K-FACTORS FOR UNBRACED FRAMES - ALIGNMENT CHART ACCURACY FOR PRACTICAL FRAME VARIATIONS

For design of steel columns in unbraced frames, the current AISC speci fication commentaries from both LRFD and ASD contain an alignment char t to determine the K factor for a particular column. The K factor is b ased on the effective length concept where K factors are used to equat e the strength of a compression member of length L to an equivalent pi n-ended member of length KL subjected to axial load only. The unbraced frame alignment chart is a graphical representation in nomograph form of a transcendental equation of a buckling solution of a subassemblag e. This solution involves several assumptions not necessarily satisfie d in a particular practical situation. The goals of this paper are to point out the practical limitations of the unbraced frame alignment ch art effective length approach and to encourage the use of story-based effective length factors. This is done in the following manner. A para metric study shows the limits of accuracy and applicability of the unb raced frame alignment chart results from a linear buckling analysis. T he benefits of the AISC Commentary equation C-C2-5 story-based effecti ve length factor are shown through comparison to the alignment chart a nd linear buckling analysis results. The parametric study shows suffic ient examples to encourage practitioners to use story-based techniques for stability assessment. The parametric study investigates variation s in bay width, column moment of inertia, loading, and column height. All nomograph results reported include correction of girder factors to account for unequal girder end moments with non-centerspan inflection point.The alignment chart performance is relatively insensitive to ba y width variation. Variations in column moment of inertia and column l oading lead to large inaccuracies in the alignment chart K factor valu es but C-C2-5 better handles these cases. The alignment chart performa nce is most sensitive to column height variation. Configurations with large variation in column height require frame stability analysis to o btain accurate K factors.