Berkeley-3SB (NCBF-B-3SB): Strongback Retrofit of Non-Seismically Detailed Concentric Braced Frame (NCBF)

The NCBF-B-3SB test was the third in a series of experimental tests done at the NEES@Berkeley lab that explored the vulnerabilities of existing braced frames designed prior to the 1988 Uniform Building Code. The NCBF-B-3SB test represented a potential retrofit strategy of two previously tested older braced frames designed to the 1982 Uniform Building Code (NCBF-B-1 and NCBF-B-2). Both older tests exhibited soft story behavior, concentrating almost all damage in the second and first story respectively. This third test attempted to mitigate this soft story behavior through the use of a retrofit "strongback" (SB) system. The beams, columns, and shear tabs were considered to be from the original NCBF design and were not modified for the strongback retrofit. The original "chevron" braces were re-oriented in a "lambda" configuration, and the new gusset connections were designed using current capacity design principles. The new lambda configuration consisted of two halves. One half was designed to remain essentially elastic throughout the test, acting like a strong backbone for the system and distributing story drifts over the entire height of the frame. The other half consisted of a buckling-restrained brace (BRB) and half of the beam as the main energy dissipating devices in the system, controlling the inelastic behavior of the frame.