PSI - Issue 78

S. Cattari et al. / Procedia Structural Integrity 78 (2026) 1577–1584 S.Cattari et al. / Structural Integrity Procedia 00 (2025) 000–000

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Similarly, for RC buildings, the intervention categories include the strengthening of the various structural components forming the seismic-resistant system – namely columns, beam–column joints, beams, and, where applicable, shear walls. Regarding horizontal structural elements, in RC buildings the interventions typically focus on either structural strengthening or improving the connection between slabs and vertical structural elements. In URM buildings, the strengthening of floor diaphragms – such as through the addition of a reinforced concrete overlay slab – not only improves structural performance but also increases in-plane stiffness, enabling a more uniform redistribution of seismic forces across all load-bearing elements and preventing the concentration of damage in specific weak points. Additionally, in many URM buildings, interventions frequently included the demolition and reconstruction of the roof structure.

Table 1. Intervention strategies designed for the analysed sample

Masonry structures (URM) – Vertical structures

Reinforced concrete structures (RC) – Vertical load bearing system

No intervention

Increased strength - piers

Increased strength - spandrels

Steel closing

frame of openings OOP failure prevention

Wall-to-wall connection

Masonry

monolithicity

No intervention

Increased strength - columns

Increased strength - walls

Increased strength - joints 19% 44%

Increased strength - beams

No intervention

-

8% 8%

8% 8%

- -

- -

8%

8%

- -

52% 11%

- -

F-Wall-to-diaphragm connection

8%

-

-

11% 19%

F- Structural strengthening / Diaphragm stiffening with RC slab F-Elimination of horizontal thrusts R-Wall-to-diaphragm connection R- Structural strengthening /Diaphragm stiffening with RC slab

8%

58% 58%

8%

8%

17%

-

-

19%

-

15% 26%

- -

8%

8%

8% 8%

-

8% 8%

-

- -

- -

- -

- -

- -

33% 33%

8%

8%

8%

8%

-

8%

-

-

-

4%

-

4%

7%

-

Horizontal structures

R-New roofing system

17% 42% 42%

8%

8%

8%

8%

-

-

-

-

-

An important aspect in the assessment of retrofit strategies is the extent of intervention, i.e., whether the intervention targets limited and specific portions of the structure, or if it has been implemented extensively and systematically across all structural elements. Certain types of interventions are designed to address localized vulnerabilities and do not act at the global structural level. These include interventions aimed at improving masonry monolithicity, such as stitching and replacement techniques (used to remove and replace damaged portions of masonry), or interventions that enhance wall-to-wall connections through the insertion of steel bars to increase cohesion between orthogonal walls. In the analysed URM sample, out-of-plane failure prevention interventions were also generally designed to address specific weaknesses, such as slender masonry elements. In the only two cases involving slender walls, the adopted techniques included either a traditional approach – increasing the wall section thickness – or a more modern one, wrapping with composite materials. Each intervention strategy presented in Table 1 can be implemented using different execution techniques in real world practice. In URM buildings, load-bearing wall strengthening may be performed either through traditional methods, such as reinforced plaster, or through more advanced solutions using FRP (Fiber Reinforced Polymer) or FRCM (Fiber Reinforced Cementitious Matrix) system. The use of electrowelded steel mesh anchored to the masonry surface and embedded in a concrete layer (traditional reinforced plaster) is increasingly being replaced by more innovative techniques. These include the substitution of steel mesh with alternative reinforcement meshes (CRM – Composite Reinforced Mortar), such as glass or carbon fiber meshes, or the use of composite materials made of high strength fibers with organic (FRP) or inorganic (FRCM) matrices. Fig. 5a presents a detailed overview of the