Numerous cable-stayed bridges have been designed with two planes of stay cables suspending a steel grillage comprising edge girders, floorbeams and substringers and completed with a prestressed-concrete deck slab. Precast deck slab panels are made continuous with the steel grillage with in-place closures over floorbeams, substringers and built-up edge I-girders. The cables are anchored over the edge girders or in anchor pipes bolted to their outer face.
A composite grillage is lighter than a prestressed-concrete deck, and this generates cost savings in cables, towers and foundations. The deck is typically erected as balanced cantilevers. When the area under the bridge is accessible, low-level decks are erected with ground or floating cranes, and high-level decks are erected with on-deck derrick platforms that handle modules of edge girders and floorbeams and precast deck panels individually. Light erection equipment and small load unbalance diminish longitudinal bending of staged construction in towers and foundations. The drawbacks are the cost of the steel grillage, its maintenance cost with time, the need for an efficient temporary restraint between the deck and the tower during construction, and the need for a wide deck to assure lateral stability of long cantilevers during construction.
Prestressed-concrete ribbed slabs comprising edge girders and floorbeams are also used in cable-stayed decks with two planes of cables. The stay cables resist most of the negative bending that characterizes cantilever construction, and the use of many cables with closely spaced anchor points diminishes the demand for flexural stiffness at the deck level with both harp and fan arrangements.
A ribbed slab is easier to cast than a box girder, the cables are anchored at the bottom of recess pipes embedded into the edge girders, the edge girders directly resist the longitudinal component of the pull in the stay cables, and maintenance is less expensive than for a steel-composite grillage. A prestressed-concrete ribbed slab, however, is heavier than a composite grillage, larger load unbalance during staged construction increases longitudinal bending in towers and foundations, and two planes of stay cables are still necessary because of the poor torsional constant of the open section.
Several cable-stayed bridges have been designed with prestressed-concrete box girder deck. The box girder often has single-cell section, its width may reach 18-20m, and diagonal struts have been successfully used to further widen the top slab. The torsional strength and stiffness of a wide box girder integral with a central pylon are typically enough to support the deck with a single central plane of stay cables.
Many cable-stayed bridges carrying separated highways have been designed with a single plane of cables to simplify the tower and diminish the number of anchorages. A central tower, however, further increases the width of the deck, and so heavy cross-sections soon lead to excessive foundation loads and expensive bridges. In addition, prestressed-concrete decks are conceptually unfit to the use of a single central plane of cables because negative transverse bending in the deck requires expensive transverse post-tensioning that weakens top-slab regions directly exposed to traffic.
Different solutions have been tested to lighten the deck of prestressed-concrete cable-stayed bridges and to use lighter erection equipment that diminishes load unbalance in towers and foundations during construction. The use of twin box girders suspended from a central plane of cables through precast delta-frames has found a handful of applications because of the complexity and poor structural efficiency of the solution. Modern solutions based on streamlined edge girders suspended at the edges from two planes of cables and connected by precast T-beams at deck centerline improve the aerodynamic stability of the deck, diminish the drag coefficient, assure better aesthetics and structural efficiency, and minimize the demand for transverse top-slab post-tensioning.
Cable-stayed box girders are made of steel, prestressed-concrete or composite construction. Streamlined steel box girders erected with floating cranes or lifting frames are used for the longest spans because of the higher strength-to-density ratio of steel. Prestressed-concrete is the typical choice for shorter spans due to the lower cost of materials and less maintenance with time. Single-cell prestressed-concrete box girders supported from a central plane of cables are too wide and heavy for segmental precasting and are mostly cast in-place with form travelers. Prestressed composite box girders with steel corrugated-plate webs are earning popularity in the 100-200m span range because of weight saving, high flexural efficiency, and the web capability of not interfering with post-tensioning and the longitudinal component of the pull in the stay cables.
