Finite Element Comparison of Design Methods for Locally Slender Steel Beams and Columns

The objective of this paper is to describe and analyze a series of nonlinear finite element analyses used to compare three design methods for locally slender, braced, steel beams and columns. The three design methods are: (i) the hot-rolled steel AISC Specification, (ii) the coldformed steel AISI Specification which uses the effective width method throughout, and (iii) the cold-formed steel Direct Strength Method which was recently adopted as an alternative design method in the AISI Specification. To aid the comparison of the available methods the design strength formulas, for locally slender W-section beams and columns, are provided in a common notation. The resulting design expressions highlight the prominent role of elastic cross-section stability as the key parameter for strength prediction. The comparison for beams shows that the AISC and AISI methods take very different approaches to predicting the strength of locally slender sections. A nonlinear finite element analysis parameter study, using ABAQUS, is initiated for the purpose of understanding and highlighting the parameters that lead to the divergence between the capacity predictions of the different design methods. Particular attention is placed on understanding the regimes where the AISC methods give divergent results, from either the other Specifications, or the nonlinear finite element analysis results. 1 Graduate Research Asst., Johns Hopkins University, [email protected] 2 Associate Professor, Johns Hopkins University, [email protected] INTRODUCTION Typically, locally slender (noncompact or slender) cross-sections are avoided in design of hot-rolled steel structural members. However, this strategy becomes inefficient with the advent of high and ultra-high yield strength steels, as the increased yield stress drives even standard shapes from locally compact to locally slender. The effect of increasing the yield strength on local buckling is a topic that has seen some study in recent years (see e.g., Earls 1999). Initial analysis of the AISC provisions (Schafer and Seif 2008) indicates geometric regions where the existing AISC design approach may be excessively overly conservative, and other regions where it may be moderately unconservative. Efficient and reliable strength predictions are needed for hot-rolled steel cross-sections to take advantage of more locally slender sections. Since the design of locally slender cross-sections is common practice in cold-formed steel, the two design methods in current use for cold-formed steel are compared with the AISC approach herein. DESIGN METHODS The design of locally slender steel cross-sections may be completed by a variety of methods, three of which are examined here: (1) The AISC method, as embodied in the 2005 AISC Specification, labeled AISC herein, (2) The AISI Effective Width Method from the main body of the 2007 AISI Specification for cold-formed steel, labeled AISI herein, and, (3) The Direct Strength Method as given in Appendix 1 of the 2007 AISI Specification, labeled DSM herein. For each of these three design methods the expressions for strength prediction of braced columns and beams have been provided in a common notation in Table 1 and Table 2, respectively. Previous work provided and examined the design expressions for unbraced columns with locally slender cross-sections (Schafer and Seif 2008). In this paper the design expressions for both columns and beams are provided, but only for the braced case, as this provides a focus on the local buckling strength predictions alone, and may be readily compared to the subsequently conducted nonlinear finite element analysis. Table 1 Braced column strength for locally slender I-shaped section