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CONTENTS
Volume 55, Number 6, September25 2015
 

Abstract
In this paper, the effect of functionally graded material (FGM) coatings on the fracture behavior of semi-elliptical cracks in cylinders is assessed. The objective is to calculate the stress intensity factor (SIF) of a longitudinal semi-elliptical crack on the wall of an aluminum cylinder with FGM coating. A threedimensional finite element method (FEM) is used for constructing the mechanical models and analyzing the SIFs of cracks. The effect of many geometrical parameters such as relative depth, crack aspect ratio, FG coating thickness to liner thickness as well as the mechanical properties of the FG coating on the SIF of the cracks is discussed. For a special case, the validity of the FE model is examined. The results indicated that there is a particular crack aspect ratio in which the maximum value of SIFs changes from the deepest point to the surface point of the crack. Moreover, it was found that the SIFs decrease by increasing the thickness ratio of the cylinder. But, the cylinder length has no effect on the crack SIFs.

Key Words
fracture, functionally gradient materials (FGMs); cylinder; stress intensity factor (SIF); semielliptical crack; finite element method (FEM)

Address
Peyman Farahpour, Faculty of Industrial and Mechanical Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran
Vahid Babaghasabha, Young Researchers and Elite Club, Qazvin Branch, Islamic Azad University, Qazvin, Iran
Mahdi Khadem, Department of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea


Abstract
External prestressing has been applied to both new construction and retrofitting of existing reinforced and prestressed concrete structures. Continuous beams are preferred to simply supported beams because of economy, fewer movement joints and possible benefits from moment redistribution. However, this paper argues that continuous prestressed concrete beams with external unbonded tendons demonstrate different full-range behaviour compared to reinforced concrete (RC) beams. Applying the same design approach for RC to external prestressing may lead to design with a lower safety margin. To better understand the behaviour of continuous prestressed concrete beams with unbonded tendons, an experimental investigation is performed in which nine such specimens are tested to failure. The full-range behaviour is investigated with reference to moment-curvature relationship and moment redistribution. The amounts of moment redistribution measured in the experiments are compared with those allowed by BS 8110, EC2 and ACI 318. Design equations are also proposed to estimate the curvature ductility index of unbonded prestressed concrete beams.

Key Words
continuous beams; ductility; full-range behaviour; moment redistribution; partial prestressing; prestressed concrete; unbonded prestressing tendons

Address
K.H. Enoch Chan, CH2M HILL, Burderop Park, Swindon, United Kingdom
Francis T.K. Au, Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong

Abstract
High powered computers and engineering computer systems allow designers to routinely simulate complex physical phenomena. The presented work deals with the analysis of two finite element method optimization techniques (First Order Method-FOM and Subproblem Approximation Method-SAM) implemented in the individual Design Optimization module in the Ansys software to analyze the behavior of real problems. A design optimization is a difficult mathematical process, intended to find the minimum or maximum of an objective function, which is mostly based on iterative procedure. Using optimization techniques in engineering designs requires detailed knowledge of the analyzed problem but also an ability to select the appropriate optimization method. The methods embedded in advanced computer software are based on different optimization techniques and their efficiency is significantly influenced by the specific character of a problem. The efficiency, robustness and accuracy of the methods are studied through strictly convex two-dimensional optimization problem, which is represented by volume minimization of two bars‟ plane frame structure subjected to maximal vertical displacement limit. Advantages and disadvantages of the methods are described and some practical tips provided

Key Words
design optimization; First Order Method; Subproblem Approximation Method; feasible/infeasible design space; robustness; accuracy

Address
Filip Fedorik and Mikko Malaska, Structural Engineering and Construction Technology Research Group, University of Oulu, P.O. Box 4200, FI-90014, Oulu, Finland
Jiří Kala, Institute of Structural Mechanics, Brno University of Technology, Veveří 331/95, 60200 Brno, Czech Republic
Antti Haapala, Wood Material Science, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland

Abstract
In this study, a new retrofitting method for improving the seismic performance of reinforced concrete column was presented, in which prestressed steel strips were utilized as retrofitting stuff to confine the reinforced concrete column transversely. In order to figure out the seismic performance of concrete column specimen retrofitted by such prestressed steel strips methods, a series of quasi-static tests of five retrofitted specimens and two unconfined column specimen which acted as control specimens were conducted. Based on the test results, the seismic performance including the failure modes, hysteresis performance, ductility performance, energy dissipation and stiffness degradation of all these specimens were fully investigated and analyzed. And furthermore the influences of some key parameters such as the axial force ratios, shear span ratios and steel strips spacing on seismic performance of those retrofitted reinforced concrete column specimens were also studied. It was shown that the prestressed steel strips provided large transverse confining effect on reinforced concrete column specimens, which resulted in improving the shearing bearing capacity, ductility performance, deformation capacity and energy dissipation performance of retrofitted specimens effectively. In comparison to the specimen which was retrofitted by the carbon fiber reinforced plastics (CFRP) strips method, the seismic performance of the specimens retrofitted by the prestressed steel strips was a bit better, and with much less cost both in material and labor. From this research results, it can be concluded that this new retrofitting method is really useful and has significant advantages both in saving money and time over some other retrofitting methods.

