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The European building stock consists of a large number of unreinforced masonry structures. Masonry structures proved themselves to be fragile if subjected to horizontal loads, particularly out-of-plane. The complexity of existing structures during an earthquake increases if they are arranged in aggregates. This research deals with the definition of vulnerability and fragility curves for Croatian existing masonry buildings in aggregates. The curves were derived following the European macroseismic scale (EMS–98). The adopted case study includes three aggregates located in Lower Town in Zagreb, Croatia. The capital of Croatia was hit by a series of earthquakes in March 2020. Until these recent earthquakes, the damage database from previous earthquakes was non-existent. The structural vulnerability was mainly assessed by analytical methods, which were not a proper solution for identifying the seismic risk on a larger scale, ultimately at the state level. The database from this earthquake series enabled the application, validation and calibration of empirical methods. The macroseismic index-based method was applied and based on the observed damage, new formulations to assess the vulnerability are proposed.

The procedure of a detailed condition assessment of the building under heritage protection in Zagreb is presented. A detailed historical background of the case study building is shown, and observed damage and conducted in situ tests are discussed. The nonlinear static seismic analysis performed in the 3Muri software is extensively elaborated. Four different levels of reconstruction according to new Croatian law are briefly presented. Additionally, several strengthening scenarios are proposed with various strengthening techniques. The renovation of the case study building is presented through extensive photo documentation. The problems in renovations of culturally protected buildings in a specific case study are raised.

After the Petrinja earthquake in December 2020, 442 individual immovable cultural assets were damaged, including 124 cultural heritage buildings that suffered severe structural damage. The largest number of damaged cultural heritage assets concern sacral buildings, mainly churches and chapels. In this paper, a brief overview of the damage to several sacral buildings after the earthquakes in Croatia is presented. The failure mechanism and typical damages are presented graphically. In addition, special emphasis is placed on the complete retrofitting of a damaged church. The condition assessment, numerical modelling and full seismic retrofit of the selected church is shown.

This paper presents the preliminary results of the project called Establishment of the national training centre for nearly Zero Energy Buildings (nZEB). The project is funded by EEA Grants (Energy and Climate Change Programme). Seismic condition assessment and upgrading of existing RC structures are presented with a case study building in Zagreb. New technologies were applied and followed by numerical modeling and verifications. A strategy for energy retrofitting will be presented. As the integrated approach should be respected in the renovation of existing buildings, this case study can represent an example of good practice in seismic and energy retrofitting.

After a long period of inactivity, strong seismic events occurred in Croatia. Due to the extensive damage caused by the recent earthquakes, a significant phase of reconstruction and strengthening follows. Determining the actual seismic behavior of existing masonry structures is of great importance for future management and the economical and purposeful strengthening of the load-bearing structure. Modern software solutions and design methods are an essential part of the assessment, but they are only as useful as long as the input parameters are reliable. This is where in-situ test methods come into play, providing useful information about the mechanical properties of the structure. In the paper, the flat-jack method and sonic pulse velocity method are described in more detail and the results of certain tests are presented and compared.

The paper describes a novel Adriseismic method for expeditious assessment of seismic risk associated with unreinforced masonry buildings. The methodology was developed for the Adriseismic project of the Interreg ADRION programme, with the aim to develop and share tools for increasing cooperation and reducing seismic risk for six participating countries within the region surrounding the Adriatic and the Ionian Seas. The method is applicable to unreinforced masonry buildings characterised by three main seismic failure mechanisms, namely masonry disintegration, out-of-plane failure, and in-plane damage/failure. Depending on the input parameters for a specific structure, the assessment yields a qualitative output that consists of the masonry quality index, the index of structural response, the level of seismic risk, and the most probable collapse mechanism. Both input and output of the method are applied in the spreadsheet form. The method has so far been applied in urban areas of participating countries in the project, including Mirandola, Italy; Kaštela, Croatia; Belgrade, Serbia. In parallel, the methodology has been validated by performing a detailed seismic assessment of more than 25 buildings, and the results have been compared with the results of the proposed expeditious method. The results show a good correlation between the two methods, for example, the structural response index obtained from the expeditious method and the capacity/demand ratio obtained from the conventional assessment method.

