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CONTENTS
Volume 77, Number 1, January10 2021
 


Abstract
The influence of prestress force on the fundamental frequency and static deflection shape of uncracked Prestressed Concrete (PC) beams with a parabolic bonded tendon was examined in this paper. Due to the conflicts among existing theories, the analytical solutions for properly considering the dynamic and static behavior of these members is not straightforward. A series of experiments were conducted for a total period of approximately 2.5 months on a PC beam made with high strength concrete, subsequently and closely to the 28 days of age of concrete. Specifically, the simply supported PC member was short term subjected to free transverse vibration and three-point bending tests during its early-age. Subsequently, the experimental data were compared with a model that describes the dynamic behavior of PC girders as a combination of two substructures interconnected, i.e., a compressed Euler–Bernoulli beam and a tensioned parabolic cable. It was established that the fundamental frequency of uncracked PC beams with a parabolic bonded tendon is sensitive to the variation of the initial elastic modulus of concrete in the early-age curing. Furthermore, the small variation in experimental frequency with time makes doubtful its use in inverse problem identifications. Conversely, the relationship between prestress force and static deflection shape is well described by the magnification factor formula of the “compression-softening” theory by assuming the variation of the chord elastic modulus of concrete with time.

Key Words
concrete beam; deflection shape; elastic modulus; frequency; parabolic bonded tendon; prestress force

Address
Marco Bonopera: Bridge Engineering Division, National Center for Research on Earthquake Engineering, Taiwan
Kuo-Chun Chang: Department of Civil Engineering, National Taiwan University, Taipei, Taiwan
Tzu-Kang Lin: Department of Civil Engineering, National Chiao Tung University, Hsinchu City, Taiwan
Nerio Tullini: Department of Engineering, University of Ferrara, Ferrara, Italy

Abstract
Existing methods to estimate the probability of seismic pounding occurrence of adjacent buildings do not account for nonlinear behavior or only apply to simple lumped mass systems. The present study proposes an efficient method based on subset simulation for fragility and risk assessment of seismic pounding occurrence between nonlinear adjacent buildings neglecting pounding effects with application to finite element models. The proposed method is first applied to adjacent buildings modeled as elastoplastic systems with substantially different dynamic properties for different structural parameters. Seismic pounding fragility and risk of adjacent frame structures with different floor levels is then assessed, paying special attention to modeling the non-linear material behavior in finite element models. Difference in natural periods and impact location are identified to affect the pounding fragility simultaneously. The reliability levels of the minimum code-specified separation distances are also determined. In addition, the incremental dynamic analysis method is extended to assess seismic pounding fragility of the adjacent frame structures, resulting in higher fragility estimates for separation distances larger than the minimum code-specified ones in comparison with the proposed method.

Key Words
Seismic pounding; nonlinear adjacent buildings; fragility assessment; risk assessment; pounding occurrence

Address
School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

Abstract
In this study, a method for free vibration analysis of wall-frame systems built on weak soil is proposed. In the development of the method, the wall-frame system that constitutes the superstructure was modeled as flexural-shear beam. In the study, it is accepted that the soil layers are isotropic, homogeneous and elastic, and the waves are only vertical propagating shear waves. Based on this assumption, the soil layer below is modeled as an equivalent shear beam. Then the differential equation system that represented the behavior of the whole system was written for both regions in a separate way. Natural periods were obtained by solving the differential equations by employing boundary conditions. At the end of the study, two examples were solved and the suitability of the proposed method to the Finite Element Method was evaluated.

