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The Multnomah County Board of Commissioners gave the go-ahead Thursday for further exploration of four final alternatives for either seismic retrofitting or replacing the Burnside Bridge.

Commissioners unanimously approved a resolution that identifies the four project possibilities, including three full replacement options. In addition, the resolution adopts a purpose-and-need statement developed over the course of two years of public outreach carried out by the county.

鈥淭he four options we’re presenting to you today really have pretty broad support,鈥� county public information officer Mike Pullen told commissioners. 鈥淭he tunnel suggestion comes up occasionally, usually from people who don’t know about the cost of a tunnel, which is usually about four to five times more than a bridge.鈥�

The decision marks the end of a two-year feasibility study that whittled down the number of options from over 100 to four. Three of the final options involve construction of a new replacement bridge while the fourth involves a full seismic retrofit and replacement of a section of the bridge spanning Interstate 5 and Interstate 84 on the east side of the Willamette River.

The estimated project costs vary significantly depending on whether Burnside Street would be kept open to vehicle traffic during construction 鈥� a process that would require a temporary bridge structure and add more than $100 million to the cost of each option.

For example, the cheapest option is the enhanced seismic retrofit, which would cost roughly $575 million with Burnside Street closed to traffic or almost $700 million with traffic retained.

Two other options involve a movable bridge much like the one now. One of those options, however, would involve building two ramps on the east side of the river 鈥� one connecting the bridge with Burnside Street eastbound, and the other with Couch Street heading westbound. The second iteration would simply connect Burnside Street on each side of the river.

鈥淚t’s kind of an in-kind replacement of what’s there today with the exception that it would be widened,鈥� explained Heather Catron, a project manager with HDR Engineering, which led the feasibility study. 鈥淚t would be in the existing alignment that is there today.鈥�

Both of those options would cost more than $650 million each with traffic halted during construction or nearly $850 million with traffic retained.

The fourth alternative would feature a fixed-span bridge up to 97 feet in height. This is the most expensive option and likely would be the most disruptive to existing structures and roadways on both sides of the Willamette River. The cost estimate is almost $700 million with traffic stopped, or $900 million with traffic retained.

鈥淲hen we kicked off the capital improvement plan in 2015 we set out a 12-year timeline to have a resilient crossing in place,鈥� county Engineering Services Manager Megan Neill said. 鈥淭wo years ago we started two projects to achieve the vision. One is a general maintenance project for the Burnside Bridge 鈥� repairs that will prevent interruptions of service. It’s a 92-year-old bridge, so that project addresses a lot of crumbling concrete and rusting steel, and a lot of mechanical components inside the bridge that have reached the end of their service lives.鈥�

The second, of course, is the bridge’s replacement or seismic upgrade. And if the four preferred alternatives ultimately prove infeasible because of costs or other variables, the county retains the option of doing nothing at all.

But looming over project planning is a one-in-three chance of a sizable earthquake occurring sometime during the next 50 years.

鈥淲e have many constraints that we’ll be dealing with as we deliver this project,鈥� Neill said. 鈥淭his is a project of a size that the county has never attempted before, so there will be a lot of firsts.鈥�

The next step involves the start of an estimated three-year environmental review at the federal level. At the same time, local stakeholders led by the county will work to identify any possible funding sources. If plans come together, design work could start in late 2021 and construction could begin by mid-2024. If the project still is on schedule at that point, construction could wrap up by the end of 2028.

鈥淚t is an incredibly complex project and it’s going to be a major change for Portland,鈥� Commissioner Jessica Vega Pederson said.