The paper focuses on the analysis of actual bending moments in RC walls of high-rise structures. A comprehensive analysis was carried out on three concrete buildings in order to evaluate the accuracy of the procedure for obtaining bending moments proposed by Eurocode 8 (EC8). The buildings are designed according to EC8 for medium ductility class (DCM) and high ductility (DCH). Nonlinear 3D models of the selected structures were subjected to 60 strong earthquake ground motions recorded on rock and stiff soil. One hundred and eighty nonlinear time-history analyses (NTHA) are carried out to compare the actual bending moments against design EC8 bending moments. Based on the findings of the conducted study, the possible improvement of EC8 bending moments design envelope for RC walls of high-rise buildings is indicated. A simple proposal for the correction of the existing EC8 bending moment envelope is introduced by its modification over the height of the wall to account bending moment increase due to the higher mode effects and by substituting EC8 design moment with wall resistance moment at the base. This improved envelope better matches the actual bending moments in RC walls of high-rise buildings. Further calibration is clearly required.

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.

Using a significant number of transverse hoops in the joint’s core is one recognized way for achieving the requirements of strength, stiffness, and ductility under dynamic loading in a column joint. The shear capacity of a joint is influenced by the concrete’s compressive strength, the anchoring of longitudinal beam reinforcement, the number of stirrups in the joint, and the junction’s aspect ratio. Seismic motion on the beam may produce shear capacity and bond breaking in the joint, causing the joint to fracture. Furthermore, due to inadequate joint design and details, the entire structure is jeopardized. In this study, the specimens were divided into two groups for corner and interior beam–column joints based on the joint reinforcement detailing. The controlled specimen has joint detailing as per IS 456:2000, and the strengthened specimen has additional diagonal cross bars (modified reinforcement technique) at the joints detailed as per IS 456:200. The displacement time history curve, load-displacement response curves, load-displacement hysteretic curve, and load cycle vs. shear stress were used to compare the results of the controlled and strengthened specimens. The findings show that adding diagonal cross bars (modified reinforcing techniques) to beam–column joints exposed to cyclic loads enhances their performance. The inclusion of a diagonal cross bar increased the stiffness of the joint by giving an additional mechanism for shear transfer and ductility, as well as greater strength with minimum cracks.

The global objective of sustainable development has been greatly directed toward the preservation of existing structures. Therefore, condition assessment and reconstruction of existing timber structures have been gaining importance in recent times. This is particularly evident on timber roofs whose elements are exposed to degradation, either because of rheological effects or due to the direct influence of moisture and biological factors. In case of accidental events, such as an earthquake, the question of the structure’s condition is essential for the condition of the entire building. In order to prove the load-bearing capacity and serviceability of existing structures, as well as to check the need for reconstruction, it is necessary to define crucial parameters that are influencing the condition of materials, elements, and systems. Although there are many non destructive testing methods, the frequency and scope of their use, as well as the decision-making approach, have not been defined. In the paper, non-destructive and semi-destructive methods frequently used for timber structures are explained. A systematic review of criteria to be used in the assessment of load-bearing timber structures in a seismic active area was the main objective of this paper as well as the illustration of non-destructive and semi-destructive test methods through a case study involving roof construction of a hundred-year-old building in Zagreb, Croatia. Pre- and post-earthquake inspection was made. The overall condition of the roof structure after two significant earthquakes can be assessed as satisfactory given that the observed system is a large-span and massive roof structure. The presented results and identification of typical damages after the earthquake are presented in order to facilitate policy makers and for the future implementation of development strategies in the renovation of the city.

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.

In the wake of recent strong earthquakes in Croatia, there is a need for a detailed and more comprehensive post-earthquake damage assessment. Given that masonry structures are highly vulnerable to horizontal actions caused by earthquakes and a majority of the Croatian building stock is made of masonry, this field is particularly important for Croatia. In this paper, the "regular" post-earthquake assessment procedures of damaged buildings are given and new technologies such as photogrammetry, laser scanning and UAV are shown in order to preserve the built heritage. As Zagreb was severely damaged by the earthquakes in 2020, there is an urgent need to make, at least, digital twins of representative buildings. In this manuscript, different case studies with different demands for preservation are shown.

