Different cross-sections may be used for precast segmental cable-stayed decks. Balanced Cantilever Construction of Precast Segmental Bridges (81 pages) explores the pros and cons of three alternative solutions:
- Twin box girders with symmetrical side wings, connected by central delta-frames (precast trusses) and supported by a central plane of stay cables. This solution is still used in a few bridges in the USA despite its poor structural efficiency and aesthetics.
- Twin edge box girders connected by precast T-crossbeams and supported by two outer planes of cables (see photograph). This solution offers numerous advantages ranging from higher structural efficiency to streamlined surfaces and excellent aesthetics.
- Single-cell box girders with a central plane of stay cables and steel diagonals within the box cell connecting the top-slab cable anchor points with the bottom web-slab nodes of the cross-section. This solution offers several advantages over the first solution, especially when the deck is not very wide or the segments are cast in-place to save the cost and time constraints of a precasting facility.
The structural conception of a precast segmental cable-stayed deck has significant impacts on the construction cost of the bridge and the cycle time of segment erection, and the three alternatives have different efficiency. Modern cable-stayed bridges have wide decks, and transverse bending in the deck is a primary parameter in measuring the structural efficiency and cost-effectiveness of different options.
Set b the total width of the deck and q the deck self-weight per unit area of deck surface, the first solution (twin box girders and delta-frames cantilevering out from a central plane of cables) generates a peak negative transverse self-weight bending per unit deck length equal to -(qb^2)/8. The second solution (streamlined edge box girders and central T-crossbeams supported at the edges by two planes of cables) generates a peak positive transverse self-weight bending equal to +(qb^2)/8. In absolute values, transverse self-weight bending is similar in the two solutions, but opposite in sign.
Positive transverse bending offers many advantages in a precast segmental bridge, as it induces axial compression in the top slab that saves or avoids transverse post-tensioning. Even if the peak transverse bending is equal in absolute value, positive bending can be resisted with a few powerful tendons in the T-crossbeams, while negative bending requires top-slab tendons. Flat-duct top-slab tendons contain a small number of strands, and this leads to a huge number of duct splicing, pressure-testing, tendon fabrication, stressing, and grouting operations. Horizontal top-slab tendons are more prone to grouting defects that may jeopardize deck durability, and reinforcement congestion in the presence of so many transverse tendons may cause additional quality defects.
Draped tendons in precast T-crossbeams are anchored close to the cable anchorages for direct load transfer, and varying-depth crossbeams further enhance the efficiency of transverse post-tensioning and the aesthetic appeal of the deck. Wet joints for webs and top flanges simplify segment alignment and splicing of transverse tendon ducts, and avoid match casting of the T-crossbeams. The top flanges of precast T-crossbeams complete the deck surface.
The top slab of the deck is a rectangular matrix of precast plates with similar curing time and time-dependent behavior, while the delta-frames require in-place casting of the central top-slab strip to facilitate splicing of the transverse tendon ducts, so that young top-slab segments are directly loaded by the anchor force of the stay cables, and the entire top slab is prone to time-dependent stress redistribution. Minimal time-dependent stress redistribution and the absence of transverse top-slab tendons are important advantages in decks certified for 75- or 100-year service life.
The stay cables are directly anchored in precast box girders with weeks of curing, and two planes of stay cables are not necessarily more expensive than one central plane as the total number of strands is similar and the central cables of the delta-frame decks must often be separated into twin cables because of their huge number of strands.
Photo: Wadi Abdoun Bridge, Jordan.