Second-order inelastic and modified elastic analysis and design evaluation of planar steel frames

This research addresses two methods developed to simplify the design evaluation of steel framing systems, advanced analysis and the “modified elastic” approach. In the advanced analysis approach, the primary member limit states are modeled in the analysis to the extent that the corresponding design checks are superceded. In the modified elastic approach developed in this work, the phenomena that affect member strength and system interaction, specifically the effects of nominal initial frame nonverticality and inelasticity due to residual stresses, are modeled to the extent possible within an elastic analysis.; The proposed modified elastic approach is closely tied to the underlying physical behavior of the system. This method is based on the assertion that the best determination of system strength can be achieved by making an accurate estimate of the second-order inelastic forces in the members and other structural components, within the context of an elastic analysis, which can then be checked against simplified equations for the actual internal member resistances. The limit of the structural capacity is thereby established as the load level at which the limit state of the most critical member is reached. By providing a more accurate assessment of both system behavior and the distribution of forces within the system, both the modified elastic and advanced analysis methods eliminate the need for elastic buckling based solutions or the corresponding effective length factors required in the current AISC LRFD Specification for steel design.; The methods are extended to include the effects of connection behavior and the inelastic response of fully-composite beams. A nonlinear, fully-composite beam element is developed which accounts for the effects of construction loading on the section constitutive behavior. The element tracks the spread of yielding in the steel beam due to stresses present after the application of construction loads and the consequent effect on member behavior. In addition, a trilinear connection model is proposed for use with both the advanced analysis and modified elastic approaches. When used in conjunction with the modified elastic approach, a rational assessment of the effects of nonlinear connection response may be included within an elastic analysis-design method without the need to calculate either appropriate secant stiffness values or effective length factors.