A severe earthquake hit Zagreb on the March 22, 2020 (magnitude ML = 5.5, with an epicenter 7 km north of the city center). During the COVID-19 lockdown, the event occurred and caused significant damage to the built environment and enormous disruption in everyday life. Brief facts about the Zagreb earthquake, the typology of buildings in the city, and the data collection after the quake itself are described. The paper focuses on post-earthquake assessment, damage classification, and failure patterns of residential buildings in Zagreb's old historical town, where masonry structures prevail. The earthquake critically damaged buildings that are important architectural achievements and ruptured Zagreb's historically recognizable city center. Furthermore, the data collected in the rapid post-earthquake assessment were analyzed and discussed. Graphical representations of damages are detailed and accompanied by photographs. This earthquake exposed Croatian building stock's vulnerability that should be mitigated as efficiently as possible in the coming years.

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.

Post-earthquake assessment can be defined as a procedure to define the given structure’s safety and usability after an earthquake. The seismic behaviour of buildings depends on numerous parameters such as the construction, building material, the geometry of the structure, stiffness properties and non-linear aspects, conceptual design, etc. The main importance of seismic design is to protect property and life in buildings and infrastructure in case of earthquake events. However, an appropriate seismic design approach must necessarily develop on knowledge and feedback from existing structures. Past events showed that seismic loads usually cause significant damage in masonry buildings due to their large mass and stiffness compared to other constructional typologies and materials.The last decade has seen a growth in the technological development of various tools that can greatly help predict structural safety and seismic behavior of existing structures. The applicability of the thermal camera for building diagnostics and detection of energy-related building defects has been investigated by numerous authors. Nowadays, technology and sensor minimization enable the development of the multi-sensor costume-made UAV (Unmanned Aerial Vehicle) for multi-sensor aerial mapping and post-earthquake damage assessment. When doing on-site assessment from a street-level, an engineer cannot see what happened to a building on higher floors or roof of a structure. The main problems (especially for moderate-intensity earthquakes) are chimneys, damaged roof structures, damaged gable walls, etc. With a UAV device, all of the mentioned information can be easily assessed.

Designing a timber connection which is buildable with minimum errors has been considered the most difficult aspect of a timber design. Although many studies have been conducted in the past on various aspects of the usability of Eurocode 5, the authors felt that with the impending revisions to the Eurocodes, it was pertinent to ask the users for their opinions in a more focused manner, with questions aligned with the revision objectives. As part of the activities of the working group 3, “Connection” of the COST Action FP1402, a Europe-wide survey was thus conducted among the practitioners, where a number of questions related to Eurocode 5 in general and the connections chapter in particular was asked. The majority of respondents felt there were improvements to be made both to the technical content, as well as the usability of the standard in terms of clarity and ease of navigation. The results of the survey are summarized in this paper with a proposal for reorganization of the current technical content, which the authors feel answer the majority of the concerns raised by the users.

Ocjena stanja i kontinuirano praćenje ponašanja postojećih drvenih konstrukcija u posljednje vrijeme sve više dobivaju na značaju. Globalni cilj održivog razvoja u velikoj mjeri usmjeren je na očuvanje postojećih konstrukcija i zaštitu graditeljske baštine. U radu su prikazane metode ocjenjivanja stanja postojećih drvenih konstrukcija. Glavni naglasak stavljen je na prikaz nedestruktivnih i poludestruktivnih metoda za drvene konstrukcije koje su često u uporabi. U radu su navedene metode na konkretnom primjeru ocjene stanja Tehničkog muzeja Nikole Tesle u Zagrebu., Condition assessment and continuous monitoring of the existing timber structures have been gaining in importance in recent times. The global objective of sustainable development has been greatly directed toward preservation of the existing structures and protection of architectural heritage. Methods used for assessing condition of the existing timber structures are presented in the paper. The main emphasis is placed on the presentation of non-destructive and semi-destructive methods for frequently used timber structures. The methods are presented through the example of condition assessment of the Nikola Tesla Technical Museum in Zagreb.

