Steel concentrically braced frames are common lateral force resisting systems in both new construction and existing buildings. However, the seismic behavior of braced frames designed prior to the adoption of capacity design principles in the 1980s is generally not well understood despite their widespread presence. These older braced frames, termed non-seismic concentrically braced frames (NCBFs) are the subject of a research project titled “NEESR: Collaborative Developments for Seismic Rehabilitation of Vulnerable Braced Frames” that seeks to evaluate NCBFs and determine retrofit strategies. The National Science Foundation (NSF), through the Network for Earthquake Engineering Simulation (NEES), funds testing at three sites—the University of Washington (UW), the University of California Berkeley (UCB), and the National Center for Research on Earthquake Engineering (NCREE) laboratory in Taiwan. Finite element models will be developed and validated using this experimental data.

Steel concentrically braced frames are common lateral force resisting systems in both new construction and existing buildings. However, the seismic behavior of braced frames designed prior to the adoption of capacity design principles in the 1980s is generally not well understood despite their widespread presence. These older braced frames, termed non-seismic concentrically braced frames (NCBFs) are the subject of a research project titled “NEESR: Collaborative Developments for Seismic Rehabilitation of Vulnerable Braced Frames” that seeks to evaluate NCBFs and determine retrofit strategies. The National Science Foundation (NSF), through the Network for Earthquake Engineering Simulation (NEES), funds testing at three sites—the University of Washington (UW), the University of California Berkeley (UCB), and the National Center for Research on Earthquake Engineering (NCREE) laboratory in Taiwan. Finite element models will be developed and validated using this experimental data.

The first test at the NCREE laboratory (NCBF-INV-1) examined a full-scale two-story NCBF in the chevron (inverted-V) configuration subjected to quasistatic cyclic loading. In the chevron configuration, large unbalanced forces are expected to develop at the middle gusset plate connection of the beam due to the difference between the compressive buckling and tensile yield strengths of the braces. This specimen contained a beam at the second floor that is considered weak by AISC capacity design standards, as it is unable to develop the resultant downward force elastically. The size of the beam was selected to be representative of many NCBFs based on an extensive survey of existing structures. Significant inelastic deformation of such a beam has the potential to accelerate brace buckling and, subsequently, brace fracture at low drifts. The ductility of the frame was further inhibited by the use of older connection detailing and non-seismically compact braces.

The second NCREE test (NCBF-INV-2) examined a post-earthquake repair scenario using the damaged NCBF-INV-1 frame. Damage in the previous test was localized to the first story. The first story braces and gusset plates were removed and replaced with seismically compact braces with connections designed for in-plane buckling. This was achieved through the use of knife plates with a 3tkp clearance between the gusset plates and braces. The new frame components were designed using the balanced design procedure, which seeks to promote ductility by permitting a controlled amount of yielding in the connections while suppressing undesirable failure mechanisms.

The third test (NCBF-INV-3) examined another repair by replacing the first story braces and connections with wide flange braces and connections designed for out-of-plane buckling. As in the second test, the balanced design procedure was used to design the new components. Per the recommendations of the procedure, the lower (corner) gusset plates were designed with 8tp elliptical clearances, while the upper (middle) gusset plate was designed with a 6tp vertical clearance.

The fourth test (NCBF-INV-4) will examine an inverted-V braced frame with details consistent with modern design except for the second floor beam. The exterior frame (roof beam and columns) is the same as the previous tests, but all of the interior elements were removed. The new second floor beam is the same size as that used in the previous three tests and the frame will be braced with seismically compact HSS braces. The balanced design procedure was employed to design the gusset plates for out-of-plane brace buckling as in NCBF-INV-3.

All tests at NCREE will be performed under the supervision of National Taiwan University (NTU) Professor Keh-Chyuan Tsai. Finite element models developed in Abaqus will be validated using data collected from these tests. The outcomes of this research will be a greater understanding of the seismic performance of NCBFs, the development of rehabilitation strategies for vulnerable NCBFs, and a robust technique for modeling NCBFs.