Reliability of built-up cold-formed steel columns designed by the direct strength method

Resumo

Built-up cold-formed steel (CFS) columns are composed of two or more sections that are joined together
with welds, bolts or screws. The use of built-up sections may be an option at certain locations in a CFS-framed
building when higher axial capacity or local frame rigidity is required. This paper presents a study of the reliability
of built-up cold-formed steel columns. Reliability indexes are evaluated by First Order Reliability Method (FORM
method) for usual nominal live-to-dead load ratios. The reliability analysis used to assess the safety level of design
specifications included the model error and other random variables such as strength parameters, geometric
parameters, dead and live loads. For the statistical study of the model error variable, column test results obtained
from literature were compared to the resistant capacities obtained by the direct strength method (DSM). A total of
266 column tests were selected in order to ensure representativeness of the buckling modes and section types. A
lack of uniformity of reliability indices was observed in the analyzes organized by instability mode or by section
type. It was found that the all-data case leads to unsatisfactory results, with the reliability index lower than the
target value.