Influence of ductility on the structural behavior of cold-formed steel members
The main purpose of this investigation has been to study the feasibility of using low carbon, low ductility, high strength steel for cold-formed structural load carrying members. The present investigation is limited to determining the influence of two factors, viz. ductility and the spread between the yield strength and the ultimate strength, on the behavior of cold-formed members and connections under static loading. In particular, an attempt is made to answer the following two questions: (1) Is the measure of ductility by standard tension coupon test as specified by ASTM specification A370-68 acceptable for static load conditions? (2) What is the minimum ductility that is required of a material to be considered suitable for the conventional structural steel design? For this investigation, two manufacturers supplied specially cold-rolled low ductility steel sheets and a third manufacturer supplied a commercially available low ductility steel. The elongation in a 2 inch gage length, in direction parallel to rolling, for these low ductility steels ranged from 4 to 8 percent and the tensile-yield strength ratio varied from 1.0 to 1.1. A detailed investigation of material properties along with a survey of pertinent literature indicated that a modified form of the standard tension coupon test would be sufficient to define separately local elongation and uniform elongation capabilities of the material. Present procedures do not separate these two parameters. Tests were made on rectangular plates with holes and perforated open rectangular tubes. They showed that most of the specimens developed their full tensile strength based on the net section, in spite of the stress concentrations and of the low ductility of the material. The ultimate load carrying capacity of most of the tested low ductility steel bolted and welded connections can be predicted by equations similar to those for high ductility steels, with little loss in strength compared to normal-ductility steels. A few perforated and notched plates were analyzed theoretically, using the finite element method in the elastic-plastic range. From these analyses and the overall experimental performance of low ductility steels, certain simplified minimum ductility criteria were established. A limited investigation of the weldability and formability indicated that these low ductility steels could be: (1) Welded without using any special welding process with little strength loss from annealing. (2) Formed into 90° corners in a press-brake with a radius greater than four times the thickness (t) of the material. For corner radii less than 4t most of the specimens showed fine surface cracks at the extreme tension fiber. A simplified empirical formula is suggested for the minimum radius to thickness ratio above which no surface cracks nor localized elongation would be obtained in the extreme tension fiber of a corner. Finally a few compression tests conducted on unstiffened compression elements showed no adverse effects on the structural behavior of compression members, due to the lower ductility of the steel used.
Civil, Architectural and Environmental Engineering
American Iron and Steel Institute
Wei-Wen Yu Center for Cold-Formed Steel Structures
Report - Technical
Technical Report Number
Report No. 336
Winter, George and Dhalla, A. K., "Influence of ductility on the structural behavior of cold-formed steel members" (1971). Center for Cold-Formed Steel Structures Library. 80.