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The objective of this paper is to provide the results from an experimental program to measure global imperfections and dimensional variations in cold-formed steel-lipped channel members. Measurements were conducted at seven different manufacturing plants in the United States. A total of 210 cross sections were measured for global imperfections. Each cross section was measured at three locations, and every 305 mm (12 in.) along the length. The data is used to characterize global bow, camber, and twist. The majority of the data exhibits global imperfections well approximated by a half sine-wave. Further, global bow imperfections are shown to have a systematic bias in the direction of bow. The measured data is compared with manufacturing tolerances (e.g., L/384) and assumed imperfection sizes from column curves (e.g., L/960) as utilized in design practice in the United States. The measured imperfections served as justification for tightening manufacturing tolerances in the United States (e.g., to L/960). Dimensional variation of typical cold-formed steel members is also measured and compared with tolerances. The measured data provides much needed input for nonlinear analysis of cold-formed steel structures. [ABSTRACT FROM AUTHOR]

This paper presents a simplified approximate analysis of the overall collapse of the towers of World Trade Center in New York on September 11, 2001. The analysis shows that if prolonged heating caused the majority of columns of a single floor to lose their load carrying capacity, the whole tower was doomed. Despite optimistic simplifying assumptions, the structural resistance is found to be an order of magnitude less than necessary for survival. [ABSTRACT FROM AUTHOR]