In recent years, energy savings have gained strategic importance due to requirements for less environmental pollution, the use of renewable energy sources, and ensuring energy independence (UN (United Nations) 2015). The latter was indicated by the adoption of measures to reduce the use of energy in the building sector, which constitutes a significant share of the total use of energy. Due to a lack of primary energy sources in Europe, most measures and details to increase energy efficiency in buildings originate from Northern and Central Europe with a cold continental climate, where energy savings for heating are great and the initial investment in better energy efficiency is repaid quickly. From the environmental and energy-efficiency aspect, solutions to reduce energy consumption also apply to other parts of Europe to attain the objectives of the European Union, e.g. the Directive on the energy performance of buildings (DEUS 2010/31/EU) and the European Green Deal (EU Commission 2019). Many existing solutions spread to other parts of the world. They include the transfer of details to provide energy efficiency to areas with increased seismic risk, where the established construction practice differs from non-earthquake-prone areas in many ways. Therefore, when using developed details and solutions to increase energy efficiency, it must be additionally verified in seismically active areas whether they can directly or indirectly affect the earthquake resistance of a building. Since there are no systematic and standard solutions for earthquake-prone areas, we studied the most common details used to reduce energy consumption in the monograph, which appear in practice in modern energy-efficient buildings.

The primary purpose of the monograph was to explore special structural details in energy-efficient buildings in the context of their earthquake resistance. The hypothesis that the principles of energy-efficient buildings (particularly requirements to prevent thermal bridges) can reduce the earthquake resistance of structures in comparison with conventional earthquake-resistant construction was put forward. In the first part of the monograph, we selected and presented structural details and solutions on the assumption that they are critical to earthquake resistance (Chap. 4). Various structural details on the building envelope for preventing thermal bridges and their interaction with the load-bearing structure were analysed, such as foundations on thermal insulation under the foundation slab, the building connection between an outer wall and the foundation slab, the building connection between the load-bearing balcony structure and an outer wall, the building connection between an outer wall and the unheated basement, the building connection between an outer wall and the roof structure, and others.

For the foundations on thermal insulation boards from extruded polystyrene (XPS), preliminary detailed numerical and experimental analyses of the seismic response were carried out (Azinović et al. 2014, 2015, 2016; Kilar et al. 2014). The results of the research show that foundations on thermal insulation under the foundation slab elongates the basic fundamental period of the structure. This is not always favourable. For instance, the stiff structures with short fundamental periods could be consequently shifted to the response spectrum plateau, where the seismic forces are larger. Many energy-efficient buildings would fit exactly in this category and therefore their shift to earthquake-prone areas could bring larger seismic forces and deformations. It was also established that foundations on thermal insulation may lead to the undesired rocking of a building on a flexible XPS layer and (uncontrolled) horizontal displacements at the connection between the foundation slab and thermal insulation or between individual layers of thermal insulation. The analyses results showed that rocking may result in exceeded elastic compressive deformation in XPS in three to four-storey slender and heavier buildings in areas with design ground acceleration higher than 0.25 g. In addition, such buildings (i.e. buildings higher than four storeys with a slender floor area and greater mass) may have instability issues as a result of rocking, which is why foundations on thermal insulation boards are not recommended in such cases. Greater carefulness and individual consideration are crucial to special cases of more complex buildings, irregular buildings in terms of floor plan or height and asymmetrical buildings, where the maximum number of storeys may be reduced. Such cases occur in buildings with large cantilevers or other height-related irregularities, and require the supervision of compressive stresses in thermal insulation under the foundation slab due to vertical and horizontal seismic forces. Based on the results of the analysed structures, we also find that lightweight buildings with less storeys are most exposed to the horizontal displacements on thermal insulation layer. The shift largely depends on the static friction coefficient in the selected foundation slab structural assembly and other boundary conditions (underground, non-underground building, etc.). In addition to the structural characteristics, the effect of insulation under the foundation slab on the use of energy and the prevention of thermal bridges was also evaluated.

Solutions including base insulation blocks for masonry structures in earthquake-prone areas were also analysed. Based on the literature review and the characteristics of base insulation blocks used as masonry in non-earthquake-prone areas, we find that most of them are inappropriate due to the insufficient normalised compressive strength. The limitation to the minimum compressive strength of masonry is provided in standards, such as Eurocode 8, and is intended to prevent undesired masonry failure mechanisms in earthquake-prone areas. We also analysed the effect of the thermal conductivity of base insulation blocks on the extent of the thermal bridge and surface temperatures of the building connection detail with the unheated basement. The effect of load-bearing thermal insulation elements on resolving the thermal bridge at the location where cantilever structures are fixed, was also analysed and demonstrated. All the performed analyses and reviewed literature support the importance of energy-efficient details, reflected in higher surface temperatures on the inner surface of the envelope and lower heat losses.

