The Hindu : The ruins hold the answers

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Thursday, Jan 08, 2004

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The ruins hold the answers

By James Glanz

Research on why the twin towers of the World Trade Center collapsed like they did could help improve the safety of future high rises.

THE JET fuel, of course, caught fire first. As traces of the fuel's stench hung ominously in the air, a fiery burst quickly became a sullen orange glow, then grew until flames were licking the ceiling 11 feet above. The flames fed on a cluster of office workstations lying smashed, littered with collapsed ceiling tiles and office papers.

Within minutes, a maelstrom of heavy black soot, gyrating sparks and brilliant flame was leaping through the tops of four narrow 7-foot-high windows, warming the nearby faces of men wearing bunker coats, oxygen tanks and fire helmets.

The World Trade Center was burning again — at least one small part of it — in this fuel-soaked, full-size re-creation of a Marsh & McLennan office on the 96th floor of the north tower, where the first plane struck on September 11.

This office is inside a towering laboratory in America's National Institute of Standards and Technology at Gaithersburg. And it is only one fragment of the disaster being replayed for a federal investigation. In other studies, steel recovered from the twin towers is being ripped apart at high speeds, rebuilt structural supports are baking in artificial infernos, and fireproofing insulation is being pounded with simulated plane impacts to see how well it sticks to the steel it was meant to protect.

Initial research has already turned up major findings involving the surprising ease with which heat leaks through damaged insulation into the steel, and the ways in which isolated parts of the WTC's structure may have been prone to fail in a major fire — even without structural damage from the planes.

Some of those findings could lead to safety recommendations for hundreds of ordinary high-rises around the country when investigators issue their final report, expected by next fall. The sprawling forensic analysis is also expected to advance the science of fire and how it affects structure, an area that has remained a backwater in engineering, sometimes relying on techniques a century old.

Dozens of researchers are folding their data into a series of advanced computer simulations in an effort to re-create the destruction that was hidden within the twin towers on September 11. They are re-creating, in a digital world, the shape and extent of the dark holes gouged into the heart of the towers by the planes, the sweep of the deadly fire from floor to floor, the spots where the heat weakened the steel structure enough to set off the total collapse of each tower.

Dr. Frank W. Gayle, a metallurgist said the investigation had obtained samples of all 14 grades, or strengths, of steel used in the twin towers for analysis. All together, investigators have collected 236 major pieces of WTC steel, Dr. S. Shyam Sunder, leader of the investigation team at the National Institute of Standards and Technology, said.

One lab here is taking tiny medallions of the steel, no larger than electronic watch batteries, and crushing them at high speed with a high-tech battering ram called a Kolsky bar. Because steel, like taffy, becomes stronger when torn or deformed quickly, the measurements are critical for assessing how components behaved when the planes struck them.

Ordinary buildings, Dr. Gayle said, generally use no more than two or three grades of steel, but that is only one measure of the astonishing complexity of the towers' structure. Each relied on a cluster of relatively heavy steel columns in its core, connected by lightweight floor supports called bar-joist trusses to a tight palisade of columns around the facade — 59 per face, producing those tall, narrow windows.

Even though the exterior columns all looked identical, both the grade and thickness of their steel varied from place to place, said Fahim Sadek, a researcher at the institute's building and fire research lab, who is producing a detailed structural model of the towers on a computer using the original blueprints. So there were actually more than 130 different column types, he said, each having to be accounted for in his model. From there, it gets only more intricate. One of Dr. Sadek's detailed models for a single floor — the 96th floor of the north tower, considered typical — contains 40,000 separate elements. A coarser representation of the entire tower contains 90,000 elements.

One thing the investigation has not done yet is find a specific sequence of failures that could have brought the towers down. But early studies of how isolated parts of the buildings might fail have produced their own unexpected twist.

Suspicion has long centred on the floor trusses, which some experts believe to have been poorly fireproofed and especially susceptible to buckling in a fire. Those suspicions have been bolstered by videos and photographs that show the trusses sagging and possibly giving way in the minutes before each tower collapsed. But preliminary calculations suggest that long before they buckle, the trusses are likely to expand in the heat and bow the exterior columns perilously outward. The highly simplified calculations have not been carried beyond that point, but presumably the columns themselves could then have buckled under the tremendous weight they carried. It all goes to show what investigators are up against in the most complex analysis of a building failure ever carried out, said Dr. Sunder. One possible outcome is that several different plausible explanations for the collapse could emerge, he said.

Although that may not be the clean answer that historians, surviving family members and other scientists would like, it may be all that the fires of the WTC have left to give. — New York Times News Service.

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