University of Structural Engineering & Architecture "Lyuben Karavelov"
Recent publications
The building is a masterpiece of the industrial architecture in the thirties of XX century. The original design is the work of the architect Rudolf Fischer. The construction of the building is realized in the period 1924–1925. The combination of different construction materials: masonry, reinforced concrete and timber, gives a unique structure. This contribution introduces a structural investigation, using MFE software, into the stability and efficiency of the design of the roof truss of the storage heritage building in Kustendil. A two dimensional model was used in numerical analysis to discuss and highlight structural sense of Austrian architect R. Fischer in the period of 1925–1925. The modeling of timber construction and the final conclusions are basic for the decision concerning the possible interventions in the building and the possibility to extend the service life of the roof structure.
The investigation discusses the history of construction of the Orthodox Christian architectural complex St Stefan in Constantinople, comprising a steel church and a convent. The paper pays special attention to the third stage of the creating of the complex: the steel structure of prefabricate elements and the ornamented cast-iron cladding manufactured and fitted by the Vienna-based company Rudolph Philipp Waagner in 1896–1898 on a design of Turkish architect Hovsep Aznavur. The church is a monument of cultural heritage of Bulgaria and Turkey. The study is a result of in situ work and of an analysis of unpublished records kept in the Central State Archives in Sofia.
The advanced tendencies in building materials development are related to the design of cement composites with a reduced amount of Portland cement, contributing to reduced CO2 emissions, sustainable development of used non-renewal raw materials, and decreased energy consumption. This work deals with water cured for 28 and 120 days cement composites: Sample A—reference (white Portland cement + sand + water); Sample B—white Portland cement + marble powder + water; and Sample C white Portland cement + marble powder + polycarboxylate-based water reducer + water. By powder X-ray diffraction and FTIR spectroscopy, the redistribution of CO32−, SO42−, SiO44−, AlO45−, and OH− (as O-H bond in structural OH− anions and O-H bond belonging to crystal bonded water molecules) from raw minerals to newly formed minerals have been studied, and the scheme of samples hydration has been defined. By thermal analysis, the ranges of the sample’s decomposition mechanisms were distinct: dehydration, dehydroxylation, decarbonation, and desulphuration. Using mass spectroscopic analysis of evolving gases during thermal analysis, the reaction mechanism of samples thermal decomposition has been determined. These results have both practical (architecture and construction) and fundamental (study of archaeological artifacts as ancient mortars) applications.
The major issue studied in this paper is a natural mineral aggregate of quartz, calcite, and fluorapatite (the raw material originated from Bulgaria) before and after high energy milling and thermal treatment, in the order to investigate the properties of natural CaO-SiO2-P2O5 ceramic system. The activation effects are monitored by chemical analysis, X-ray powder diffraction, Fourier transformed infrared spectroscopy, and Thermal analysis (TG/DTG). The activation effect study shows: (i) change of chemical bond strength; (ii) deformation of structural polyhedrons with the formation of new isomorphic phases; (iii) the prolonged time of HEM activation leads to lower raw mineral stability and to the formation of new phases; iv) increased SiO2 reactivity resulting in solid-phase crystallization. The obtained results can be used in the study of ceramic and cement materials (ancient and modern), soil conditioners, etc.
Failures occur in the structures of reinforced concrete buildings and facilities during their continuous exploitation, without being overloaded or exposed to extreme impacts, the most common being cracks. Their detection and change in time are related to the assessment of the state of the structures, their safety, and reliability during their construction and especially for their safety exploitation. This paper describes the results of the experimental studies conducted by authors aiming to verify the possibility of using the non-destructive ultrasonic pulse velocity method (NDUPVM) for detection and evaluation of cracks. Results of an experimental study of 12 reinforced concrete beams are presented. In previous experiments, some of them were subjected to bending until the maximum crack width of 0.3 mm was reached and others until yielding of the longitudinal reinforcement. The results obtained from the measurements of the depths of the normal cracks with different widths with NDUPVM were compared with the visually measured ones. In the present research cracks with the same width and with a similar depth were chosen. The influence of extreme external conditions to the accuracy of the measured crack depths by the NDUPVM was investigated. Non-destructive ultrasonic research was done by a portable device Proceq TICO.
Comparative analysis is made of different finite element models of a buried arch bridge which is constructed from precast concrete elements. Two static schemes of the bridge structure are considered - a static scheme of rigid connection between all structure elements and a static scheme of hinged connection between the lateral retaining walls and the vault plate. The modeling is carried out by the Plaxis 2D software. Two nonlinear constitutive models for soil materials are used - linear elastic-perfectly plastic Mohr-Coulomb's model and advanced elastic-plastic Hardening-Soil model. The Hardening-Soil model is a state-of-the-art scientific achievement which approximates the mechanical behavior of the soil materials with high accuracy using 15 material parameters. Two types of finite elements are used for modeling of the bridge structure: beam and plate elements. The soil-structure contacts are modeled by interface elements of thickness closed to zero. Traffic loads on a road situated above the bridge are assumed according to the model LM1 of Eurocode 1. Mechanical behavior of the bridge is studied by analyses of states in construction and service periods. Analysis of the ground bearing capacity is performed using the shear strength reduction technique.
