The physics & engineering behind the Ravenel Bridge

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It has several names. The Cooper River bridge, the Ravenel, or simply “the bridge.” 

Connecting Downtown & Mt. Pleasant, it is a stunning backdrop for photos, a favorite for runners, and is one of the defining landmarks of Charleston.

Built to replace the old parallel bridges that had become outdated and unsafe, ground broke on the Ravenel Bridge nearly 20 years ago with construction finishing a year ahead of schedule in 2005. Millions of visitors and commuters have passed under its double diamonds since then… likely not considering engineering and science principles at work to keep the bridge stable! 

Let’s first break down the numbers. The bridge is 13,200 feet long with two 575 foot tall towers. 128 cables are anchored to these iconic double diamonds which suspends the bridge nearly 180 feet above the water.

It’s those cables where this bridge design gets its name- a cable-stayed bridge.

The Golden Gate Bridge, a suspension bridge, in San Francisco compared to the Ravenel Bridge, a cable stayed bridge in Charleston.

At a glance, a cable stayed bridge like the Ravenel looks similar to a suspension bridge like the Golden Gate, however, their designs are quite different.

Suspension bridges have large main cables that stretch between two towers with smaller vertical cables connecting the main cable to the bridge deck. Compare this to our cable-stayed bridge with cables running directly from the tower to the bridge deck. In this design, the towers are the main load-bearing structures, compared to a suspension bridge in which the weight is distributed between massive main cable anchors at both ends of the bridge and the towers.

In the end, it’s two different ways to balance out the same forces at work- tension & compression. Let’s take a closer look at what these forces are, by looking at something we all use. Or rather, what’s inside some pens- a spring.  If you squeeze a spring, it compresses and shortens, when you pull it apart, it creates tension and the spring lengthens.

In both bridge designs, the towers take care of the downward compression force caused by the weight of the bridge & everyone on it, while the cables, under tension, balance the vertical forces at work to keep the bridges strong and stable. Too much compression, weight- and the bridge will buckle, too much tension in the cables and they’ll deform, or even snap. Smart engineering ensures this doesn’t happen by spreading and dissipating tension among many cables and transferring incredible amounts of force caused by the weight of the roadway to reinforced towers.

With these principles in mind, our iconic bridge has been built to last and withstand everything that could be thrown at it, including wind gusts in excess of 300 mph and earthquakes above 7.0 on the Richter scale. That’s something to consider on your commute or on your weekly run over this Lowcountry landmark. 

Storm Team 2 Meteorologist David Dickson

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