Cable-Stayed Bridges - Cooper River Bridge (4)

We are continuing to study different aspects of the Cooper River Bridge.
The cables are arranged in what's called a semi-fan arrangement around the towers.

In a harp arrangement, all of the cable are parallel. In a fan, the cable on one side of the tower has the same angle on the other side and they are all congregated near the top of the tower.

This bridge has a semi-fan arrangement which means each cable has the same angle on both sides of the tower, but the cables are spread along the top of the tower.

Since the cables become less effective at carrying vertical load as their angle becomes less steep, the semi-fan arrangement seems optimal. More cables can be directed to the center of the span where because they are less effective at lifting while fewer cables can be used to support the span by the towers where they are more effective.

Apparently, the towers were quickly constructed using self-climbing forms and the lowest cables were hung and began supporting deck segments before the tower was even completed.

Cable-Stayed Bridges - Cooper River Bridge (4) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Cable-Stayed Bridges - Cooper River Bridge (3)

It began to rain softly as we walked along the deck of the new Cooper River Bridge.  I can see several raindrops on the camera lens obscuring this photo.

The new bridge has four lanes in each direction. In some of the computer graphics they showed an asphalt deck, but in this photo it looks like a reinforced concrete deck. I would be surprised if asphalt was used since it weighs so much. In California, an acrylic or polymer coating is often used for deck protection.

The 1929 bridge had two 10 ft lanes and the 1966 bridge had three 12 ft lanes, and the third lane was made reversable resulting in several head-on collisions. Maintenance wasn't being performed regularly on the bridges and traffic loads had to be reduced. Also, modern container ships couldn't go under the existing bridges. All of these problems were solved with the construction of the New Cooper River Bridge.

Cable-Stayed Bridges - Cooper River Bridge (3) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Cable-Stayed Bridges - Cooper River Bridge (2)

I must be standing on a platform extending out from the strut past the bridge tower. In this photo we have a nice view of one of the gravel islands built to protect the towers from ship impacts. The theory is that ships will run aground before they strike the tower. However, it looks to me that if the ship had a projecting element, it could still strike the tower. Look at that barge carrying the crane. If it was a container ship with a large prow, I could easily strike the tower.

This is another all concrete bridge like the previously studied Sunshine Skyway Bridge. However it appears that we've learned a few things about cable-stayed bridges in the intervening twenty years. The towers now have a more harmonious transition from under the deck to over the deck (if you don't mind the tower swelling around the deck). I also feel more comfortable with two planes of cable stays to support the superstructure (the bridge is designed for a M7.4 earthquake a few miles from the bridge site). And look at the fancy dampers and anchors that connect the cables to the deck. Also, the main span is somewhat longer (although still dwarfed by China's and Japan's cable-stayed bridges). The prestressed concrete superstructure is not directly supported by the towers (rubber snubbers prevent the superstructure from smashing into the tower legs during earthquakes and wind storms).

I have had nothing but trouble with design-build contracts. I don't know anyone who likes them. Usually the contractor goes broke, and the owner loses control of the project (the contractor is waiting for the owner to make a change so they can recompense their losses). I don't know if this contract was a happier experience. A friend at SCDOT told me they weren't specific enough in requiring on and off ramps at both ends of the bridge and paid dearly for that change order.

Note the bike lane sitting outside the cables. I believe this was also added after the contract began (hopefully without a painfully expensive change order). Bicyclists and pedestrians have to share this space.

Cable-Stayed Bridges - Cooper River Bridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Cable-Stayed Bridges - Cooper River Bridge

Ports can have a big impact on bridges. The new Cooper River Bridge replaced the 1929 Grace Memorial Bridge and the 1966 Silas Pearman Bridge with a structure that provides 58 meters of vertical clearance for ships heading up the Cooper River to the Port of Charleston. Although this is less than the 70 meters required by the largest container ships, it was sufficient incentive for the Port to put $50 million into the fund to replace the cantilever truss structures. Of course, that was a small fraction of the $700 million cost of this design-build project.

The truss bridges in the photo (taken while construction was being completed in 2005) are now gone. If you look closely you can see the concrete roadway suspended inside the diamond-shaped towers (and crossing over the old bridges). The Cooper River (or U. S. Senator Arthur Ravenel Jr) Bridge currently is the longest cable-stayed span in the U. S. at 471 m (1546 ft). We'll take a closer look at this interesting bridge tomorrow.

Cable-Stayed Bridges - Cooper River Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.