Masonry structures are notoriously vulnerable to horizontal actions caused by earthquakes. Given the high seismicity of the European region, and that the European building stock comprises a lot of masonry buildings, knowledge about their structural response to seismic excitation is particularly important, but at the same time difficult to determine, due to the heterogenous nature of materials and/or constructional techniques in use. An additional issue is represented by the current methods for mechanical properties assessment, that do not provide a reliable framework for accurate structural estimations of existing buildings characterized by different typological properties. Every structure, in other words, should be separately inspected in regard to its mechanical behaviour, based on dedicated approaches able to capture potential critical issues. In this review paper, an insight on the Croatian ARES project is presented (Assessment and Rehabilitation of Existing Structures), including a state-of-the-art of the actual building stock and giving evidence of major difficulties concerning the assessment of existing structures. The most commonly used techniques and tools are compared, with a focus on their basic features and field of application. A brief overview of prevailing structural behaviours and Finite Element numerical modelling issues are also mentioned. As shown, the general tendency is to ensure “sustainable” and energy-efficient building systems. The latter, however, seem in disagreement with basic principles of structural maintenance and renovation. The aim of the ongoing ARES project, in this context, is to improve the current knowledge regarding the assessment and strengthening of structures, with a focus on a more reliable design and maintenance process for existing masonry buildings.

In addition to traditional methods of strengthening shear masonry walls, some newer materials and systems, such as fibre reinforced polymers (FRP) and textile reinforced mortars (TRM), have recently been introduced. The earthquake that occurred in Zagreb and its surroundings on 22 March 2020 has revealed the sensitivity of unreinforced masonry buildings to horizontal actions, while pointing to the need to repair damage to load-bearing and non-load-bearing walls and to strengthen walls against shear failure. Existing regulations do not cover design of structures with such systems. The paper presents modern procedures for strengthening masonry with FRP or TRM, scientific research in this area, advantages and disadvantages, and calculation of such reinforcements.

Concrete beams reinforced with FRP rebars have greater durability than standard steel reinforced elements. The main disadvantage of using FRP rebars is the low ductility of elements which may be unacceptable in certain situations. There are several different ways of increasing the ductility of concrete elements, which are analyzed in this paper. They are compared based on efficiency, influence on durability and ease of construction. Less analyzed and tested methods are given more attention to try and expand the current knowledge and possibilities. For methods that lack experimental data, theoretical analysis is undertaken to assess the possible influence of that method on the increase in ductility. Ductility was obtained by calculating bending moment–curvature diagrams of cross sections for different reinforcement layouts. One method that lacks experimental data is confining the compressive area of beams with tensile FRP reinforcement. Theoretical analysis showed that confining the compressive area of concrete can significantly increase the ductility and bending capacity of beams. Since experimental data of beams reinforced with FRP rebars in tension and confined compressive area is sparse, some suggestions on the possible test setups are given to validate this theoretical analysis. Concrete beams reinforced with FRP can be detailed in such a way that they have sufficient ductility, but additional experimental research is needed.

The amount of energy dissipated during an earthquake depends on the type of failure of the concrete element. Shear failure should be avoided because less energy is spent than that due to bending failure. Compression controlled failure is usually avoided by increasing the thickness of a wall. Considering that the current code largely decreases this strength, this becomes hard to achieve in practice. Because of that, the analysis described in this paper is made to determine the reason for a large strength reduction at high curvatures. Mechanisms contributing to compression controlled shear strength are analysed. Using Rankine’s strength theorem, section equilibrium, arch mechanism and bending moment-curvature diagrams, the influence of different parameters are observed and charted. The findings are compared to the existing procedures and a new, simple and safe analytical equation is derived. Compression controlled shear strength is mainly influenced by axial force, followed by the amount of longitudinal reinforcement and the achieved confinement. Results show that the value of strength reduces significantly with the increase of ductility and that some reduction exists even for lower levels of curvature. Current code provisions may lead to unsafe design, so designers should be careful when dealing with potentially critical walls.

According to the systematic research of Statistical Yearbooks, Croatia's national building stock consists of approx. residential 800000 buildings and 125000 non-residential buildings. More than 75 % of the building stock is older than 30 years, an age which often requires renovation or modification of buildings. More than 40 % of the building stock is older than 50 years, meaning that the structure's service life is expired. In Croatia's building sector, up to 40 % of the expenses are spent for the rehabilitation, modification, and demolition of existing structures.