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CONTENTS
Volume 69, Number 1, January10 2019
 

Abstract
In the construction of flat plate slabs, which are widely used for tall buildings but have relatively low flexural stiffness, serviceability problems such as excessive deflections and cracks are of great concern. To prevent excessive deflections at service load levels, current design codes require the minimum slab thickness, but the requirement could be unconservative because it is independent on loading and elastic modulus of concrete, both of which have significant effects on slab deflections. In the present study, to investigate the effects of the construction load of shored slabs, reduced flexural stiffness and moment distribution of early-age slabs, and creep and shrinkage of concrete on immediate and time-dependent deflections, numerical analysis was performed using the previously developed numerical models. A parametric study was performed for various design and construction conditions of practical ranges, and a new minimum permissible thickness of flat plate slabs was proposed satisfying the serviceability requirement for deflection. The proposed minimum slab thickness was compared with current design code provisions and numerical analysis results, and it agreed well with the numerical analysis results.

Key Words
flat plate slab; construction load; immediate deflection; time-dependent deflection; minimum thickness

Address
Hyeon-Jong Hwang:
1) College of Civil Engineering and Hunan Provincial Key Lab on Damage Diagnosis for Engineering Structures, Hunan Univ., Yuelu Mountain, Changsha, Hunan 410082, P.R. China
2) Engineering Research Institute, Seoul National Univ., Seoul 151-744, Republic of Korea
Gao Ma: College of Civil Engineering and Hunan Provincial Key Lab on Damage Diagnosis for Engineering Structures, Hunan Univ., Yuelu Mountain, Changsha, Hunan 410082, P.R. China
Chang-Soo Kim: School of Architecture, Seoul National University of Science and Technology, 232 Gongreung-ro, Nowon-gu, Seoul 01811, Republic of Korea

Abstract
The biaxial failure mechanism of transversally bedding concrete layers was numerically simulated using a sophisticated two-dimensional discrete element method (DEM) implemented in the particle flow code (PFC2D). This numerical modelling code was first calibrated by uniaxial compression and Brazilian testing results to ensure the conformity of the simulated numerical model\'s response. Secondly, 21 rectangular models with dimension of 54 mmx108 mm were built. Each model contains two transversely bedding layers. The first bedding layer has low mechanical properties, less than mechanical properties of intact material, and second bedding layer has high mechanical properties, more than mechanical properties of intact material. The angle of first bedding layer, with weak mechanical properties, related to loading direction was 0o, 15o, 30o, 45o, 60o, 75o and 90o while the angle of second layer, with high mechanical properties, related to loading direction was 90o, 105o, 120o, 135o, 150o, 160o and 180o. Is to be note that the angle between bedding layer was 90o in all bedding configurations. Also, three different pairs of the thickness were chosen in models, i.e., 5 mm/10 mm, 10 mm/10 mm and 20 mm/10 mm. The result shows that in all configurations, shear cracks develop between the weaker bedding layers. Shear cracks angel related to normal load change from 0o to 90o with increment of 15o. Numbers of shear cracks are constant by increasing the bedding thickness. It\'s to be noted that in some configuration, tensile cracks develop through the intact area of material model. There is not any failure in direction of bedding plane interface with higher strength.

Key Words
transversely bedding layer; biaxial strength; PFC2D

Address
Hadi Haeri and Zheming Zhu: MOE Key Laboratory of Deep Underground Science and Engineering, School of Architecture and Environment, Sichuan University, Chengdu 610065, China
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Ehsan Moosavi: Department of Mining Engineering, Islamic Azad Unviersity, South Tehran Branch, Tehran, Iran

Abstract
Optimizing composite structures to exploit their maximum potential is a realistic application with promising returns. In this research, simultaneous maximization of the fundamental frequency and frequency separation between the first two modes by optimizing the fiber angles is considered. A high-fidelity design optimization methodology is developed by combining the high-accuracy of finite element method with iterative improvement capability of metaheuristic algorithms. Three powerful nature-inspired optimization algorithms viz. a genetic algorithm (GA), a particle swarm optimization (PSO) variant and a cuckoo search (CS) variant are used. Advanced memetic features are incorporated in the PSO and CS to form their respective variants-RPSOLC (repulsive particle swarm optimization with local search and chaotic perturbation) and CHP (co-evolutionary host-parasite). A comprehensive set of benchmark solutions on several new problems are reported. Statistical tests and comprehensive assessment of the predicted results show CHP comprehensively outperforms RPSOLC and GA, while RPSOLC has a little superiority over GA. Extensive simulations show that the on repeated trials of the same experiment, CHP has very low variability. About 50% fewer variations are seen in RPSOLC as compared to GA on repeated trials.

