On the Seismic Performance of Skewed Special Moment Frame Reduced Beam Section Connections

Special Moment Frames (SMFs) are frequently used in high seismic areas for architecturally constrained designs, as they provide lateral system stiffness without the use of braces which often obstruct views and architectural features. Reduced beam section (RBS) connections are popular SMF connection details developed following the Northridge earthquake to limit brittle fractures within connection welds. Current American Institute of Steel Construction (ASIC) provisions (i.e. AISC 341-16) provide prequalified SMF RBS connection details (including welding requirements); however, all prequalified details only consider orthogonal connections between the beam and column. This dissertation investigates the effect of adding skew within SMF RBS connections and provides insights into allowable skew levels for design, widening the application of SMF RBS connections. The study presented herein involves parametric component-level analyses and system-level dynamic time-history analyses of skewed SMF RBS connections. The component-level parametric study involves detailed finite-element analysis of 64 SMF RBS connections and 48 SMF Welded Unreinforced Flange-Welded Web (WUF-W) connections (as a typical connection alternative to the RBS). The component-level investigation considers 3 skew angles, 4 column axial load levels and 3 beam-to-column connection geometries (shallow, medium, and deep column geometries). Connection capacity, column twist/yielding, connection response and fatigue performance are all investigated. Additional component-level composite (concrete-steel) connection simulations are conducted to investigate the effects of composite concrete slabs on the behavior of the skewed connections. In addition to the component-level analyses, system-level time-history analyses are used to investigate skewed SMF RBS connection demands during dynamic seismic loading. To investigate system-level effects on skewed connection behavior, a six-story building containing various levels of skew at the SMF connection is designed, simulated using detailed finite element procedures, and loaded using a scaled earthquake ground motion to represent both design basis and maximum considered earthquake demands. In addition to the detailed finite element investigations, an experimental testing program is designed and initiated to allow prequalification of skewed SMF RBS connections within the AISC provisions. Specimen fabrication, experimental setup (including instrumentation, loading, and boundary conditions), and initial results for the prequalification testing are described herein. Results from the component-level parametric research work indicate that SMF RBS connection capacity decreases when increasing the skew angle; however, all performance levels achieved would satisfy current AISC requirements for prequalification. Additionally, as skew angle is increased within the SMF RBS connection, the resulting column twist increases. Column flange-tip yielding is also observed at beam bottom-flange levels of the skewed geometries, and this yielding does increase for skewed connections having medium and deep column geometries when increasing the skew angle; however, the yielding is rather localized on the column flange. Local damage (indication of low-cycle fatigue fracture susceptibility) within the RBS section decreases when increasing the column axial load but does increase when increasing the skew angle. When a concrete slab is included, the connection’s positive moment capacity increases due to composite action, but the result is increased column twist for medium and deep column geometries at rather large skew (30° skew relative to the column). A column twist prediction formula is developed and proposed.