Key Words
prestressed steel strip; retrofitted methods; reinforced concrete column; seismic performance; quasi- static tests; experimental research

Address
Bo Zhang: College of Civil Engineering, Xi\'an University of Architecture & Technology, Xi\'an 710055, China; Department of civil Engineering & Architecture, Shaanxi University of Technology, HanZhong 723001, China
Yong Yang, Yuan-feng Wei, Ru-yue Liu: College of Civil Engineering, Xi\'an University of Architecture & Technology, Xi\'an 710055, China
Chu Ding: Department of Civil & Environmental Engineering, Virginia Tech, Blacksburg, VA 24060, USA
Ke-qiang Zhang: Department of civil Engineering & Architecture, Shaanxi University of Technology, HanZhong 723001, China


Abstract
This study presents optimizing structural topology patterns using regularization of Heaviside function. The present method needs not filtering process to typical SIMP method. Using the penalty formulation of the SIMP approach, a topology optimization problem is formulated in co-operation, i.e., couple-signals, with design variable values of discrete elements and a regularized Heaviside step function. The regularization of discontinuous material distributions is a key scheme in order to improve the numerical problems of material topology optimization with 0 (void)-1 (solid) solutions. The weak forms of an equilibrium equation are expressed using a coupled regularized Heaviside function to evaluate sensitivity analysis. Numerical results show that the incorporation of the regularized Heaviside function and the SIMP leads to convergent solutions. This method is tested using several examples of a linear elastostatic structure. It demonstrates that improved optimal solutions can be obtained without the additional use of sensitivity filtering to improve the discontinuous 0-1 solutions, which have generally been used in material topology optimization problems.

Key Words
optimization; topology patterns; SIMP; filtering process, regularized Heaviside function

Address
Dongkyu Lee, Department of Architectural Engineering, College of Engineering, Sejong University, 143-747, Seoul, Korea
Soomi Shin, Research Institute of Industrial Technology, Pusan National University, 609-735, Busan, Korea

Abstract
In this paper, a new intensity measure of earthquakes for probabilistic seismic analysis is presented for skewed highway bridges. Three different cases of skewed bridges with different skew angles (0o, 30o and 45o) are considered. Well-known intensity measures (e.g., PGA, Sa) are evaluated and critically discussed based on sensitivity analysis: efficiency, practically, proficiency and sufficiency of intensity measures are considered in detail. The analyses demonstrated that the intensity measures have to take into account structural acceleration on a wide range of periods so that a new seismic intensity measure is proposed showing that it has less dispersion compared to others. Since the proposed intensity represents the average value of the Sa (between a lower and upper structural period) it has been called Averaged Spectral Acceleration (ASA). Based on performed incremental dynamic analysis (IDA), the seismic analytical fragility curves of typical skewed highway bridges have been evaluated for different states of damage controlling the low dispersion of the ASA index as well as its proficiency and sufficiency.

Key Words
skewed highway bridges; analytical fragility curve; spectral intensity measures; incremental dynamic analysis (IDA)

Address
M. Bayat and F. Daneshjoo, Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran, Iran
N. Nisticò, Department of Structural and Geotechnical engineering, Universita degli Studi di Roma \"La Sapienza\", Roma, Italy

Abstract
By taking a cable-stayed-suspension hybrid bridge with main span of 1400 m as example, seismic response of the bridge under the horizontal and vertical seismic excitations is investigated numerically by response spectrum analysis and time history analysis, its seismic performance is discussed and compared to the cable-stayed bridge and suspension bridge with the same main span, and considering the aspect of seismic performance, the feasibility of using cable-stayed-suspension hybrid bridge in super long-span bridges is discussed. Under the horizontal seismic action, the effects of structural design parameters including the cable sag to span ratio, the suspension to span ratio, the side span length, the subsidiary piers in side spans, the girder supporting system and the deck form etc on the seismic performance of the bridge are investigated by response spectrum analysis, and the favorable values of these design parameters are proposed.

