PSI - Issue 64

Alessandro Prota et al. / Procedia Structural Integrity 64 (2024) 1041–1048 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Seismic strengthening intervention 3.1. Design of steel exoskeleton system

The need for a strengthening intervention arises from the assessment performed on the as-built school building. 2D parallel steel exoskeleton (S-EXO-2D-//) was selected as the strengthening solution. Steel exoskeletons offer a range of advantages in the context of seismic retrofitting of existing buildings (Prota et al., 2024). Firstly, their implementation significantly contributes to enhancing the seismic safety of buildings by improving overall stiffness and resistance to lateral actions. A crucial point is that this outcome is achieved without the need to disrupt the daily activities of the building, thus avoiding the necessity to relocate occupants, as exoskeletons can be applied externally with minimal impact on ongoing operations. The speed of execution, especially with highly industrialized dry solutions, represents an additional advantage in terms of intervention timelines. While exoskeleton applications offer numerous advantages, certain critical issues must be considered in the decision-making process. Firstly, the application of these systems, when arranged orthogonally to the existing building facade, is limited to isolated structures, a circumstance that may reduce their applicability in densely developed urban contexts. The need for space along the building perimeter is a peculiar concern for parallel exoskeletons, as it may lead to interference with balconies or other architectural elements. Finally, it is crucial to conduct a thorough verification of the diaphragm's capacity (assumption of a rigid floor), as the addition of new seismic-resistant elements can influence the behavior and stability of this structural component (Marini et al., 2022). The design of exoskeleton systems adhered to a systematic process outlined by some of the authors in a prior publication (Di Lorenzo et al., 2023). This process involved multiple steps, initiating with the evaluation of the seismic performance of the existing structure (Step 1) and the selection of suitable materials for the new lateral-load resisting system (Step 2). Subsequently, the design procedure encompassed defining the exoskeleton's shape (Step 3), specifying cross-section types (Step 4), and ultimately determining the dimensions of exoskeleton elements through a displacement-based approach (Step 5). The evaluation of the existing structure's seismic performance is comprehensively addressed in previous section. The chosen steel for the exoskeleton elements is S355J2 (Step 2). The EXO-2D-//, oriented parallel to the structure, features a configuration with X-shaped concentric bracing. The width of individual walls was determined based on aspect ratios relative to the total building height, as outlined by Di Lorenzo et al. (2023). The overall performance required by the exoskeleton system was defined using a displacement-based procedure. In particular, the intervention was designed in order to get a safety ratio equal or greater than 1, with reference to the LS limit state, as prescribed from the code. The displacement-based procedure was carried out by equating the capacity point of the as- built structure with the target displacement (Δ * target ), which represents the demand point of the retrofitted structure for that specific limit state, it is possible to derive stiffness and strength values that the retrofitted structure must ensure (K Ed-LSi and R Ed-LSi ) such that the verifications for that limit state are met. Then, it is possible to define the required lateral stiffness and resistance of the external strengthening system (K EXO and R EXO , respectively) as the difference between K Ed-LSi and R Ed-LSi with the strength and stiffness of the as-built structure (K ab and R ab , respectively). The most demanding values from all the considered load patterns were taken into consideration for the design. Before initiating the wall sizing process, the selection of the total number of shear walls and their layout in both directions was undertaken. This phase holds significant importance as the exoskeleton system needs to be distributed along the exterior perimeter to facilitate the effective transfer of horizontal forces from the existing structure. Furthermore, the positioning of the walls is critical to minimize the eccentricity between the center of stiffness and the resultant seismic force. A total of 24 shear walls were taken into consideration, with 12 in each main direction (see Figure 6a). These walls were symmetrically arranged concerning the center of mass of the structure. Consequently, the required stiffness and strength values for an individual wall can be determined by dividing the overall required stiffness and strength by the number of walls. As result of the design process steel profile HE 280 B, HE 220 B and CHS 139.7x10 mm were selected for columns, beams and diagonals of the shear walls.