Evaluation of the Effective Buckling Length of Columns in Structural Steel Frames

Abstract

Stability analysis of steel rigid frames is one of the important sections in the design of compression members. Stability of a frame or a compression member can be expressed by the evaluation of K-factor. Three different methods are used in this study to evaluate the K-factor, direct method using the stability functions, alignment charts and the finite element method. The evaluated K-factors are collected in tables and diagrammatically presented. In addition, deviation percentages between the different methods are graphically presented. The analysis using the aforementioned methods is implemented on three families that covers the commonly used structures considering the column base type, number of stories and sway permission for five different beam to column bending stiffness ratios. In addition to the study of those standard frame families, the study included the influence of using different vertical loads to frame columns and the influence of using mixed column bases in the same frame. Moreover, more special cases based on the single story frames are presented as these structures are widely spread in usage. These special cases included the case of single story frame having three identical columns and the pitched roof frames. The special cases of the single story frames included all the parameters of the boundary conditions and the K-factor values are evaluated for each case. Pitched roof frames are divided into frames with crane loads and frames without crane loads. Each load pattern included three different roof slopes. All other boundary conditions are included. Stiffness method using large deformation technique presented the behaviour of each case of the pitched roof frames and the rectangular frames. 7.2 Conclusions In this study, a practical method consists of the generated tables and graphs can be used to determine the buckling length factor of columns (K-factor) in rigid frames is presented. The study considered the sway conditions, the column base types, beam-to-column stiffness ratios, the influence of axial forces distribution and the mixed column bases. The main conclusions achieved from this study can be summarized as follows: 1. The nomographs such as alignment charts show major deviations compared to the exact values, and that may lead to inaccurate design even in the most commonly used frames especially in case of Hinged permitted to sway frames. However, the results were found to be acceptable in the case of prevented from sway hinged frames and conservative in the cases of fixed frames even if they were permitted or prevented from sway. 2. All the K-factor values representing the elastic buckling length factor which are stated in the generated FE graphs are the most accurate and realistic values to be considered in the actual design. 3. The study of the full structural system incorporating all its members an