PSI - Issue 78

Alessandro Fulco et al. / Procedia Structural Integrity 78 (2026) 2046–2053

2051

according to the "orthogonal endoskeleton" configuration. The planned external portals are similar to "buttresses" systems designed to significantly increase the overall rigidity of the structure. The single frame is a lattice wall composed of 140 mm diameter tubular elements that form three St. Andrew's cross bracing fields over a total height of 4,0 m. The bracing portals are distributed along the entire perimeter of the building with variable spacing of approximately 6,0 m. At the top, the portals are connected by a lattice plane, forming a rigid connection between all bracing systems. The connection between the external portals and the existing structure is created at the floor level using a steel system that defines a constraint condition attributable to a connecting rod, therefore with pinned ends. This constraint system therefore ensures the congruence of horizontal movements between the existing building and the external portals, avoiding the exchange of shears and flexures. This constraint configuration avoids concentrations of parasitic stresses on the existing structure due to coupling with the external portals, particularly axial tensile and compressive stress. Fig. 5 shows the structure's structural framework and details of the individual bracing portal. Fig. 5 a, b, and c show excerpt of drawings of the connection system. The overall bracing interventions on the building resulted in an overall increase in stiffness that resulted approximately four times greater than in the existing conditions: this mitigated the seismic actions acting on the existing structure, delaying its collapse mechanisms and allowing a risk index value of 0,80 to be achieved. The structure's natural periods were reduced from 0,30 s (pre-existing building) to 0,15 s (strengthened building).

Fig. 5 Building C: design carpentry (left); connecting system (right): a) plan view; b) side elevation; c) planimetric detail

3.2. Endoskeleton-type design solutions Below is a design example of a single masonry building subject to a planning constraint requiring its architectural preservation. Therefore, given the structure's vulnerability and the client's needs, which required open-plan spaces and the demolition of internal walls, the endoskeleton strategy was adopted. Building D The building, dating back to 1930, has a rectangular plan measuring 8,50 x 6,00 m and consists of two stories plus an additional pavilion-type roof. From the analyses carried out, the structure in its current state shows critical issues both locally and globally, identifying capacity/demand coefficients largely insufficient: close to 0,25 for local out-of plane and global mechanisms. The seismic retrofit project involves the installation of an independent endoskeleton within the existing envelope to which both seismic and static action will be fully attributed. The new internal structural system is defined by main steel frames connected transversally by a secondary framework. In particular, the main frames (in the Y direction) feature HEB300 columns and HEB200 beams. The existing perimeter walls (envelope) are connected to the floors of the internal steel structure using a connection system that acts as a constraint against out of-plane loads but allows for decoupling from both vertical and horizontal loads. Connecting rods spaced 1,50 m apart are provided at the perimeter beams of the steel structure, which are hinged on a web welded to the perimeter beams of the endoskeleton. A continuous steel plate is provided around the perimeter of the existing walls. Essentially, the seismic mass of the decks and therefore the associated seismic stresses have been removed from the existing structure. Furthermore, the new foundations planned for the endoskeleton have also been used as foundation strengthenong for the existing walls using through-connections made of steel profiles. Fig. 6 shows the carpentry and the project cross section along with a detail of the connection system.