Conventional methods for seismic retrofitting of concrete columns include reinforcement with steel plates or steel frame braces, as well as cross-sectional increments and in-filled walls. However, these methods have some disadvantages, such as the increase in mass and the need for precise construction. Fiber-reinforced polymer (FRP) sheets for seismic strengthening of concrete columns using new light-weight composite materials, such as carbon fiber or glass fiber, have been developed, have excellent durability and performance, and are being widely applied to overcome the shortcomings of conventional seismic strengthening methods. Nonetheless, the FRP-sheet reinforcement method also has some drawbacks, such as the need for prior surface treatment, problems at joints, and relatively expensive material costs. In the current research, the structural and material properties associated with a new method for seismic strengthening of concrete columns using FRP were investigated. The new technique is a sprayed FRP system, achieved by mixing chopped glass and carbon fibers with epoxy and vinyl ester resin in the open air and randomly spraying the resulting mixture onto the uneven surface of the concrete columns. This paper reports on the seismic resistance of reinforced concrete columns controlled by shear strengthening using the sprayed FRP system. Five shear column specimens were designed, and then strengthened with sprayed FRP by using different combinations of short carbon or glass fibers and epoxy or vinyl ester resins. There was also a non-strengthened control specimen. Cyclic loading tests were carried out, and the ultimate load carrying capacity and deformation were investigated, as well as hysteresis in the lateral load-drift relationship. The results showed that shear strengths and deformation capacities of shear columns strengthened using sprayed FRP improved markedly, compared with those of the control column. The spraying FRP technique developed in this study can be practically and effectively used for the seismic strengthening of existing concrete columns.

Cross-bracing (X-bracing) is one of the most popular methods of seismic retrofitting, and has been shown to significantly increase the structural stiffness and strength of buildings. Conventional steel X-bracing methods typically exhibit brittle failure at the connection between the brace and the building, or buckling failure of the braces. This study investigated the structural properties of a new type of non-compression X-bracing system using carbon fiber composite cable (CFCC). This non-compression X-bracing system uses CFCC bracing and bolt connections between structural members and the terminal fixer of the CFCC, instead of conventional steel bracing. The aim is to overcome the brittle and buckling failures that can occur at the connection and bracings with conventional steel X-bracing methods. We carried out cyclic loading tests, and the maximum load carrying capacity and deformation were investigated, as well as hysteresis in the lateral load–drift relations. The test results revealed that the CFCC X-bracing system installed in reinforced concrete frames enhanced the strength markedly, and buckling failure of the bracing was not observed.

We describe a novel technique for restoration of reinforced concrete (RC) structures that have sustained damage during an earthquake. The reinforcement scheme described here is a hybrid seismic retrofitting technique that combines noncompression X-bracing using CF with externally bonded GF sheets to strengthen RC structures that have sustained damage following an earthquake. The GF sheet is used to improve the ductility of columns, and the noncompression CF X-bracing system, which consists of CF bracing and anchors to replace the conventional steel bracing and bolt connections, is used to increase the lateral strength of the framing system. We report seismic restoration capacity, which enables reuse of the damaged RC frames via the hybrid CF X-bracing and GF sheet wrapping system. Cyclic loading tests were carried out to investigate hysteresis of the lateral load-drift relations, as well as the ductility. The GF sheet significantly improved the ductility of columns, resulting in a change in failure mode. The strengthening effect of conventional CF sheets used in columns is not sufficient with respect to lateral strength and stiffness. However, this study results in a significant increase in the strength of the structure due to the use of CF X-bracing and inhibited buckling failure of the bracing. This result can be exploited to develop guidelines for the application of the reinforcement system to restore damaged RC structures.

In this study, we propose a new equation that evaluates the shear strength of beam–column connections in reinforced concrete and steel beam (RCS) composite materials. This equation encompasses the effect of shear keys, extended face bearing plates (E-FBP), and transverse beams on connection shear strength, as well as the contribution of cover plates. Mobilization coefficients for beam–column connections in the RCS composite system are suggested. The proposed model, validated by statistical analysis, provided the strongest correlation with test results for connections containing both E-FBP and transverse beams. Additionally, our results indicated that Architectural Institute of Japan (AIJ) and Modified AIJ (M-AIJ) equations should be used carefully to evaluate the shear strength for connections that do not have E-FBP or transverse beams.

The purpose of this study was to develop a new strengthening material that increases the shear strength and improves the deformation capacity of reinforced concrete (RC) beam members. A new composite material named epoxy mortar with steel fiber (EMSF) was prepared by inserting steel fibers into an epoxy mortar to provide the new strengthening material with high strength-toughness characteristics. In this study, materials and structural tests were employed. The materials tests assessed the influence of the volume percentage and the length of steel fiber on the tensile strength and toughness capacity of the composite material, EMSF. The structural tests help determine the influence of the EMSF panel type and thickness on the shear capacity and shear failure mode of strengthened RC beam members. The results of the two test groups are summarized as follows: 1) the results from the material tests indicated that the EMSF panel with 35 mm steel fibers exhibited high tensile strength, and the EMSF panel with both 35 mm and 50 mm steel fibers showed good toughness capacity; 2) the RC beam strengthened by an EMSF panel with short (35 mm) fibers possessed the highest shear strength, 3) the RC beam strengthened by an EMSF panel with both short (35 mm) and long (50 mm) fibers exhibited improved deformation capacity; and 4) a shear strength equation was derived from the equilibrium of the cracked RC beam members and is in good agreement with the test results.

At present, information on foundation–structure interactions of buildings is lacking. As a result, seismic performance evaluations of buildings seldom consider the effect of foundation performance on the building response. Recent destructive earthquakes, such as the Hokkaido Nansei-oki earthquake (magnitude M = 7.8, Japan, 1993), Northridge earthquake ( M = 6.7, U.S.A., 1994), Hyogoken-Nambu earthquake ( M = 7.2, Japan, 1995), Chi-Chi earthquake ( M = 7.6, Taiwan, 1999), Niigata Chuetsu earthquake ( M = 6.8, Japan, 2004), Sichuan earthquake ( M = 7.9, China, 2008), and Tohokuchiho Taiheiyo earthquake ( M = 9.0, Japan, 2011), have shown that building damage is significantly affected by the degree of damage sustained by the building foundation, and by the interaction between the building and its foundation. This paper presents a new methodology for nonlinear dynamic analysis of the seismic performance of reinforced-concrete (RC) buildings considering the pile foundation–structure interaction. The proposed method is applied to an actual example building that was severely damaged during the Hokkaido Nansei-oki earthquake. The results reveal that the method proposed in this study is able to predict the seismic performance of an RC building with a pile foundation and can provide fundamental data for the seismic design of such buildings.