| Geometric imperfections play a crucial role in the behavior and strength prediction of thin-walled members. Geometric imperfections influence the sensitivity in observed strength, the sensitivity in observed failure mechanism, and the demands on all bracing elements. All of these facts make modeling of imperfections an important issue in the analysis of structures comprised of thin-walled members. In this study, the preferred approach in modeling is to simulate the geometric imperfections as a physical reality, as opposed to a mathematical convenience. For such a goal, the distribution and magnitude of the imperfection should be investigated and be tied to available data on measured geometric imperfections.;In this thesis, imperfection sensitivity of structures is discussed and different classes of imperfection sensitivity introduced and explored. A program has been conducted to measure geometric imperfections in cold-formed steel manufacturing plants. The results from this program and available geometric imperfection data on cold-formed steel members are employed to categorize geometric imperfections in cold-formed steel members. Simulation models are introduced that more closely marry simulated imperfection distribution and magnitude to physical reality.;Three methods are presented to generate and simulate imperfection fields: the first is the classical approach employing a superposition of eigenmode imperfections, but scaled to match peaks in the measured physical measurements. The second is a method based on the multi-dimensional spectral representation method, in which imperfections are considered as a two-dimensional random field and simulations are performed taking a spectra-based approach. The third is a novel combination of modal approaches and spectral representation that directly considers the frequency content of the imperfection field, but employs a spectral representation method driven by the cross-sectional eigenmode shapes to generate the imperfection fields. The effect of these different approaches on the simulated strength and collapse behavior of members is investigated using material and geometric nonlinear finite element collapse modeling. The third imperfection generation method, termed the 1D Modal Spectra Method, provides an intriguing new tool in the simulation of thin-walled members. |