Key Words
cuckoo search; finite element method; genetic algorithm; multiobjective optimization; particle swarm optimization

Address
Kanak Kalita: Department of Aerospace Engineering and Applied Mechanics, Indian Institute of Engineering, Science and Technology, Howrah, West Bengal 711103, India
Uvaraja Ragavendran: Department of Electronics and Telecommunication Engineering, MPSTME SVKM\'S Narsee Monjee Institute of Management Studies,
Shirpur, Maharashtra 425405, India
Manickam Ramachandran: Department of Mechanical Engineering, MPSTME SVKM\'S Narsee Monjee Institute of Management Studies, Shirpur, Maharashtra 425405, India
Akash Kumar Bhoi: Department of Electrical & Electronics Engineering, Sikkim Manipal Institute of Technology, Sikkim Manipal University, Majhitar, Sikkim 737136, India

Abstract
Thermal cracks are cracks that commonly form at early ages in mass concrete. During the concrete pouring process, the elastic modulus changes continuously. This requires the time domain to be divided into several steps in order to solve for the temperature, stress, and displacement of the concrete. Numerical simulations of thermal crack propagation in concrete are more difficult at early ages. To solve this problem, this study divides crack propagation in concrete at early ages into two cases: the case in which cracks do not propagate but the elastic modulus of the concrete changes and the case in which cracks propagate at a certain time. This paper provides computational models for these two cases by integrating the characteristics of the extended finite element algorithm, compiles the corresponding computational programs, and verifies the accuracy of the proposed model using numerical comparisons. The model presented in this paper has the advantages of high computational accuracy and stable results in resolving thermal cracking and its propagation in concrete at early ages.

Key Words
thermal crack; extended finite element; propagation; concrete; early age

Address
Zhenyang Zhu, Guoxin Zhang, Yi Liu and Zhenhong Wang: State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, P.R. China

Abstract
In this research, the effect of bedding layer on the tensile failure mechanism of rocks has been investigated using PFC2D. For this purpose, firstly calibration of PFC2d was performed using Brazilian tensile strength. Secondly Brazilian test was performed on the bedding layer. Thickness of layers were 5 mm, 10 mm and 20 mm. in each thickness layer, layer angles changes from 0o to 90o with increment of 15o. Totally, 21 model were simulated and tested by loading rate of 0.016 mm/s. The results show that when layer angle is less than 15, tensile cracks initiates between the layers and propagate till coalesce with model boundary. Its trace is too high. With increasing the layer angle, less layer mobilizes in failure process. Also, the failure trace is very short. It\'s to be noted that number of cracks decrease with increasing the layer thickness. Also, Brazilian tensile strength is minimum when bedding layer angle is between 45o and 75o. The maximum one is related to layer angle of 90o.

Key Words
bedding layer; Brazilian test; anisotropy; crack; PFC2D

Address
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Hadi Haeri: Young Researchers and Elite Club, Bafgh Branch, Islamic Azad University, Bafgh, Iran
Mohammad Fatehi Marji: Department of Mining Engineering, Yazd University, Yazd, Iran

Abstract
In this paper, the anisotropy of tensile behaviours of layered rocks consisting internal notch has been investigated using particle flow code. For this purpose, firstly calibration of PFC2D was performed using Brazilian tensile strength. Secondly Brazilian test models consisting bedding layer was simulated numerically. Thickness of layers was 10 mm and layered angularity was 90o, 75o, 60o, 45o, 30o, 15o and 0o. The strength of bedding interface was too high. Each model was consisted of one internal notch. Notch length is 1 cm, 2 cm and 4 cm and notch angularities are 60o, 45, 30o, 15o and 0o. Totally, 90 model were tested. The results show that failure pattern was affected by notch orientation and notch length. It\'s to be noted that layer angle has not any effect on the failure pattern. Also, Brazilian tensile strength is affected by notch orientation and notch length.

