Restraint Demand Factors and Effective Lengths of Braced Columns

The effective length approach for column stability evaluation has long played an important role in approximate methods of analysis. Methods for determination of effective lengths of compression members in frame systems are examined and discussed in terms of exact results and general principles of mechanics of buckling. Conventional rotational restraint factors in the general case are found to be less than accurate. Such factors include the well known G (or ψ) term, defined as the relative joint stiffness ratio. The vertical interaction between compression members below and above a joint, as implied by the conventional G-term, are shown to be conceptually flawed. The novel concept of a restraint demand factor is introduced, to allow for improved development of vertical interaction in member stability terms, including effective length predictions. Conventional and alternative restraint demand factors are compared to exact solutions for a realistically wide range of frame parameters. Sources of inaccuracies are identified and quantified, and limits of applicability are suggested.