Repair of the earthquake-damaged Washington Monument will be a complex project that could require some kind of external scaffolding as well as months of work that will probably keep the monument closed until 2013.

National Park Service officials detailed some preliminary work plans Thursday as they announced a private donation of $7.5 million toward the project. The federal government is paying the rest of the estimated $15 million tab.

The 555-foot-tall monument, which is entered by about 600,000 visitors each year, has been closed since the 5.8 magnitude earthquake struck the area Aug. 23.

Bob Vogel, superintendent of the National Mall and Memorial Parks, said at a news conference Thursday at the monument that the Park Service has awarded a contract for the planning phase of the job.

The firm of Wiss, Janney, El­­stner Associates, which used workers suspended on ropes to conduct the exterior inspection of the monument after the quake, already is designing the plan, Vogel said.

The contract for the execution of the repairs will probably not be awarded until late summer, with work starting sometime after that.

“This is a complex job,” he said. “This is a one-of-a-kind structure that poses challenges for repair that other buildings don’t.”

The monument shook violently during the quake, especially near the top, and many of its stone blocks were cracked and chipped on both the interior and exterior.

It was not clear if the work will require extensive scaffolding like that which encased the monument during repairs in the late 1990s, or the kind used by high-rise window washers, said Sean Kenneally, the park’s chief of facilities maintenance.

Park officials said they were not sure if the monument would remain closed the whole time, but Vogel said that was likely.

The news conference was called to mark the donation to the monument by David M. Rubenstein, a billionaire Bethesda philanthropist, through the nonprofit Trust for the National Mall.

“I would suggest that [the monument] hadn’t even stopped shaking before David Rubenstein came to me and asked if he could help,” said Jonathan B. Jarvis, director of the National Park Service.

Rubenstein, 62, is co-founder and managing director of the Carlyle Group, a global asset-management firm that handles $148 billion in assets, according to its Web site.

He also is a member of the board of regents at the Smithsonian Institution and chairman of Washington’s John F. Kennedy Center for the Performing Arts, where he is the single largest donor in its history, with gifts totaling $25 million. Forbes magazine estimates his worth at $2.7 billion

Rubenstein, the son of a Baltimore postal worker, said he has been fortunate in life and wants to donate to worthy causes. “I don’t want my executor to give it all away,” he said. “I’d like to have the pleasure of giving away to things that I think are good while I’m alive. The country’s been wonderful to me. The city’s been wonderful to me and my family.”

He said he toured the monument last week, riding the elevator up but taking the stairs down. “It’s fortunate that they didn’t have me walk up,” he joked at the news conference Thursday.

Jarvis said he was not sure how long the monument would be closed. “Our hope is to get it back open as quickly as possible,” he said. “I think it’s going to be some time in ’13, though, before it’s fully back operational.”