As the United States undertakes efforts to upgrade its ageing infrastructure, the cable-stayed methods fast becoming the preferred design option for new bridge construction.
The cable-stayed method involves the use of cables suspended from one or more pylons to support the bridge deck. The design has a lengthy history, with the earliest examples dating back to 16th century Italy.
The pylons themselves serve as the chief load-bearing structures, transmitting the bridge loads to the ground. The cables directly connect the pylon to the bridge deck, with no need for firm anchorage.
Cable-stayed designs are considered by many modern engineers to be the optimal design for bridges of moderate dimensions, those with measurements between the lengths suited to the shorter cantilever bridge or the larger suspension bridge.
The cable-stayed method best suits bridges whose longest road deck measures between 150 and 915 metres in length. Suspension designs are preferable for much longer bridges, as when the main span is greater than 915 metres physics requires that the towers of cable-stayed bridges rise to twice the height of suspension bridges.
Within the appropriate span range, the cable-stayed method permits the use of thinner decks than cantilever bridges, which become far heavier as their spans increase in length, while making use of fewer cables than the more complex suspension bridge.
This makes the cable-stayed design a far more economical option for many bridges due to their reduced material requirements and shorter construction time.
The design has thus become a popular choice for the development of new bridge, with Andrew Hermann, former president of the American Society of Civil Engineers, calling the cable-stayed design the current “go-to type” in the US.
The method is also being applied to bridges of greater dimensions via the deployment of multiple spans, as exemplified by the use of a cable-stayed design to replace New York’s Tappan Zee Bridge across the second-widest point of the Hudson River.
The original cantilevered bridge, which was built in the early 1950s, has already long gone past its original 50-year service life, and incurs maintenance costs of as much as US$50 million a year.
The replacement for the Tappan Zee is the New NY Bridge – a cable-stayed dual span twin bridge that is currently being built on the northern side of its predecessor.
The new bridge will extend five kilometres across the Hudson River, with the longest stretch of roadway slated for installation measuring 365 metres in length. According to David Capbianco, a NY Bridge project manager, this length puts it well within “the sweet spot” of what is permitted by cable-stay designs.
The bridge is expected to cost between $5 to 6 billion, and will feature a total of eight lanes as well as a shared-use bicycle and pedestrian path upon its expected completion date of 2017.
Cable-stayed bridges are not without their shortcomings. Cable corrosion and vibrations can be major issues for this type of bridge design, and both of these problems will be especially acute for the New NY Bridge as it is situated in a high wind area above the lower Hudson’s briny waters.
The New NY Bridge is nonetheless expected to enjoy an operating life of as long as a century – more than twice that of its deteriorating predecessor. Engineers hope to overcome the problems traditionally associated with cable-stayed designs via the installation of wireless sensors, which will enable them to collect and analyse data on the corrosion of cables and monitor vibration levels.
Australia is no stranger to the cable-stayed bridge, with prominent examples including Sydney’s iconic ANZAC Bridge – Australia’s longest cable-stayed bridge with a main span measuring 345 metres in length; the Eleanor Schonell Bridge in Brisbane and the West Gate Bridge that crosses the Yarra River in Melbourne.