Development of a closed-form 3-D RBS beam finite element and associated case studies

The 1994 Northridge and the 1995 Kobe earthquakes demonstrated that steel moment resisting frames (SMRF) which were traditionally considered to provide an ideal structural system in resisting lateral earthquake loads could indeed suffer brittle fractures at the beam–column connections. To remedy this, competing connection design improvements have been adopted, among which is the radius-cut Reduced Beam Section (RBS) moment connection. As the design of a vast majority of these SMRF buildings is drift-controlled, the reduction in lateral stiffness and hence the increase in drifts in general 3-dimensional SMRF buildings with RBS moment connections–particularly those with torsional irregularity–is investigated here through development and verification of a new closed-form 3-D RBS finite element based on Timoshenko’s shear deformation theory for beams. The study shows that the increase in story drifts in 3-dimensional torsionally irregular SMRF buildings with RBS moment connections could be as high as 15% under the action of lateral and gravity loads. This is in contrast to the current recommendation of increasing elastic story drifts by a maximum of 9%–10% for SMRF buildings to account for stiffness reduction in the presence of RBS connections. The study also shows that the decrease in weak axis joint rotational and shear stiffness coefficients of beams with RBS connections on both sides–and unsupported laterally–could be as high as 67% for flange area reductions of only 30%.