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Thin-walled cold-formed steel beams are investigated with existing experimental data and shell finite element simulations to characterize their in-plane moment-rotation behavior, up to and past peak strength, in local or distortional failure modes. Although ultimate strength prediction of cold-formed steel members is generally well addressed in design codes, pre- and post-peak member stiffness is only partially addressed; while member ductility and post-peak moment-rotation response suffers from a lack of any clear guidance. Without fundamental information on cold-formed steel moment-rotation/curvature response, i.e. the backbone curve, system modeling for cold-formed steel structures to collapse remains severely hampered. Existing data on cold-formed steel beams are used as the basis for the study conducted herein. Simplified moment-rotation models, motivated from ASCE 41 characterizations, defined with pre-peak flexural rigidity degradation, post-peak plateau and strength drop are explored by equating the area under the backbone moment-rotation response (energy) between the available data and simplified models. In-plane response of cold-formed steel beams is parametrized with new design expressions depending on local and distortional cross-section slenderness. This research provides work for potential incorporation into design standards such as ASCE 41 and AISI S100. Out-of-plane response of cold-formed steel beams, including lateral-torsional buckling, remains as needed future work. [ABSTRACT FROM AUTHOR]

Experimental study on thin-walled welded steel beam-to-column connections under cyclic large plastic strain loading was conducted. Severe shear buckling in panel zones was observed during the testing. The connections have stable and excellent seismic performance, and finally failed due to the post-buckling ductile fracture. This paper aims to numerically simulate the post-buckling ductile cracking process using a proposed micro-mechanism based ductile fracture model, and further investigate critical factors, such as equivalent panel zone width-to-thickness ratio, axial load ratio and initial geometrical imperfection, that affect cracking behaviors of the connections. The cyclic ductile fracture model is verified with the experimental data, and employed to conduct the parametric analyses. Deterioration of stress-carrying capacity is also considered, so the fracture model can successfully simulate the load decrease in the load-displacement curves of the experimental results. [ABSTRACT FROM AUTHOR]

In designing a steel beam, one of the limit states is lateral torsional buckling. The value of the bending moment limit is called the critical moment. AISC specification only provides formulas for calculating the critical moment in prismatic beams, but not for non-prismatic beams like castellated beams. The objective of this paper is to study the lateral torsional buckling of castellated beams. The method used for this study is the collapse analysis using the finite element method, analyzed by ADINA 8.9. In the collapse analysis, a geometrically imperfect beam is loaded with an increasing load. At a certain value of the load, the lateral displacement increases progressively. The value of the maximum load is considered to be the critical load. In this study, the imperfection is taken 1/1500 of the unbraced length of the beam. The critical moment of castellated beam obtained from the finite element analysis is up to 42.31% lower than the critical moment of full beams with the same dimension calculated by using the AISC formula. Therefore, the correction factor for AISC formula to calculate the critical moment of the castellated beam is established. [ABSTRACT FROM AUTHOR]