Nearly 45 years after BART started serving downtown San Francisco, Market Street stations will be getting covered and enclosed entrances to protect their escalators from the elements — and from becoming repositories of human waste, garbage and used hypodermic needles.

Two dozen modernistic glassy coverings — BART calls them canopies — will be built over the open-air entrances to the Embarcadero, Montgomery, Powell and Civic Center stations under a plan expected to be approved Thursday by the transit system’s Board of Directors.

Two entrance enclosures — at Powell Station, where Market meets Ellis and Stockton streets, and outside Civic Center Station on Seventh Street — are already under construction as test projects and are expected to open this fall.

Contracts for the remaining enclosures will be awarded next year, with construction expected to be completed in 2025. The estimated cost of the project is $45 million.

Each of the station-entrance coverings will feature a white metal roof perched on steel columns, hovering above 10-foot-high glass walls and — most important — a motorized screen that will be lowered and locked when the stations are closed to keep people from doing their dirty deeds in the stairwells and escalator landings.

“This is going to be a big, important change,” said Bevan Dufty, a BART director from San Francisco, whose district includes the Powell and Civic Center stations. “It’s a big part of the transformation of our downtown stations. Having the ability to close off station entrances at night is going to be a game changer.”

The street-level entrances have been open to the fog, rain, wind and occasional sunshine since late 1973, when BART opened its Market Street subway stations. In recent years, as the city’s homelessness and drug problems have grown, the bottoms of the entrances have become popular hideaways for people to sleep, urinate and defecate or shoot up, leaving behind human waste, debris and used needles that are not only disgusting to clean but damage escalators.

The early-morning habitants have presented an unsafe environment to BART and Muni employees who open the stations or report to work early. Some station agents have been assaulted and others harassed during the morning openings.

Protecting escalators from rain and wind-whipped debris is expected to pay off, too. In the three years since BART installed a curved glass-and-steel shelter over one of its 19th Street Station entrances in Oakland, escalator breakdowns have been down by about 30 percent, said Tim Chan, BART’s acting stations planning manager.

San Francisco officials, who have opposed populating Market Street with entrance enclosures in the past, didn’t object this time around but decided they wanted their own design, which was selected in 2015.

“What we’re trying to do with the canopy design is blend BART’s vision with the city’s vision for Market Street,” Chan said. “This was a very close partnership with the city of San Francisco. They provided a lot of input into the designs, and they’re helping to fund the canopies and the replacement escalators.”

The entry coverings coincide with BART’s plan to overhaul or replace all 40 of the breakdown-prone escalators in its downtown and Mission stations by 2027 as well as the city’s slow-moving efforts to redesign Market Street, a plan that could eliminate cars from most of the boulevard and make more room for transit, bikes and pedestrians.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan