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CONTENTS
Volume 65, Number 1, January10 2018
 

Abstract
This paper reviews the mechanical effects produced by reinforcement corrosion of prestressed concrete beams. Specifically, modifications in the bonding of the tendon to the concrete that reduce service life and load bearing capacity are studied. Experimental information gathered from previous works has been used for the theoretical analysis. Relationships between bond stress loss and reinforcement penetration in the concrete, and concrete external cracking were established. Also, it was analysed the influence that has the location of the area affected by corrosion on the loss magnitude of the initial prestress.

Key Words
corrosion; prestressed concrete; bond; cracking

Address
Nestor F. Ortega:
1) Engineering Institute, Engineering Department, Universidad Nacional del Sur, Av. Alem 1254, (8000) Bahia Blanca, Argentina
2) Comisión de Investigaciones Cientificas de la Provincia de Buenos Aires, Argentina
Juan M. Moro:
1) Engineering Institute, Engineering Department, Universidad Nacional del Sur, Av. Alem 1254, (8000) Bahía Blanca, Argentina
2) Consejo Nacional de Investigaciones Científicas y Tecnologicas, (C1033AAJ) CABA, Argentina
Romina S. Meneses: Engineering Institute, Engineering Department, Universidad Nacional del Sur, Av. Alem 1254, (8000) Bahía Blanca, Argentina

Abstract
This study concerns about calculating exact natural frequencies of frames using a single variable shear deformation theory (SVSDT) which considers the parabolic shear stress distribution across the cross section. Free vibration analyses are performed for multi-bay, multi-storey and multi-bay multi-storey type frame structures. Dynamic stiffness formulations are derived and used to obtain first five natural frequencies of frames. Different beam and column cross sections are considered to reveal their effects on free vibration analysis. The calculated natural frequencies are tabulated with the results obtained using Euler-Bernoulli Beam Theory (EBT) and Timoshenko Beam Theory (TBT). Moreover, the effects of inner and outer columns on natural frequencies are compared for multi-bay frames. Several mode shapes are plotted.

Key Words
dynamic stiffness; multi-bay frame; multi-storey frame; natural frequency; single variable shear deformation

Address
Baran Bozyigit and Yusuf Yesilce: Department of Civil Engineering, Dokuz Eylul University, 35160, Buca, Izmir, Turkey

Abstract
In this paper, a new quasi-3D sinusoidal shear deformation theory for functionally graded (FG) plates is proposed. The theory considers both shear deformation and thickness-stretching influences by a trigonometric distribution of all displacements within the thickness, and respects the stress-free boundary conditions on the upper and lower faces of the plate without employing any shear correction coefficient. The advantage of the proposed model is that it posses a smaller number of variables and governing equations than the existing quasi-3D models, but its results compare well with those of 3D and quasi-3D theories. This benefit is due to the use of undetermined integral unknowns in the displacement field of the present theory. By employing the Hamilton principle, equations of motion are obtained in the present formulation. Closed-form solutions for bending and free vibration problems are determined for simply supported plates. Numerical examples are proposed to check the accuracy of the developed theory.

Key Words
quasi 3D theory; bending; vibration; functionally graded plate

Address
Mamia Benchohra, Hafida Driz and Ahmed Bakor: Material and Hydrology Laboratory, Civil Engineering Department, Faculty of Technology, University of Sidi Bel Abbes, Algeria
Abdelouahed Tounsi:
1)Material and Hydrology Laboratory, Civil Engineering Department, Faculty of Technology, University of Sidi Bel Abbes, Algeria
2) Department of Civil and Environmental Engineering, King Fahd University of Petroleum & Minerals,
31261 Dhahran, Eastern Province, Saudi Arabia
3) Laboratory of Modeling and Multi-scale Simulation, Department of Physics, Faculty of Exact Sciences, University of Sidi Bel Abbes, Algeria
E.A. Adda Bedia: Material and Hydrology Laboratory, Civil Engineering Department, Faculty of Technology, University of Sidi Bel Abbes, Algeria
S.R. Mahmoud: Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

