Yuri Ribakov

Experimental Investigation of Full Scale Two-Layer Reinforced Concrete Beams

Two-layer beams (TLB), consisting of steel fibered high strength concrete (SFHSC) in the compression zone and normal strength concrete (NSC) in the tensile one, are studied experimentally. The current study is based on results of previous theoretical investigations and tests that showed high efficiency of such beams, carrying rather big bending moments. According to the previously developed concept, the fiber content for TLB should be calculated, corresponding to the required ductility level of an RC element. Following the results of a previous experimental study that were carried out to select the optimal fiber ratio, the current research is focused on testing full scale TLB. The study is aimed at experimental verification of the data related to interaction of concrete layers in TLB and proving the efficiency of two-layer bending element from beginning of loading and up to the ultimate load stages, including collapse. Development of Poisson deformations was studied to compare them with those obtained in the previous study for cylindrical specimens. The SFHSC layer was cast after the NSC hardening to study the influence of separate casting technology that is more convenient for TLB production in real construction. Three identical specimens with constant fiber content, corresponding to the proper ductility level, were prepared and tested by four-point loading. The results demonstrate the role of fibers in a high strength concrete layer and form a basis for development of TLB design provisions.

Estimation of RC slab-column joints effective strength using neural networks

The nominal strength of slab-column joints made of highstrength concrete (HSC) columns and normal strength concrete (NSC) slabs is of great importance in structural design and construction of concrete buildings. This topic has been intensively studied during the last decades. Different types of column-slab joints have been investigated experimentally providing a basis for developing design provisions. However, available data does not cover all classes of concretes, reinforcements, and possible loading cases for the proper calculation of joint stresses necessary for design purposes. New numerical methods based on modern software seem to be effective and may allow reliable prediction of column-slab joint strength. The current research is focused on analysis of available experimental data on different slab-to-column joints with the aim of predicting the nominal strength of slabcolumn joint. Neural networks technique is proposed herein using MATLAB routines developed to analyze available experimental data. The obtained results allow prediction of the effective strength of column-slab joints with accuracy and good correlation coefficients when compared to regression based models. The proposed method enables the user to predict the effective design of column-slab joints without the need for conservative safety coefficients generally promoted and used by most construction codes.

Retrofitting Heritage Buildings by Strengthening or Using Seismic Isolation

Many heritage buildings in the Mediterranean area include stone domes as a structural and architectural element. Present stage of these buildings often requires strengthening or retrofitting in order to increase their seismic resistance. Strengthening is possible by casting above existing dome a thin reinforced concrete shell with a support ring. It yields reduction of stresses and strains in the dome. This paper deals with examples of actual restoration and strengthening of three structures in Georgia, two of them damaged by an earthquake in 1991, (a temple in Nikortzminda and a synagogue in Oni, built in 11th and 19r century, respectively) and a mosque in Akhaltzikhe, built in 18th century. Retrofitting of these structures was aimed at preservation of initial geometry and appearance by creating composite (stone—reinforced concrete, or stone—shotcrete) structures, which were partially or fully hidden. Further improving of seismic response may be achieved by using hybrid seismic isolation decreasing the s...

Optimal control synthesis for the constrained bilinear biquadratic regulator problem

Optimal process control with control constraints is a challenging task related to many real-life problems. In this paper, a single input continuous time constrained linear quadratic regulator problem, is defined and fully solved. The constraints include both bilinear inequality constraints and customary control force bounds. As a first step, the problem is reformulated as an equivalent constrained bilinear biquadratic optimal control problem. Next, Krotov’s method is used to solve it. To this end, a sequence of improving functions suitable to the problem’s new formulation is constructed and the corresponding successive algorithm is derived. The required computational steps are arranged as an algorithm and proof outlines for the convergence and optimality of the solution are given. The efficiency of the suggested method is demonstrated by numerical example.

Optimal Control of a Constrained Bilinear Dynamic System

In this paper, an optimal feedback, for a free vibrating semi-active controlled plant, is derived. The problem is represented as a constrained optimal control problem of a single input, free vibrating bilinear system, and a quadratic performance index. It is solved by using Krotov’s method and to this end, a novel sequence of Krotov functions that suits the addressed problem, is derived. The solution is arranged as an algorithm, which requires solving the states equation and a differential Lyapunov equation in each iteration. An outline of the proof for the algorithm convergence is provided. Emphasis is given on semi-active control design for stable free vibrating plants with a single control input. It is shown that a control force, derived by the proposed technique, obeys the physical constraint related with semi-active actuator force without the need of any arbitrary signal clipping. The control efficiency is demonstrated with a numerical example.

Efficient Newton method for optimal viscous dampers design

Passive control is a known method for vibrations damping in many mechanical systems, including civil structures. The simplicity and reliability of passive damping devices makes them a worthy candidate in many practical problems. However, despite of its practical simplicity, the optimal design of passive controller is quite hard computational problem. In this work, an enhanced optimal viscous passive dampers design method is proposed. The optimization is done with relation to performance index consists of ||H||2 system norm and quadratic gains norm. An algorithm is suggested for the look after a candidate optimum. It is based on Newtons optimization method with a new and effective calculation method for the Hessian matrix. Numerical tests of the suggested method shows a very fast convergence rate with relation to known ones.

Using canonical models for LQR and H-infinity control of structures

According to the seismic modal approach, optimal modal control design should derive a control signal affecting only the dominant modal coordinates. However, existence of an optimal state feedback, affecting only a selected set of modes, is questionable. This study proves the existence of optimal modal feedback for the case of infinite horizon LQR and H∞ control methods. It defines a modal state space model by using a new state-space similarity transformation and solves the corresponding algebraic Riccatis equation. The solution is based on the selection of LQR weighting and H∞ output matrices. A modal design method is formulated and the spillover effect for the controlled structure is introduced.