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FRP lacks ductility so its application as reinforcement of new structures in seis-mic areas is not common. Seismic actions can produce a large capacity and ductil-ity demand. Although large capacity is readily available in FRP reinforced con-crete elements, ductility capacity is often lacking. A large amount of building stock consists of low to mid-rise buildings with a significant number of walls. In such structures, most of the capacity is usually provided by walls. This means that the behaviour of the whole structure depends on the behaviour of the walls, more specifically on their load bearing and lateral drift capacity. For squat walls, there is usually an excess of load bearing capacity but for more slender walls, some amount of ductility is required. This ductility is provided by bending of an element, so drift capacity depends on curvature capacity. In this paper, curvature ductility of FRPRC walls with different reinforcement configurations is theoreti-cally and numerically analysed. There is a large number of different FRP rebar products available on the market, each with different strength and ultimate strain. Using only one type of rebar in a wall can provide some amount of ductility, depending on the diameter and spac-ing of bars. However, combining different products can lead to a significant change in the overall behaviour of an element. This paper provides a quick, pre-liminary analysis of FRPRC slender walls to provide a basis for developing more comprehensive research and guide future experimental efforts.

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.

Serviceability limit states are very important in the design of reinforced concrete elements but they are complex to calculate. Simplified serviceability calculations are provided in EN 1992-1-1 (2013) for steel reinforced elements. The crack widths are assumed to be acceptable if the bar diameters or bar spacings are not too large, while deflections are acceptable if the slenderness is not too large. In recent decades, FRP bars have become an adequate replacement for steel bars, especially in aggressive environments. The calculation procedures for FRP-reinforced concrete elements (FRPRC) were developed from calculation methods for steel reinforced elements. The first part of this paper demonstrates the procedures and parametric investigation for calculating the maximum bar diameter and bar spacing for the purpose of controlling the crack width, focusing on calculations for the maximum bar diameter for which cracks widths are acceptable. The second part of the paper demonstrates the procedures and parametric calculations for the slenderness limits for concrete elements reinforced with FRP bars in order to satisfy the usual deflection limits. Due to the different modulus of elasticity values of FRP and steel, the tables used for steel cannot be used for concrete beams reinforced with FRP bars. Therefore, new tables and diagrams are proposed in the paper. The new tables and diagrams for the maximum allowable bar diameters for the different modulus of elasticity values of FRP can be useful for the rapid control of the crack width in FRPRC elements. They are conservative compared to the exact calculations because some assumptions taken in the calculations are different to those taken in the exact calculation procedure for the crack width. The results of parametric calculations for the slenderness limits for FRPRC elements are provided in the form of a diagram for different concrete classes. Satisfying the slenderness from these curves will result in a smaller deflection than that allowed for each parameter related to that class of concrete.

Horizontal loads such as earthquake and wind are considered dominant loads for the design of tall buildings. One of the most efficient structural systems in this regard is the tube structural system. Even though such systems have a high resistance when it comes to horizontal loads, the shear lag effect that is characterized by an incomplete and uneven activation of vertical elements may cause a series of problems such as the deformation of internal panels and secondary structural elements, which cumulatively grow with the height of the building. In this paper, the shear lag effect in a typical tube structure will be observed and analyzed on a series of different numerical models. A parametric analysis will be conducted with a great number of variations in the structural elements and building layout, for the purpose of giving recommendations for an optimal design of a tube structural system.

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 earthquake in Zagreb on March 22, 2020 damaged many buildings, most of which were masonry and constructed before the use of earthquake-resistant construction regulations. That quake indicated the need for strengthening masonry buildings for seismic action. Seismic forces are mostly carried by the in-plane shear walls. Therefore, the main problem is proper calculation of the required strengthening of the in-plane shear walls and solving the strengthening details, but not only locally, at the level of individual walls, but also at the level of the whole building, to properly cover all mechanisms of masonry failure and transferring seismic forces to the foundation soil. The paper presents the most common methods of calculation of in-plane shear wall strengthening with a review of details that are not sufficiently well clarified or are neglected, i.e., poorly used in practice. In addition to the calculation of strengthening of masonry with the reinforced concrete layer (shotcrete) and dowels, modern methods of strengthening are considered, using FRP fabrics or plates and FRCM systems. As these calculation methods are not implemented in the existing codes, various recommendations are used based on similar assumptions, but with different input parameters and safety factors that affect the calculation result and that should be properly interpreted and used in the calculation.