Key Words
soil structure interaction; wall frame; continuum model; free vibration; finite element method

Address
Dondu Kara and Erdinc Keskin: Department of Civil Engineering, Engineering Faculty, Kirklareli University,39000, Kirklareli , Turkey
Kanat Burak Bozdogan: Department of Civil Engineering, Canakkale Onsekiz Mart University, 17020, Canakkale, Turkey

Abstract
This paper deals with damage detection using a Gapped Smoothing Method (GSM) combined with deep learning. Convolutional Neural Network (CNN) is a model of deep learning. CNN has an input layer, an output layer, and a number of hidden layers that consist of convolutional layers. The input layer is a tensor with shape (number of images) × (image width) × (image height) × (image depth). An activation function is applied each time to this tensor passing through a hidden layer and the last layer is the fully connected layer. After the fully connected layer, the output layer, which is the final layer, is predicted by CNN. In this paper, a complete machine learning system is introduced. The training data was taken from a Finite Element (FE) model. The input images are the contour plots of curvature gapped smooth damage index. A free-free beam is used as a case study. In the first step, the FE model of the beam was used to generate data. The collected data were then divided into two parts, i.e. 70% for training and 30% for validation. In the second step, the proposed CNN was trained using training data and then validated using available data. Furthermore, a vibration experiment on steel damaged beam in free-free support condition was carried out in the laboratory to test the method. A total number of 15 accelerometers were set up to measure the mode shapes and calculate the curvature gapped smooth of the damaged beam. Two scenarios were introduced with different severities of the damage. The results showed that the trained CNN was successful in detecting the location as well as the severity of the damage in the experimental damaged beam.

Key Words
damage detections; vibration based; gapped smoothing method (GSM); machine learning; deep learning; convolutional neural network; Finite Element Method (FEM)

Address
Duong Huong Nguyen: 1Department of Electrical energy, metals, mechanical constructions and systems,
Faculty of Engineering and Architecture, Ghent University, Belgium
2National University of Civil Engineering, Hanoi, Vietnam
T. Bui-Tien: University of Transport and Communications, Hanoi, Vietnam
Guido De Roeck: KU Leuven, Department of Civil Engineering, Structural Mechanics, B 3001 Leuven, Belgium
Magd Abdel Wahab: 5Division of Computational Mechanics, Ton Duc Thang University, Ho Chi Minh City, Vietnam
6Faculty of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam

Abstract
The thermal eigenvalue responses of the graded sandwich shell structure are evaluated numerically under the variable thermal loadings considering the temperature-dependent properties. The polynomial type rule-based sandwich panel model is derived using higher-order type kinematics considering the shear deformation in the framework of the equivalent single-layer theory. The frequency values are computed through an own home-made computer code (MATLAB environment) prepared using the finite element type higher-order formulation. The sandwich face-sheets and the metal core are discretized via isoparametric quadrilateral Lagrangian element. The model convergence is checked by solving the similar type published numerical examples in the open domain and extended for the comparison of natural frequencies to have the final confirmation of the model accuracy. Also, the influence of each variable structural parameter, i.e. the curvature ratios, core-face thickness ratios, end-support conditions, the power-law indices and sandwich types (symmetrical and unsymmetrical) on the thermal frequencies of FG sandwich curved shell panel model. The solutions are helping to bring out the necessary influence of one or more parameters on the frequencies. The effects of individual and the combined parameters as well as the temperature profiles (uniform, linear and nonlinear) are examined through several numerical examples, which affect the structural strength/stiffness values. The present study may help in designing the future graded structures which are under the influence of the variable temperature loading.

Key Words
FG curved panels; HSDT; thermal vibration; FEM; MATLAB

Address
Brundaban Sahoo, Bamadev Sahoo: Departepartment of Mechanical Engineering, IIIT, Bhubaneswar, 751003, Odisha, India
Kulmani Mehar: Department of Mechanical Engineering, Madanapalle Institute of Technology and Science, 517325, Andhra Pradesh, India
Nitin Sharma: School of Mechanical Engineering, KIIT, Bhubaneswar, 751024, Odisha, India
Subrata Kumar Panda: Department of Mechanical Engineering, NIT, Rourkela, 769008, Odisha, India

Abstract
The present study pertains to the introduction of two new types of grip adaptor for universal testing machines, namely Polyvinyl Chloride (PVC) and Polyoxymethylene (POM) grip adaptors, and their application to tension testing of FRP bars with different fiber and surface finish types. The tabs are connected to the FRP bar sample with the help of mechanical anchors, i.e. bolts. These new adaptors offer vital superiorities over the existing end tab designs (anchors with filling material or mechanical anchorage), including the reduction in the time and labor for production, reusability and the mild nature, i.e. low hardness of the tab material, which retards and even prevents peeling and crushing in the gripping regions of an FRP sample. The methods were successfully applied to FRP bars with different types of fiber (CFRP, GFRP and BFRP) and different types of surface texture (ribbed, wrapped, sand-coated and wound). The test results indicated that the both types of end caps prevented slip of the bar, crushing and peeling in the gripping zone. The mechanical properties from the material tests with the new caps were in perfect agreement with the ones from the material tests with steel tubular caps.

