Twin Box Girders for Precast Segmental Cable-Stayed Bridges?

twin box girders for precast segmental cable-stayed bridges

Different cross-sections may be used for precast segmental cable-stayed decks. Balanced Cantilever Construction of Precast Segmental Bridges (81 pages in full A4/letter format) explores the pros and cons of twin box girders with symmetrical side wings, delta-frames and a central plane of stay cables; single-cell box girders with a central plane of cables; and twin edge box girders connected with T-crossbeams and supported with two outer planes of cables.

Twin box girders with symmetrical side wings, delta-frames and a central plane of cables are a complex, dated solution for precast segmental cable-stayed decks. The effort of using in the main cable-stayed span precast segmental solutions adopted for the approach ramps may have architectural merits, but the structural weak points of the solution are undeniable.

In the cross-sectional plane, the two halves of the deck cantilever out from the central plane of cables. Negative transverse bending increases transverse post-tensioning in the top slab and complicates control of cracking and chloride penetration in deck surfaces directly exposed to traffic. Self-weight reduction limits the thickness of the top slab, longitudinal top slab post-tensioning is necessary at the root of the cantilevers and in midspan, and providing substantial amounts of transverse post-tensioning with small flat tendons that cross the longitudinal tendons leads to a huge number of pressure testing, post-tensioning, and grouting operations.

For a given longitudinal spacing of the deck anchor points of the stay cables, the pull in the central plane of cables is twice as much as the pull in two edge planes of cables, which complicates transfer of the anchor forces. The cables are also anchored outside the deck cross-section, and delta-frames are needed to provide vertical support to the twin box girders at each cable anchor point.

The delta-frames are exposed to atmosphere, and stiffness and durability reasons suggest the use of precast concrete members. The delta-frames are typically heavier than the precast segments of the twin box girders, which increases the cost of the cranes that handle the segments in the precasting facility and on-site, and also increases the weight of the deck and the cost of the stay cables. The delta-frames also require special transportation saddles and lifting beams and their center-of-gravity is high above the bottom chord, which further complicates handling and diminishes stability.

The diagonals of the delta-frames are very inclined for geometry reasons, and this leads to huge tensile forces. For equilibrium, the bottom chord of the delta-frames is subject to significant transverse compression, which suggests the use of labor-intensive vertical braces at deck centerline to control vertical out-of-plane buckling. The varying inclination of the cable anchorages further complicates the forming and casting operations for the delta-frames.

The tensile force in the diagonals requires powerful post-tensioning tendons, control of cracking is difficult with so heavily congested sections, and the solutions of continuity at the lower inner web-slab nodes of the twin box girders (needed to anchor the delta-frames with in-place pours) complicate segment production and site assembly.

The stay cables are anchored at the bridge centerline, and having access to the anchorages is complex during construction and for maintenance. The central top-slab strip between the inner wings of the box girders is cast in-place, which involves complex, labor-intensive forming operations, and splicing of a huge number of transverse tendons.

The longitudinal component of the pull in so powerful stay cables is applied to a slab system that includes young cast-in-place concrete, older precast segments, epoxy joints, and wet joints. Two full-length construction joints between the twin box girders and the cast-in-place median slab are additional disadvantages.

Neither does this solution offer cost savings in terms of erection equipment. As explained in Balanced Cantilever Construction of Precast Segmental Bridges, the precast segmental cable-stayed bridges are typically erected with lifting frames, long precast segmental approaches are erected with self-launching gantries, and multiple sets of erection equipment are therefore necessary anyways. Segment geometry is also different for main span and approaches.

Wide single-cell box girders with one central plane of stay cables are also subject to negative transverse bending that increases top slab post-tensioning and complicates control of cracking, but their erection is simpler and faster and requires less in-place casting operations. When the precast segments are too heavy for ground transportation, these bridges are cast in-place with form travelers.

Two tension diagonals connect the cable anchor points to the bottom web-slab nodes of the box girder to provide direct vertical support to the webs. Steel diagonals are used within the box cell to save weight and simplify inner forming. Less in-place casting operations, a lighter deck, and less expensive stay cables are major advantages of wide single-cell box girders over the twin box girders with delta-frames. The time-dependent stress redistribution within the deck is also more predictable as the top slab concrete has uniform elastic modulus and creep coefficient.

The use of twin edge box girders connected by precast T-crossbeams and supported by two outer planes of stay cables offers numerous additional advantages, ranging from higher structural efficiency to streamlined surfaces and excellent aesthetics, and is leading to a new generation of precast segmental cable-stayed bridges.