<p>The residential building fund of Bulgaria is relatively young. About 30% is under age 30, almost half is under 40, and only 4% of fund is inherited from before 1919.</p><p>At present the Global development requires a sustainable growth. In order to respond to the key challenges outlined above, a European strategy has been defined for the next decade - develop or strengthen appropriate programs for repair, maintenance and renovation projects with ambitious sustainability targets.</p><p>The paper presents detailed analysis of residential buildings in Bulgaria, their type and structural conditions; principles of sustainable development; the general aspect of National Sustainable Development Strategy (NSDS) of Bulgaria, the National Programme for renovation of the residential buildings in Republic of Bulgaria and different kind of possibilities to bring the existing residential buildings in Bulgaria to the Global /European/ requirements for structural sustainability.</p>

<p>A comparative study of codes from seismically active regions of various countries is presented covering US, European, Italian, Greek, Romanian, Brazilian and Bulgarian Standards. The study focuses on the comparison of certain critical points: recurrence periods; seismic zonation and design ground motion parameter values; shape of the response spectrum; soil amplification; importance levels; seismic force-resisting systems; behavior factors; structural irregularities; story drift limits; procedures for seismic analysis. Following the comparison of the text of the codes, their application on the seismic design of an ordinary reinforced concrete structure is presented. The structure is subjected to the seismic input according to the above set of codes and obtained results are compared highlighting the differences between the codes. Overall this study aims to assist to the future improvement of the various seismic standards.</p>

For super high-rise buildings, the analysis and selection of suitable structural solutions are very important. The structure has not only to carry the gravity loads (self-weight, live load, etc.), but also to resist lateral loads (wind and earthquake loads). As the buildings become taller, the demand on different structural systems dramatically increases. The article considers the division of the structural systems of tall buildings into two main categories - interior structures for which the major part of the lateral load resisting system is located within the interior of the building, and exterior structures for which the major part of the lateral load resisting system is located at the building perimeter. The basic types of each of the main structural categories are described. In particular, the framed tube structures, which belong to the second main category of exterior structures, seem to be very efficient. That type of structure system allows tall buildings resist the lateral loads. However, those tube systems are affected by shear lag effect - a nonlinear distribution of stresses across the sides of the section, which is commonly found in box girders under lateral loads. Based on a numerical example, some general conclusions for the influence of the shear-lag effect on frequencies, periods, distribution and variation of the magnitude of the internal forces in the structure are presented.

<p> In the 70s of the last century an investment project for a plant for wastewater of the city of Gabrovo (Bulgaria) starts. The project is partially completed and is put into operation in 1984-85. Once built, the tanks for the treatment of the wastewater never operated, respectively not maintained. In 2012- 2013, analysis to already constructed equipments to assess their technical compliance with current regulations, grade of completion and operation in order fully construction and modernization of the equipments is done. That investigation aims to answer about actual state of the tank’s structure and to conclude on the suitability of the structure for the future use</p><p> The paper focuses on assessment, numerical modelling, analysis and recommendations for the rehabilitation of existing reinforced concrete tanks according the requirements of the new technology for the treatment of the wastewater of the city.</p>

<p>This article reviews the possibilities of the application of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) for strengthening existing steel and concrete structures in Bulgaria. Although the application of UHPFRC in structural engineering is increasing worldwide in the last decades, both for strengthening and for new designs, in Bulgaria it is still highly restricted. This is mainly due to the lack of experience and knowledge in design engineers, contractors and clients.</p><p>In order to investigate the possibilities of its local production in concrete plants and to gain experience in its application for strengthening existing structures a research project in the University of Architecture, Civil Engineering and Geodesy in Sofia has started recently. Under this project strengthening of steel orthotropic bridge decks as well as reinforced concrete beams is envisaged. The strengthened specimens will be tested in laboratory under static loading and the results are going to be compared with the test from non- strengthened ones. All specimens are full scaled as for the orthotropic deck only a segment between two cross girders is considered.</p><p>The results and experience gained from the research project will be used in the future to promote the application of UHPFRC in Bulgaria for durable reinforced concrete repairs and strengthening of existing steel bridges with orthotropic decks.</p>

<p>In the University of Architecture, Civil Engineering and Geodesy in Sofia a research project has started in 2018 aiming to investigate the possibilities of local production and application of high- strength concrete (or even UHPFRC) for strengthening existing structures in Bulgaria. Under this project laboratory tests of steel orthotropic bridge deck specimens as well as reinforced concrete beams, strengthened with high-strength concrete are performed. All elements are strengthened with an additional layer of high-strength concrete with thickness of 50mm on top.</p><p>The results obtained from the tests are summarized in this article. Comparative analysis showing the effect of this strengthening method is also presented. At the end summary and conclusions are drawn. Future steps for enhancing and promoting this strengthening technique in Bulgaria are outlined.</p>

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