Boston Building Babble: Zakim Bridge

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Zakim and the Garden

Ok so… It may not be a building… But it is one of my favorite structures in Boston.

Up this week, Leonard P. Zakim Bunker Hill Memorial Bridge.

Zakim Bridge #2

I think we can all agree this bridge is cool.  It is cool in looks, and the procession into the city, but also in the geeky structural engineering sense.

Charlestown & Zakim Bridge _IMG_ 3451

The Zakim holds the title for the widest cable-stayed bridge in the world.  It is also the first cable-stayed bridge with an asymmetrical deck. The main deck has 4 lanes of traffic running north and southbound and has two lanes (Leverett Connector) cantilevered off the main form. The structure in the end is not only asymmetrical in plan, but also elevation and section.

Zakim Bridge

So what is a cable-stayed bridge, you may ask? Isn’t the Zakim a suspension bridge? Yes, the design of the Zakim Bridge appears to be a suspension bridge, but other than in basic appearance, the two bridge designs have very little in common.

In suspension bridges, the cables merely rest on the central towers.  The load is supported by tying back and anchoring the cables at either end of the bridge. The cables are then embedded in solid rock or man-made concrete block structures, and pulled taut.

In a cable-stay bridges, the design does not required anchorage into an embankment. Cable-stay bridges are used when the ground cannot support this type of connection, and can also produced some very creative forms depending on the connection method.

This design provides a more cost effect bridge than a suspension bridge because the two halves of the bridge are cantilevered from the towers and not requiring this embankment. The load of the bridge is supported by a central tower or towers that acts in compression.  The attached cables act in tension pulling the deck into compression. The tower becomes the main support with everything else essentially cantilevered off. This assembly, allows the bridge to be constructed building out from the towers, which is not so in suspension bridge design.  Once the towers have been constructed in a suspension bridge, steel cables have to be strung across the entire length of the bridge.

Boston's North End and Greenway

Ok so I may have really geek-ed out within this installment of Boston Building Babble… Hope you enjoyed.

Boston Building Babble gives you weekly insight on buildings across Beantown. Feel free to use the random facts you learn to impress your hairstylist, bartender, or seatmate on the T.
Any suggestions for next week, feel free to contact me!