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CONTENTS
Volume 48, Number 6, December25 2013
 

Abstract
An integrated simulation tool for multilayer stepped pyramidal structures is presented. The tool, based on a semi-analytical mathematical strategy, is able to calculate the temperature distributions and thermal stresses at the interfaces between the layers of such structures. The core of the thermal solver is the analytical simulator for power electronic devices, DJOSER, which has been supplemented with a mechanical solver based on the finite-element method. To this end, a new ele-ment is proposed whose geometry is defined by its mean surface and thickness, just as in a plate. The resulting mechanical model is fully three-dimensional, in the sense that the deformability in the direction orthogonal to the mean surface is taken into account. The dedicated finite element code developed for solving the equilibrium problem of structures made up of two or more superimposed plates subjected to thermal loads is applied to some two-layer samples made of silicon and copper. Comparisons performed with the results of standard finite element analyses using a large number of brick elements reveal the soundness of the strategy employed and the accuracy of the tool developed.

Key Words
multilayer structures, thermal stresses, finite element method, power electronic devices

Address
Paolo Emilio Bagnoli : Department of Information Engineering, University of Pisa, Via G. Caruso 16 56122, Pisa, Italy
Maria Girardi, Cristina Padovani and Giuseppe Pasquinelli : Institute of Information Science and Technologies

Abstract
The current paper proposes a boundary element formulation, applicable to 2-D and 3-D elastostatics problems using a unified approach for transformations of the domain integrals into boundary integrals. The method is applicable to linear problems encompassing both finite and infinite multi-region domains allowing non-vanishing body forces. Numerical results agree quite well with the analytical solutions; while the present method offers easy formulation with less numerical efforts in comparison to FEM or some BEM which need interior points to treat arbitrary body forces. It is demonstrated that the method has the potential to have profound impact on engineering design, notably in dam-foundation interaction.

Key Words
boundary element method; elastostatics; multi-zone structures; body force

Address
Mohammad Ghiasian and Mohammad Taghi Ahmadi : Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran 14115-397, Iran

Abstract
Ultra High Performance Cementitious Composites with compressive strength 200MPa (UHPCC-200) is proposed for the structural design of super high hybrid wind turbine tower to gain durability, ductility and high strength design objectives. The minimal wall thickness is analyzed using basic bending and compression theory and is modified by a toque influence coefficient. Two cases of wall thickness combination of middle and bottom segment including varied ratio and constant ratio are considered within typical wall thickness dimension. Using nonlinear finite element analysis, the effects of wall thickness combinations with varied and constant ratio and prestress on the structural stress and lateral displacement are calculated and analyzed. The design limitation of the segmental wall thickness combinations is recommended.

Key Words
hybrid tower; simulation; geometrical parameter; post tension; ultra high performance cementitious composites

Address
Xiangguo Wu, Issa B. Mpalla : Key Lab of Structures Dynamic Behavior and Control (Harbin Institute of Technology), Ministry of Education, Heilongjiang, Harbin, 150090, China; School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China
Jing Yang : School of Architecture Engineering, Harbin Engineering University, Harbin 150001, China

Abstract
Base isolation is widely used in seismic resisting buildings due to its low construction cost, high reliability, mature theory and convenient usage. However, it is difficult to design the isolation layer in highrise buildings using the available bearings because high-rise buildings are characterized with long period, low horizontal stiffness, and complex re-distribution of the internal forces under earthquake loads etc. In this paper, a simple and innovative isolation bearing, named Teflon-based lead rubber isolation bearing, is developed to address the mentioned problems. The Teflon-based lead rubber isolation bearing consists of friction material and lead rubber isolation bearing. Hence, it integrates advantages of friction bearings and lead rubber isolation bearings so that improves the stability of base isolation system. An experimental study was conducted to validate the effectiveness of this new bearing. The effects of vertical loading, displacement amplitude and loading frequency on the force-displacement relationship and energy dissipation capacity of the Teflon-based lead rubber isolation bearing were studied. An analytical model was also proposed to predict the force-displacement relationship of the new bearing. Comparison of analytical and experimental results showed that the analytical model can accurately predict the force-displacement relationship and elastic shear deflection of the Teflon-based lead rubber isolation bearings.

Key Words
teflon-based lead rubber isolation bearing; force-displacement relationship; bearing testing; restoring force; analytical model

Address
Lu Wang, Jin Ou, Weiqing Liu and Shuguang Wang : College of Civil Engineering, Nanjing University of Technology, Nanjing, China

Abstract
In this paper, it has been attempted to present a powerful analytical approach called Homotopy Perturbation Method (HPM). Free vibration of an electrostatically actuated microbeam is considered to study analytically. The effect of important parameters on the response of the system is considered. Some comparisons are presented to verify the results with other researcher\'s results and numerical solutions. It has been indicated that HPM could be easily extend to any nonlinear equation. We try to provide an easy method to achieve high accurate solution which valid for whole domain.