U radu je dan kratak osvrt na trenutno projektiranje konstrukcija otpornih na potres. U ovom radu prikazane su i sažete najnovije spoznaje u pogledu proračuna potresno otpornih građevina, s najosnovnijim primjerima i kritičkim osvrtom na trenutno stanje propisa i budućnosti projektiranja na potres. Fokus rada bio je prikazati novije metode kao što su metode zasnovane na trošenju energije. Predstavljeni su trenutačni zahtjevi i problemi pri proračunu konstrukcija energetskim metodama te predstavljeni dijagrami toka vezanim za spomenutu metodu., This paper gives a brief overview of the current design of earthquake resistant structures. The paper presents and summarizes the latest insights regarding the design of earthquake resistant buildings, with the most basic examples and a critical review of the current state of regulations and the future of seismicdesign. The focus of the paper was to present newer methods such as energy-based design. Current requirements and problems in the design of structures by energy-based design are presented. Shown flow-diagrams serve to understand the procedures more clearly

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.

Assessment and structural health monitoring of existing timber structures has experienced huge interest in last decades. The main reasons are clear messages that sustainable development is a long term goal of the global policy which results in modifications or substitutions or extensions of existing buildings and engineering works (Kyoto protocol 1997, all further World Climate Summits) beside protection and conservation of the heritage buildings built in timber and assessment and collection of crucial data for the design and long-term behavior of new structures in timber. At the moment, European norms (Eurocodes) cover engineering principles that could be used to form the basis of assessment of structures or structural elements but basically they are concepted for the design of new structures. In this paper assessment methods for timber structures are summarized and the most common ones are briefly explained. The main focus of the paper is to present non-destructive and semi-destructive test methods for timber structures and use of unmanned vehicle for gathering the data which were inputs for numerical model and structural analysis of the structure. The whole protocol is shown on actual case study of the H2020 Project INCEPTION, Technical Museum Nikola Tesla in Zagreb, Croatia.

Use of timber as a construction material has entered a period of renaissance since the development of high-performance engineered wood products, enabling larger and taller buildings to be built. In addition, due to substantial contribution of the building sector to global energy use, greenhouse gas emissions and waste production, sustainable solutions are needed, for which timber has shown a great potential as a sustainable, resilient and renewable building alternative, not only for single family homes but also for mid-rise and high-rise buildings. Both recent technological developments in timber engineering and exponentially increased use of engineered wood products and wood composites reflect in deficiency of current timber codes and standards. This paper presents an overview of some of the current challenges and emerging trends in the field of seismic design of timber buildings. Currently existing building codes and the development of new generation of European building codes are presented. Ongoing studies on a variety topics within seismic timber engineering are presented, including tall timber and hybrid buildings, composites with timber and seismic retrofitting with timber. Crucial challenges, key research needs and opportunities are addressed and critically discussed.

The COST Action FP1402 aims to overcome the existing gap in the timber construction world between broadly available scientific results and the specific information needed by designers, industry, authorities and code committees, providing transfer for practical application in timber design and innovation. Its results will provide some background knowledge for the development of the so-called Second Generation of the Eurocodes, aimed at 2020. Within the Working Group 3 of the Action, it was decided to develop a questionnaire to get the opinion of the practitioners about the content and structure of the Eurocode 5. The idea of this questionnaire research therefore was to get feedback about the problems faced by practitioners in using the code to make our engagements with the code writers more meaningful.

The conservation of historical timber structures requires both the knowledge of structural behavior of complete building and behavior of any single element in the structure. Due to poor communication between the institutions dealing with conservation and restoration of buildings and institutions that urgently require structure renovation, renovation of historic buildings is often performed inappropriately with respect to engineering and architectural approach. Buildings are frequently subjected to inappropriate interventions, replacements of structural elements, or even worse, they are demolished. First case study in this paper is Cathedral from Gospić, which was subjected to inappropriate reconstruction in the 1990s. The cathedral was demolished during the war and the tower and roof structure were subjected to reconstruction. The second case study is a small traditionally constructed timber chapel in the village Lijevi Štefanki, which represents a valuable monument of rural cultural heritage. Visual inspection of the chapel was done and recommendations for reconstruction were proposed.