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Aleksandrov Y.,Higher School of Civil Engineering
Bulgarian Journal of Agricultural Science | Year: 2017

In this paper are reviewed several solutions for panel walls and panel connections, representing a part of the enclosing structure of the chambers of fruit storehouses, which are often subject to hits coming from various sources, including during extreme situations. The solutions represent patented inventions, representing an integral part of the dissertation of the author [1]. BG 401 (Y1)-"Connection between panels"; This solution integrates an elastic protective screen, which can be dismounted in case of premature runout; the hits produced by root fruits and vegetables, stored in bulk are absorbed by the screen, which is suspended over pins, which transfer the energy of the hits to the bearing construction of the wall panels. BG 62742 (B1)-“Wall panel”; the hit absorption is ensured with the help of elastic screens and coverings, suspended and connected to the sides of the panel. BG 63218 (B1)-“Multilayer panel with impact-protection and connection between panels.”{ The covering of the panels is made of elastic materials and it integrates star-shaped elements, which are inserted into the surface layers of the panel. Membranes, stretched over tensegrity-structures are able to offer protection against harmful impact, e.g. acid rain, volcanic ash, overheating, etc., whereas the refrigerators for fruits and vegetables are situated under these membranes. The membranes are made of transparent materials, e.g. polyketone, reinforced by carbon fiber, whereas the hothouse effect occurring beneath them can be used to maintain positive temperatures inside the chambers of the refrigerators for fruits and vegetables. Additional protection against premature runout of the external covering of the enclosing structure is ensured by transparent protective membranes. Positive temperature inside of the chambers of the refrigerators is a result of the use of solar energy. The protection of the refrigerators for fruits and vegetables with transparent membranes is especially important in case of extreme situations of various origin. © 2017, National Centre for Agrarian Sciences. All rights reserved.


Partov D.,Higher School of Civil Engineering | Kantchev V.,HSCE L.Karavelov
Central European Journal of Engineering | Year: 2011

The paper presents analysis of the stress and deflections changes due to creep in statically determinate composite steel-concrete beam. 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 for the concrete part. On the basis of the theory of the viscoelastic body of Arutyunian-Trost-Bažant for determining the redistribution of stresses in beam section between concrete plate and steel beam with respect to time "t", two independent Volterra integral equations of the second kind have been derived. Numerical method based on linear approximation of the singular kernal function in the integral equation is presented. Example with the model proposed is investigated. The creep functions is suggested by the model CEB MC90-99 and the "ACI 209R-92 model. The elastic modulus of concrete Ec(t) is assumed to be constant in time 't'. The obtained results from the both models are compared. © Versita sp. z o.o.


Bozic B.,University of Belgrade | Fan H.,KTH Royal Institute of Technology | Milosavljevic Z.,Higher School of Civil Engineering
Survey Review | Year: 2013

This paper deals with the estimation of the quality of the baseline for the calibration of distance measurement devices which was established by Serbian Military Geographic Institute for military use. The basic characteristics of the baseline are explained, and a plan for the checking of the baseline quality is proposed. The measurements realised so far can be grouped into two phases. The measurements have been processed, and the estimates of the distances of this length standard have been obtained. The standard deviations of the least squares estimates of the lengths were better than 0?3 mm in each epoch. This precision offers the possibility to check all measurement devices with a minimum uncertainty of the calibrations, of ¡(1 mmz1 ppm). The stability of the pillars is also analysed. The conventional deformation analysis method was applied to three datasets and the results obtained by evaluating them are shown. © 2013 Survey Review Ltd.


Partov D.,Higher School of Civil Engineering | Partov D.,Brno University of Technology | Kantchev V.,Higher School of Civil Engineering | Kantchev V.,Sofia University
ACI Structural Journal | Year: 2014

The paper presents an analysis of the stress and deflections changes due to creep in statically determinate composite steelconcrete beams. The mathematical model involves the equation of equilibrium, compatibility, and constitutive relationship, that is, an elastic law for the steel part and an integral-type creep law of Boltzmann-Volterra for the concrete part. Based on the theory of the viscoelastic body of Arutyunian-Trost-Ba'ant for determining the redistribution of stresses in beam sections between a concrete plate and steel beam with respect to time t, two independent Volterra integral equations of the second kind have been derived. A numerical method based on linear approximation of the singular kernal function in the integral equation is presented. An example with the proposed model is investigated. The creep functions are suggested by the Gardner and Lockman model. The elastic modulus of concrete Ec(t) is assumed to be constant in time t. Copyright © 2014, American Concrete Institute.