Key Words
grip; axial tension test; end cap; grip adaptor; end tab; FRP bar

Address
Bogachan Basaran: Deparment of Construction, Vocational School of Technical Sciences, Amasya University, Amasya, Turkey
Harun Yaka: Deparment of Mechanical Engineering, Faculty of Technology, Amasya University, Amasya, Turkey
Ilker Kalkan: Department of Civil Engineering, Faculty of Engineering and Architecture, Kirikkale University, Kirikkale, Turkey

Abstract
Rapid construction of prefabricated bridges requires minimizing the field work of precast members and ensuring structural stability and constructability. In this study, we conducted experimental and analytical investigations of transverse joints of prefabricated T-girder bridge superstructures to verify the flexural performance and serviceability. In addition, we conducted parametric studies to identify the joint parameters. The results showed that both the segmented and continuous specimens satisfied the ultimate flexural strength criterion, and the segmented specimen exhibited unified behavior, with the flexural strength corresponding to that of the continuous specimen. The segmented specimens exhibited elastic behavior under service load conditions, and the maximum crack width satisfied the acceptance criteria. The reliability of the finite element model of the joint was verified, and parametric analysis of the convexity of the joint section and the compressive strength of the filler concrete showed that the minimum deflection and crack width occurred at a specific angle. As the strength of the filler concrete increased, the deflection and crack width decreased. However, we confirmed that the reduction in the crack width was hardly observed above a specific strength. Therefore, a design suitable for prefabricated bridges and accelerated construction can be achieved by improving the joint specifications based on the required criteria.

Key Words
prefabricated T girder bridges; transverse joint; flexural performance; serviceability; parametric studies

Address
Seungkyung Kye: Applied Science Research Institute, Korean Advanced Institute for Science and Technology,
291 Daehak ro, Yuseong gu, Daejeon 34141, Republic of Korea
Hyung-Jo Jung: Department of Civil and Environmental Engineering, Korean Advanced Institute for Science and Technology,
291 Daehak ro, Yuseong gu, Daejeon 34141, Republic of Korea
Sun-Kyu Park: School of Civil, Architectural and Environmental Engineering, Sungkyunkwan University,
Seobu ro, Jangan gu, Suwon si, Gyeonggi do, Republic of Korea

Abstract
Ultra high strength concrete (UHSC) originally proposed by Richards and Cheyrezy (1995) composed of cement, silica fume, quartz sand, quartz powder, steel fibers, superplasticizer etc. Later, other ingredients such as fly ash, GGBS, metakaoline, copper slag, fine aggregate of different sizes have been added to original UHSC. In the present investigation, the combined effect of coarse aggregate (6mm – 10mm) and steel fibers (0.50%, 1.0% and 1.5%) has been studied on UHSC mixes to evaluate mechanical and fracture properties. Compressive strength, split tensile strength and modulus of elasticity were determined for the three UHSC mixes. Size dependent fracture energy was evaluated by using RILEM work of fracture and size independent fracture energy was evaluated by using (i) RILEM work of fracture with tail correction to load – deflection plot (ii) boundary effect method. The constitutive relationship between the residual stress carrying capacity (σ) and the corresponding crack opening (w) has been constructed in an inverse manner based on the concept of a non-linear hinge from the load-crack mouth opening plots of notched three-point bend beams. It was found that (i) the size independent fracture energy obtained by using above two approaches yielded similar value and (ii) tensile stress increases with the increase of % of fibers. These two fracture properties will be very much useful for the analysis of cracked concrete structural components.

