Seismic design spectra considering nonlinear connections between the existing building and the retrofit system

Holistic retrofit solutions for RC buildings carried out from the outside through structural exoskeletons, have been widely proposed in recent years. Based on the need to further investigate this type of intervention, the interaction between the existing building and the external exoskeleton was studied and discussed using a simplified 2DOF model. Both elastic and nonlinear retrofit solutions were considered. In the nonlinear case, the nonlinearity of the retrofit was lumped into the connection between the elements representing the existing building and the exoskeleton, yet the simplified model could be slightly adjusted to account for the nonlinearity in all its elements. The main contribution of the paper is the definition of design spectra aimed at the preliminary proportioning of elastic and nonlinear seismic retrofit solutions. Based on existing building characteristics, the developed design spectra provide the minimum retrofit stiffness required to achieve a given target displacement and, in the case of a nonlinear retrofit, the connection stiffness and yielding force that minimizes such displacement. The proposed design spectra allow for a direct comparison of the response of different retrofit solutions thus defining the optimal parameters for the retrofit system; the advantages and disadvantages of introducing elastic and nonlinear retrofit solutions may be discussed. A simplified design method was proposed and validated through nonlinear analyses applied to a 3D case study resembling an existing RC building that requires seismic retrofit. The case study validates the design spectra findings highlighting how the use of nonlinear connection could minimize the displacement. Furthermore, it highlights the beneficial effects of nonlinear connections in reducing the burden on the foundation of the new retrofit system and in reducing the seismic actions in the existing floors.