http://www.nytimes.com/2008/04/09/nyregion/09freedom.html?ref=nyregion
Replicas of New Tower Endure Nature’s Fury and a Test Blast
A mock-up of the Freedom Tower at the World Trade Center, in Ontario, Calif., was built to be exposed to extreme conditions.
By DAVID W. DUNLAP
April 9, 2008
One World Trade Center has not yet emerged from below ground, but its facade has already survived earthquakes, hurricanes and an explosion that shook the earth a quarter-mile away.
In recent months, two full-size mock-ups of a few floors of the glass and aluminum facade have been built and tested. One is outside Los Angeles, in Ontario, Calif. The other was at a site in central New Mexico that can be reached only over dirt roads in four-wheel-drive vehicles.
At 1,368 feet, with 23 acres of glass-clad surface area, 1 World Trade Center will be subject to tremendous natural forces. The building, also known as the Freedom Tower (at a symbolic 1,776 feet, when its mast is counted), will be the tallest in New York City and as the skyscraping phoenix on the site of ground zero, it may be the target of terrorist attacks, too.
The Port Authority of New York and New Jersey, which is building 1 World Trade Center, and Skidmore, Owings & Merrill, which designed it, said both mock-ups performed well. The facade, called a curtain wall, is being made by Benson Industries of Portland, Ore. The engineering firm Weidlinger Associates is the consultant in blast-resistant design.
“Physical testing is a confirmation that curtain-wall contractors are in fact meeting performance requirements,” said Carl Galioto, a Skidmore partner. “Full fabrication of the curtain wall cannot begin until the mock-up specimen passes these tests.”
Almost invisible to passers-by, the foundations of 1 World Trade Center are rising every day toward street level.
The first mock-up was subjected to a blast test in Socorro, N.M., at the Energetic Materials Research and Testing Center, a division of the New Mexico Institute of Mining and Technology. Because details might arm a prospective attacker — providing information like how much force the curtain wall is designed to withstand — officials would say almost nothing about the test of this mock-up.
“The simple answer is, yes, it passed,” said John McCullough, the project executive for the Port Authority.
He was more forthcoming about the tests last month at Construction Consulting Laboratory West in California. There,
a $537,000 mock-up was built to represent a corner of three typical tower floors, with laminated glass panes one and a half inches thick. The largest are 5 feet by 13 feet and weigh half a ton. An enclosed steel chamber was constructed behind the glass and aluminum cladding.
The goal was to find out how much air and water leakage could be expected under storm conditions that could be expected at least once in 50 years.
Water jets simulating winds of 74 miles per hour were sprayed at the facade. During the 15-minute test cycle, each square foot of glass was hit with more than a gallon of water.
In another test, a dismounted airplane propeller was switched on to simulate even-stronger and more-scattered winds. “It’s pretty colorful,” said Mr. Galioto, who witnessed the test. “It’s very noisy. Water is blowing in every direction and smoke is blowing from the engine.”
Air infiltration is measured by gauges. Water infiltration is measured by witnesses who are inside the chamber.
“Water is coming into the face of the curtain wall with such intensity that you can’t see,” said Bruce Fox, the deputy project executive for the Port Authority. “Then you’re looking into and opening up all the different pieces to see if there’s any evidence of leakage.” There was none.
Hydraulic jacks were used to simulate the different horizontal sway of various floors, both fully occupied and empty. The surface was also chilled to 10 degrees (refrigerated piping was applied to the glass) and baked at 100 degrees (by heat lamps).
Gusts up to 167 m.p.h. were simulated by using pumps to pull air out of the chamber, creating a condition in which the external air pressure was far greater than the internal pressure. The process was reversed, too, by pumping air into the chamber, simulating conditions on the side of the tower away from the wind.
An earthquake was simulated by jacks pulling the mock-up in different directions. Finally, a much stronger earthquake was simulated. At this point, the designers no longer expected the mock-up to remain airtight and watertight. But the criteria required that no glass could crack and no panes could be dislodged.
Mr. McCullough of the Port Authority said the mock-up met all the performance criteria.
And Mr. Fox marveled: “Sometimes on these tests, you have to do forensics and do corrections. Here, we had no failure at all.”
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