The Sunshine Skyway Bridge is one of the largest clear-span, stage-constructed concrete bridges in North America. The bridge consists of trestle, approach spans, low-level approach spans, high-level approach spans, and 366 m of cable-stayed main span. While the low-level approach spans were constructed with twin box girders, the main span was constructed with a single-cell box girder.
The reinforced-concrete pylons supporting the cable stays are supported by two main piers, each consisting of twin pier shafts. The pylons, established at a height of 132.67 m above sea level, support 21 cable stays for each half of the main span. These cables are anchored along the centerline of the box sections.
The span-by-span construction method was used for the twin box girders in the low-level approach spans. Whereas the balanced cantilever method was used in the high-level approach and the main cable-stayed span.
1. Basic Design Considerations
The following considerations governed the girder design:
- In order to facilitate the construction, an open cross-section was used instead of a box girder.
- The two main girders are located at the very outside by anchoring the cables directly on to them.
- The roadway slab is made of concrete instead of the usual steel orthotropic deck, to reduce costs.
- By using precast slabs for the deck, the redistribution of compression forces onto the steel girders due to shrinkage and creep was minimized, and forming costs were reduced.
- The roadway spans longitudinally between the floor beams. Thus, the tensile stresses from local loads are superimposed onto compression stresses from the overall system.
- The deck in the center of the main span was not prestressed, although the overall compression decreased there. For crack control, the reinforcement was increased and lapped in cast-in-place joints.
- Greater resistance against rotation to the torsionally weak bridge deck system was achieved by anchoring the stay cables to the outside main girders and converging them at the tower top, thereby creating a stiff space truss.
- Above the roadway, an A-frame tower was created. The legs converge from the deck downwards to fit the requested common foundation with the concrete alternate.
- The two main channel tower foundations had been designed to receive the towers for either the concrete or the steel alternate. As a compression member, a concrete tower is less expensive than a steel tower.
- In order to avoid auxiliary stays during girder erection, a small cable spacing (equal to the girder-segment length) was chosen.
- The girder was erected in units of small weight, i.e., the steel grid and the precast slabs were erected separately.
- The governing components from an esthetical point of view were the continuous fascia girder and the diamond-shaped towers connected by the transparent fan of stay cables.
2. Structural System
The main structural system of a bridge consists of girders, cables, and towers. These structural systems are discussed below.
The main girders have an asymmetrical cross-section with inclined webs. Girders were fully welded with the exception of the bolted field splices. In order to keep the secondary stresses due to the eccentrically (with respect to the web) positioned stays at a minimum, the stay anchorages were placed as close as possible to the floor beams.
The floor beams were welded plate girders with 9/16-in webs without transverse stiffeners to simplify fabrication. They were connected to the main girders with high-strength bolts.
Each 9-in thick conventionally reinforced precast roadway slab has a weight of approximately 294 KN. The slabs rest on thin layers of mortar between neoprene strips on top of the floor beams, with the shear studs protruding into pockets.
The longitudinal joints run continuously over the full length of the bridge. They are under compression from the bending of the floor beams. The transverse joints are staggered across the width of the bridge and are located at the points of counter-flexure between the floor beams. The joints have sufficient width to lap the rebars. The slab faces are keyed for better shear transfer. The roadway slab reinforcement was epoxy coated since the bridge is located over saltwater.
2.2 Stay Cables
The stay cables consist of parallel wires with high amplitude anchorages in polyethylene pipes, injected with cement grout after installation for prevention of corrosion. High amplitude anchorages transfer the tensile forces from the wires into the sockets gradually by elastic arch action of the steel balls between the socket walls.
The fatigue strength of the wires is not impaired by this anchorage. The fatigue range for 2 X 106 cycles with an upper stress is in excess of 200 N/mm2 for a remaining ultimate strength of at least the guaranteed ultimate strength.
The tower foundations for both the steel and concrete alternates consist of cylinder shafts, with 23 m diameters supported on piles. Governing the tower design was the hurricane loading, for which, instead of the regular American Association of State Highway and Transportation Officials (AASHTO) safety factor of 1.3, the increased value of 1.6 was applied.
In the lower part of the tower, where the distinct advantage of A-towers carrying horizontal loads by tension and compression is lost, the transverse horizontal forces are carried by bending.
Wind-tunnel tests confirmed the shape factor of 1.9 in accordance with Deutsches Institut für Normung (DIN) -1055 and the shielding at the leeward tower leg. The increase of moments due to non-linearity amounted to 6% longitudinally and 2.5% transversely.
The horizontal tie beams between the tower legs were to be prestressed with a total force of 37 MN each. The forces from the overlapping cables were introduced into a wall at the tower head, which connects the two tower legs. The horizontal cable-force components balance one another; the cable inclinations in the plan create unbalanced horizontal deviating forces.
The walls were, therefore, prestressed transversely with loop tendons. The big moment capacity of the towers required during the final stage permitted a free cantilever erection without temporary staying of the tower or the girder.
2.4 Live Load and Wind Load Consideration
Live Loads: HS20-44 (as per AASHTO code) with military loading. One lane is designed for military loading and other lanes are designed for HS20-44 loading combinations.
Wind loads: As the bridge is located in a hurricane-prone area, the basic wind pressure up to girder level was determined to be 322 kN/m, increasing to 4 kN/m at the top of the towers (100-year probability).
3. Design Codes
The design was in accordance with the AASHTO bridge specifications. In addition, the CEB-FIP Model Code for concrete structures was applied for shrinkage and creep, the German DIN Code-1055 was used for determining the shape factors for stays and towers, DIN Code-1073 was applied for permissible stay-cable stresses, and DIN Code-1075 was applied to calculate the effective girder widths in bending (shear lag).
All concrete members were sized by the load-factor method and checked under working loads for crack widths, fatigue, and deflections. The structural steel members were designed for permissible stresses under service loads. For the roadway and towers, a 5,000-psi concrete was used. All reinforcement was grade 60, and the structural steel was A 572 of grade 50.
4. Construction Details
It was planned to construct the two bridge halves staggered from the outward towers. The tower was built by jumping forms, whereby three auxiliary struts were required between the upper inclined legs. The main girders and floor beams were preassembled to complete girder grids of 16 m in length. The stepwise installation of the beam elements is shown in Figure-5(A) and 5(B).
The erection of the first girder grid of weight 123 tons was done by floating crane. After placement of the concrete slab, the first derrick had to install the next two elements on the same side to make room for the second derrick. During the run of erection, the permissible out-of-balance between side span and main span decreased from one complete section to one steel grid or three precast slabs.
The erection of a new element was generally only to begin after complete composite action between the steel grid, and the concrete slabs of the previous section had been established. In the case of an approaching hurricane, the girder was also to be tied down. The anchors and tie ropes required were to be kept on site until such an event might occur.
The Sunshine Sky bridge is a highway bridge located in Tampa Bay of Florida, US.
It is the longest highway bridge in Florida and more than 50,000 vehicles cross this bridge daily.
The approach line of the Sunshine sky bridge is sloping towards the main towers of the bridge. Thus, when a vehicle approaches the bridge, the tower of the bridge appears to be mounted from the sky.
The main towers of the Sunshine Sky Bridge are located at a height of 131 m from the sea level.