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CONTENTS
Volume 61, Number 6, March25 2017
 

Abstract
Concrete cover cracking due to the corrosion of steel reinforcing bars is one of the main causes of deterioration in Reinforced Concrete (RC) structures. The oxidation level of the bars causes varying levels of expansion. The rebar expansions could lead to through-thickness cracking of the concrete cover, where depending on the cracking characteristics, the service life of the structures would be affected. In this paper, the effect of geometrical and material parameters, i.e., concrete cover thickness, reinforcing bar diameter, and concrete tensile strength, on the required pressure for concrete cover cracking due to corrosion has been investigated through detailed numerical simulations. ABAQUS finite element software is employed as a modeling platform where the concrete cracking is simulated by means of eXtended Finite Element Method (XFEM). The accuracy of the numerical simulations is verified by comparing the numerical results with experimental data obtained from the literature. Using a previously proposed empirical equation and the numerical model, the time from corrosion initiation to the cover cracking is predicted and then compared to the respective experimental data. Finally, a parametric study is undertaken to determine the optimum ratio of the rebar diameter to the reinforcing bars spacing in order to avoid concrete cover delamination.

Key Words
corrosion; concrete cover cracking; reinforced concrete structures; eXtended finite element method; service life

Address
Mohammad Javad Mirzaee, Farshid Jandaghi Alaee: Department of Civil Engineering and Architecture, University of Shahrood, Shahrood, Iran
Mohammad Hajsadeghi: School of Engineering Sciences, University of Liverpool, Liverpool L69 3BX, UK
Tadeh Zirakian: Department of Civil Engineering and Construction Management, California State University, Northridge, CA, USA

Abstract
Seismic dissipation devices can play a crucial role in mitigating earthquake damages, loss of life and post-event repair and downtime costs. This research investigates the use of ring springs with high-force-to-volume (HF2V) dissipaters to create damage-free, recentring connections and structures. HF2V devices are passive rate-dependent extrusion-based devices with high energy absorption characteristics. Ring springs are passive energy dissipation devices with high self-centring capability to reduce the residual displacements. Dynamic behaviour of a system with nonlinear structural stiffness and supplemental hybrid damping via HF2V devices and ring spring dampers is used to investigate the design space and potential. HF2V devices are modelled with design forces equal to 5% and 10% of seismic weight and ring springs are modelled with loading stiffness values of 20% and 40% of initial structural stiffness and respective unloading stiffness of 7% and 14% of structural stiffness (equivalent to 35% of their loading stiffness). Using a suite of 20 design level earthquake ground motions, nonlinear response spectra for 8 different configurations are generated. Results show up to 50% reduction in peak displacements and greater than 80% reduction in residual displacements of augmented structure compared to the baseline structure. These gains come at a cost of a significant rise in the base shear values up to 200% mainly as a result of the force contributed by the supplemental devices.

Key Words
spectral design; self-centring; nonlinear structure; high force damper

Address
Farzin G. Golzar, Geoffrey W. Rodgers and J. Geoffrey Chase: Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

Abstract
This study investigates the effects of reinforcing bar diameter and cover depth on the shrinkage behavior of restrained ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs. For this, twelve large-sized UHPFRC slabs with three different rebar diameters (db=9.5, 15.9, and 22.2 mm) and four different cover depths (h=5, 10, 20, and 30 mm) were fabricated. In addition, a large-sized UHPFRC slab without steel rebar was fabricated for evaluating degree of restraint. Test results revealed that the uses of steel rebar with a large diameter, leading to a larger reinforcement ratio, and a low cover depth are unfavorable regarding the restrained shrinkage performance of UHPFRC slabs, since a larger rebar diameter and a lower cover depth result in a higher degree of restraint. The shrinkage strain near the exposed surface was high because of water evaporation. However, below a depth of 18 mm, the shrinkage strain was seldom influenced by the cover depth; this was because of the very dense microstructure of UHPFRC. Finally, owing to their superior tensile strength, all UHPFRC slabs with steel rebars tested in this study showed no shrinkage cracks until 30 days.

Key Words
ultra-high-performance fiber-reinforced concrete; slab; shrinkage; degree of restraint; steel rebar; cover depth; rebar diameter

Address
Doo-Yeol Yoo: Department of Architectural Engineering, Hanyang University, Seoul, Korea
Ki-Yeon Kwon: Structural Systems & Site Evaluation Department, Korea Institute of Nuclear Safety, Daejeon, Korea
Jun-Mo Yang: Steel Structure Research Group, POSCO, Incheon, Korea
Young-Soo Yoon: School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, Korea

Abstract
In this paper, free vibration characteristics of functionally graded (FG) nanobeams embedded on elastic medium are investigated based on third order shear deformation (Reddy) beam theory by presenting a Navier type solution for the first time. The material properties of FG nanobeam are assumed to vary gradually along the thickness and are estimated through the power-law and Mori-Tanaka models. A two parameters elastic foundation including the linear Winkler springs along with the Pasternak shear layer is in contact with beam. The small scale effect is taken into consideration based on nonlocal elasticity theory of Eringen. The nonlocal equations of motion are derived based on third order shear deformation beam theory through Hamilton\'s principle and they are solved applying analytical solution. According to the numerical results, it is revealed that the proposed modeling can provide accurate frequency results of the FG nanobeams as compared to some cases in the literature. The obtained results are presented for the vibration analysis of the FG nanobeams such as the influences of foundation parameters, gradient index, nonlocal parameter and slenderness ratio in detail.