For the detail of cantilever structures with load-bearing thermal insulation elements, preliminary detailed numerical and experimental analyses of the seismic response were carried out (Azinović et al. 2015). It was established that the analysed load-bearing thermal insulation elements could be used for cantilevers of up to 300 cm in length in earthquake-prone areas on the basis of limit deflection (w < l/150) and without additional measures. The results of the analysis of these elements also confirmed the hypothesis that the cantilever uplift and tensile stress in the bottom edge of the cross-section could occur in an earthquake. Since most load-bearing thermal insulation elements on the market originate from non-earthquake-prone areas, such precast elements are designed exclusively for vertical loads with an installed asymmetrical steel reinforcement (only in the top edge of the cross-section). Based on the analyses, we believe that the elements must be improved for earthquake-prone areas (e.g. by installing reinforcement also in the bottom edge of the cross-section or other measures) to avoid potential severe damage to them resulting from the cantilever uplift in severe earthquakes.

In addition to the shown importance of using modern energy-efficient details, the monograph also focuses on the effect of these solutions on the earthquake resistance of buildings. The assumptions of their effect on earthquake resistance were presented for all the analysed details. Preliminary studies show that earthquake resistance is most significantly affected by the requirement of the continuous thermal envelope, which is a condition necessary to prevent thermal bridges and their adverse consequences. Therefore, we proposed the detail evaluation methodology based on guidelines for energy-efficient and earthquake-resistant construction. Two parts of evaluation are defined, whereby one part is about the quality of a detail from the technical and structural aspect and the other from the environmental and energy-efficiency aspect. Each detail must be assessed on the basis of seven criteria in the environmental and energy-efficiency evaluation, and seven criteria in the technical and structural evaluation. The final score may also be affected by weighting factors and six external factors, which affect evaluation less than the assessment of the primary criteria. All the parameters and the prepared evaluation criteria are described in detail, and guidelines for the design of structural energy-efficient buildings in earthquake-prone areas are provided. The evaluation methodology facilitates the separation of details that are more critical from the aspect of earthquake resistance and energy efficiency on the basis of simple engineering approaches (the review of designs and geometry, analyses of heat transfer, etc.). On the other hand, individual parameters can be assessed on the basis of detailed experimental analyses and (or) numerical simulation to precisely determine the quality of a detail in terms of its structural resistance (with a focus on seismic loads), energy efficiency, and environmental protection.

In the last section of the monograph, the proposed evaluation methodology was used in practical cases of four different types of details: the building connection between an outer wall and the foundation slab, the building connection between an outer wall and the unheated basement, the building connection between the load-bearing balcony structure and an outer wall, and the building connection between an outer wall and the roof. The assessment results of analysed details showed that the proposed methodology can be used to separate better solutions from poorer ones in the conceptual design. In this way, significant changes to the load-bearing capacity, stiffness, interruptions in the load-bearing structure, asymmetrical solutions, and other important characteristics that affect the quality of a detail to be used in earthquake-resistant structures are recognised. Analyses of heat transfer and environmental impact scores can also be used to choose from alternative detail solutions and decide, on the basis of the evaluation results, on the measures to prevent thermal bridges.

It can be concluded that the proposed methodology is generally not intended for the absolute evaluation of a selected detail, but rather for a comparison of details and to assist designers in the conceptual phase of designing the building envelope, serving as a tool for the selection of the best solutions. The limit values of the evaluation criteria could be determined by users in view of the conditions of the location and local regulations (rules and criteria applicable to the construction of energy-efficient and earthquake-resistance buildings in the analysed area).

The appendix to the monograph includes a catalogue of the selected structural details most frequently used in energy-efficient buildings, focusing on both problematic details as well as details that contribute to better solutions in practice. Various unsuitable solutions used to appear, in which energy efficiency was not crucial, resulting in energy-wasting buildings and, as experience shows, other adverse effects on thermal comfort and health of users of the building (e.g. consequences of thermal bridges—condensation and mould). In our opinion, one of the advanced options is also to use the methodology in computer programmes used for building information modelling (BIM). Some of these programmes already facilitate the analysis of thermal bridges, the environmental analysis, and the analysis of the energy use, which is required in the environmental and energy-efficiency evaluation. The latter could be upgraded with the technical and structural evaluation of the most common details, facilitating the selection of the most appropriate details for earthquake-prone areas during design.

The monograph is a result of over ten years of research by the authors in the field of the earthquake resistance of modern energy-efficient buildings. It aims to promote a wider interest in, and awareness of, the importance of structural details and their effect on the structural safety of energy-efficient buildings. At the same time, the monograph is the basis for further in-depth and interdisciplinary studies in this field. The cooperation of all professions included in the design of energy-efficient buildings and their end users is crucial.