The depth of the cross-section, the shear area of the webs and the thickness of the bottom slab diminish along the cantilever to decrease the cross-sectional flexural and shear capacity according to the decreasing demand of cantilever construction. Varying-depth precast segmental box girders have been used on 80-180m spans and are mostly competitive on 100-120m spans. The deck may have constant or varying depth. When the deck is not cable-stayed, balanced cantilever bridges typically have box section. Ribbed slabs comprising solid edge girders and crossbeams are used for balanced cantilever erection of cable-stayed bridges with two planes of stay cables because the stay cables resist most of negative bending and torsion. The center of gravity of a ribbed slab is closer to the top slab, which further complicates the transmission of negative bending. For a given cross-sectional area, the moment of inertia, radius of gyration, and flexural efficiency of a ribbed slab are smaller than those of a box girder, which ultimately requires more post-tensioning. Ribbed slabs with double-T section are simpler to form and cast than box girders and have been used for some balanced cantilever bridges in the past. Single-cell box girders may reach 18-20m of width transverse ribs in the top slab, diagonal struts propping the edges of the side wings from the bottom web-slab nodes of the cross-section, and combinations of ribs and struts have been successfully used to further widen the top slab of single-cell box girders. This is particularly true when the deck is curved in plan. The torsional strength and stiffness of a hollow section assure stability of the cantilevers on the long spans where this construction method is typically employed. The bottom slab of a box girder lowers the cross-sectional center of gravity, which increases the flexural capacity in the negative bending regions.
The advantage of balanced cantilever erection is that the segments are handled individually, and the erection equipment is therefore lighter and less expensive than a self-launching gantry for span-by-span construction, which sustains the weight of an entire span of segments during span assembly and application of post-tensioning.īalanced cantilever bridges typically have box section. For a given span length, balanced cantilever erection of precast segmental bridges is often more expensive than span-by-span erection in terms of quantities of structural materials and associated labor.
Negative bending governs balanced cantilever construction, and this often requires box sections with a thick bottom slab at the root of the cantilever and many longitudinal tendons in the top slab. This construction method is particularly advantageous on long spans, in marine operations, and where access beneath the deck is difficult. The deck is self-supporting during construction, and it also supports erection equipment and construction materials stored on the cantilever. The deck is erected segmentally on each side of the pier in a balanced sequence to minimize load unbalance and longitudinal bending in piers and foundations. Balanced cantilever construction is suited to precast segmental and cast-in-place bridges.
0 kommentar(er)
