Performance of Compression-free turnbuckle braces using partially fibered CFRP under cyclic load

Turnbuckle braces have been widely used as the seismic-resistant member of steel structures. Being a very slender member, the compressive resistance capacity is not considered in the structural design, but these braces buckle easily due to compressive forces, and the buckling deformation damages the finishing material and causes local plastic strain concentration and plastic fatigue failure. Based on this, the authors of this study developed a compression-free brace that can be easily installed on existing steel turnbuckle braces. We focused on the use of lightweight, high-strength carbon fiber reinforced polymer (CFRP) as a brace material and joined the CFRP rod to the existing steel turnbuckle braces through a pipe style turnbuckle body. The proposed compression-free brace uses partially fibered CFRP (PFCFRP) rods to absorb compressive deformation so that only tensile forces are applied to the brace, and out-of-plane deformation due to buckling is significantly reduced. In addition, the combined use of steel turnbuckle braces can be expected to absorb seismic energy by the plastic deformation of steel. PFCFRP rods are only fiberized in the middle position and the unfiberized position (CFRP rods positions) at both ends are used for the joints. They are molded by pultrusion because of their excellent cost-effectiveness, superior mechanical performance, and stability. The partially fibered sections are created without resin injec-tion. PFCFRP rods are connected to the braces using a pipe-style turnbuckle body. The method used to fix the PFCFRP rod and the pipe-style turnbuckle body involves widening the end of the PFCFRP rod to drive a wedge and insert it into the turnbuckle body. Then, it is filled with steel balls and epoxy resin to prevent it from pulling out. Furthermore, we use the pipe-style turnbuckle body to connect the steel structure like a sleeve joint to provide easy and useful assembly. Two different tests were carried out to confirm the effectiveness of the compression-free braces. Firstly, testing of materials used in compression-free braces was conducted. The results of the tests were used to compare the strength of steel turnbuckle braces to clarify the scope of the application of compression-free braces. Next, cyclic loading tests were carried out on a full-scale steel structural frame to simulate the actual seismic bracing situation. The results show that the proposed compression-free braces have similar historical behavior to existing steel turnbuckle braces. Because the PFCFRP rod absorbs the compressive axial deformation, the out-of-plane deformation can be significantly reduced. These results demonstrate that the proposed compression-free braces using PFCFRP can provide a safe and applicable seismic bracing system.