Compared to a composite grillage or a ribbed slab, the constant of torsion of a box girder is 2-3 orders of magnitude higher, the moment of inertia is one order of magnitude higher, and the cross-sectional area is similar. A box girder is therefore perfectly fit for incremental launching over temporary piers and suspension from the towers on launch completion. When the area under the bridge can be partially disrupted during construction to erect and dismantle the temporary piers, deck launching offers several advantages over conventional balanced cantilever construction:
- Construction is faster and less expensive. Approaches and main span are constructed in a fixed facility behind the abutment with less investment and simpler logistics. When a steel main span is combined with prestressed-concrete approaches, a portion of the main span can be used as a launch nose during incremental launching.
- The towers can be erected out of the critical path during deck launching, as the cables are fabricated and stressed on launch completion.
- The deck can be used as a working platform for cable fabrication. Construction materials are directly downloaded on the deck, and heavy cranes may be operated on the deck as well. The number of cable stressing operations diminishes, and control of geometry and the pull in the stay cables is much simpler.
- The deck is unaffected by aeroelastic disturbance during construction.
When the deck is over water, the drawbacks include the number of temporary piers and their foundations and the interference with the navigation channel. The 21.8m wide, 527m prestressed-concrete box girder of the Wandre Bridge in Belgium was launched over temporary piers placed in the Meuse River and a parallel channel. With 18m deck segments, launching the 11800t deck required a 35m launch nose. On launch completion, the deck was suspended from a 95.5m A-tower to attain the final static system with two cable-stayed spans of 144m and 168m.
Launching a low-level deck over temporary piers is particularly advantageous when the area under the bridge can be partially disrupted. In wide railway crossings, some tracks can often be temporarily deactivated or their spacing may be compatible with the presence of temporary piers. The dual-track LRT Palizzi Bridge in Italy was launched over 6 electrified railway tracks with the help of two temporary piers. The main span is 66m.
When no temporary piers can be used for launching, the pylon may be supported on the deck and used to deviate temporary cables that support the front cantilever during full-span launching. This construction method was used for a pedestrian bridge on top of an 80m intake tower within a reservoir close to Granada in Spain.
Multispan cable-stayed bridges with integral pylons are optimal candidates for low-level launching over railways. The 23.8m wide, 580m composite deck of the Coast Meridian Overpass in Canada was launched over 50 parallel tracks without the use of temporary piers. The deck includes four 30m steel pylons, a single central plane of stay cables, and five cable-stayed spans of length ranging from 111m to 125m. During full-span incremental launching, leading pylon and stay cables were used to support the 125m front cantilever in combination with a long launch nose.
Multispan cable-stayed bridges with integral pylons are optimal candidates also for high-level launching, as the deck establishes a working platform for the activities to be performed above the deck on launch completion. The 32m-wide, 2460m streamlined steel box girder of the Millau Viaduct in France includes seven 87m inverted-V steel pylons over the deck and a single central plane of stay cables. The length of the eight cable-stayed spans of the bridge is 204m for the end spans and 342m for the interior spans. Two long deck sections were launched from the opposite abutments with the help of one temporary pier per span for central closure. The leading pylons were used to support the front cantilevers during launch with permanent stay cables. On launch completion, the rear pylons were delivered on the deck, rotated to vertical and completed with the stay cables.
The streamlined steel box girder of the 260m main span of the self-anchored-suspension Hangzhou Jiangdong Bridge in China was also launched over temporary piers. The stiffening girder of a self-anchored suspension bridge must be completed prior to applying the cables, and the temporary piers needed to support the girder can also be used for launching. Incremental launching of the 47m-wide deck simplified construction and avoided the use of floating cranes. Because of the cambered profile of the deck, the launch bearing alignment was adjusted vertically during launch.
A deck launched over temporary piers can also serve as a working platform for rib erection of a tied arch. This solution was adopted for the 218m span of the Reggio Emilia Bridge in Italy. After launching the 27m-wide single-cell steel box girder over three temporary piers, the latter were extended over the deck into three strand-jacking towers needed to lift arch rib sections and hold them in-place during welding of the field splices. After completion of the arch rib and removal of the strand-jacking towers, locked-coil strand hangers were applied to suspend the box girder from the arch and remove the launch piers. A similar solution was used for the arch bridge over the River Loire in Orleans.