NEW YORK - Since Sept. 11, the owners of signature skyscrapers have been trying to find ways to calm jittery tenants and make the buildings more durable if catastrophe strikes. Perhaps nowhere has anyone thought longer and harder about that challenge than at Citigroup Center, the 59-story tower on East 53rd Street in Manhattan with the distinctive triangular top and a troubled history involving secretive structural repairs, which were begun 24 years ago to keep the building from toppling in hurricane-force winds.

Now the building's owner, Boston Properties, has undertaken an ambitious - and similarly secretive - project to strengthen the most exposed of the four stiltlike legs that support the bulk of the building nine stories above a plaza and, in one corner, St. Peter's Church.

The leg, composed of interconnected steel columns that are normally wrapped in a burnished aluminum facing, sits in a spot that looks highly vulnerable - only a few feet from traffic rumbling along 53rd Street.

A plywood enclosure, discreetly painted to match the aluminum, now shrouds the leg on three sides. Permits filed at the Department of Buildings in February and March indicate that over a 42-foot segment within the enclosure, crews are making major structural improvements designed to ward off the kind of forces that would be produced not by wind, but by a street-level explosion, according to experts who have seen the plans.

Vertical steel-and-copper shields are being put in place to fend off such a blast, and heavy structural plates are being fitted into place to help keep the columns from buckling in the concussion a blast would produce.

For all its secrecy, the project is a vivid example of a highly focused, structurally sophisticated and quite expensive effort - the cost could be several million dollars - to buttress a building against threats that suddenly seem to be everywhere, from the sky to the street, in the post-Sept. 11 world.

Citigroup, which is still a principal tenant of the building and remains one of the premier international symbols of American finance, may be especially sensitive to those threats. Branches of the company have suffered rocket and bomb attacks by terrorists in Europe, including in Greece in 1995, 1998 and 2000.

'Strengthening'

Engineers at LZA Technology, whose seal appears on the plans at the Buildings Department, would not comment on what was taking place behind the painted plywood box in the middle of Manhattan. But a structural engineer who is not involved in the work, Ramon Gilsanz, a managing partner at Gilsanz Murray Steficek, inspected the plans and confirmed that they involve structural reinforcements. "This is a strengthening for a blast or an impact load," Gilsanz said.

The design, which includes clever details such as copper sleeves behind the shields which would presumably crumple in an explosion and act like shock absorbers, "is a nice job," Gilsanz said.

A senior vice president and manager of the New York office at Boston Properties in New York, Robert Selsam, would say only, "We have an ongoing program to improve the safety and security of all of our buildings." When asked specifically about Citigroup Center, Selsam said, "It doesn't serve the interest of us or the tenants or the city to go into details."

Several company spokesmen emphasized that despite earlier structural questions, the building was entirely stable against ordinary natural forces like wind or the rare, mild earthquakes felt in New York. Selsam would not say whether the carefully constructed enclosure was part of a plan to keep the work quiet. But the plywood's mimetic coloring seems to have succeeded in doing so when it comes to office workers in the building, few of whom were aware of the nature of the work when asked about it by a reporter outside the building on Tuesday afternoon.

Nigel Hughes, an employee at Sun Microsystems, which moved its offices to Citigroup Center from the World Trade Center, said he had not even noticed the construction. Though Hughes said he was glad the owner was reinforcing the building, he added that office workers should have been told what was being done.

The problems that the 1.6 million-square-foot Citigroup Center faced in the summer of 1978 were frightening. A year after tenants moved into the new skyscraper, its main structural engineer discovered a design flaw that could have caused the entire structure to topple like a rotten oak tree in high winds. The engineer, William J. LeMessurier, made the discovery in July, just as the peak of the hurricane season was approaching.

Air rights

That was hardly the ideal that Citigroup had envisioned when it hired Hugh Stubbins, an architect, and LeMessurier to design what was to be a striking, even radical, architectural and structural vision for the company's headquarters. The company bought the air rights above St. Peter's in a deal that involved constructing a brand new church on the northwest corner of the site. Because of the church, Stubbins and LeMessurier put the building, in effect, on stilts.

The designers did not place the legs of the stilts at the corners of the tower, a move that would have pierced the church. "There couldn't be a column in that corner," LeMessurier said in an interview.

"My solution was to move the columns to each side," he said. That left the four corners of the tower beetling into space over the plaza.

To give the soaring 914-foot tower steadiness against the wind and keep it from toppling over, LeMessurier outfitted its outer walls with powerful, eight-story, V-shaped steel supports. He also placed a 400-ton concrete block, called a tuned mass damper, in the crown of the building. When the building began to sway in the wind, machinery would float the block on a film of oil and hydraulically slide it back and forth to counteract the sway.

But in the summer of 1978, LeMessurier and his colleagues discovered that their original estimates of wind forces on the building had been far too low. He also learned that, despite specifications to the contrary, the steel structural members had been bolted together rather than welded, seriously weakening the building's ability to withstand the wind.

LeMessurier realized with horror the devastation that a tipped-over Citigroup Center could wreak in a section of Manhattan that is thickly forested with skyscrapers.

With the company and the city, he worked out a secret evacuation plan for the entire area, and brought in an army of welders to reinforce the connections at night. "We came in after 5 o'clock, when all the secretaries had gone home and most of the executives," said Arthur Nusbaum, who led the work for HRH Construction. By 3 a.m., Nusbaum said, workers were tidying up for the night.

Aided by a city newspaper strike that started on Aug. 9, the work remained largely secret. The hurricane season fizzled, and by the time the welders were finished, the tower was as stout in a gale as any skyscraper in New York, LeMessurier and other structural engineers say. And even though those troubles are thought to be over, the decision to protect the exposed leg against terrorist attacks is "a marvelous idea," he said. "I should have told them to do it a long time ago," he said.

A source familiar with the work said major structural changes had already been finished. Less intensive work involving fireproofing, the exterior facing and other details remained, the source said.

"From the very beginning, we met regularly and closely with representatives of all of the tenants," Selsam said. But he conceded that "there is no doubt that there are individual employees who did not know about the work."