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CONTENTS
Volume 79, Number 4, August25 2021
 


Abstract
In this paper, in various boundary conditions, the vibration behavior of the two types of porous FG truncated conical sandwich shells is investigated based on the improved high order sandwich shells theory. Two types of porosity are considered in the power law rule to model the FGM properties. In the first type, FG face sheets cover a homogeneous core, and in the second one, the FG core is covered by the homogeneous face sheets. All materials are temperature dependent. By utilizing the Hamilton's energy principle, using the nonlinear von Karman strains in the layers and considering the in-plane stresses and thermal stresses in the core and the face sheets, the governing equations are obtained. A Galerkin method is used to solve the equations with clamped-clamped, clamped-free, and free-free boundary conditions. To validate the results, a FEM software is used and some results are validated with the results in the literatures. Also, Some geometrical parameters, temperature variations and porosity effects are studied. By increasing the length to thickness ratio, temperature, the semi-vertex angle and the radius to thickness ratio, the fundamental frequency parameter decreases in all boundary conditions. In both types of sandwiches for both porosity distributions, by increasing the porosity volume fraction, the fundamental frequency parameters increase. Frequency variation of type-II is lower than type-I in the thermal conditions. And the fundamental frequencies of the clamped-clamped (CC) and clamped-free (C-F) boundary conditions have the highest and lowest values, respectively.

Key Words
boundary condition; conical shell; FGM; free vibration; porosity; sandwich structure

Address
Mohsen Rahmani: Department of Mechanics, Tuyserkan Branch, Islamic Azad University, Tuyserkan, Iran
Younes Mohammadi: Faculty of Industrial and Mechanical Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran

Abstract
This study aimed to develop a compact high-efficiency vibration isolator. It was proposed to use force characteristic with quasi-zero stiffness. To avoid a number of design problems, the isolator was designed in a dome shape. This study features a mathematical model of the vibration isolator with quasi-zero stiffness. It allows calculating the isolator properties by geometrical parameters. Stability analysis giving advanced formulas for achieving the maximum workload at certain dimensions was made. For an experimental study, the prototypes were made of shock-absorbing rubbers IRP1346, IRP1347, IRP1348, and fluoroelastomer SKF-32. Force characteristic in static condition was studied, which showed the high efficiency of the compact vibration isolator with quasi-zero stiffness: natural frequency equals 0.8-1.2 Hz. An experimental study in dynamic condition was done using load cell sensors to measure dynamic force transmitted with and without the vibration isolator. The experiment shows a vibration isolation coefficient equal to 244, corresponding to the natural vibration frequency of 2.17 Hz. The study shows the vibration isolator with quasi-zero stiffness as highly efficient, compact, and very perspective for industrial application.

Key Words
base-isolation; control; designing; force characteristic; negative stiffness; quasi-zero stiffness; strain gauge sensors; vibration isolator

Address
Anvar Valeev, Radmir Tashbulatov and Boris Mastobaev: Transportation and Storage of Oil and Gas, Ufa State Petroleum Technological University, Kosmonavtov Street 1, Ufa, 450062, Russia

Abstract
Currently, prominent energy balance concept can efficiently be used to calculate the yield base shear force of structures. Energy balance equation is an additional constraint for the balance of seismic input energy by the energy absorption of the structure. This equation can be defined as the sum of elastic and plastic energies of structural systems using elastic and plastic load-deformation characteristics and the total energy dissipation capacity can finally be equated to the seismic energy demand. The objective of this paper is to present a procedure for the determination of energy-based yield force coefficient of reinforced concrete (RC) frame structures considering P-delta effects. The total of elastic and plastic energies are computed by means of elastic spectral velocity and energy modification factor, which is originally derived for structural systems with geometric nonlinearity. Total inelastic energy of single-degree-of-freedom (SDOF) system is transformed into the total inelastic energy of the multi-degree-of-freedom (MDOF) system using the conversion based on structural dynamics. Plastic energy of MDOF system is formulated after total seismic energy demand is obtained. It is accordingly equated to the total work done by inelastic MDOF structure with P-delta effects and consequently, a dimensionless yield force coefficient is derived. Nonlinear static analyses are performed for selected multi-story RC frames and yield force coefficients are compared with the results of the energy-based formulation. The results show that the proposed formulation taking P-delta effects into account can be effectively used to estimate the yield force coefficient of RC frame structures.