Key Words
electrostatically actuated microbeam; nonlinear vibration; homotopy perturbation method

Address
M. Bayat : Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran
I. Pakar : Young Researchers and Elites Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran
A. Emadi : Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran

Abstract
Nine rectangular-section of High Strength Concrete(HSC) beams were designed and casted based on the American Concrete Institute (ACI) code provisons with varying of tensile reinforcement ratio as (pmin, 0.2pb, 0.3pb, 0.4pb, 0.5pb, 0.75pb, 0.85pb, pb, 1.2pb). Steel and concrete strains and deflections were measured at different points of the beam\'s length for every incremental load up to failure. The ductility ratios were calculated and the moment-curvature and load-deflection curves were drawn. The results showed that the ductility ratio reduced to less than 2 when the tensile reinforcement ratio increased to 0.5pb. Comparison of the theoretical ductility coefficient from CSA94, NZS95 and ACI with the experimental ones shows that the three mentioned codes exhibit conservative values for low reinforced HSC beams. For over-reinforced HSC beams, only the CSA94 provision is more valid. ACI bending provision is 10 percent conservative for assessing of ultimate bending moment in low-reinforced HSC section while its results are valid for overreinforced HSC sections. The ACI code provision is non-conservative for the modulus of rupture and needs to be reviewed.

Key Words
High Strength Concrete (HSC); ductility; tensile reinforcement ratio; compressive strain

Address
Mohammad Mohammadhassani, Meldi Suhatril, Mahdi Shariati and Farhad Ghanbari : Department of Civil Engineering, University of Malaya, Malaysia

Abstract
This paper consists of two parts, which broadly examines solution techniques abilities for the structures with geometrical nonlinear behavior. In part I of the article, formulations of several well-known approaches will be presented. These solution strategies include different groups, such as: residual load minimization, normal plane, updated normal plane, cylindrical arc length, work control, residual displacement minimization, generalized displacement control, modified normal flow, and three-parameter ellipsoidal, hyperbolic, and polynomial schemes. For better understanding and easier application of the solution techniques, a consistent mathematical notation is employed in all formulations for correction and predictor steps. Moreover, other features of these approaches and their algorithms will be investigated. Common methods of determining the amount and sign of load factor increment in the predictor step and choosing the correct root in predictor and corrector step will be reviewed. The way that these features are determined is very important for tracing of the structural equilibrium path. In the second part of article, robustness and efficiency of the solution schemes will be comprehensively evaluated by performing numerical analyses.

Key Words
nonlinear solution techniques; equilibrium path; condition equation

Address
M. Rezaiee-Pajand, M. Ghalishooyan and M. Salehi-Ahmadabad : Department of Civil Engineering, School of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

Abstract
In part I of the article, formulation and characteristics of the several well-known structural geometrical nonlinear solution techniques were studied. In the present paper, the efficiencies and capabilities of residual load minimization, normal plane, updated normal plane, cylindrical arc length, work control, residual displacement minimization, generalized displacement control and modified normal flow will be evaluated. To achieve this goal, a comprehensive comparison of these solution methods will be performed. Due to limit page of the article, only the findings of 17 numerical problems, including 2-D and 3-D trusses, 2-D and 3-D frames, and shells, will be presented. Performance of the solution strategies will be considered by doing more than 12500 nonlinear analyses, and conclusions will be drawn based on the outcomes. Most of the mentioned structures have complex nonlinear behavior, including load limit and snap-back points. In this investigation, criteria like number of diverged and complete analyses, the ability of passing load limit and snap-back points, the total number of steps and analysis iterations, the analysis running time and divergence points will be examined. Numerical properties of each problem, like, maximum allowed iteration, divergence tolerance, maximum and minimum size of the load factor, load increment changes and the target point will be selected in such a way that comparison result to be highly reliable. Following this, capabilities and deficiencies of each solution technique will be surveyed in comparison with the other ones, and superior solution schemes will be introduced.

Key Words
nonlinear solution techniques; benchmark problems; path-tracing ability; geometrical nonlinear behavior; comparison study; load limit points; snap-back points

Address
M. Rezaiee-Pajand, M. Ghalishooyan and M. Salehi-Ahmadabad : Department of Civil Engineering, School of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran


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