World Trade Center/NYC/v1/twin towers/steel

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Issuepedia is trying to determine the exact types of steel used in constructing the World Trade Center twin towers. The exact type of steel used apparently makes a great deal of difference in determining how the steel structural elements in the towers would have responded to the internal fires caused by the jet airplane impacts on 9/11.

NIST apparently did extensive thermal tests on actual steel samples from the twin towers, and the results of these tests are presumably somewhere in their report.

A 2007 paper commenting on the NIST analysis says this: Because of their high strength, the steels used in the exterior wall columns are not ordinary construction steels. A typical high-rise building might use steel of only three strength grades, based on minimum yield strength (FY). In contrast, the WTC structural plans specified steels that began at a minimum yield strength FY = 36 ksi and increased from FY = 40 ksi to FY = 85 ksi in 5 ksi (34.5 MPa) increments. Corner elements in the exterior wall often used FY = 100 ksi steels. Contemporaneous construction documents indicate that the lowest strength exterior wall column steels were supplied to the ASTM A 36 standard, but all the steels with strengths above that value conformed to proprietary grades that the Port Authority of New York and New Jersey, the building owner, authorized. Yawata Iron and Steel, now Nippon Steel, supplied most of the steel plate for the exterior wall columns. The plate that faced the interior of the building usually came from a domestic mill, however.

Japanese and British mills supplied most of the steel for the core columns. These plates and hot-rolled, wide-flange shapes were mostly FY = 36 ksi ASTM A 36. Little information survived about which steel mills supplied the core beams.

The floor truss angles and webs were specified to a mixture of ASTM A 36 and ASTM A 242. The latter is a high-strength, low-alloy (HSLA) steel, though the composition limits in the WTC construction era differ from those of the standard today. Even when the plans called for A 36, the mill often supplied an HSLA steel with substantially higher yield strength.