Seismic Retrofitting Effects of Steel Brace on Capacity of Shear-Critical RC Frame Using Round Bars

Abstract eng:
After the Hyogo-KEN Nanbu Earthquake, existing R/C buildings in Japan, violating the current criteria of design code, have been seismically retrofitted with various reinforcing components. In most cases, among the available retrofitting systems, the steel bracing system was preferably adopted, because the weight of this system is lighter than the other systems and it can provide moderate openings. However, in order to assess the effect of seismic retrofitting by steel bracing system correctly, there are still remained three issues to be solved as follows: 1) Up to now, the reinforcing effect of steel bracing has been mostly investigated through the test on R/C sub-frame specimens with single story and span. Thus, the impact of single and/or multiple steel bracing systems incorporated in R/C framed structures on the overall capacity of building with multiple stories and spans is not clarified. 2) Some critical aspects involved in the existing R/C buildings designed according to the old seismic design code in Japan are the shear failure of columns and the bond-slip behavior of round longitudinal bars in columns. The influence of these behaviors on the capacity of R/C structures is not fully understood. 3) Effect of the structural slits, usually made for the non-structural walls during the retrofit work, on the seismic retrofitting performance has not been clearly investigated. The short columns with non-structural walls tend to cause brittle shear failure. Thus, it is recommended to make structural slits such that the height of column becomes longer and leads to ductile flexural failure. However, the effect of the structural slits is not clearly verified. Three single-story and two-span R/C framed specimens were made; the first one is the reference specimen with non-structural walls, designed according to the old seismic code, and the other two are the retrofitted specimens by the steel bracing unit and/or the structural slits, and the static cyclic loading tests were performed. The test results demonstrated that the superior retrofitting effect of the steel bracing system, enhancing the capacity of R/C framed specimen, could be evaluated quantitatively. Furthermore, the test results indicated that the structural slits were effective for preventing the shear failure of column. On the other hand, it could be pointed out that the structural slits might not improve deformability of the retrofitted framed specimens. The source of insufficient deformability may be partly due to severe fractures at the indirect joints of mortar between the R/C frame and the steel frame along the columns. It seems that this joint fracture might be due to the stiffness degradation caused in column by introducing the structural slits. Also, the bond-slip of longitudinal round bars in the columns might contribute to the additional stiffness reduction of the columns. Finally, in order to investigate the development of stress resultants such as shearing and axial force in columns and steel braces and the seismic resisting mechanism of the steel brace-frame system, the nonlinear finite element analyses were carried out.

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