Cold formed profiles
Introduction
The use of cold-formed members in buildings began in the United States and Great Britain around the 1950s, but they were not widely used until around 1940. It is now very common for the main structure of tall buildings to be formed from heavy, hot-rolled sections; On the other hand, secondary elements such as joists, panels or covers "Deck (construction)") and mezzanines are generally made up of cold-formed members.
History and approach to analysis
During the 1930s, the development and acceptance of cold-formed profiles faced real difficulties, among other things due to the small number of design specifications and publications covering these topics. Knowing its great importance, in 1939 the AISI Committee (then called “Committee on Building Codes”) sponsored a research project directed by Professor George Winter of Cornell University (Ithaca "Ithaca (New York)"), which had as its objective the development of design methods that satisfactorily model the behavior of this type of members.
The developed theory was based on the detailed study of each of the elements that make up a section and its behavior under the action of compression forces. It was then discovered that these elements experience local buckling mainly depending on their joining conditions at the ends and on the existing plane width to thickness ratio.
With the objective of obtaining a practical method of analysis, the problem was simplified to extract (not consider) the cross-sectional area of the compression zone that is considered dented when the maximum tension that said zone can request is reached. In this way, the problem of finding an effective section that behaves stably in the face of the external actions considered is reduced.
The scope of this method extends only to cold-formed profiles with a thickness less than 1 inch (25.4mm), and although the use of ASTM-designated steels listed in Section A3.1 of the Specification is encouraged, other steels may also be used, provided they satisfy the requirements established in that section.
Studies prove (studies carried out by Chajes, Britvec, Winter, Karren and Uribe at Cornell University in the 1960s) that the mechanical properties of steel are modified by cold forming, especially yield stress, tensile strength, and elongation; These are increased by the strain hardening experienced by the curves of the sections; the AISI specifications allow this effect to be taken into account under a series of conditions.
On the other hand, cold forming generates a decrease in ductility[1].
References
- [1] ↑ Herrera, Ricardo (2007). Perfiles Delgados. Santiago, Chile: Departamento de Ingeniería Civil, Universidad de Chile.