Monday, May 23, 2011

It’s a bird… it’s a plane…no, it’s the super girder

By guest blogger Hilary Bingman

Driving south along the SR 99 Alaskan Way Viaduct, you may have noticed a lot of really big things in our construction zone: gigantic cranes, towering columns, huge machinery, and colossal trucks. These are the tools needed to build sections of the new bridge that will replace the viaduct’s south end. On May 19, something even bigger arrived and was added to this gargantuan list – “super” girders.

For a quick lesson in Bridge Building 101, girders are simply support beams used to support the roadway of a bridge. They serve as the heavy duty backbone connecting the bridge columns and form the base underneath the new roadway you’ll soon be driving on.

Girders vary in length, depending on the amount of space between the columns. The smallest girders we use are about the length of an articulated bus – the double-length ones that bend in the middle.  However, several of the girders we’re using to replace the south end of the Alaskan Way Viaduct are the largest ever produced in Washington state. Weighing in at 127 tons a piece, these “super” girders stretch to a length of 205 feet.  That’s as long as a Boeing 787 Dreamliner.

Each “super” girder is custom manufactured in Tacoma and transported on a truck specially-designed to hold the weight.  Due to their size, the girders must be transported to the construction area and set into place during the middle of the night to avoid shutting down major downtown routes. Once the girders arrive on site, crews use two cranes to set them on top of the new bridge column.

The girders will span the intersection of S. Atlantic Street and Alaskan Way S. and the railroad tail track, near the entrance of the Port of Seattle’s Terminal 46. This intersection sees a lot of activity – including freight trucks, trains, commuters, bikes and pedestrians.  Using “super” girders to span the intersection avoids placing new columns where they may affect traffic flow through the intersection.

We’re proud of our “super” girders. On the viaduct, they make all the difference.

For more information on the Alaskan Way Viaduct and Seawall Replacement program, visit