The oldest man-made false-arch stone bridges are presented and briefly described. It is shown that this construction technique was based on the experiences of the first builders, formed at the junction of ancient Egyptian, Mycenaean as well as Assyrian and Babylonian cultures. Arches in such bridges have not yet been constructed in a classical manner, i.e. one that was later prevalent by the Romans, but these were only the primitive arch-like structures, with a false needle vault, that were shaped mainly by corbelling. This type of the structure, if it was used in bridges, turned out to be much more stable than the well-known at that time and commonly used in gateway passages oval “true-arch” built from sun-dried mud bricks.
This paper describes the construction and behavior of a large steel double twin truss launching gantry with total length 168m, designed by Dr. Eng. Borislav Bankov (1912÷1992), used for erection of prestressed concrete bridge girders with span up to 60m (weight up to 220ton) along the Hemus highway, Republic of Bulgaria. It consists of two twin frame steel girders and rails on the top and on the bottom flanges for moving the special girder carrying devices. Experimental investigations are performed by the Road Agency and vertical deflections of the mentioned twin truss under different loads and support conditions are measured. 3D FEM of the existing steel twin truss launching gantry considering and neglecting the the increased axial stiffness of the lower chord at truss joint zones are developed by the authors to investigate its behavior under different load and support conditions. Results from these FEM are analyzed, compared with the test results and discussed.
The paper presents analysis of the stress-strain behaviour due to creep in statically determinate composite steel-concrete beam according to AAEM method of Bažant in comparison with numerical method. The analysis is based on the results obtained by numerical solution with Volterra integral equations derived for determining the redistribution of stresses in beam section between concrete plate and steel beam with respect to time “t” and creep of concrete according EC2 provisions. On the basis of the theory of the viscoelastic body of Arutyunian–Trost-Bažant it is analysed the migration of stresses from concrete plate to steel beam using two independent Volterra integral equations of the second kind and two independent algebraic equations, during the period of 70 years. The closeness of the results obtained by the two methods is shown with an example from the bridge practice.
Mapping the landslides is one of the main activities in the overall spectrum of geological hazards and geological risk. Landslides form the most serious part of the geological hazard in Bulgaria after the earthquakes. They are widespread in the all country’s territory. All known landslide types of internationally accepted classification of Varnes (Landslides, analysis and control, 1978) are represented there. The main attention is paid to the level of activity and the territorial scope of landslides according to the criteria involved in the National Programme for Landslide Prevention of Bulgaria (2014) aimed at prioritizing landslides depending on the level of activity and the risk they carry. Thus 5 degrees of landslide hazard are accepted. Hazards and vulnerabilities are considered to be weighed, i.e. they can be assessed in advance in terms of what will be the weight of the probability of landslide hazard of the research area and what will be the results (vulnerability) due to its occurrence. The vulnerability is represented by indicators of GDP per capita and density of population. The intensity of the hazard can be very low, low, medium, high and very high, which correspond respectively to classes from I to V. The third stage is the determining factor of landslide hazard. This stage is determined by the intensity of the landslide hazard for the study area. It is obtained according to the district class of intensity for landslide hazard. The results express the weighted average hazard. They are obtained through a combination of the weight of the landslide hazard and its intensity that is accepted for the zone concerned. The weights for landslide hazard and its ratios derived from the potential intensity are multiplied to obtain individually weighted hazard score.
Open image in new windowThe investigated landslide occurred after mass excavation at the toe of the slope due to construction of an aqua park and heavy rainfall in November 2014. The induced slope instability affected the only road linking the village of Gorni Voden with the town of Asenovgrad. Immediately after the landslide occurrence an engineering-geological site investigation had been carried out. It was followed by proposal for remediation measures. The landslide has an elongated shape with a maximum width of up to 60 m, length 140 m and area of approximately 8000 m2. The volume is above 20,000 m3. The damaged length of the road was 35 m. The investigation established the geological composition of the slope, key elements of the landslide, its mechanism, physical and engineering properties of the soil and the groundwater conditions. The reinforcement design consists of anchored pile retaining wall and drainage system. Due to the delay in construction, during the following warm and rainy winter the landslide increased its area more than two times. Subsequently, after the anchored pile retaining walls were constructed, the builder of the new aqua park made a new deep excavation downslope from the reconstructed road section. This excavation destabilized the landslide once again leading to cracks formation on the slope and increasing the depth of the sliding surface. The landslide body has been saturated through the open cracks by the following rainfall, soil consistency has been changed and the shear strength has decreased. The landslide process has progressed upwards and the new sliding surface reached the anchored pile retaining wall and compromised it locally. This paper discusses the remediation design proposed after the landslide occurrence, its implementation and the factors influencing the consequent structural damages on retaining wall due to loss of overall stability.