Abstract
In this paper, using modified couple stress theory in place of classical continuum theory, and using shell model in place of beam model, vibrational behavior of nanotubes is investigated via the finite element method. Accordingly classical continuum theory is unable to correctly compute stiffness and account for size effects in micro/nanostructures, higher order continuum theories such as modified couple stress theory have taken on great appeal. In the present work the mass-stiffness matrix for cylindrical shell element is developed, and by means of size-dependent finite element formulation is extended to more precisely account for nanotube vibration. In addition to modified couple stress cylindrical shell element, the classical cylindrical shell element can also be defined by setting length scale parameter to zero in the equations. The boundary condition were assumed simply supported at both ends and it is shown that the natural frequency of nano-scale shell using the modified coupled stress theory is larger than that using the classical shell theory and the results of Ansys. The results have indicated using the modified couple stress cylindrical shell element, the rigidity of the nano-shell is greater than that in the classical continuum theory, which results in increase in natural frequencies. Besides, in addition to reducing the number of elements required, the use of this type of element also increases convergence speed and accuracy.

Key Words
modified couple stress theory; FEM; cylindrical shell element; size dependent; Thin Shell Theory

Address
Iman Soleimani: Mechanical Engineering Department, Shahrekord University, Shahrekord, Iran
Yaghoub T. Beni and Mohsen B. Dehkordi: Faculty of Engineering, Shahrekord University, Shahrekord, Iran


Abstract
Structural modal parameters i.e. natural frequencies, damping ratios and mode shapes are dynamic features obtained either by measuring the vibration responses of a structure or by means of finite elements models. Over the past two decades, modal parameters have been used to detect damage in structures by observing its variations over time. However, such variations can also be caused by environmental factors such as humidity, wind and, more importantly, temperature. In so doing, the use of modal parameters as damage indicators can be seriously compromised if these effects are not properly tackled. Many researchers around the world have found numerous methods to mitigate the influence of such environmental factors from modal parameters and many advanced damage indicators have been developed and proposed to improve the reliability of structural health monitoring. In this paper, several vibration tests are performed on a simply supported steel beam subjected to different damage scenarios and temperature conditions, aiming to describe the variation in modal parameters due to temperature changes. Moreover, four statistical methodologies are proposed to identify damage. Results show a slightly linear decrease in the modal parameters due to temperature increase, although it is not possible to establish an empirical equation to describe this tendency.

Key Words
structural health monitoring; modal parameters; thermal effect; damage detection; structural dynamics

Address
Fabricio A. Ortiz Moralesand Ricardo A. Fiorotti Peixoto: Department of Civil Engineering, Federal University of Ouro Preto, Ouro Preto, MG, Brazil
Alexandre A. Cury: Department of Applied and Computational Mechanics, Federal University of Juiz de Fora, Juiz de Fora, MG, Brazil

Abstract
In this paper, an innovative procedure is proposed for the seismic design of reinforced concrete frame structures. The main contribution of the proposed procedure is to minimize the construction cost, considering the uniform damage distribution over the height of structure due to earthquake excitations. As such, this procedure is structured in the framework of an optimization problem, and the initial construction cost is chosen as the objective function. The aim of uniform damage distribution is reached through a design constraint in the optimization problem. Since this aim requires defining allowable degree of damage, a damage pattern based on the concept of global collapse mechanism is presented. To show the efficiency of the proposed procedure, the uniform damage-based optimum seismic design is compared with two other seismic design procedures, which are the strength-based optimum seismic design and the damage-based optimum seismic design. By using the three different seismic design methods, three reinforced concrete frames including six-, nine-, and twelve-story with three bays are designed optimally under a same artificial earthquake. Then, to show the effects of the uniform damage distribution, all three optimized frames are used for seismic damage analysis under a suite of earthquake records. The results show that the uniform damage-based optimum seismic design method renders a design that will suffer less damage under severe earthquakes.