Key Words
free vibration; third order shear deformation beam theory; functionally graded nanobeam; nonlocal elasticity theory

Address
Farzad Ebrahimi and Mohammad Reza Barati: Mechanical Engineering Department, Faculty of Engineering, Imam Khomeini International University, Qazvin, P.O.B. 16818-34149, Iran


Abstract
The touch down zone (TDZ) and top connection point of the vessel are most critical part of fatigue damage in the steel catenary riser (SCR). In general, the linear soil model has been used to evaluate fatigue performance of SCRs because it gives conservative results in the TDZ. However, the conservative linear soil model shows the limitation to accommodate real behavior in the TDZ as water depth is increased. Therefore, the riser behavior on soft clay seabed is investigated using a nonlinear soil model through time domain approach in this study. The numerical analysis considering various important parameters of the nonlinear soil model such as shear strength at mudline, shear strength gradient and suction resistance force is conducted to check the adoptability and applicability of nonlinear soil model for SCR design.

Key Words
steel catenary riser; touch down zone; nonlinear soil; vortex-induced vibration; fatigue damage

Address
Y.T. Kim: Environmental and Plant Engineering Research Team, Daewoo Institute of Construction Technology, 16297 Suwon, Republic of Korea
D.K. Kim: Ocean and Ship Technology, Deepwater Technology Mission Oriented Research, Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia; Graduate School of Engineering Mastership, Pohang University of Science and Technology, 37673 Pohang, Republic of Korea
H.S. Choi: Graduate School of Engineering Mastership, Pohang University of Science and Technology, 37673 Pohang, Republic of Korea
S.Y. Yu: Ocean and Ship Technology, Deepwater Technology Mission Oriented Research, Department of Civil and Environmental Engineering, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia
K.S. Park: Steel Structure Research Group, POSCO Global R&D Center, 21985 Incheon, Republic of Korea

Abstract
Exact solutions for stresses for an infinite rectangular plate perforated by two circular holes of different radii subjected to uni-axial or bi-axial uniform loads are investigated using the Airy stress function. The hoop stresses occurring at the edge of the circular hole are computed and plotted. Comparisons are made for the stress concentration factors for several types of loading conditions.

Key Words
perforated plate; circular hole; multi-holes; biaxial tension; stress concentration factor; airy stress function; exact solution

Address
Yeong-Bin Yang: Department of Civil Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan; School of Civil Engineering, Chongqing University, 83 Shabei Street, Shapingba District, Chongqing, 400045, China
Jae-Hoon Kang: Department of Architectural Engineering, Chung-Ang University, 221 Heuksuk-Dong, Dongjak-Ku, Seoul 156-756, Republic Korea

Abstract
In this study, the susceptibility of different symmetric steel buildings with dual frame system to Progressive Collapse (PC) was assessed. Some ten-story dual frame systems with different type of braced frames (concentrically and eccentrically braced frames) were considered. In addition, numbers and locations of braced bays were investigated (two and three braced bays in exterior frames) to quantitatively find out its effect on PC resistance. An Alternate Path Method (APM) with a linear static analysis was carried out based on General Services Administration (GSA 2003) guidelines. Maximum Demand Capacity Ratio (DCR) for the elements (beams and columns) with highest DCRs (DCRmoment and DCRshear) is given in tables. The results showed that the three braced bays with concentric braced frames especially X-braced and inverted V-braced frame systems had a lower susceptibility and greater resistance to PC. Also, the results represented that the beams were more critical than columns against PC after the removal of column.

Key Words
Alternate Path Method (APM); Concentrically and Eccentrically Braced Frames (CBFs and EBFs); two and three braced bays; Demand Capacity Ratio (DCR); Progressive Collapse (PC)

Address
Reza Jalali Larijani, Heydar Dashti Nasserabadi: Department of Civil Engineering, Islamic Azad University, Chaloos Branch, Iran
Iman Aghayan: Department of Civil Engineering, Shahrood University of Technology, Shahrood, Iran

Abstract
The quadratic B-spline finite element method yields mass and stiffness matrices which are half the size of matrices obtained by the conventional finite element method. We solve the free vibration problem of a rotating Rayleigh beam using the quadratic B-spline finite element method. Rayleigh beam theory includes the rotary inertia effects in addition to the Euler- Bernoulli theory assumptions and presents a good mathematical model for rotating beams. Galerkin\'s approach is used to obtain the weak form which yields a system of symmetric matrices. Results obtained for the natural frequencies at different rotating speeds show an accurate match with the published results. A comparison with Euler-Bernoulli beam is done to decipher the variations in higher modes of the Rayleigh beam due to the slenderness ratio. The results are obtained for different values of non-uniform parameter ( n ).

