Stiffness modeling of reinforced concrete beam-columns for frame analysis

For the purposes of defining member stress coefficients, simple distinction s between beams or columns can be misleading, particularly for frames gover ned by earthquake or wind loads where column compression loads may be small relative to column strengths. Factors influencing beam-column stiffness in frame analysis are reviewed and simple formulas are proposed to determine effective flexural and shear stiffness coefficients of beam-columns as a fu nction of the applied axial compression. The proposed stiffness coefficient s represent conditions at incipient yield, and are applicable for linear (e lastic) analyses and the linear preyield region of nonlinear (inelastic) an alyses. Proposed stiffness coefficients are compared to test data and alter native recommendations from several sources, including a CEB state-of-the-a rt report on seismic analysis of reinforced concrete frames and three major design codes (ACI 318-95, New Zealand Standard, and Architecture Institute of Japan Standard). More test data are needed for refinement of available models, especially in the area of shear stiffness.