PSI - Issue 78

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

2049

2.2. Endoskeleton-type solution The endoskeleton solution is typically used to comply with the principles of structural conservation. This conservation can be either historical or administrative, in compliance with local regulations. Therefore, this solution is most applicable to masonry structures, although its use for reinforced concrete buildings is not excluded. The design strategy was based on the definition of a new seismic-resistant steel system within the existing masonry envelope. The new internal steel system (endoskeleton) performs both the load-bearing functions with respect to static and seismic conditions. In fact, the existing floors at all levels are demolished and rebuilt, along with the load-bearing steel structure. Essentially, the perimeter walls will be "unloaded" from vertical forces and will be less affected by seismic conditions as they are connected to the endoskeleton through a retaining system that allows for decoupling from vertical loads (which would translate into seismic mass) and horizontal forces. This solution provides that all the seismic action is attributed to the new endoskeleton, while the envelope will be affected only by the seismic action rate associated with its dead load and local mechanisms. 3. Design examples 3.1. Exoskeleton-type design solutions It is important to note that the exoskeleton design solution can be achieved through different arrangements. Two typical configurations are described in the following: the first one is called "parallel exoskeletons," the second one is called "orthogonal exoskeletons." The first configuration involves the installation of steel braces positioned parallel to the seismic-resistant alignments, while the second involves a widespread distribution of the bracing systems orthogonal to the seismic-resistant alignments to form buttresses. For the first configuration type, two design examples are shown, called "Building A" and "Building B ” . Building A The building, built in the 1950s and located in Perugia, Italy, is used for teaching and laboratory purposes. The building comprises five floors, a semi-basement, and an attic. The building's height above ground is 26.50 m, with inter-story heights varying between 3,00 m and 3,80 m. The structure's seismic capacity is very low. The first collapse mechanisms occur for seismic demands with a return period of less than 30 years. The causes of column shear failures depend on the low amount of transverse reinforcement, but, above all, depend on the torsional effects caused by the in-plan irregularity, i.e., the significant eccentricity between the mass and stiffness centers. The exoskeleton system adopted consists primarily of steel braced frames, arranged externally to the building close to the elevations. The external bracing is made of large-section steel profiles (HE400B and HEB500) to ensure the required stiffness and is based on r/c thick slabs with deep foundations. The bracing system is shown in Fig. 2a (plan layout) and Fig. 2b (elevations). The connection system is composed of two parts, one fixed to the exoskeleton and the other constrained to the existing building. The two parts are interconnected by the shear-resistant connector profile. The part constrained to the building consists of a beam on which there are contrast boxes to house one end of the connector. The contrast box has a vertical slotted hole allowing the connector to slide vertically without generating compression or traction in the building's columns. The rectangular steel connector profiles are placed at 800 mm intervals and form a sort of "comb". Fig. 3 shows some details from the executive drawings of to the connection system. The upgrading intervention allowed to achieve a seismic improvement corresponding to a Capacity/Demand ratio of 0,60, that is it increased the building seismic capacity up to 60% of the adaptation level (level of a new built building).

Fig. 2 Building A: a) positioning of external bracing (in red); b) longitudinal and transverse elevation