The ingenuity of man in tackling societal problems has, over time, led to the construction of magnificent engineering structures, one of which are bridges. Bridges, by definition, refer to engineering artefacts that traverse over physical objects with a view to allow the passage of diverse objects such as ships, vehicles and trains underneath. As such, they are constructed over water bodies, roads, valleys and even railway networks.
Over the past few centuries, the techniques adopted in the construction of bridges have evolved (Lamb and Morrissey). The authors note that, while in the past, bridges were made by simply placing timber logs in between river beds, over time, the spans covered by the bridges, their designs and construction materials used have been enhanced significantly. As a result, types of bridges range from the simple beam, truss and arch bridges to the complex suspension, arch, cantilever and cable-stayed bridges (Lamb and Morrissey).
The current report focuses on the engineering concepts of cable-stayed bridges and in particular, the Sunshine Skyway bridge, built across Tampa bay in Florida. This report is structured into five sub-sections with the first providing an overview of how cable-stayed bridges work and their difference from suspension bridges. In the second sub-section, the history of the bridge is described while in the third, design considerations factored in during its constructions are expounded. The fourth sub-section describes its structural behaviour while the last outlines various analytical concepts associated with cable-stayed bridges. These include load calculations, equilibrium and load paths among others.
Overview of cable-stayed and suspension bridges
A misconception often arises in distinguishing suspension and cable-stayed bridges, particularly due to the fact that the two structures are associated with towers and cables (Bell). However, the author highlights that several notable differences observed between the two; First, is in the overall interconnection of cables, towers and the deck where it is noted that with suspension bridges, cables traverse across towers freely and in effect, transmit the anchorage and load at both ends. Lamb and Morrissey add that while the cables are pulled by gravitational forces, the towers at the central section of the deck hold them above the surface. Figure 1 below illustrates a suspension bridge.
Figure 1. Suspension bridge (source: Hasan)
From the figure, it is observed that the layout of the suspension bridge is such that the the deck is held in place by cables which run freely over towers at the central section of the bridge.
On the contrary, cable-stayed bridges are designed in such a way that the cables are attached to the central towers which then bear the entire load. As a result, the bridges do not require anchorage points or two towers, as required by suspension bridges. Instead, cables run from both sides of the deck to the singular tower thereby providing stability (Lamb and Morrissey). In addition, the authors highlight that the cable-stayed bridges can be configured in diverse layouts. For instance, cables can be radially attached to a single point in the tower or can be attached to the roadway and tower at different separate points. Figure 2 below illustrates a cable-stayed bridge.
Figure 2. Cable-stayed bridge (source: Hasan)
From the figure, it is observed that the different cables are attached to each tower and the deck separately and in effect, generating stability.
A second difference noted between the two is that suspension bridges traverse longer distances in comparison to the cable-stayed alternative (Bell). The author highlights that this arises because, with suspension bridges, an increase in span length is proportional to the tower height. As such, higher towers support longer bridge lengths. Conversely, Bell notes that cable-stayed bridges are more suited for shorter spans and for the construction of individual segments of bridges at remote locations.
Thirdly, it is also observed that suspension bridges require anchorages at the ends of their cable lines whereas cable-stayed bridges do not require them (Lamb and Morrissey). As such, there is an added design consideration for suspension bridges in that, the concrete anchorages used should exceed the weight of the vehicular load and the bridge. Consequently, cable-stayed bridges are constructed within a shorter time period while consuming less material compared to the suspension bridges.
From the next section onwards, the report focuses on Sunshine Skyway bridge, a cable-stayed bridge built in Florida.
History of Sunshine Skyway bridge
The Sunshine Skyway bridge spans a distance of 29,040 feet from Bradenton through to St. Petersburg in the West coast of Florida. The bridge, which is considered as one of the longest cable-stayed bridges in the world, was constructed over a three-year period (1984 to 1987), consuming an expenditure of $220 million (Goldberger). The underlying motivation for its construction arose from the need to replace its predecessor, the original Sunshine Skyway which was a cantilever bridge.