Davidov J.,Bulgarian Academy of Science | Davidov J.,Higher School of Civil Engineering
International Journal of Geometric Methods in Modern Physics | Year: 2014

Every almost Hermitian structure (g, J) on a four-manifold M determines a hypersurface ΣJ in the (positive) twistor space of (M, g) consisting of the complex structures anti-commuting with J. In this paper, we find the conditions under which ΣJ is minimal with respect to a natural Riemannian metric on the twistor space in the cases when J is integrable or symplectic. Several examples illustrating the obtained results are also discussed. © 2014 World Scientific Publishing Company.


Davidov J.,Bulgarian Academy of Science | Davidov J.,Higher School of Civil Engineering | Grantcharov G.,Florida International University | Mushkarov O.,Bulgarian Academy of Science | Yotov M.,Florida International University
Communications in Mathematical Physics | Year: 2011

In this paper we consider pseudo-bihermitian structures - pairs of complex structures compatible with a pseudo-Riemannian metric. We establish relations of these structures with generalized (pseudo-) Kähler geometry and holomorphic Poisson structures similar to that in the positive definite case. We provide a list of compact complex surfaces which could admit pseudo-bihermitian structures and give examples of such structures on some of them. We also consider a naturally defined null plane distribution on a generalized pseudo-Kähler 4-manifold and show that under a mild restriction it determines an Engel structure. © 2011 Springer-Verlag.


Ganchev G.,Bulgarian Academy of Science | Milousheva V.,Bulgarian Academy of Science | Milousheva V.,Higher School of Civil Engineering
Journal of Mathematical Physics | Year: 2012

A marginally trapped surface in the four-dimensional Minkowski space is a spacelike surface whose mean curvature vector is lightlike at each point. We associate a geometrically determined moving frame field to such a surface and using the derivative formulas for this frame field we obtain seven invariant functions. Our main theorem states that these seven invariants determine the surface up to a motion in Minkowski space. We introduce meridian surfaces as one-parameter systems of meridians of a rotational hypersurface in the four-dimensional Minkowski space. We find all marginally trapped meridian surfaces. © 2012 American Institute of Physics.


Ganchev G.,Bulgarian Academy of Science | Milousheva V.,Bulgarian Academy of Science | Milousheva V.,Higher School of Civil Engineering
International Journal of Geometric Methods in Modern Physics | Year: 2013

A marginally trapped surface in the four-dimensional Minkowski space is a spacelike surface whose mean curvature vector is lightlike at each point. We introduce meridian surfaces of parabolic type as one-parameter systems of meridians of a rotational hypersurface with lightlike axis in Minkowski 4-space and find their basic invariants. We find all marginally trapped meridian surfaces of parabolic type and give a geometric construction of these surfaces. © World Scientific Publishing Company.


Partov D.,Higher School of Civil Engineering | Kantchev V.,Higher School of Civil Engineering
ACI Structural Journal | Year: 2014

The paper presents an analysis of the stress and deflections changes due to creep in statically determinate composite steel-concrete beams. The mathematical model involves the equation of equilibrium, compatibility, and constitutive relationship, that is, an elastic law for the steel part and an integral-type creep law of Boltzmann-Volterra for the concrete part. Based on the theory of the viscoelastic body of Arutyunian-Trost-Bažant for determining the redistribution of stresses in beam sections between a concrete plate and steel beam with respect to time t, two independent Volterra integral equations of the second kind have been derived. A numerical method based on linear approximation of the singular kernal function in the integral equation is presented. An example with the proposed model is investigated. The creep functions are suggested by the Gardner and Lockman model. The elastic modulus of concrete Ec(t) is assumed to be constant in time t.


Davidov J.,Bulgarian Academy of Science | Davidov J.,Higher School of Civil Engineering | Grantcharov G.,Florida International University | Mushkarov O.,Bulgarian Academy of Science | Yotov M.,Florida International University
Nuclear Physics B | Year: 2012

We study the problem of existence of geometric structures on compact complex surfaces that are related to split quaternions. These structures, called para-hypercomplex, para-hyperhermitian and para-hyperkähler, are analogs of the hypercomplex, hyperhermitian and hyperkähler structures in the definite case. We show that a compact 4-manifold carries a para-hyperkähler structure iff it has a metric of split signature together with two parallel, null, orthogonal, pointwise linearly independent vector fields. Every compact complex surface admitting a para-hyperhermitian structure has vanishing first Chern class and we show that, unlike the definite case, many of these surfaces carry infinite-dimensional families of such structures. We provide also compact examples of complex surfaces with para-hyperhermitian structures which are not locally conformally para-hyperkähler. Finally, we discuss the problem of non-existence of para-hyperhermitian structures on Inoue surfaces of type S 0 and provide a list of compact complex surfaces which could carry para-hypercomplex structures. © 2012 Elsevier B.V.

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