Key Words
Galerkin method; quadratic B-spline basis function; rotating beam; free vibration; conventional finite element method

Address
Vijay Panchore and Ranjan Ganguli: Aerospace Engineering, Indian Institute of Science, Bangalore, India

Abstract
This study is devoted to developing many new substructure models for ballasted railway track by using the pyramid model philosophy. As the effect of railway embankment has been less considered in the previous studies in the field of vehicle/track interaction, so the present study develops the pyramid models in the presence of railway embankment and implements them in vehicle/track interaction dynamic analyses. Considering a moving car body as multi bodies with 10 degrees of freedom and the ballasted track including rail, sleeper, ballast, subgrade and embankment, two categories of numerical analyses are performed by considering the new substructure systems including type A (initiation of stress overlap areas in adjacent sleepers from the ballast layer) or type B (initiation of stress overlap areas in adjacent sleepers from the subgrade layer). A comprehensive sensitivity analyses are performed on effective parameters such as ballast height, sleepers spacing and sleeper width. The results indicate that the stiffness of subgrade, embankment and foundation increased by increasing the ballast height. Also, by increasing the ballast height, rail and ballast vertical displacement decreased.

Key Words
new substructure models; developed pyramid models; railway embankment; vehicle/track interaction; dynamic analysis

Address
Seyed-Ali Mosayebi, Jabbar-Ali Zakeri and Morteza Esmaeili: School of Railway Engineering, Iran University of Science and Technology, Tehran, Iran

Abstract
This paper is concerned with the comparative numerical and experimental study on the natural behavior and the motion responses of a 1/75 moored scale model of a 2.5 MW spar-type floating offshore wind turbine subject to 1-D regular wave. Heave, pitch and surge motions and the mooring tensions are investigated and compared by numerical and experimental methods. The upper part of wind turbine which is composed of three rotor blades, hub and nacelle is modeled as a lumped mass and three mooring lines are pre-tensioned by means of linear springs. The numerical simulations are carried out by a coupled FEM-cable dynamics code, while the experiments are performed in a wave tank equipped with the specially-designed vision and data acquisition system. Using the both methods, the natural behavior and the motion responses in RAOs are compared and parametrically investigated to the fairlead position, the spring constant and the location of mass center of platform. It is confirmed, from the comparison, that both methods show a good agreement for all the test cases. And, it is observed that the mooring tension is influenced by all three parameters but the platform motion is dominated by the location of mass center. In addition, from the sensitivity analysis of RAOs, the coupling characteristic of platform motions and the sensitivities to the mooring parameters are investigated.

Key Words
spar-type floating platform; scale model; natural frequencies; response amplitude operator (RAO); mooring tension; center of mass; fairlead position; spring constant

Address
Sin-Pyo Hong: Global Core Research Center for Ships and Offshore Plants, Pusan National University, Busan 609-735, Republic of Korea
Jin-Rae Cho: Department of Naval Architecture and Ocean Engineering, Hongik University, Sejong 339-701, Republic of Korea

Abstract
According to the characteristics of continuous beam bridges, the relative displacement is too large to collision or even girder falling under earthquakes. A device named Cable-sliding Modular Expansion Joints(CMEJs) that can control the relative displacement and avoid collision under different ground motions is proposed. Working principle and mechanical model is described. This paper design the CMEJs, establish the restoring force model, verify the force model of this device by the pseudo-static tests, and describe and analyze results of the tests, and then based on a triple continuous beam bridge that has different heights of piers, a 3D model with or without CMEJs were established under Conventional System (CS) and Seismic Isolation System (SIS). The results show that this device can control the relative displacement and avoid collisions. The combination of isolation technology and CMEJs can be more effective to achieve both functions, but it need to take measures to prevent girder falling due to the displacement between pier and beam under large earthquakes.

Key Words
continuous girder bridge; the effect in limiting relative displacement; cable-sliding modular expansion joints (CMEJs); conventional system; seismic isolation system; pseudo-static tests

Address
Kang Gao: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University,1239 Siping Road, Shanghai 200092, China; School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
Wan C. Yuan and Xin Z. Dang: State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University,1239 Siping Road, Shanghai 200092, China

Abstract
In this paper, we introduce an efficient new model reduction method, named the automated static condensation method, which is developed for the local analysis of large finite element models. The algebraic multilevel substructuring procedure is modified appropriately, and then applied to the original static condensation method. The retained substructure, which is the local finite element model to be analyzed, is defined, and then the remaining part of the global model is automatically partitioned into many omitted substructures in an algebraic perspective. For an efficient condensation procedure, a substructural tree diagram and substructural sets are established. Using these, the omitted substructures are sequentially condensed into the retained substructure to construct the reduced model. Using several large practical engineering problems, the performance of the proposed method is demonstrated in terms of its solution accuracy and computational efficiency, compared to the original static condensation method and the superelement technique.

Key Words
finite element model; local analysis; model reduction method; static condensation method; superelement technique; algebraic multilevel substructuring

Address
Seung-Hwan Boo: Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
Min-Han Oh: Offshore Engineering Research Department, Hyundai Heavy Industries, 1000 Bangeojinsunhwan-doro, Dong-gu, Ulsan 682-792, Republic of Korea


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