WASHINGTON -- Repairs to the Washington Monument will require massive scaffolding to be built around the 555-foot obelisk and may keep it closed into 2014 after it was damaged by an earthquake last year, the National Park Service said Monday.

A damage assessment by engineers who inspected the stone structure by rappelling from the top last September found scaffolding is necessary to provide workers access to the top of the monument, said park service spokeswoman Carol Johnson. The engineers determined the heaviest damage is between 475 feet and 530 feet on the structure, but panels are damaged from top to bottom.

“It is difficult to get to, and it’s difficult to repair,” Johnson said. “Panels stressed during the earthquake will have to have anchors installed.”

The park service offered the $15 million project up for bids from contractors on June 29. The agency hopes to award a contract and begin mobilizing the work in September. From there it will take 12 to 18 months to complete the repairs.

Robert Vogel, superintendent of the National Mall and Memorial Parks, said the $15 million price tag remains a firm estimate for the repair project. Reopening the monument as soon as possible remains a top priority for the park service, he said.

“We’re pretty much on track and trying to keep it moving as fast as it can,” Vogel said. “We obviously will have some greater clarity on the timeframe as soon as the contract is actually awarded in September.”

The monument usually has about 700,000 visitors a year who take turns riding an elevator or climbing stairs to the top.

A 5.8-magnitude earthquake centered in Virginia shook the marble and granite monument on Aug. 23, 2011, causing deep cracks to form near the top and loosening some pieces of stone and mortar. Visitors were inside the monument when the earthquake struck and fled down flights of stairs. No one was seriously injured. The monument has been closed to visitors ever since.

One cracked corner of the monument is held together by a lead joint and a lightning protection line that was also bent out of shape in the earthquake, according to the damage assessment. In some places, masonry anchors will be installed to strengthen the damaged stone.

The scaffolding that will be built for the repair project will be similar to one used when the monument’s exterior was refurbished in 1999 and 2000. Officials hope to use a decorative cover for the metal scaffolding, similar to a covering used 12 years ago that proved popular with the public when it was lit at night.

A circular stone plaza at the base of the monument and some flagpoles and benches will have to be removed to install scaffolding.

Plans for the earthquake repair project were released Monday for public comment. Engineers have not recommended further strengthening of the monument against future earthquakes, according to the repair plan.

Park service officials may install a web camera near the top of the monument to show the work being done online. They are also looking at other ways to keep visitors to the National Mall informed about the project.

Congress has allocated $7.5 million for the repair project. In January, Washington businessman David Rubenstein pledged another $7.5 million to complete the work.

Construction of the monument took from 1848 through 1884. The completed monument was the world’s tallest structure until 1889, when the Eiffel Tower was built.

•  Washington Monument Repair Project: http://parkplanning.nps.gov/projectHome.cfm?projectID40394