-Concrete filled steel tube(CFT), which has superior ductility and strength, is used for building column, bridge piers of ocean structure. Shear design equations of CFT existing in structural design provisions are excessively conservative. It has an effect on constructability and the economics of CFT. However, to suggest the reasonable shear design equation, experimental studies on the shear capacity of CFT have been rarely conducted. This study is analytical research to suggest improved shear design equations of large-diameter concrete-filled steel tubes. This analytical research was conducted to apply finite element analysis model of CFT based on the prior research. It was verified by comparison with prior test results. The verified model was used for parameter studies to estimate the influence of overhang length, concrete compressive strength and diameter-thickness ratio on shear strength.

SCBFs (Special Concentrically Braced Frames) have been praised for considerable strength, high lateral stiffness, economic feasibility and good performance of seismic loads, which make engineers widely use SCBF as lateral-load resisting systems in buildings and have researchers develop design requirements. Compared to the extensive research of SCBF, comparatively little information is currently available on the performance of SCBFs designed and constructed before the early 1990’s. Before the early 1990’s, CBFs (Concentrically Braced Frames) design requirements were substantially less restrictive. As a result, many existing structures designed to these requirements may not exhibit desirable behavior in a seismic event. Existing CBFs that do not meet modern SCBF criteria shall be referred to herein as NCBFs (Non-Seismic Concentrically Braced Frames). In this study, FE (Finite Element) analysis was carried out. The detailed inelastic FE models were verified, which are based on the preceding research. Finally, these models were compared to evaluate the seismic performance of NCBFs.

-Special concentrically braced frames(SCBFs) have distinctive advantages in considerable seismic performance, which make engineers widely use SCBFs as lateral-load resisting systems in buildings and have researchers to develop SCBFs design methods. Compared to the extensive research of SCBF, comparatively little information is currently available on the performance of SCBFs designed and constructed before the early 1990's. Prior to 1988, concentrically braced frames(CBFs) design requirements were substantially less restrictive. As a result, many existing structures designed to these requirements may not ensure ductility and pose a significant concern in current buildings. In this study, these older frames are referred as non-seismic braced frames(NCBFs). In order to investigate the seismic behavior of NCBFs, finite-element(FE) models of SCBF and NCBF were suggested and verified using case investigation of NCBF conducted on the University of Washington. Using these models, the seismic behavior of NCBF with shared welding shear tab, which is the representative of the types of connections, was established and compared with the seismic performance of SCBF. Keywords -Non-Seismic concentrically braced frames, Special concentrically braced frames, Finite element analysis, Seismic behavior

-Concrete filled steel tube(CFT) has outstanding deformation capacity and strength in comparison with reinforced concrete or steel tube. CFT drilled shaft, which is developed large shear force due to seismic load and soil liquefaction, is designed as large diameter. However, shear design equations of the current standards do not consider bond stress of CFT and it results in extremely conservative design. Currently, previous studies for improving shear equations scarcely exist and are impossible applied to large CFT drilled shafts since these studies focus on only small scale experimental research. In this study, eventually to propose improving shear equation of large diameter CFT, it is preliminary research to compare and investigate the previous studies and current standards.