Stuck halfway between Hong Kong and Montreal, Melanie Pinard sat on one of her bags at the United Airlines departure area at San Francisco International Airport Monday morning wondering how to get home.

When her original flight was canceled, United rebooked her on a flight to Calgary, Alberta, Tuesday morning — but didn’t give her a hotel. An airline spokeswoman said United covers hotels when cancellations are within their control, but this one wasn’t since the airport was responsible for construction delays.

“I’m tired, it’s frustrating,” Pinard said. “San Francisco is nice but ... that’s not my plan. I have things to do.

“They should have told me if they already knew,” she added.

Over the weekend, SFO turned into travel purgatory for hundreds of travelers delayed or stranded because of canceled flights after the airport closed one runway Saturday to fix its crumbling pavement. The three-week construction project was announced six months ago, and airlines reduced flight schedules by 13% to prepare. But the delays left some wondering whether more changes could have been made to reduce the headaches travelers encountered.

More than 800 flights — 1 in 5 scheduled — were delayed between Saturday morning and Monday afternoon, according to flight tracking site FlightAware. Another 375 flights were canceled. The average delay over a 24-hour period was 60 minutes, SFO spokesman Doug Yakel said, which was expected. SFO is the nation’s seventh-busiest airport, with more than 53 million passengers in 2016, according to Federal Aviation Administration data.

Officials knew that closing one of the busiest runways at the airport would cause major disruptions and leave some travelers marooned for hours. But they had no choice, said Yakel. In 2017, airport engineers saw that the 1,900-foot strip on Runway 28L was in bad shape, with pavement deteriorating in the concrete base layer between the runway surface and the soil. Crossed by two other runways, the half-century-old section was built at a time when planes were lighter and flights less frequent.

In March, SFO announced it would close the runway for construction from Sept. 7-26, warning of two-hour delays. The construction is expected to finish the morning of Sept. 27.

“This has become a massive mess in an awfully short time,” said Henry Harteveldt, a travel industry analyst with Atmosphere Research Group. “This is clearly something that was not planned and executed as well as it needed to be.

“Did FAA at SFO fully understand the implications? Did the airport share everything needed to with the parties involved? And did the airlines listen? Should the airlines have cut more flights?” Harteveldt asked.

New York’s John F. Kennedy International Airport, the nation’s fifth-busiest, is in the middle of a six-month project that closed one runway that normally handles a third of arrivals. As of Monday, JFK was not reporting unusual delays, according to FlightAware.

But SFO is unusually constrained; any expansion of its runways would require filling in the bay, and a plan to do so faltered against strong environmental opposition in 2001.

SFO said it chose September because construction requires dry weather and it avoided the busy summer and holiday travel seasons. But the airport closure still falls during the Oracle OpenWorld convention at the Moscone Center, which draws roughly 60,000 attendees, as well as several health care conferences. Observers said it’s hard to predict the economic impact of the delays and cancellations. “There’s no doubt this construction project and resulting delays will leave a very bad taste in people’s mouths of San Francisco as a place to do business and visit,” said Harteveldt.

The airport has no authority to tell airlines how many flights to operate, “so all schedule changes were voluntary,” Yakel said. The FAA, similarly, has little ability to tell airlines how to handle an interruption. Its role is safety and air-traffic control; it can ground flights when there’s not capacity for them to take off or land, but it can’t dictate airline schedules.

“Ultimately, when someone buys an airline ticket they’re essentially entering a contract with the airline,” Yakel said. “At that point, the onus for notification is on the airline.” He noted, however, that airports share a responsibility to advise travelers of work that could cause delays.

United, which has a major hub at SFO, accounted for 45% of the airport’s traffic last year, the most of any airline. In July, United reduced the number of flights scheduled for the September construction period by 10% and rebooked customers on the canceled flights, said airline spokeswoman Maddie King. On Aug. 26, the airline offered waivers to all passengers with flights scheduled from Sept. 7-26, allowing them to change flights or connect through a different airport without paying a change fee.

If you’re traveling On United: Flights to or from San Francisco for travel Sept. 7-26 booked on or before Aug. 25 can be rescheduled without a change fee or extra fare. For flights booked after Aug. 25, the change fee is waived. 800-864-8331, united.com On American: Customers whose flights were rescheduled can change flights without fees or seek a full refund. 800-433-7300, aa.com On Alaska: The airline recommends signing up for flight notifications and checking flight status. 800-654-5669, alaskaair.com On Southwest: The airline is allowing passengers to change flights to other Bay Area airports. 800-435-9792, southwest.com For other airlines: Go to flysfo.com and navigate to Flight Info > Airlines at SFO for contact information. Sources: San Francisco International Airport, Chronicle research

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United introduced a larger 777 aircraft between SFO and Los Angeles International Airport daily to reduce flight numbers. It is also diverting some flights to the San Jose airport and busing customers to SFO. They’re also sending text notifications and updates to affected passengers.

“We’re constantly monitoring the conditions and our team at SFO is working around the clock to provide relief to customers who are experiencing longer delays,” said King in an email Monday. “We knew the airport closing one of the runways would be a challenging period which is why we offered the waiver in advance and started communicating to customers about the closure and the impact it would have on our operations.”

Alaska Airlines, the second-largest SFO operator, said it trimmed its flight schedule and added extra time between flights. The airline is not offering travel waivers but said it will closely monitor the impact of the runway work and make adjustments if needed.

American Airlines, which operated 8% of SFO flights last year, reduced its flight schedule by 15%. The airline issued a travel alert and let customers whose flights were canceled or rescheduled make additional changes or request a refund without additional fees, said spokesman Curtis Blessing.

Southwest reduced its schedule by about 10%, said spokesman Dan Landson, and proactively canceled 11 departures Monday. The airline is working to get customers to destinations as quickly as possible and offering flexible accommodations to change to other California destinations.

Airport Commission President Larry Mazzola shrugged off the delays and inconvenience to passengers.

“There’s always delays when you shut a runway down,” Mazzola said. “It’s something that has to be done for safety.”

Other commissioners said there was only so much airlines and airports could do to deal with construction.

“We don’t run the airlines, we only run the airport,” said Commissioner Eleanor Johns, who was among the travelers affected by the shutdown.

She said she called JetBlue a few days before a flight Saturday from Long Beach to San Francisco on the first day of the runway closure. The customer service representative from the airline didn’t know about the closure. Johns’ flight was delayed 4½ hours. JetBlue did not respond to a request for comment; a page on its website for airport-specific conditions does not mention delays at SFO.

“When I landed at 5:30 p.m., I saw all the people waiting,” she said. “I really felt for them.”

Still stuck at SFO Monday afternoon, waiting passenger Pinard was scanning the terminal, looking for somewhere to spend the night.

“Is there even somewhere to sleep?” she said.

San Francisco Chronicle staff writer Alejandro Serrano contributed to this report.

Mallory Moench and Rachel Swan are San Francisco Chronicle staff writers. Email: mallory.moench@sfchronicle.com, rswan@sfchronicle.com Twitter: @mallorymoench, @rachelswan