Key Words
structural damage; collapse mechanism; seismic damage analysis; reinforced concrete; construction cost; optimization

Address
Sadjad Gharehbaghi: Department of Civil Engineering, Behbahan Khatam Alanbia University of Technology, Behbahan, Iran

Abstract
This paper is devoted to two new techniques for free vibration analysis of concrete arch dam-reservoir systems. The proposed schemes are quadratic ideal-coupled eigen-problems, which can solve the originally non-symmetric eigen-problem of the system. To find the natural frequencies and mode shapes, a new special-purpose eigen-value solution routine is developed. Moreover, the accuracy of the proposed approach is thoroughly assessed, and it is confirmed that the new scheme is very accurate under all practical conditions. It is also concluded that both decoupled and ideal-coupled strategy proposed in the previous works can be considered as special cases of the current more general procedure.

Key Words
arch dam; fluid-structure interaction; decoupled method; ideal-coupled method; quadratic ideal-coupled method; subspace iteration method

Address
Mohammad Rezaiee-Pajand, Ahmad Aftabi Sani and Mohammad Sadegh Kazemiyan: Department of Civil Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

Abstract
Stone column installation is a convenient method for improvement of soft ground. In very soft clays, in order to increase the lateral confinement of the stone columns, encasing the columns with high stiffness and creep resistant geosynthetics has proved to be a successful solution. This paper presents the results of three dimensional finite element analyses for evaluating improvement in behaviour of ordinary stone columns (OSCs) installed in soft clay by geotextile encasement under monotonic and cyclic loading by a comprehensive parametric study. The parameters include length and stiffness of encasement, types of stone columns (floating and end bearing), frictional angle and elastic modulus of stone column\'s materials in comparison with OSCs. Also, encasing at the top portion of stone column up to triple the diameter of column is found to be adequate in improving its residual settlement and at all loading cycles, end bearing columns provide much higher resistance than floating columns.

Key Words
stone column; geotextile encasement; cyclic loading; residual settlement; lateral deformation; finite element analyses

Address
Alireza Ardakani and Naeem Gholampoor: Faculty of Engineering and Technology, Imam Khomeini International University, Qazvin, Iran
Mahdi Bayat: Department of Civil Engineering, Islamic Azad University, Roudehen Branch, Roudehen, Iran
Mahmoud Bayat: Young Researchers and Elite Club, Islamic Azad University, Roudehen Branch, Roudehen, Iran

Abstract
Stone column installation is a convenient method for improvement of soft ground. In very soft clays, in order to increase the lateral confinement of the stone columns, encasing the columns with high stiffness and creep resistant geosynthetics has proved to be a successful solution. This paper presents the results of three dimensional finite element analyses for evaluating improvement in behaviour of ordinary stone columns (OSCs) installed in soft clay by geotextile encasement under monotonic and cyclic loading by a comprehensive parametric study. The parameters include length and stiffness of encasement, types of stone columns (floating and end bearing), frictional angle and elastic modulus of stone column\'s material and diameter of stone columns. The results indicate that increasing the stiffness of encasement clearly enhances cyclic behaviour of geotextile encased stone columns (GESCs) in terms of reduction in residual settlement. Performance of GESCs is less sensitive to internal friction angle and elasticity modulus of column\'s materials in comparison with OSCs. Also, encasing at the top portion of stone column up to triple the diameter of column is found to be adequate in improving its residual settlement and at all loading cycles, end bearing columns provide much higher resistance than floating columns.

Key Words
stone column; geotextile encasement; cyclic loading; residual settlement; lateral deformation; finite element analyses

Address
Alireza Ardakani and Naeem Gholampoor: Faculty of Engineering and Technology, Imam Khomeini International University, Qazvin, Iran
Mahdi Bayat: Department of Civil Engineering, Islamic Azad University, Roudehen Branch, Roudehen, Iran
Mahmoud Bayat: Young Researchers and Elite Club, Islamic Azad University, Roudehen Branch, Roudehen, Iran

Abstract
Three structure-dependent integration methods with no numerical dissipation have been successfully developed for time integration. Although these three integration methods generally have the same numerical properties, such as unconditional stability, second-order accuracy, explicit formulation, no overshoot and no numerical damping, there still exist some different numerical properties. It is found that TLM can only have unconditional stability for linear elastic and stiffness softening systems for zero viscous damping while for nonzero viscous damping it only has unconditional stability for linear elastic systems. Whereas, both CEM and CRM can have unconditional stability for linear elastic and stiffness softening systems for both zero and nonzero viscous damping. However, the most significantly different property among the three integration methods is a weak instability. In fact, both CRM and TLM have a weak instability, which will lead to an adverse overshoot or even a numerical instability in the high frequency responses to nonzero initial conditions. Whereas, CEM possesses no such an adverse weak instability. As a result, the performance of CEM is much better than for CRM and TLM. Notice that a weak instability property of CRM and TLM might severely limit its practical applications.