Key Words
energy balance; energy modification factor; nonlinear static analysis; P-delta effects; plastic energy; yield force coefficient

Address
Taner Ucar: Department of Architecture, Dokuz Eylul University, 35390, Buca, Izmir, Turkey
Onur Merter: Department of Civil Engineering, Izmir University of Economics, 35330, Balcova, Izmir, Turkey

Abstract
A new method is presented to study the free vibrations of multi-cracked beams with arbitrary boundary conditions. In the literature, the method based on changes in modal strain energy was used to perform the dynamic analysis of beams with just one crack. In this paper, the changes in modal strain energy are used iteratively to study the dynamic behavior of beams with multiple cracks. The iterative method consists in finding the dynamic frequencies in steps by considering the effects of cracks one by one. First, the beam is assumed intact, for it a single crack is taken into account via the method based on changes in modal strain energy. Then, this procedure is repeated iteratively by taking at steps (i+1), the frequencies obtained at step (i). The end is detected when the total number of cracks is reached. This developed iterative approach is used to analyze the effect of multiple open cracks on the modal parameters of a cantilever beam subjected to free vibration. The results are in good agreement with finite element and experimental methods. This developed method may also be used to generate training data for pattern recognition approaches to health monitoring.

Key Words
multi-cracked beam; sequential method; steel; vibration

Address
Abdellatif Selmi: College of Engineering, Department of Civil Engineering, Prince Sattam bin Abdulaziz University,
Saudi Arabia, Alkharj, 16273, Saudi Arabia; Ecole Nationale d'Ingénieurs de Tunis (ENIT), Civil Engineering Laboratory. B.P. 37, Le belvédère 1002, Tunis, Tunisia

Abstract
To study the axial compression behavior of carbon fiber reinforced plastics (CFRP) confined steel reinforced recycled concrete (CSRRC) columns, 11 specimens of CSRRC columns were manufactured and tested under axial compression loading. The design variables in the experiments included the replacement percentage of recycled coarse aggregate (RCA), layers of CFRP, strength of recycled aggregate concrete (RAC), profile steel ratio and slenderness ratio. Subsequently, the failure process and modes, load-displacement curves, stress-strain curves, transverse deformation coefficient and stiffness degradation of the specimens were obtained and analyzed in detail. The experimental results showed that the profile steel yielded before the steel rebars in the columns, then the RAC was crushed, and finally the CFRP broke under axial compression loading. The axial bearing capacity of CSRRC columns decreased with the increase of replacement percentage of RCA and slenderness ratio, respectively. However, the CFRP can give full play to its high-strength confinement performance and effectively improve the axial bearing capacity and deformability of columns. Moreover, the profile steel ratio and strength of RAC have significant effects on the initial stiffness of CSRRC columns, and the stiffness degradation rate of columns decreases with the increase of these parameters. Overall, the CSRRC columns exhibit high axial bearing capacity and good ductility deformation ability. Based on ACI 440.2R-08, the modified formula on the nominal axial bearing capacity of CSRRC columns was proposed in this study. The accuracy on the modified formulae was evaluated by the comparison between the calculated values and test values.

Key Words
axial bearing capacity; axial compression columns; CFRP confined recycled concrete; mechanical behavior; steel reinforced recycled concrete

Address
Hui Ma: School of Civil Engineering and Architecture, Xi'an University of Technology, Xi'an, China; State Key Laboratory of Eco-hydraulics in Northwest Arid Region, Xi'an University of Technology, Xi'an, China
Yanan Wu, Cheng Huang: School of Civil Engineering and Architecture, Xi'an University of Technology, Xi'an, China
Yanli Zhao: Research and Design Institute of Water Conservancy and Hydropower, Xi'an University of Technology, Xi'an, China

Abstract
Recently an implicit time integration method based-on Cubic B-spline has been presented for solving the problems in structural dynamics. This method benefits from high accuracy and desire stability. In this paper, the presented method is developed for analyzing the wave propagation problems and the results are compared to the methods in the literature including a series of Bathe family methods (Noh-Bathe, Standard Bathe p1/B2-Bathe and pq-Bathe methods) and Newmark trapezoidal rule method. A numerical dispersion analysis is presented to evaluate the method in one and two wave propagation problems. Results indicate that the method under study has adequate performance in wave propagation as good as structural dynamic problems.

Key Words
CFL number; cubic B-spline; numerical dispersion; time integration; wave propagation

Address
S. Rostami: Department of Civil Engineering, Technical and Vocational University (TVU), Tehran, Iran
B. Hooshmand, S. Shojaee and S. Hamzehei-Javaran: Department of Civil Engineering, Shahid Bahonar University of Kerman, Kerman, Iran

Abstract
The main aim of this study is to propose an effective approach to conduct the stability analysis of rock slopes. Based on the strength reduction technology, the recent developed adaptive finite element limit analysis is adopted to investigate the stability of rock slopes for the first time. The rock mass strength is described by the latest generalized Hoek-Brown failure criterion. To better estimate the rock slope stability, the rock mass disturbance D and slope angle B are also incorporated into the determination of factor of safety by implementing the disturbance weighting factor s and the slope angle weighting factor fB, respectively. The factors of safety obtained from this study are validated by those from existing works based on two real slope cases. For practical use, the factors of safety for the cases of B=45o in undisturbed rock masses are provided in the form of design charts. Finally, a set of design charts for s and a fitting formula for fB are presented and discussed.

