EFFECT OF STRAIN RATE ON COLD-FORMED STEEL STUB COLUMNS

The material properties of steel and the strength of steel members are affected by strain rate. To investigate this characteristic for compression members, 49 stub columns fabricated from 35XF sheet steel and 48 stub columns fabricated from 50XF sheet steel are studied experimentally and analytically under different strain rates. The strain rate ranged from 10(-5) to 0.1 in./in./sec (10(-5) to 0.1 mm/mm/s). The material properties of 35XF and 50XF sheet steels developed from previous tests are used for the evaluation of the test data obtained from the member tests using specimens fabricated from the same sheet steel. The results show that the strength of stub columns increased with the strain rate. The amount of increase is found to be dependent on the type of material, the F(u)/F(y) ratio, the width-to-thickness ratio (w/t) of the compression element, and the strain rate used in the tests. The effective width approach included in the American Iron and Steel Institute (AISI) specification for cold-formed steel members and in the AISI Automotive Steel Design Manual is utilized for the evaluation of stub column strengths using static and dynamic yield stresses corresponding to the strain rates used in the tests. It is found that better agreement can be achieved between the predicted and tested stub column strengths when using the dynamic yield stresses and considering the effect of cold work.