The last construction barriers have been removed, the pneumatic hammers have fallen silent, and the final displaced employees have returned.

After 17 years, the job of renovating the Pentagon is complete. Little remains to be done but

the paperwork closing out the $4.5 billion program, which when it began was the world’s largest reconstruction project.

Constructing the building took just 17 frenetic months during World War II and remains one of the great engineering feats in U.S. history.

The Pentagon renovation, however, went on for so long that the first parts completed are showing their age, and some equipment — including fire alarms and electrical and mechanical systems — are already being upgraded.

Yet the remaking of the 6.5 million-square-foot, 29-acre site is considered such a success in industry circles that its “design-build” techniques have influenced other federal projects, including the rebuilding of levees by the U.S. Army Corps of Engineers in New Orleans as well as the construction of a new research facility for the National Renewable Energy Laboratory in Colorado.

Why did the renovation take so long?

Lee Evey, who oversaw the effort for five of those 17 years — including after the attack on Sept. 11, 2001 — has an answer:

“We took the building apart and put it together again, with 20,000 people sitting in it.”

The building had to be stripped down to concrete columns and rebuilt from slab to ceiling, yet still operate as the Defense Department headquarters 24 hours a day, 365 days a year. Constructed for an era of manual typewriters, the Pentagon needed to be entirely reconfigured to meet modern communications and technology demands. The Sept. 11 attack, which occurred during the renovation, highlighted major safety deficiencies.

Workers installed about 177 miles of cable tray to carry wiring through the building.

“It’s like taking apart a black-and-white TV and putting it back together again in color, without missing any of your favorite programs,” Evey said.

Old-timers accustomed to marching up ramps and stairs marvel at the 70 passenger elevators in the new Pentagon.

The institutional cafeterias with kitchen mixing bowls the size of Volkswagens are gone, too, replaced by an airy two-story dining atrium of terrazzo, stainless steel and glass. The hot dog stand in the center courtyard was rebuilt and is now known as the Center Court Cafe, offering panini and quesadillas.

The Pentagon has been “built for the next 50 years,” according to a renovation-program slogan, but officials concede that it is hard to project all future IT needs.

At the height of the renovation — during the Phoenix Project after the terrorist attacks — more than 3,500 workers were on the job. Close to 15,000 people have worked on the program over the years.

Now only a handful remain, finishing the paperwork.

Contractors and workers will be honored at a ceremony Wednesday, and a ribbon-cutting event is planned for this summer.

“The mood around here today is kind of bittersweet,” said Mike Daigler, a group leader with the program from the start. “A great sense of accomplishment mixed with seeing a lot of people leaving.”

The renovation is ending so quietly that many Pentagon employees are unaware of its conclusion. There was no fanfare when the final displaced workers moved back in late April. “It was more like a non-event,” said Sajeel Ahmed, the current director of the renovation.

Board of horrors

Inside the Pentagon’s Integrated Emergency Operations Center on a recent morning, workers sitting at banks of desks monitored sensors scanning the building and its surroundings for any signs of a chemical, biological or radiation attack. Screens on the walls flashed images from the 500 security cameras panning across parking lots, corridors and entrances. One screen showed the status of every fire alarm in the building.

It is a far cry from what Steve Carter discovered when he retired from the Navy and took a job in the Pentagon maintenance shop in 1984. Expecting to find sophisticated technology, Carter learned the building manager’s office did not have a single computer. There were few fire sprinklers anywhere in the building.

Dr. Strangelove fantasies aside, the Pentagon had not met National Electrical Code standards since 1953. The building averaged 20 to 30 power failures a day.

Corridors were dingy and dark. The only passenger elevator was reserved for the defense secretary.

Nonetheless, it took years for building supervisors to persuade the Defense Department leadership and Congress to approve a renovation. Carter’s boss, David O. “Doc” Cooke, the legendary unofficial mayor of the Pentagon, wrested control of the building in 1990 from the General Services Administration, which had neglected its upkeep for decades.

Carter put together a “horror board,” on which rusted pipe, ancient wiring and pieces of loose asbestos were mounted and carried to offices across Capitol Hill. When resistance continued, Carter took senior officials on what he called “armpit tours” into the depths of the Pentagon, showing off the sagging floors and leaking pipes. Approval was finally secured in 1993, with the renovation to be done over 14 years to spread out costs and minimize disruption.

Work began on the basement in October 1994, but it did not go well.

Senior Pentagon officials were so upset by the teeth-shaking vibrations and incessant racket that they threatened to shut down the project. New sewer lines, covered with concrete, were leaking. Cost overruns were eating the entire budget.

Evey took over the project in 1997, instituting design-build techniques in which contractors and designers worked as a team rather than rivals. The work was mapped out with the precision of a military campaign, and the building was divided into five chevron-shaped wedges to be renovated one by one.

Employees moved as work progressed, first out of the building and later to newly renovated space. Sometimes tenants refused to go. Cutting off power usually did the trick; other times, jackhammers on the ceiling helped. In extreme cases, it took notices warning of airborne asbestos particles.

Once emptied, the wedge would be gutted. Tons of refuse was hauled out, including about 70,000 cubic yards of asbestos. All the mechanical systems were replaced. Then the offices were rebuilt to modern standards.

Exposed vulnerabilities

The first wedge had just been completed in September 2001 when terrorists flew a hijacked American Airlines jet into the Pentagon’s west wall, killing 125 people in the building and 59 passengers and crew members. The plane struck the newly renovated section, which had blast-resistant windows, structural supports and fire sprinklers. These saved lives, as did the fact that many of the destroyed offices were still vacant.

After the attack, the renovation program began the Phoenix Project, a race to rebuild the destroyed portions of the building within one year. By the first anniversary, employees were back working in the offices where the plane struck.

The renovation expanded to address vulnerabilities exposed by the attack. In the emergency operations center, for example, building workers can close off water valves from computers, safeguards against the conditions that nearly shut down the Pentagon after the attack.

Although much construction had to be redone, the attack speeded up the renovation. The project was originally scheduled to be completed by 2007, but by 1999, the date had slipped to 2014. After the attack, however, Congress added funding so that the project could be completed by 2011.

The “legacy Pentagon,” as it is now called, is mostly a memory. Apart from the concrete and the limestone facade, very little of the original building remains. At the last minute, officials preserved a 1,600-square-foot section to show visitors the distinctive World War II-era Pentagon decor.

“The program came in ahead of schedule and below cost,” said Ahmed, the renovation director. “With all the mission changes, and two wars going on, we stayed on course.”