Why bridges (and overpasses) are like butterflies

Over the weekend, crews closed State Route 99 through downtown Seattle for an important reason: the South Atlantic Street overpass needed space to emerge from its cocoon. Don’t worry, it wasn’t flying anywhere. And it isn’t nearly as colorful as a butterfly. Still, you’ll notice a huge difference the next time you take SR 99 through SODO.

For nearly a year, hundreds of steel and timber beams, and acres of plywood and foam spanned the highway in roughly the same shape as the future overpass, which is located to the west of the stadiums. Collectively, this temporary structure is known as falsework. It’s the “cocoon” that holds the permanent rebar and concrete in place until the overpass is strong enough to stand on its own.

For the Atlantic Street overpass, that time came over the weekend, when crews removed the falsework above the highway to reveal the shape of the overpass. You might not find it beautiful, but the difference is striking.

When it opens late this year, the overpass will allow traffic to bypass a busy railroad track that crosses Atlantic. Crews still have to complete sidewalks and railings, install lighting and utilities, stripe the pavement and connect the overpass to the streets below. You can track their progress on our construction cameras