The light-rail bridge being built in south Bellevue seems to soar unsupported over I-90. Here’s why.

BELLEVUE — Maybe you’ve wondered if Sound Transit’s unfinished light-rail bridge extending over Interstate 90 will fall at the exact moment your car or bus rolls beneath.

Contractors will vanquish any fear by mid-September, when they pour the final concrete to fuse those growing decks in the middle. Bridge sections currently end in midair above the westbound freeway and the onramp from Bellevue Way to eastbound I-90, where 85,000 motorists travel per day. One section was substantially completed in March.

So far, so good.

Sound Transit’s bridge design is known as a balanced cantilever. Hundreds have been built worldwide, including some along Sound Transit’s 2009 corridor from Rainier Beach to Tukwila.

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“It’s no problem doing work over live traffic, over rivers, you name it. It’s all done from the top down,” said Tony Gargaro, senior engineer with McNary Bergeron, which devised I-90-area work plans for the Shimmick/Parsons construction partnership.

For that reason, a wave of U.S. transit projects presents a growing market for elevated trackways, according to the American Segmental Bridge Institute. Sound Transit and L.A. Metro each won multibillion-dollar ballot measures in 2016.

The south Bellevue decks are held aloft by constant compression from each end. Every time a pair of segments is poured, a pair of steel cables are inserted and tightened at 200,000 pounds of pressure from either end, known as post-tensioning. New cables are pulled through conduits and cinched through all deck segments then in place — so that at the finish, more than 20 cable pairs compress the bridge, each pair longer than those installed the week before.

The other trick is extending the bridge simultaneously on either side of a column, like a weightlifter who adds a steel plate alternately on each side of the barbell. That way, the structure stays in balance.

The construction team uses a pair of horizontal gantries called “form travelers” — painted in Seahawks midnight blue and rave green — that are pushed ahead 15 feet for each segment. Workers adjust the forms and weave new rebar. Then the concrete is poured and hardens.

In Tukwila, transit contractors also used a gantry but hoisted 2,500 concrete segments fabricated near Wenatchee, instead of pouring fresh concrete.

Sound Transit deemed balanced cantilever to be the safest and simplest method at I-90, where the deck width and curvature don’t allow space for temporary columns or frames that would prop up a conventional girder or poured bridge.

“It minimizes our footprint on the ground, so that on a busy roadway like I-90, it allows us to maintain our construction schedule, with minimal impacts to motorists,” said executive project director Mike Bell.

The challenging part is to maneuver the traveler through curves, he said. The traveler is straight, of course, so every 15 feet it’s shifted at a slight angle. From afar, the 600-foot-long structure looks like an arc. In addition, unfinished bridge decks sag slightly as they’re extended; therefore the structure must be angled slightly higher than its final target, and constantly surveyed.

“You have to really start planning for the end at the beginning,” Bell said.  For instance, new weight exerted a 1.5-inch sag after the most recent segment pour, the engineering staff says.

Elevated bridge-building can be hazardous labor. In May 2018, Walter Burrows fell to his death near the future Overlake Village Station, while positioning a girder on a column. Three years earlier, Joe Arrants, a foreman on the state’s Highway 520 bridge, suffered a fatal fall while moving concrete forms.

Some 35 construction workers in Washington have died by falling in the past four years, the state Department of Labor and Industries says.

In response, Sound Transit and contractors imposed pre-shift inspections of safety lanyards, Bell said. Wood railings are built along overhead decks. The steel travelers provide tie-off rings overhead — safer than a low tie-off, where the exposed edge of a column or deck might break a cable.

To protect the driving public, nets are mounted below work zones, and certain jobs such as moving the traveler and pouring concrete occur during overnight or weekend lane closures. Sound Transit surveyors stood on I-90 during a closure to map exactly which bridge portion would be built over which traffic lanes. Tools are tethered. Even hard hats are connected to worker vests, by a short cable.

Overall, construction of the $3.7 billion East Link route from Chinatown International District Station to Overlake is on schedule and budget to open in mid-2023, serving a projected 50,000 daily passengers. Trains will travel 30 to 35 mph through the south Bellevue curve.