DANBURY — Improvements to a pair of small Danbury bridges over the Still River could cause large headaches for some commuters this summer.

The plans call for the closure of the Triangle Street bridge, just north of the Department of Motor Vehicles office, for about a month in mid- to late summer to make repairs to the innards of the short span.

Water intrusion has slowly damaged some of the concrete around the piers of the bridge over many decades and it needs a new drainage system to alleviate some of that pressure, said city engineer Thomas Altermatt.

The upgrades include repaving the bridge, but that will require a detour at a chokepoint on the heavily used road, he conceded.

“A lot of the concrete work underneath the bridge can be done while the road’s open, but some of the work just has to be done when the road’s closed,” he said.

The detour is expected to turn cars on Triangle away from the bridge at Lee Mac Avenue to the south, where the DMV is, and Taylor Street to the north, where the enormous electrical transformers are, according to the plans. Drivers would be expected to get around the work using Sheridan and Casper streets just west of the river.

The detour should last no longer than a month — “worst case scenario” — but it could extend the life of the bridge another 30 or even 35 years, said Rifat Saleh, president and chief engineer at RHS Consulting, who designed the project.

At the same time in the heart of downtown, crews also will entirely rebuild the connector bridge that links the north end of the White Street shopping center parking lot to Crosby Street.

State inspections of the bridge over the past two years have found the bridge is in “very poor condition,” Altermatt said. The decades-old span simply needs to be replaced with new concrete beams and a new slab, instead of further rehabilitation, he added.

That will not require lane closures on Crosby, but the bridge will be completely closed to both cars and pedestrians during the three-month construction. Traffic and in out of the shopping center will be limited to its White Street driveway and the narrow exit onto Main Street for Webster Bank customers.

Crews have planned the project, also slated to start in mid- to late summer, with the help of businesses in the center to reduce the project’s footprint and keep almost all of the parking lot open, Saleh and Altermatt said.

The Triangle Street project will cost $500,000 and the Crosby Street project is estimated at $1.5 million, with state funding picking up half the costs for each. Both projects are in their final design and permitting phases and will be bid early this spring, Altermatt said.

The projects pale in comparison to an upcoming plan to rebuild a portion of the Kennedy Avenue bridge, which is hardly noticed as a bridge at all.

State inspectors have found that bridge and channel — taking the Still River past Kennedy Flats and underneath Kennedy Avenue and Kennedy Park — also need repairs after more than 50 years.

That work is still in its early design phases, but it will require opening up both the park and Kennedy Avenue to repair the structure and put a protective slab over it — causing major traffic snarls in the process, not only for the thousands of drivers who pass through there but for the regional bus transfer station that sits directly on top of the work site.

The project is slated for construction in spring 2020 and much more work needs to be done to plan the timing and traffic detours necessary to complete it as quickly as possible, once it does start, Saleh said. The work is estimated to cost $2.9 million, half of which the state will fund.

“We know there will be substantial impacts to the commuters and public, but it needs to be done,” Altermatt said. “This bridge was built back in 1964. It’s time to repair it so we get another 20, 25 years out of it. It’s a major structure to the city.”