Before planes rammed the twin towers of the World Trade Center Tuesday morning, the 110-story structures stood as symbols of American financial might and engineering prowess. Their structural engineer even said as recently as last week that he had designed them to withstand the impact of a Boeing 707 jetliner.

But then, one after the other, the enormous skyscrapers collapsed in a giant cloud of dust and rubble, stunning engineers who had expected them to remain standing and raising anew the question: How can Americans strike a balance between security against terrorism and the openness that is a hallmark of a democratic society?

Coincidentally, the attacks occurred on the same day that final construction bids were due for a new, four-story building in Oklahoma City that will replace the Alfred P. Murrah Building. A truck bomb detonated at the Murrah Building on April 19, 1995, killing 168 people, including 19 children, in what was then the worst terrorist attack on American soil.

The attacks illustrate "that you can defeat any [security] system," said Carol Ross Barney, the Chicago architect who has designed the new Oklahoma City structure.

Added Joseph Burns, a partner at the Chicago firm of Thornton-Thomasetti Engineers: "No one designs buildings for bombs the size of 767s."

Engineer's boast

Burns said he attended a conference last week in Frankfurt, Germany, at which Leslie Robertson, the World Trade Center's chief structural engineer, was asked after a presentation to describe what he had done to protect the twin towers from terrorist attacks.

Burns said Robertson replied: "I designed it for a 707 to smash into it."

Burns, whose firm did the structural engineering for the Petronas Twin Towers in Malaysia--the world's tallest buildings--said that Robertson did not elaborate on the remark.

A spokesman for Robertson's firm, Leslie E. Robertson Associates of New York City, declined comment, but confirmed that the structural engineer had publicly stated that the towers would not buckle if hit by a 707.

Like the Robertson firm, the architects of the World Trade Center, Minoru Yamasaki Associates of suburban Detroit, said they would work with law-enforcement authorities.

Completed in 1972 and 1973, the twin towers of the World Trade Center were the fifth- and sixth-tallest buildings in the world. The north tower, One World Trade Center, was finished in 1972 and was 1,368 feet high. It was briefly the tallest building before Sears Tower assumed that title in 1973. The south tower, Two World Trade Center, stood 1,362 feet tall.

When the towers were finished, architecture critics ridiculed them as banal boxes, but the buildings ultimately were accepted by the public as urban symbols of lower Manhattan. Resembling twin pillars, they have been called "the monumental gate to New York and the United States."

Innovative structure

The buildings were supported by an innovative, economical structural system consisting of a central concrete core and a series of closely spaced steel columns that ringed the load-bearing perimeter of the towers. Structural engineers call this support system a "tube."

The system did not break down on Feb. 26, 1993, when a bomb-laden van exploded in one of the underground parking structures at the World Trade Center, killing six people and injuring more than 1,000. But Tuesday's attack was different, introducing a new weapon--the hijacked commercial jetliner--into the terrorist's arsenal.

That weapon also destroyed a 47-story building in the commercial complex. The building, Seven World Trade Center, was not hit by the planes but ignited and collapsed about 71/2 hours after the first tower buckled.

The attacks marked the second time that planes have smashed into New York City skyscrapers. In 1945, a B-25 flying at 200 m.p.h. in fog slammed into the 78th and 79th floors of the Empire State Building. Fourteen people were killed.

In the first moments after the attacks, engineers predicted that the towers would stand because of their tightly bunched perimeter columns. The notion was that several columns could be lost, but that the structures would remain standing.

Fuel flames inferno

After the collapse, Burns and other structural engineers said the planes spawned immense fires with highly flammable jet fuel stoking blazes that would have reached more than 1,000 degrees Fahrenheit. The fires would have melted steel columns and knocked out sprinkler systems.

"It takes a while for the steel [columns] to heat up. It's encased in a little coat" of fire retardant, said Jeff Garrett, a senior managing engineer in the Chicago office of Exponent Failure Analysis Associates, based in Menlo Park, Calif. "But as the fire heated up, the structure got soft, and it collapsed. The extra weight caused the whole building to pancake down on itself."

Two World Trade Center, the south tower and the second to be hit, collapsed 57 minutes after impact. One World Trade Center gave in 1 hour 45 minutes after being struck.

Chicago structural engineer R. Shankar Nair argued that the collapse could have resulted from the very structural feature that engineers at first thought would protect the towers: their closely spaced perimeter columns.

In his view, the planes' impact destabilized the tops of the towers, causing their lower portions to eventually fall out of balance.

"It's like taking a post sticking out of the ground and you hammer it sideways near the top. The place where it's most likely to break would be near the base," said Nair, the former chairman of the Council on Tall Buildings, an international group that studies skyscrapers.

Whatever the cause of the disaster, the attacks are likely to prompt a national debate about a wide range of security measures against terrorism.

Protective measures

In the wake of the 1993 World Trade Center blast and the 1995 Oklahoma City bombing, public officials and business executives have closed city streets, including a stretch of Pennsylvania Avenue in front of the White House, and have erected concrete bollards to keep bomb-laden trucks away from key government and office buildings.

But "you look at this and you think of Oklahoma City and the silly little bollards we have in front of our federal buildings, and you say, `So what?'" said Ross Barney, the architect of the new building in Oklahoma City.