A vast majority of buildings built in Croatia are masonry buildings. A fair amount of these buildings was built before the development of seismic codes. Also, most of the structures that are considered to have great historical and cultural value are in fact built in masonry. If all the above is taken into account with the Zagreb earthquake being just the most recent reminder of the seismic fragility of these types of structures, it is safe to say that the assessment and rehabilitation of masonry structures should be one of the greatest priorities in the civil engineering community. When talking about the assessment process, the visual inspection of a structure is one of the first and most important phases. For the purposes of the visual assessment of masonry structures, a specific analytical method was developed in Italy called the Masonry Quality Index (MQI) method. In this paper an insight into the said method will be given. Also, the use of this method on a typical case study of a masonry building in Zagreb will be shown. Concluding remarks about the quality and usability of the method will be drawn and discussed.

During the design of reinforced concrete elements, satisfying serviceability limit state conditions is very important. To simplify the calculations of crack width, for the elements subjected to bending, the EN 1992-1-1 (2013.) gives a restriction of bar diameter or bar spacing which satisfy allowable crack width (tables 7.2(N) and 7.3(N)). During the last decades, FRP reinforcement became a good replacement for steel reinforcement, especially in an aggressive environment. Calculation methods for reinforced concrete elements with FRP bars (FRPRC) are developed from calculation methods for reinforced concrete elements with steel bars. Now, more and more, these rules are implemented in some national codes, and they will be implemented in Eurocodes soon. Procedures for the calculation of maximum bar diameter and bar spacing to control crack width are shown in the paper. These two values depend on each other and the focus of the paper is set on calculations of the bar diameter. Due to different modulus of elasticity between FRP and steel, the tables used for steel cannot be used for concrete beams reinforced with FRP bars. Therefore, the parametric calculations of maximum FRP bar diameter are described in this paper and new tables and diagrams are shown. The calculations were made according to rules from EN 1992-1-1 (2013.), paragraph 7.3.4 and equations (7.8) and (7.9) but instead of the material properties for steel reinforcement, those for FRP reinforcement are used. For additional explanations and considerations, EC2 Commentary (2008.) is used. Also, due to many types of FRP reinforcement, one table for different materials will not be sufficient. Therefore, for easier usage, diagrams for maximum value of bar diameter are developed and shown in the paper.

2020 was a tough year for Croatia, which experienced two significant earthquakes during an ongoing global pandemic. Damage from those earthquakes was extensive and mostly concentrated in the old masonry buildings. Many of the mentioned buildings exceeded the usual service life span, with some existing for over a century. Earthquakes exposed Croatian building stock’s vulnerability that should be mitigated as efficiently as possible in the coming years. Novel approaches in assessment procedures should be applied together with state-of-the-art repair and reconstruction of the damaged buildings.

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&rsquo, 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.

U novije vrijeme uz tradicionalne metode pojačanja ziđa na posmik, koriste se i noviji materijali i sustavi kao što su polimeri armirani vlaknima (FRP) i tekstilom armirani mortovi (TRM). Potres koji se dogodio u Zagrebu i okolici 22. ožujka 2020. pokazao je osjetljivost nearmiranih zidanih zgrada na horizontalna djelovanja, potrebu za sanacijom oštećenja na nosivom i nenosivom ziđu i pojačanjem ziđa na posmik. Postojeći propisi ne pokrivaju proračune pojačanja s takvim sustavima. U radu su prikazani suvremeni postupci pojačanja ziđa FRP-om ili TRM-om, znanstvena istraživanja iz tog područja, prednosti i nedostaci te proračun takvih pojačanja., 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.