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Mechanical Properties of Structural Steels. Federal Building and Fire Safety Investigation of the World Trade Center Disaster.


pdf icon Mechanical Properties of Structural Steels. Federal Building and Fire Safety Investigation of the World Trade Center Disaster. (6748 K)
Luecke, W. E.; McColskey, J. D.; McCowan, C. N.; Banovic, S. W.; Fields, R. J.; Foecke, T.; Siewert, T. A.; Gayle, F. W.

NIST NCSTAR 1-3D; 322 p. September 2005.

Keywords:

World Trade Center; high rise buildings; building collapse; disasters; fire safety; fire investigations; terrorists; terrorism; steel structures; mechanical properties; high temperature; impact; creep; strain rate; modulus; tensile strength; yield strength

Abstract:

This report provides five types of mechanical properties for steels from the World Trade Center (WTC): elastic, room-temperature tensile, room-temperature high strain rate, impact, and elevated-temperature tensile. Specimens of 29 different steels representing the 12 identified strength levels in the building as built were characterized. Elastic properties include modulus, E, and Poisson's ratio, v for temperatures up to 900 DGC. The expression for E(T) for T < 723 DGC is based on measurements of WTC perimeter column steels. Behavior for T > 723 DGC is estimated from literature data. Room temperature tensile properties include yield and tensile strength and total elongation for samples of all grades of steel used in the towers. The report provides model stress-strain curves for each type of steel, estimated from the measured stress- strain curves, surviving mill test reports, and historically expected values. With a few exceptions, the recovered steels, bolts, and welds met the specifications they were supplied to. In a few cases, the measured yield strengths of recovered steels were slightly lower than specified, probably because of a combination of mechanical damage, natural variability, and differences in testing methodology. High- strain-rate properties for selected perimeter and core column steels include yield and tensile strength, total elongation and strain rate sensitivity for rates up to 400 s-1. Measured properties were consistent with literature reports on other structural steels. Impact properties were evaluated with Charpy testing. Properties for perimeter and core column steels were consistent with other structural steels of the era. The impact toughness at room temperature of nearly all WTC steels tested exceeded 15 ft-lbf at room temperature. Elevated-temperature stress-strain curves were collected for selected perimeter and core column and truss steels. The report presents a methodology for estimating high-temperature stress-strain curves for the steels not characterized based on room-temperature behavior and behavior of other structural steels from the literature. The measured elevated-temperature stress-strain behavior of WTC steels is consistent with other structural steels from that era. For the truss steels, the report presents a complete constitutive law for creep deformation based on experimental measurements. For the steels not characterized, the report presents a methodology for estimating the creep deformation law.