Verdict Traffic reports that the original idea to build a bridge over Tampa bay arose out of a need to address the high transport requirement observed in the bay. The authors note that the only available mode of transport across the bay was the Bee Line ferry. However, as high traffic volumes increased in the 1960s, the Florida department of transport identified the need to build a bridge. As a result, construction of the original bridge commenced in 1967 for a period of four years until 1971 when it was completed (Verdict Traffic). In addition, the bridge cost an average of $22 million.
The bridge spanned four miles and had a vertical clearance of 150ft with a channel clearance level of 869ft (Verdict Traffic). Subsequently, it facilitated transport across the bridge efficiently, for southbound traffic whereas a northbound span catered for the northbound traffic. Nonetheless, tragedy struck in May 1980 when the Summit Venture freight ship struck the southbound bridge causing more than 1000ft of the bridge to sink into the bay and killing about 35 people (Goldberger).
A few weeks after the tragedy, the transport authority contemplated between repairing the existent bridge, as it was only nine years old, and replacing the entire structure (Goldberger). The latter option was adopted in favour of building a safer and longer bridge structure. This new bridge would be developed over a three-year period (1984-1987) based on designs proposed by Figg " Muller, an engineering firm that specialized in the design of concrete bridges (Verdict Traffic). The older bridge was in the meantime demolished with its ends being converted to fishing piers. At the end of the demolition, up-to 9000 tonnes of steel had been brought down from the bridge’s deck trusses, approach thru trusses and its towers.
However, it is important to note that the development of the project was divided among several other engineering firms. Verdict Traffic reports that while Figg " Muller were tasked with the responsibility of designing the main span and high level approach spans of the bridge, the low level spans were developed by several other engineering firms such as Quade " Douglas, Parsons Brinckerhoff and both the New York and FDOT design structure bureaus among others (Verdict Traffic).
Design considerations for the bridge
In developing the new bridge, several design considerations were prioritised. First, was connection to the interstate 275 road network that was under construction around the time period. One of the concerns that the transport department had, was handling the amount of increased traffic that was using the northbound and southbound spans, before the tragedy occurred (Verdict Traffic).
However, with the destruction of the latter and the construction of the new interstate traversing through St. Petersburg, the department realised that the northbound span had to be demolished as well as it did not comply to interstate standards. Subsequently, the first key consideration was to ensure the new bridge would be compatible with the interstate 275 under construction in order to enable smooth flow of traffic.
A second aspect pertained to safety level of the bridge and its capacity to withstand tragic events such as ships ploughing into its towers, as was the case with the 1980 accident destroyed the southbound span. One of the high priorities was to ensure the bridge would absorb any unintended impact. To achieve this, the engineers implemented dolphins (large concrete islands) around each of the six piers associated with the bridge (PBS). As a result, these structures would be in a position to absorb impact from ships of up-to 87,000 tons thereby eliminating impact on the piers. In addition, the main towers were reinforced with 9-inch diameter steel pipes to enhance their strength (Verdict Traffic).
A third aspect regarded the interaction of the new bridge with the existent shipping activities at Tampa bay. Engineers had to factor in the aspect that the bay facilitated busy shipping activity that generated significant revenue amounts. In order to ensure the activities were not tampered with, engineers designed the bridge to have a vertical clearance of 193ft in order to allow ships to easily navigate underneath (PBS). The channel where shipping occurred was also increased by 50% in order to guarantee safety (Verdict Traffic).
A fourth design consideration was the aesthetic appeal (Goldberger). The architects and engineers were also interested in creating an iconographic structure that would serve as a landmark of the Florida state. To this effect, the cables of the bridge were painted in a brilliant yellow colour in reflection of its location; the Sunshine State. The road-ways were also setup in a way that they were on either side of the cables in order to allow drivers to have an unobstructed view of the water (PBS). As a result, the bridge has grown to become a significant landmark in the state that attracts tourists.
Structural behaviour
Goldberger notes that one of the structural features that made the bridge unique from its predecessors was the use of slender pylon towers. The author notes that in order to enable the use of light and slender pylons, engineers decided to centralize the pylons and use a single row of cables from the centre of the roadway rather than attaching the cables to the outer edges of the roadway. As a result, the pylons used would be free of attachments thereby being suitably used in the bridge.