Key Words
notch; bedding layer; anisotropy; crack; Brazilian test; PFC2D

Address
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Hadi Haeri: Young Researchers and Elite Club, Bafgh Branch, Islamic Azad University, Bafgh, Iran
P. Ebneabbasi: Civil Engineering Department, Islamic Azad University, Hamedan, Iran
Kourosh Bagheri: Department of Civil Engineering, Malard Branch, Islamic Azad University, Malard, Iran

Abstract
The plates made of two materials joined to each other having the different coefficient of thermal expansions are frequently encountered in the industrial applications. The stress analysis of these members under the effect of high-temperature variation has great importance in design. In this study, the stress analysis of the experimental model developed for the problem considered here was performed by the method of photothermoelasticity. The thermal strains were formed by the mechanical way and these were fixed by the strain freezing method. For the stress measurements, the method of slicing is applied which provides three-dimensional stress analysis. The analytical solution in the literature was compared with the related stress distribution obtained from the model. Moreover, the axisymmetric finite element model developed for the problem was solved by ABAQUS and the results obtained here compared with those of the experimental model and the analytical solution. As a result of this study, this experimental method and numerical model can be used for these type of thermal stress problems which have not been comprehensively analyzed yet.

Key Words
coefficient of thermal expansion; thermal stress analysis; stress concentration factor; photothermoelasticity; the finite element analysis (FEA)

Address
Tugba Baytak and Osman Bulut: Faculty of Civil Engineering, Istanbul Technical University, Buyukdere St., Sar

Abstract
Although much research has been carried out using recycled aggregates sourced from normal strength concrete, most of the buildings to be demolished are constructed with low strength concrete. Therefore, the properties of the concrete incorporating recycled aggregates, sourced from the waste of structural elements cast with low strength concrete, were investigated in this study. Four different concrete mixtures were designed incorporating natural and recycled aggregates with and without fly ash. The results of the mechanical and durability tests of the concrete mixtures are presented. Additionally, full-scale one-way reinforced concrete slabs were cast, using these concrete mixtures, and subjected to bending test. The feasibility of using conventional reinforced concrete theory for the slabs made with structural concrete incorporating recycled aggregates was investigated.

Key Words
bond; durability; flexural behavior; low strength concrete; recycled aggregate; reinforced concrete; slab; three-point bending test

Address
Caglar Goksu, Ilyas Saribas, Ergun Binbir, Yilmaz Akkaya and Alper Ilki: Istanbul Technical University, Civil Engineering Faculty, Maslak, Istanbul, Turkey

Abstract
The Standard ASCE 61-14 proposes the Substitute Structure Method (SSM) as a Nonlinear Static Procedure (NSP) to estimate nonlinear displacement demands at the center of mass of piers or wharves under seismic actions. To account for bidirectional earthquake excitation according to the Standard, results from independent pushover analyses in each orthogonal direction should be combined using either a 100/30 directional approach or a procedure referred to as the Dynamic Magnification Factor, DMF. The main purpose of this paper is to present an evaluation of these NSPs in relation to four wharf model structures on soil conditions ranging from soft to medium dense clay. Results from nonlinear static analyses were compared against benchmark values of relevant Engineering Design Parameters, EDPs. The latter are defined as the geometric mean demands that are obtained from nonlinear dynamic analyses using a set of 30 two-component ground motion records. It was found that SSM provides close estimates of the benchmark displacement demands at the center of mass of the wharf structures. Furthermore, for the most critical pile connection at a landside corner of the wharf the 100/30 and DMF approaches produced displacement, curvature, and force demands that were reasonably comparable to corresponding benchmark values.

Key Words
pushover analysis; nonlinear static procedure (NSP); ASCE 61 standard; marginal wharves; performance-based design

Address
J. Paul. Smith-Pardo: Department of Civil and Environmental Engineering, Seattle University, Seattle, USA
Juan C. Reyes and Juan D. Sandoval: Department of Civil and Environmental Engineering, Universidad de los Andes, Bogota, Colombia
Wael M. Hassan: Department of Civil Engineering, University of Alaska, Anchorage, USA

Abstract
In this paper, a new efficient method for structural modal reanalysis is proposed, which can handle large finite element (FE) models requiring frequent design modifications. The global FE model is divided into a residual part not to be modified and a target part to be modified. Then, an automated matrix permutation and substructuring algorithm is applied to these parts independently. The reduced model for the residual part is calculated and saved in the initial analysis, and the target part is reduced repeatedly, whenever design modifications occur. Then, the reduced model for the target part is assembled with that of the residual part already saved; thus, the final reduced model corresponding to the new design is obtained easily and rapidly. Here, the formulation of the proposed method is derived in detail, and its computational efficiency and reanalysis ability are demonstrated through several engineering problems, including a topological modification.

Key Words
structural modal reanalysis; finite element method; model reduction; substructuring; eigenvalue problem

Address
Seung-Hwan Boo: Division of Naval Architecture and Ocean Systems Engineering, Korea Maritime and Ocean University,
727 Taejong-ro, Yeongdo-gu, Busan 49112, Republic of Korea



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