Key Words
weak instability; numerical instability; overshoot; structure-dependent integration method

Address
Shuenn-Yih Chang: Department of Civil Engineering, National Taipei University of Technology, 1, Section 3, Jungshiau East Road, Taipei 106-08, Republic of China

Abstract
The focus of this paper is on the elements with stable open cracks. To analyze plane problems, two triangular elements with three and six nodes are formulated using force method. Flexibility matrices of the elements are derived by combining the non-cracked flexibility and the additional one due to crack, which is computed by utilizing the local flexibility method. In order to compute the flexibility matrix of the intact element, a basic coordinate system without rigid body motions is required. In this paper, the basic system origin is located at the crack center and one of its axis coincides with the crack surfaces. This selection makes it possible to formulate elements with inclined cracks. It is obvious that the ability of the suggested elements in calculating accurate natural frequencies for cracked structures, make them applicable for vibration-based crack detection.

Key Words
triangular cracked element; released strain energy; fracture mechanics; stress intensity factor; local flexibility approach

Address
Mohammad Rezaiee-Pajand and Nima Gharaei-Moghaddam: Department of Civil Engineering, School of Engineering, Ferdowsi University of Mashhad, Iran

Abstract
This paper presents a new and simple solution for determining the natural frequencies of framed tube combined with shear-walls and tube-in-tube systems. The novelty of the presented approach is based on the bending moment function approximation instead of the mode shape function approximation. This novelty makes the presented solution very simpler and very shorter in the mathematical calculations process. The shear stiffness, flexural stiffness and mass per unit length of the structure are variable along the height. The effect of the structure weight on its natural frequencies is considered using a variable axial force. The effects of shear lag phenomena has been investigated on the natural frequencies of the structure. The whole structure is modeled by an equivalent non-prismatic shear-flexural cantilever beam under variable axial forces. The governing differential equation of motion is converted into a system of linear algebraic equations and the natural frequencies are calculated by determining a non-trivial solution for the system of equations. The accuracy of the proposed method is verified through several numerical examples and the results are compared with the literature.

Key Words
tall structure; batural frequency; shear-flexural deformation; axial force; weak form of integral equations; bending moment approximation; shear lag

Address
Mehrdad Mohammadnejad: Department of Civil Engineering, Faculty of Engineering, Birjand University of Technology, Birjand, Iran
Hasan Haji Kazemi: Department of Civil Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

Abstract
Three structure-dependent integration methods with no numerical dissipation have been successfully developed for time integration. Although these three integration methods generally have the same numerical properties, such as unconditional stability, second-order accuracy, explicit formulation, no overshoot and no numerical damping, there still exist some different numerical properties. It is found that TLM can only have unconditional stability for linear elastic and stiffness softening systems for zero viscous damping while for nonzero viscous damping it only has unconditional stability for linear elastic systems. Whereas, both CEM and CRM can have unconditional stability for linear elastic and stiffness softening systems for both zero and nonzero viscous damping. However, the most significantly different property among the three integration methods is a weak instability. In fact, both CRM and TLM have a weak instability, which will lead to an adverse overshoot or even a numerical instability in the high frequency responses to nonzero initial conditions. Whereas, CEM possesses no such an adverse weak instability. As a result, the performance of CEM is much better than for CRM and TLM. Notice that a weak instability property of CRM and TLM might severely limit its practical applications.

Key Words
weak instability; numerical instability; overshoot; structure-dependent integration method

Address
Hassan Bakhshandeh Amnieh: School of Mining, College of Engineering, University of Tehran, Iran
Mohammad Saber Zamzam: Department of Mining Engineering, Faculty of Engineering, University of Kashan, Iran


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