Post-installed fastening solution for strengthening of RC frames with seismic bracing : a displacement based approach

Abstract

Steel bracing is a popular solution for strengthening of reinforced concrete (RC) frame structures against seismic hazards. The new structural brace elements can be directly connected to the existing frame structure by means of post-installed anchors. Such a connection offers a low invasive, economical, and practical solution. However, there are also certain challenges which arise when using post-installed anchors. Different types of post-installed anchors exhibit different displacement and hysteretic behavior. Since the displacement (and hysteretic) behavior of the anchorage has a marked influence on the performance of a strengthening solution, it is important to understand the actual behavior of the anchors and to take their behavior into account in the design of a strengthening solution. Therefore, it is required to assess the suitability of different types of anchors for such structural applications. This is associated with an adequate procedure for the assessment of the hysteretic and displacement behavior of the same. When post-installed anchors are used to form the connection between the steel bracing and the RC frame, they might be subjected to geometric restrictions that negatively affect the performance of the anchors. For instance, the limited dimensions of the structural members, such as beams and columns, in which the anchors are installed in. Or the arrangement of anchors in a spatial corner configuration in case the steel braces are to be fastened to the corner of a RC frame. These challenges call for a displacement based design solution for post-installed anchors, which accounts for the actual performance of the anchors and anchor connections in terms of the displacement and hysteretic behavior. The first part of this thesis investigates the hysteretic and displacement behavior of tension loaded single anchors. For this purpose, a new displacement-based testing procedure is presented. An experimental program was carried out, in which five different types of post-installed anchors were tested following the new testing procedure and the current testing approaches for qualification of anchors against seismic actions. Based on the evaluation of the single anchor behavior, a hysteretic model was developed to simulate the behavior of anchors under pulsating tension load. The model includes the unloading and reloading behavior of the anchors, strength degradation in subsequent cycles at the same displacement level, and accounts for the residual displacements of the anchors after unloading. The second part of this thesis deals with the behavior of anchor groups. Two focal points are investigated. One is the hysteretic behavior of anchors groups. For this purpose, an experimental program is carried using the displacement-controlled testing procedure previously applied to single anchors. Based on the experimental results the hysteretic model for single anchors is extended to anchor groups. Second is the behavior of tension loaded anchor groups in narrow concrete members. Thus, anchor groups in the vicinity of two parallel close edges. The results of the experimental results highlight the need for a modified analytical design approach for anchor groups in narrow members to overcome the current over-conservatism. In the third part of the thesis, the connection between steel bracing and RC frame corner is investigated. The experimental and numerical analysis of the spatial corner configuration highlights the feasibility of such a connection approach. Based on the results, an analytical and a displacement-based design solution for the connection is proposed. The displacement-based solution is based on the nonlinear spring modelling approach and allows the assessment of the complete connection.