Issue 2

JTAM, Sofia, vol. 44 Issue 2 (2014)

Identification of Pedestrian Bridge Dynamic Response trough Field Measurements and Numerical Modelling: Case Studies

George D. Manolis1, Asimina Athanatopoulou-Kyriakou1, Kosmas D. Dragos1, Argyris Arabatzis1, Alexandros Lavdas1, Christos Z. Karakostas2
1Department of Civil Engineering, Aristotle University, 54124 Thessaloniki, Greece
2Earthquake Planning and Protection Organization, 55535 Thessaloniki, Greece

In this work, we develop a technique for performing system identification in typical pedestrian bridges, using routine equipment at a minimal configuration, and for cases where actual structural data are either sparse or absent. To this end, two pedestrian bridges were examined, modelled and finally instrumented so as to record their dynamic response under operational conditions. More specifically, the bridges were numerically modelled using the finite element method (FEM) according to what was deduced to be their current operating status, while rational assumptions were made with respect to uncertain structural properties. Next, results from field testing using a portable accelerometer unit were processed to produce response spectra that were used as input to a structural identification software program, which in turn yielded the excited natural frequencies and mode shapes of the bridges. The low level of discrepancy is given between analytical and experimental results, the latter are used for a final calibration of the numerical models.

JTAM, Sofia, vol. 44 Issue 2 pp. 03-24 (2014), [Full Article]

Length-Scale Effects and Material Models at Numerical Simulations of Nanoindentation of a Metallic Alloy

N. Nikolov1, T. Avdjieva2, I. Altaparmakov3
1Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, Sofia 1113, Bulgaria
2Faculty of Physics, University of Sofia “St. Kliment Ohridski”, 5, J. Bourchier Blvd, 1164 Sofia, Bulgaria
3Faculty of Applied Mathematics and Informatics, Technical University-Sofia, 8, St. Kl. Ohridski Blvd, 1756 Sofia, Bulgaria

Some specially designed metallic alloys crystallize during process of rapid quenching which aims their amorphization. Nevertheless, change in their mechanical properties could be seen compared to these obtained during conventional technological regimes of cooling. That attracts the attention in this elaboration. Full 3-D numerical simulations of nanoindentation process of two material models are performed. The models reflect equivalent elastic and different plastic material properties. The plastic behaviour of the first one is subjected to yield criterion of Dracker-Prager and this of the second one to yield criterion of Mises. The reported numerical results depending on the nanoindentation scale length of 1000 nanometers, suggest different adequacy of the two yield criteria to the data obtained experimentally with a Zr-Al-Cu-Ni-Mo alloy. It could be speculated that the different effects developed depending on the indenter travel of 1000 nanometers and taken into account in the two yield criteria stand behind this fact and determinate three structural levels of plastic deformation.

JTAM, Sofia, vol. 44 Issue 2 pp. 25-40 (2014), [Full Article]

Pounding Effects on the Earthquake Response of Adjacent Reinforced Concrete Structures Strengthened by Cable Elements

Angelos Liolios1, Asterios Liolios1, George Hatzigeorgiou2, Stefan Radev3
1Division of Structural Engineering, Dept. of Civil Engineering, Democritus-University of Thrace, Xanthi, Greece
2Lab. of Ecological Mechanics and Technology, Dept. of Environmental Engineering, Democritus-University of Thrace, Xanthi, Greece
3Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev str. 8, Sofia 1113, Bulgaria

A numerical approach for estimating the effects of pounding (seismic interaction) on the response of adjacent Civil Engineering structures is presented. Emphasis is given to reinforced concrete (RC) frames of existing buildings which are seismically strengthened by cable-elements. A double discretization, in space by the Finite Element Method and in time by a direct incremental approach is used. The unilateral behaviours of both, the cable-elements and the interfaces contact-constraints, are taken strictly into account and result to inequality constitutive conditions. So, in each time-step, a non-convex linear complementarity problem is solved. It is found that pounding and cable strengthening have significant effects on the earthquake response and, hence, on the seismic upgrading of existing adjacent RC structures.

JTAM, Sofia, vol. 44 Issue 2 pp. 41-56 (2014), [Full Article]

Recursive Differentiation Method for Boundary Value Problems: Application to Analysis of a Beam-Column on an Elastic Foundation

Mohamed Taha
Dept. of Eng. Math. and Physics, Faculty of Engineering, Cairo University, Giza, Egypt

In the present work, the recursive differentiation method (RDM) is introduced and implemented to obtain analytical solutions for differential equations governing different types of boundary value problems (BVP). Then, the method is applied to investigate the static behaviour of a beam-column resting on a two parameter foundation subjected to different types of lateral loading. The analytical solutions obtained using RDM and Adomian decomposition method (ADM) are found similar but the RDM requires less mathematical effort. It is indicated that the RDM is reliable, straightforward and efficient for investigation of BVP in finite domains. Several examples are solved to describe the method and the obtained results reveal that the method is convenient for solving linear, nonlinear and higher order ordinary differential equations. However, it is indicated that, in the case of beam-columns resting on foundations, the beam-column may be buckled in a higher mode rather than a lower one, then the critical load in that case is that accompanies the higher mode. This result is very important to avoid static instability as it is widely common that the buckling load of the first buckling mode is always the smaller one, which is true only in the case of the beam-columns without foundations.

JTAM, Sofia, vol. 44 Issue 2 pp. 57-70 (2014), [Full Article]

Nonlinear Analysis of a Functionally Graded Beam Resting on the Elastic Nonlinear Foundation

M. Arefi
Department of Solid Mechanic, Faculty of Mechanical Engineering, University of Kashan, Kashan 87317-51167, Iran

This paper evaluates the nonlinear responses of a functionally graded (FG) beam resting on a nonlinear foundation. After derivation of fundamental nonlinear differential equation using the Euler-Bernouli beam theory, a semi analytical method has been used to study the response of the problem. The responses can be evaluated for both linear and nonlinear isotropic and FG beams individually. Adomians Decomposition and successive approximation methods have been used for solution of nonlinear differential equation. As numerical investigation, the beams with simply supported ends and linear and nonlinear foundations have been analyzed using this method.

JTAM, Sofia, vol. 44 Issue 2 pp. 71-82 (2014), [Full Article]

Effect of Matrix Viscosity on Rheological and Microwave Properties of Polymer Nanocomposites with Multiwall Carbon Nanotubes

R. Kotsilkova1, E. Ivanov1, D. Bychanok2, A. Paddubskaya2, P. Kuzhir2
1OLEM, Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, Sofia 1113, Bulgaria
2Institute of Nuclear Problems, Belarus State University, 11, Bobrujskaya St., 220030 Minsk, Belarus

Nanocomposites of multiwalled carbon nanotubes (MWCNTs) in epoxy resin and polypropylene (PP) are studied. The effect of matrix viscosity on the degree of dispersion of nanotubes is determined by rheological methods. Rheology and microwave properties are correlated to estimate the optimal limits of nanofiller content required for improving the performance of nanocomposites. Rheological percolation threshold is determined for both types nanocomposites, 'p=0.27% for the epoxy/MWCNT and; 'p=1.5% for the PP/MWCNT, as found critical for achieving a network structure of interacting nanotubes in the matrix polymer. Good electromagnetic shielding efficiency was obtained for nanocomposites at nanotube contents above the rheological percolation. Low viscosity matrix facilitates contacts between MWCNTs, resulting in appearance of electromagnetic shielding at very low percolation threshold.

JTAM, Sofia, vol. 44 Issue 2 pp. 83-96 (2014), [Full Article]