In this paper, we describe the oriented Riemannian four-manifolds M for which the Atiyah–Hitchin–Singer or Eells–Salamon almost complex structure on the twistor space Z\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {Z}}$$\end{document} of M determines a harmonic map from Z\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {Z}}$$\end{document} into its twistor space.
The notions of a twistor space of a contact manifold and a contact connection on such a manifold have been introduced by L. Vezzoni as extensions of the corresponding notions in the case of a symplectic manifold. Given a contact connection on a contact manifold one can define an almost $CR$-structure on its twistor space and Vezzoni has found the integrability condition for this structure. In the present paper it is observed that the $CR$-structure is induced by an almost contact metric structure. The main goal of the paper is to obtain necessary and sufficient conditions for normality of this structure in terms of the curvature of the given contact connection. Illustrating examples are discussed at the end of the paper.
ul> The paper presents analysis of the stress changes due to creep in statically determinate composite steel-concrete beam according to (AAEM) method of Bažant in comparison with numerical solution using Volterra integral equations based on EC2 provisions for creep of concrete. The mathematical model involves the equation of equilibrium, compatibility and constitutive relationship, i.e. an elastic law for the steel part and an integral-type creep law of Boltzmann – Volterra and algebraic-type creep law of Trost - Bažant for the concrete part considering the above mentioned models. It is analyzed the migration of stresses from concrete plate to steel beam using two independent Volterra integral equations of the second kind and two independent algebraic equations. The closeness of the results obtained by the two methods is shown with an example from the bridge practice. </ul
Following the Barnett’s approach to gcd(a(x),b(x)) based on the use of companion matrix we develop an extended algorithm that gives effectively \(d(x),\ u(x),\ v(x),\ a_1(x)\) and \(b_1(x)\), where \(a_1(x)=a(x)/d(x), b_1(x)=b(x)/d(x)\) and \(d(x)=u(x)a(x)+v(x)b(x).\) The algorithm is suitable for parallel realization on GPU, FPGA, and smart cards.
In this paper authors present analysis of limits when formulas for limiting the span/depth ratio, given in EN1992-1-1 section 7.4.2, for different reinforced concrete members subjected to bending, are applicable. Recommendations about how to take the compression reinforcement in calculations into account, when deflection of bending members should be assessed, are also proposed.
In the present article we study a special class of surfaces in the four-dimensional Euclidean space, which are one-parameter systems of meridians of the standard rotational hypersurface. They are called meridian surfaces. We show that a meridian surface has a harmonic Gauss map if and only if it is part of a plane. Further, we give necessary and sufficient conditions for a meridian surface to have pointwise 1-type Gauss map and find all meridian surfaces with pointwise 1-type Gauss map.
Metal–polymer nanocomposites possess a great potential for minimization of heat transfer losses and heat gains through the building envelope, finding application in the field of solar control coating materials. In the present paper we report the optical properties of a metal–polymer from Ag and polymethyl methacrylate (PMMA) nanocomposite fabricated by layer-by layer deposition technique. The X-ray diffraction pattern confirms the presence of silver particles in the thin films. The average size of silver particles was calculated by Debye-Scherrer formula. An absorbance band in the spectral range 350–550 nm due to the surface plasmon resonance of the silver nanoparticles was observed in transmittance spectra. The position of the absorbance band has been analyzed by the Bruggeman’s model for the effective media. On the base of the obtained results the polymer–inorganic hybrid coatings were suggested for application as sun protective coatings.
In the four-dimensional pseudo-Euclidean space with neutral metric there are three types of rotational surfaces with two-dimensional axis – rotational surfaces of elliptic, hyperbolic or parabolic type. A surface whose mean curvature vector field is lightlike is said to be quasi-minimal. In this paper we classify all rotational quasi-minimal surfaces of elliptic, hyperbolic and parabolic type, respectively. MSC: 53A35, 53B25 Keywords: Pseudo-Euclidean 4-space with neutral metric • Quasi-minimal surfaces • Lightlike mean curvature vector • Rotational surfaces of elliptic, hyperbolic or parabolic type © Versita Sp. z o.o.
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19 members
Blagovesta Ivanova
  • Urban Planning, Theory and History of Architecture
Doncho Partov
  • Mechsnics and mathematics
Veselin Slavchev
  • Department of Building Structures
Stanislav Tsvetkov
  • Department of Building Structures
175 Suhodolska Street, 1373, Sofia, Bulgaria