure 2b)
For bridges, the two main types of cross-sections are:
- two box girders, on discrete columns, connected by cross beams and a composite concrete slab. Each box girder has longitudinal stiffening on the compression flange (see Figure 3).
- a wide box girder, to facilitate fabrication, with the top flange formed by the concrete deck slab (see Figure 1)
A box girder with an orthotropic deck is closed during all stages of erection (see Figure 3a). On the other hand, temporary horizontal bracing is generally provided between the webs of box girders with a top flange formed by a concrete deck slab until the concrete has hardened.