Key Words
Ultra high strength concrete; coarse aggregate; fracture energy; RILEM work of fracture; Boundary effect method; tensile stress; crack width

Address
B. Karthick: Department of Civil Engineering, CSI College of Engineering, Ketti, The Nilgiris, 643215, Tamilnadu, India
M. P. Muthuraj: Department of Civil Engineering, Coimbatore Institute of Technology, Peelamedu, Coimbatore, 641014, Tamilnadu, India

Abstract
This paper focuses on examining the effects of span arrangements on displacement responses of plan-symmetric RC frame buildings designed using the direct displacement-based design (DDBD) method by employing non-linear analyses and the skew seismic attack. In order to show the desired performance of DDBD design approach, the force-based design approach is also used to examine the seismic performance of the selected structures. To realize this objective, 8-story buildings with different plans are selected. In addition, the dynamic behavior of the structures is evaluated by selecting 3, 7, and 12-story buildings. In order to perform non-linear analyses, OpenSees software is used for modeling buildings. Results of an experimental model are used to validate the analytical model implemented in OpenSees. The results of non-linear static and non-linear dynamic analyses indicate that changing span arrangements does not affect estimating the responses of structures designed using the DDBD approach, and the results are more or less the same. Next, in order to apply the earthquake in non-principle directions, DDBD structures, designed for one-way performance, are designed again for two-way performance. Time history analyses are performed under a set of artificial acceleration pairs, applied to structures at different angles. It is found that the mean maximum responses of earthquakes at all angles have very good agreement with the design-acceptable limits, while the response of buildings along the height direction has a relatively acceptable and uniform distribution. Meanwhile, changes in the span arrangements did not have a significant effect on displacement responses.

Key Words
direct displacement based design; span arrangement; plan symmetric RC frame buildings; non linear analyses; skew seismic attack

Address
Department of Civil Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran

Abstract
In engineering design, the axial equivalent elastic modulus of laminated FRP pipe was mostly calculated by the average elastic modulus method or the classical laminated plate theory method, which are based on relatively simplified assumptions, and may be not accurate enough sometimes. A new analytical calculation method for the axial equivalent elastic modulus of laminated FRP pipe was established based on three-dimensional stress state. By comparing the results calculated by this method with those by the above two traditional analytical methods and the finite element method, it is found that this method for the axial equivalent elastic modulus fits well not only for thin-walled pipes with orthotropic layers, but also for thick-walled pipes with arbitrary layers. Besides, the influence of the layer stacking on the axial equivalent elastic modulus was studied with this method. It is found that a proper content of circumferential layer is beneficial for improving the axial equivalent elastic modulus of the laminated FRP pipe with oblique layers, and then can reduce its material quantity under the premise that its axial stiffness remains unchanged. Finally, the meso-mechanical mechanism of this effect was analyzed. The improving effect of circumferential layer on the axial equivalent elastic modulus of the laminated FRP pipe with oblique layers is mainly because that, the circumferential fibers can restrain the rigid body rotations of the oblique fibers, which tend to cause the significant deformations of the pipe wall units and the relatively low axial equivalent elastic modulus of the pipe.

Key Words
FRP pipe; axial equivalent elastic modulus; analytical method; circumferential layer; oblique layer; laminated

Address
Li Chen, Darong Pan: School of Architecture Engineering, Nanjing Institute of Technology, 1 Hongjing Avenue, Jiangning Science Park, Nanjing 211167, China
Qilin Zhao: School of Mechanical and Power Engineering, Nanjing Tech University, 30 Puzhu South Road, Jiangbei new district, Nanjing 211816, China
Li Chen: Engineering Research Center of Safety and Protection of Explosion & Impact of Ministry of Education, Southeast University,
2 Dongnan University Road, Jiangning District, Nanjing 210089, China
Liang Chen: College of Field Engineering, Army Engineering University of PLA, 1 Haifu lane, Qinhuai District, Nanjing 210007, China
Wei Xu: Anhui Transport Consulting & Design Institute Co., Ltd, 180 Xiangzhang Avenue, High tech Zone, Hefei 230088, China


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