Key Words
adaptive finite element limit analysis; factor of safety; generalized Hoek-Brown failure criterion; stability of rock slopes; strength reduction technology

Address
Guang-hui Chen, Jin-feng Zou: School of Civil Engineering, Central South University, No.22, Shaoshan South Road, Changsha, Hunan Province, People's Republic of China
Rui Zhang: School of Civil Engineering, Changsha University, No.98, Hongshan Road, Changsha, Hunan Province,
People's Republic of China

Abstract
The moment-rotation (M-o) response of steel pallet rack (SPR) beam-to-column connections (BCCs) is naturally complex and predicted through repeated experimental investigations motivated by the variety in the geometry of commercially available beam end connectors (BECs). Past literature has shown that even finite element modeling (FEM) was somehow unable to fully capture the structural behavior of SPR BCCs in the plastic region. This study proposes an innovative Support vector machine (SVM)-Discrete Wavelet transform (DWT)-based optimized model for M-O analysis of SPR BCCs. A data set of total thirty-two experiments on SPR BCCs was used to develop the model. The experimental investigations identified the most influential parameters affecting the M-O response of SPR BCCs which are column thickness, beam depth, and depth of the BEC. Those parameters were optimized using Firefly algorithm and selected as input parameters. Reliability assessment of proposed predictive model was performed using root-mean-square error (RMSE), Pearson coefficient (r), and coefficient of determination 'r-square' (R2). The findings of predictive model were juxtaposed with experimental outcomes and FEM results, available in the literature, and a close agreement was achieved. The R2 value of 0.958 and 0.984 were achieved for moment and rotation predictions, respectively. Hence, the proposed SVM-DWT model can be efficiently used to forecast the optimum and reliable M-O response of SPR BCCs and to minimize the need of repetitive testing.

Key Words
connections; discrete wavelet transform; moment-rotation; steel pallet racks; support vector machine

Address
S.N.R. Shah: Civil Engineering Department, Mehran University of Engineering & Technology, SZAB, Khairpur Mir's -66020, Pakistan
Syed Fawad Hussain: Faculty of Computer Science and Engineering, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, Pakistan

Abstract
Post-tensioning in flat plate slabs provides the advantage of deflection and crack control under service loads, and can reduce slab thickness. However, due to the reduced slab thickness, post-tensioned flat plate slabs may be susceptible to punching shear failure under ultimate load. This study performed experimental research of the punching shear resistance of posttensioned flat plate slabs with unbonded tendons. A total of four 3,100 mm square specimens of 150 mm thick slab were fabricated with the variables of average compressive stress in concrete, tendon layout, and aspect ratio of the column stub. Based on the test results, the load-deflection relationship, patterns of cracks, strains of flexural reinforcement, and force increments in tendons were compared. In addition, the equations for punching shear strength of unbonded post-tensioned flat plate slabs specified in the design codes (ACI 318, Eurocode 2, Model Code) were evaluated in detail.

Key Words
flat plate; post-tensioned slab; punching shear; unbonded tendons

Address
Seung-Ho Choi: Department of Architectural Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-gu, Seoul, 02504, Korea
Jin-Ha Hwang: Jeonnam and Jeju Branch, Korea Conformity Laboratories, 137 Yeosusandan-ro, Yeosu-si, Jeollanam-do 59631, Korea
Sun-Jin Han, Hyo-Eun Joo, Jae-Hyun Kim and Kang Su Kim: Department of Architectural Engineering, University of Seoul, 163 Siripdaero, Dongdaemun-gu, Seoul, 02504, Korea

Abstract
In this study, a new framework of seismic resilience estimation for urban water transmission networks was developed. The proposed resilience estimation model consists of three phases: input earthquake generation, hydraulic analysis, and recovery of network facilities. In the earthquake generation phase, the uncertainty of the ground motion is determined using the spatially correlated seismic attenuation law. In the hydraulic analysis phase, a hydraulic simulation is performed in conjunction with EPANET analysis. In the recovery phase, network components are restored, and the performance of the recovered network is evaluated through hydraulic analysis. Then, the seismic resilience curve and recovery costs are calculated. For a numerical simulation, a MATLAB-based computer code was developed for pressure-driven analysis in EPANET simulation. To demonstrate the proposed model, an actual water transmission network in South Korea was reconstructed based on geographic information system data. The performance of the network system was evaluated according to two performance indices: system and nodal serviceability. Finally, the cost of repairing the network facilities and water loss are estimated according to earthquake magnitude and interdependency. Numerical results show that the recovery slope of the resilience curve tends to decrease as the earthquake magnitude and interdependency with the power facilities increase.

Key Words
critical lifeline systems; flow-based analysis; network interdependency; recovery strategy; seismic reliability analysis; seismic resilience

Address
Sungsik Yoon: Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Young-Joo Lee: Department of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea
Hyung-Jo Jung: Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea


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