John Clark was the lead designer of the current bridge (Seattle Times, though Puget Sound Engineering Council lists him as one of the consulting engineers for the high bridge and the lead designer of the bascule bridge), which opened in 1984. He’s also the man responsible for telling the city to just epoxy over the cracks and attach some strain monitoring equipment in 2014 and later to utilize carbon fiber wrap, additional epoxy grout and additional PT cables. He was employed by Anderson, Bjornstad, Kane, Jacobs from 1979-1997 and was a part of of numerous other bridge projects in Washington.
John Clark was also heavily involved in numerous other bridge projects in the PNW as well as one of the investigators in the failure of the Lacey V. Morrow I-90 bridge.
I’ve been unable to find who the contractor was but there were concerns, long ago, that there were issues with the concrete but I don’t know if those concerns were related to the constructions of the piers, which were poured in place, or if they were with the bridge beams which would’ve, likely, been poured offsite, barged in and lifted with a crane.
There have been many points of failure along the lines that point to systemic issues rather than any one single point of failure, which is not to say that, with a failure of the structure, closure wouldn’t have been the result anyway though it’s possible, not to say improbable, that improved maintenance would’ve extended the life or obviated the need for closure.