ABSTRACT
In a box beam bridge, the deck consists of one or more boxes connected by a slab as shown in Fig. 16.2 to Fig. 16.4. Box sections are good at resisting torsion and they are generally used for spans greater than about 35 m. In order to give access for inspection and maintenance, the depth of the box needs to be at least about 1.8 m. For spans up to about 50 m, the deck is usually built on false work but for longer spans, ‘cantilever form of construction’ is used as explained in section 16.1.1. It is generally preferable to minimize the number of cells in the cross section as each cell requires additional formwork, which demands at least about 6 m spacing between the webs. It is also more economical to provide a small number of thick webs rather than a large number of thin webs because of the reduction in effective thickness doe to the presence of strands. The spacing between the webs is governed by the bending resistance of the top flange to live loading. It is also important to make sure that the cantilever projections are not too long as otherwise they might induce a large bending
In-situ concrete slab
Precast beams
moment in the webs. Single boxes are preferred when the width of the deck does not exceed about 13m. If the width is between 13m and 18 m then a two-cell single box as shown in Fig. 16.3 is used, although this are not popular because of the complexities during construction. However, if the width is between 18 and 25 m then two separate boxes with a connecting deck slab as shown in Fig. 16.4 are used. These dimensions need not be strictly adhered to as there are examples of single box bridges with a deck width of nearly 30 m with the side cantilevers nearly 7 m long. Trapezoidal boxes are used for aesthetic reasons, with the inclination of the web to the vertical in the range of 10o to 15o. They have the additional advantage of reducing the width of the bottom flange, which is generally fairly thick as it is designed to resist compressive stresses induced during cantilever construction.