Key Words
cable-stayed-suspension hybrid bridge; seismic response; response spectrum analysis; time history analysis; structural design parameter

Address
Xin-Jun Zhang and Zhou-Jun Yu, College of Civil Engineering & Architecture, Zhejiang University of Technology, Hangzhou 310014, P.R. China

Abstract
A method for structural damage identification based on Chaotic Artificial Bee Colony (CABC) algorithm is presented. ABC is a heuristic algorithm with simple structure, ease of implementation, good robustness but with slow convergence rate. To overcome the shortcoming, the tournament selection mechanism is chosen instead of the roulette mechanism and chaotic search mechanism is also introduced. Residuals of natural frequencies and modal assurance criteria (MAC) are used to establish the objective function, ABC and CABC are utilized to solve the optimization problem. Two numerical examples are studied to investigate the efficiency and correctness of the proposed method. The simulation results show that the CABC algorithm can identify the local damage better compared with ABC and other evolutionary algorithms, even with noise corruption.

Key Words
damage detection; Chaotic Artificial Bee Colony algorithm; modal assurance criteria; coupled double-beam system

Address
H.J. Xu, Z.H. Ding, Z.R. Lu and J.K. Liu, Department of Applied Mechanics and Engineering, Sun Yat-sen University, Guangzhou, Guangdong, P.R. China

Abstract
Any rational approach to define the configuration and size of viscous fluid dampers in a structure should be based on the dynamic properties of the system with the dampers. In this paper we propose an alternative representation of the complex eigenvalues of multi degree of freedom systems with dampers to calculate new equivalent natural frequencies. Analytical expressions for the dynamic properties of a two-story building model with a linear viscous damper in the first floor (i.e. with a non-proportional damping matrix) are derived. The formulas permit to obtain the equivalent damping ratios and equivalent natural frequencies for all the modes as a function of the mass, stiffness and damping coefficient for underdamped and overdamped systems. It is shown that the commonly used formula to define the equivalent natural frequency is not applicable for this type of system and for others where the damping matrix is not proportional to the mass matrix, stiffness matrix or both. Moreover, the new expressions for the equivalent natural frequencies expose a novel phenomenon; the use of viscous fluid dampers can modify the vibration frequencies of the structure. The significance of the new equivalent natural frequencies is expounded by means of a simulated free vibration test. The proposed approach may offer a new perspective to study the effect of viscous dampers on the dynamic properties of a structure.

Key Words
equivalent frequencies; damping ratios; dampers; state equations; complex eigenvalues

Address
Luis E. Suarez, Department of Civil Engineering and Surveying, University of Puerto Rico at Mayaguez, Mayaguez, 00681-9000, Puerto Rico
Carlos A. Gaviria, Department of Civil Engineering and Surveying, University of Puerto Rico at Mayaguez, Mayaguez, 00681-9000, Puerto Rico ; Civil Engineering Program, Universidad de la Costa, Barranquilla, Colombia

Abstract
Due to their low intrinsic damping, stay cables in cable-stayed bridges have often exhibited unanticipated and excessive vibrations which result in increasing maintenance frequency and disruption to normal operations of the entire bridges. Mitigation of undesired cable vibration can be achieved by attaching an external damping device near the anchorage. High Damping Rubber (HDR) dampers have many advantages such as compact size, better aesthetics, easy maintenance, temperature stability, and cost benefits; therefore, they have been widely used to increase cable damping. Although a single damper has been shown to reduce cable vibrations, it is not the most effective method due to geometric constraints. This paper proposes the use of two HDR dampers to improve effectiveness and robustness in suppressing cable vibration. Oscillation parameters of the cable-dampers system were investigated in detail by modeling the stay cable as a taut string and each HDR damper as complex-valued impedance and by using an analytical formulation of the complex eigenvalue problem. The problem of two HDR dampers arbitrarily located along a cable is solved and the solution is discussed. Asymptotic formulas to calculate the damping ratios of the cable with two HDR dampers installed near the anchorage(s) are proposed and compared with the exact solutions. Further, a design example is presented in order to justify the methodology. The results of this study show that when the two HDR dampers are installed close to each other on the same end of the cable, some interaction between the dampers leads to reduced damping ratio. When the dampers are on the opposite ends of the cable, they are effective in increasing damping ratio and can provide better vibration reduction to multiple modes.

Key Words
taut cable; HDR damper; vibration reduction; damping ratio; complex eigenvalue

Address
Viet Hung Cu and Bing Han, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
Fang Wang, Engineering Management Center, China Railway Corporation, Beijing 100844, China


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