MARYSVILLE — Crews working to replace the aging Highway 529 bridge over Ebey Slough will reach a major milestone this month when they finish setting the girders that will form the backbone of the new wider, taller bridge.

Less than a year after starting construction, crews working for the state Department of Transportation began setting 49 girder sections the first week of June. They expect to have all of the girders in place by the end of the month, when they’ll shift their focus to building the road deck on the bridge.

“Regular commuters have probably noticed this very visual sign of our progress during the last two weeks,” said Janice Fahning, DOT project engineer. “We worked hard this winter and spring to get to this critical milestone. It’s the last major step in getting the new bridge ready to open to traffic next April.”

Each girder section is seven feet tall, weighs about 31 tons and is 100 to 135 feet long — about the length of three big school buses. Crews will assemble the sections to form seven massive girders that will span the slough with a new 680-foot-long bridge.

“Girders are an integral part of any bridge because they make up the spans of the structure and help distribute the traffic load to the bridge’s foundations,” said Fahning. “Once all of the girders are in place and bolted together we’ll start building the deck for the new bridge.”

Crews are building a new four-lane, fixed-span bridge next to the existing bridge. They expect to begin shifting northbound Highway 529 traffic to the new bridge by April 2012, with all traffic on the new bridge by August 2012. They’ll then demolish the old bridge. Work on the $42.3 million project is expected to be complete by the end of 2013.

Highway 529 is a main commuter route between Marysville and Everett and is used by about 17,000 drivers each day. The 86-year-old existing bridge is at the end of its service life and does not meet current traffic needs. It has only one traffic lane in each direction and one sidewalk. The new bridge will have two traffic lanes, separate bicycle lanes and sidewalks in each direction.

For project details, graphics, photos and video, go to www.wsdot.wa.gov/projects/sr529/ebeysloughbridge.