The Design and Behavior of Beam-Columns in Unbraced Steel Frames

Design methods for beam-columsn in unbraced frames were investigated both theoretically and experimentally. Theoretical studies were conducted to determine the strength of the beam-columns in symmetrical portal frames and frames containing some columns with pinned connections (leadned frames). The variables in the studies were the slenderness ratio, the relative column-to-beam stiffness, the effect of bending axis, the effect of the residual stres, and the effect of the axial load on the pinned-end column in leaned frames. Exact interaction curves were developed for these cases. The results obtained were used as the basis for the comparison with design interaction equations. Three full size two-bay unbraced frames were tested to study the interaction of beam-columns within a frame. The first two specimens were subjected to column laods only and the third specimen was tested under the combined gravity and lateral load.

Another set of tests was conducted to study the spatial behavior and the strength of restrained beam-columns in unbraced frames where there is no experimental data currently available. Three full size biaxially loaded restrained beam-column specimens were tested under the loading and support conditions simulating the actual condition in unbraced frames. The test results were compared with a theoretical prediction and design interaction equations.

The studies indicate that a proposed interaction equation provides an accurate prediction of the strength of a beam-column in planar unbraced frames. The method was also verified by comparing the predicted value with the frame test results. The test results on biaxially loaded restrained beam-columns are in good agreement with the theoretical prediction. It was found that the current design methods are satisfactory for columns subjected to uniform moment but are very conservative for moment gradient cases. Based on the theoretical and experimental investigation in this study, recommendations for the design of beam-columns in unbraced frames are outlined.