The pylons have also transformed the visual outlook of the bridge by creating an optical illusion effect. As a viewer approaches the bridge, the cables come into view, increasing steadily until the viewer is directly on the axis with the pylons, at which point, the cables disappear completely (Goldberger). However, as the viewer moves further away, they re-appear. Such a phenomenon appears due to the fact that when viewers are in a direct axis with the pylons, they are able to blend with the background thus creating the illusion of disappearance.
Analytical concepts associated with cable-stayed bridges
Loads and equilibrium
One of the most important analytical concepts associated with cable-stayed bridges pertains to how they work, that is, how loads are distributed in the structure and equilibrium achieved. However, before expounding this concept, it is important to define two forces that act on bridge structures: compression and tension. A simplistic definition of the terms delineates the former as a force that tends to squeeze or compress objects while the latter is defined as a force that tends to stretch or elongate objects (Lamb and Morrissey).
The authors further note that due to the increased gravitational forces that act on horizontal bridge decks, additional support is required in order to attain an equilibrium level and support load. One engineering concept that enables cable-stayed bridges to attain this equilibrium, is by implementing a vertical pole (tower) at its centre and attaching cables from the deck to the tower. As a result, tension forces act on the cables while compression forces act on the tower thereby generating an equilibrium. Figure 3 below illustrates the different forces acting on a cable-stayed bridge.
Figure 3. Forces in a cable-stayed bridge (source: Hasan)
From the figure, it is observed that tension forces act on the cables that are attached to the central tower due to the weight of the deck. On the other hand, compression forces act on the tower in a downwards manner. As a result, the two systems of forces cancel out thereby generating an equilibrium level.
Material considerations
A second aspect that engineers have to consider when designing cable-stayed bridges is the nature of the materials to be utilized. A key factor that helps ascertain this aspect is to determine the maximum load anticipated in the bridge. To this effect, heavy bridges are built using steel cables and concrete bridge towers to support the loads. However, there is also a need to consider aspects such as the angle at which cables should be attached to the tower, their breaking point forces and the maximum loads supported by the deck. Engineering formulas are however provided to determine each of these aspects.
Conclusion
The current report expounded on cable-stayed bridges with particular focus on the Sunshine Skyway bridge in Tampa. It also highlighted notable differences between the bridges and suspension bridges which also have cable structures. The history of Skyway bridge was then discussed and key underlying motivations for its construction identified. Design considerations for the construction of the bridge and its structural behaviours were also outlined. Finally, a discussion on some analytical concepts associated with cable-stayed bridges was carried out.
References
Bell, Ronald. "What Is the Difference Between a Suspension Bridge " A Cable-Stayed Bridge?" Career Trend. N.p., 2017. Web. https://careertrend.com/about-6721625-difference-suspension-bridge-cable-stayed-bridge-.html
Accessed 25 Oct. 2018.
Goldberger, Paul. "Architecture View; A Breathtaking Bridge Soars High Over Tampa Bay." Nytimes.com. N.p., 1988. Web. https://www.nytimes.com/1988/10/16/arts/architecture-view-a-breathtaking-bridge-soars-high-over-tampa-bay.html
Accessed 25 Oct. 2018.
Hasan, Hussein. "Suspension Bridges VS Cable-Stayed Bridges." Slideshare.net. N.p., 2016. Web. https://www.slideshare.net/HusseinZidan/suspension-bridges-vs-cablestayed-bridges
Accessed 25 Oct. 2018.
Lamb, Robert, and Michael Morrissey. "How Bridges Work." HowStuffWorks. N.p., 2000. Web. https://science.howstuffworks.com/engineering/civil/bridge7.htm
Accessed 25 Oct. 2018.
PBS. "Building Big: Databank: Sunshine Skyway Bridge." Pbs.org. N.p., 2001. Web. http://www.pbs.org/wgbh/buildingbig/wonder/structure/sunshine_skyway.html
Accessed 25 Oct. 2018.
Verdict Traffic. "Sunshine Skyway Bridge, Florida - Verdict Traffic." Verdict Traffic. Web. https://www.roadtraffic-technology.com/projects/sunshine-skyway-bridge-florida/
Accessed 25 Oct. 2018.
