The Sunshine Skyway Bridge stands as a landmark achievement in modern engineering, recognized as one of the largest clear-span, stage-constructed concrete bridges in North America. Stretching across Tampa Bay in Florida, the bridge is a vital transportation link and a testament to innovative design and construction techniques. Comprising trestles, approach spans, and a 366-meter cable-stayed main span, it is also one of the longest bridges in the United States.
Main Components of the Bridge
Pylons and Cable-Stay System
At the heart of the Sunshine Skyway Bridge are its reinforced concrete pylons, which soar 132.67 meters above sea level. These pylons, supported by twin pier shafts, hold 21 cable stays on each side of the main span. The stays are anchored along the centerline of the bridge’s box girder, efficiently distributing the load.
Span Construction Techniques
The bridge utilizes different methods for different segments:
- Low-level approach spans were constructed using the span-by-span method with twin box girders.
- High-level approaches and the main span were built using the balanced cantilever method, enabling longer reaches without intermediate supports.
Basic Design Considerations
Several key decisions defined the bridge’s design:
- An open cross-section was used instead of a full box girder to simplify construction.
- Stay cables are anchored directly onto the outer girders, enhancing stability and aesthetics.
- The roadway slab is concrete, not the conventional steel orthotropic deck, reducing costs and improving durability.
- Precast concrete slabs minimize creep and shrinkage issues while cutting down forming expenses.
- The deck’s non-prestressed center section includes extra reinforcement to control cracking.
- By converging the stay cables at the tower top and anchoring them externally, a torsionally stiff space truss was created.
- An A-frame tower structure was selected for its ability to distribute loads efficiently.
- Foundations were designed to support both steel and concrete tower options, though concrete was ultimately chosen for cost-effectiveness.
- A small cable spacing allowed segment erection without auxiliary stays.
- Structural elements were erected in lightweight segments, streamlining the construction process.
- Aesthetically, the bridge features diamond-shaped towers, a continuous fascia girder, and a transparent fan pattern of cables, contributing to its iconic silhouette.
Structural System
Girders
The main girders have an asymmetrical cross-section with inclined webs. They were fully welded, except at field splice points. Stay anchorages were placed near floor beams to minimize secondary stresses. Floor beams were fabricated as welded plate girders, simplifying production and installation.
The 9-inch thick precast concrete slabs (weighing around 294 kN each) sit on neoprene pads atop floor beams and are connected with shear studs. Longitudinal joints run the full bridge length, while transverse joints are staggered to optimize performance. Epoxy-coated reinforcement protects the slab against the corrosive saltwater environment.
Stay Cables
Made of parallel wires enclosed in polyethylene pipes, the stay cables are grouted with cement to prevent corrosion. Their high amplitude anchorages ensure gradual load transfer, preserving the wires’ fatigue strength. The cables can endure over 2 million cycles with minimal stress loss.
Towers
Tower foundations use cylindrical shafts (23 meters in diameter) on deep piles. Due to the region’s hurricane risk, the safety factor for hurricane loads was increased from the standard 1.3 to 1.6. In lower sections where horizontal load-bearing is more complex, the towers resist bending forces directly.
Wind tunnel testing helped confirm design parameters, including a shape factor of 1.9 (per DIN-1055). The towers include prestressed horizontal tie beams and transverse prestressing in the tower head wall to balance unaligned cable forces. The final tower design allowed for free cantilever erection, avoiding the need for temporary supports.
Load Considerations
- Live Loads: Designed for HS20-44 traffic loads, with one lane capable of supporting military vehicles.
- Wind Loads: The bridge withstands wind pressures up to 322 kN/m at deck level, increasing to 4 kN/m at tower tops, aligned with a 100-year storm probability.
Design Codes and Material Specifications
The bridge was primarily designed according to AASHTO bridge specifications, supplemented by:
- CEB-FIP Model Code (shrinkage and creep),
- DIN-1055 (shape factors),
- DIN-1073 (cable stress),
- DIN-1075 (effective girder width).
Structural elements were sized using the load-factor method and checked under working loads for fatigue, deflection, and crack control. Key materials include:
- 5000 psi concrete for towers and roadways,
- Grade 60 steel for reinforcement,
- A572 Grade 50 structural steel.
Construction Details
Construction was executed in staggered halves, starting from the outer towers. Towers were built using jumping forms, with temporary struts added between inclined legs. 16-meter girder grids were preassembled off-site.
Initial girder placement, weighing 123 tons, was completed via floating crane. Derricks then installed subsequent segments. A crucial detail: new elements were not erected until full composite action had been achieved between the previous steel and concrete components.
The project also accounted for hurricane threats—anchors and tie ropes were stored on-site to secure the structure in case of severe weather.
Conclusion
The Sunshine Skyway Bridge exemplifies the seamless integration of structural innovation, economic efficiency, and architectural elegance. From its cable-stayed design and asymmetrical girders to its robust hurricane resilience, every detail reflects careful planning and engineering mastery. Today, it not only connects regions but also stands as a symbol of modern infrastructure done right.