GNGTS 2021 - Atti del 39° Convegno Nazionale

345 GNGTS 2021 S essione 2.2 SEISMIC RETROFIT OF EXISTING BUILDINGS WITH STEEL EXOSKELETONS AND BASE SLIDING DEVICES L. Sancin 1 , C. Bedon 1 , C. Amadio 1 1 Department of Engineering and Architecture, University of Trieste, Trieste, Italy Introduction Existing Reinforced Concrete (RC) structures and brittle buildings are often required to withstand seismic events that may be significantly high in magnitude and thus require high resistance / displacement demand, compared to their actual capacity. An optimal retrofit intervention can ensure enhanced structural performance. Among the techniques that have been addressed for the retrofit of existing RC frames, steel exoskeletons can notoriously improve the seismic performance of existing buildings due to their stiffness, ductility and resistance (Di Sarno and Manfredi, 2012; Scuderi, 2016; Badini et al., 2019; Di Lorenzo et al., 2020; Labò et al., 2020; Martelli et al., 2020). Traditional exoskeletons are usually designed to absorb seismic forces and, in this way, lighten the seismic demand on the brittle components of the existing building. The aim of this work is to investigate how steel exoskeletons interact with RC frames and determine consequent details to achieve a more effective design of the retrofit intervention. For this purpose, a Single Degree of Freedom (SDOF) model is first analysed. With a parametric study some limit conditions are explored. A new design concept that takes advantage of steel exoskeletons with additional base sliding devices (planar steel-PTFE (Teflon) sliders) is addressed (Dolce et al., 2005; Faiella et al., 2020). The sliding devices are placed at the base or in the middle of the base columns of the existing RC building to uncouple its motion from the ground motion and transfer all the seismic load to the steel exoskeleton, maximizing its efficiency and leaving the original building weakly stressed and without the need of even local interventions, that are invasive for inhabitants. The relative displacements of sliding devices are in general very low for the presence of the exoskeleton. Parametric study on a SDOF system The parametric analysis is carried out on a Single Degree of Freedom (SDOF) system representativeof thekeymechanical featuresof anexistingRC frameandof a steel exoskeleton that work in parallel ( Fig. 1(a) ). The steel exoskeleton is modelled as a rigid cantilever that is connected at the base through a nonlinear rotational spring (with initial stiffness K 1 , plastic resistance M 1 and ductility μ 1 ). The RC building is modelled through three rigid bars that are connected by nonlinear springs with initial rotational stiffness K 2 /2 and yielding bending resistance M 2 /2. For all the rotational springs M i =K i φ i , where φ i is the spring rotation, while the shear resistance of the elements is SMIC RETROFIT OF EXISTING BUILDINGS WITH STEEL EXO KEL T NS AND BASE SLI ING DEVIC S in 1 , C. Bedon 1 , C. Amadio 1 nt of Engineering and Architecture, University of Trieste, Trieste, Italy tion. Exis ing Reinforced Concrete (RC) structures and brittle buildings are often required to seismic events that may be significantly high in magnitude and thus require high / displacement emand, compared to their actual capacity. An optimal retrofit intervention e enhanc d stru tural perfor a ce. Among the techniques that have been addressed for the existing RC frames, steel exoskeletons can notoriously improv the seismic p rformance of uildings due to their stiffness, ductility and resistance (Di Sarno and Manfredi, 2012; 016; Badini et al., 2019; Di Lorenzo et al., 2020; Labò et al., 2020; Martelli et al., 2020). al exoskeletons are usually designed to absorb seismic forces and, in this way, lighten the emand on the brittle components of the existing building. The aim of this work is to e how steel exoskeletons interact with RC frames and determine cons quent details to more effective design of the retrofit intervention. urpose, a Single Degree of Freedom (SDOF) model is first analysed. With a parametric e limit conditions are explored. A new desig concept that takes advantage of steel ons with additional base sliding device (planar st el-PTFE (Teflon) sliders is addressed al., 2005; Faiella et al., 2020). The sliding devices are placed t the base or in the middle of columns of the existing RC building to uncouple its motion from the ground motion and ll the seismic load to the steel exoskeleton, maximizing its efficiency and leaving the uilding weakly stressed and without the need of even local interventions, that are invasive tants. The relative displacements of sliding devices are in general very low for the presence skeleton. ic study on a SDOF system . The para etric analysis is carried out on a Single Degree of (SDOF) system repres ntative of the key mechanical features of an xisting RC frame and of oskeleton that work in parallel ( Figure 1( ) ). The ste l exoskeleton is m dell d as a rigid that is connected t the base through a nonlinear rotational spring (with i itial stiffne s K 1 , istance M 1 and ductility  1 ). The RC building is modelled through three rigid bars that are by nonlinear springs with initial rotational stiffness K 2 /2 and yielding bending resistance all the rotational springs M i = K i φ i , where φ i is the spring rotation, while the shear of the ele ents i R i = M i H = K i H 2 δ , where δ is the top displacement and H the frame height (a)). At the foundation level, the RC frame is equipped with a rigid restraint against possible and vertical displacements. Two additional translational springs, characterized by a linear haviour, are introduced at the base of the building to represent the level of connection of the o the foundation in horizontal direction. The stiffness of these springs can vary from K 3  0 ding devices are used) to K 3  ∞ (to represent a traditional rigid foundation). The total mass and M for the exoskeleton and the RC frame, respectively. A rigid link is finally used to echanical interaction between the RC building and the exoskeleton. e possible combinations for the mechanical parameters described above, the attention of this , where δ is the top displacem nt a d H the frame heig t ( Fig. 1(a)). At the foundation level, the R is equip ed with a rig d restraint ag inst possible rotations and vertical displacements. Two addition l translational springs, characterized linear elastic behaviour, are introduced at the base of the building to represent the level of connection of the building to the foundation in horizontal direction. The stiffness of these springs can vary from K 3 →0 (when sliding devices are used) to K 3 →∞ (to represent a traditional rigid foundation). The total mass is set in m and M for the exoskeleton and the RC frame, respectively. A rigid link is finally used to provide mechanical interaction between the RC buildi g and the exoskelet n.