Karsten Thot bridge is heavily used, old, and a victim of rust and weather.
The State Department of Transportation said inspections this summer of the 80-year-old metal truss bridge showed it needs repairs right away.
"There is a lot of rust under there. So they're going to replace the rivets and whatever steel beams need to be fixed as well," DOT spokesperson Caroline Sluyter said.
At midnight Sunday, the state closed Kamehameha Highway between Kilani Avenue and Whitmore Avenue.
North Shore neighborhood board chairman Mike Lyons said nearby communities weren't warned, and that's troubling.
"Are ambulances going to be delayed if they've got to go to Whitmore or if they've got to go someplace else? Is a fire truck going to be delayed?" he said.
"It just came out of the blue to us. And that's not good. We need to prepare for this. This is a big deal," Wahiawa neighborhood board member Robert Lormand said.
The bridge work will take six weeks to complete. During that time traffic will be detoured to Wilikina Drive and Kamananui Road.
"I'm expecting everybody going to work from the North Shore is going to have to add at least half an hour to 45 minutes to their already hour to two-hour commute," Lyon said.
"It is unfortunate that we have to completely shut it down. We do apologize to the community because we realize that it will be an inconvenience," Sluyter said.
The last major overhaul of the bridge was done in 1997.
Sluyter said a total shutdown of the bridge is necessary so repair crews can work safely and swiftly.