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CONTENTS
Volume 79, Number 3, August10 2021
 


Abstract
In this paper, a simple n-order refined theory is developed for free vibration of a simply supported sandwich beam with functionally graded porous layers. The present theory is variationally consistent, uses the n-order polynomial term to represent the displacement field, and does not require a shear correction factor. The variation of shear stress is parabolic across the thickness and the condition at the top and bottom surface are shear stress-free. Equations of motion are derived from Hamilton's principle. In the solution of the governing equations, the Navier procedure is implemented. For porosity effect, four different porosity distributions namely O, X, V, and homogeneous distribution types are modelled; power-law variation of functionally graded face sheets is considered. Results show the effects of varying gradients, thickness to length ratios, effects of the porosity parameters and porosity types on free vibration of functionally graded sandwich beams for simply supported boundary conditions. The results of the present method were validated with existing literature for both hard (ceramic) core material and soft (metal) core material, and good agreement with the benchmarks is seen. The effect of hard-core and soft-core material on natural frequency is found to be contrasting in nature.

Key Words
free vibration; functionally graded materials; nth-order four variable refined theory; porosity; sandwich beam

Address
Lazreg Hadji: Laboratory of Geomatics and Sustainable Development, Ibn Khaldoun University of Tiaret, Algeria; Department of Mechanical Engineering, University of Tiaret, BP 78 Zaaroura, Tiaret,14000, Algeria
Royal Madan: Department of Mechanical Engineering, National Institute of Technology Raipur, India
Shubhankar Bhowmick: Department of Mechanical Engineering, National Institute of Technology Raipur, India
Abdelouahed Tounsi: YFL (Yonsei Frontier Lab), Yonsei University, Seoul, Korea; Department of Civil and Environmental Engineering, King Fahd University of Petroleum & Minerals, 31261 Dhahran, Eastern Province, Saudi Arabia

Abstract
The considerable capacity of friction dampers in energy dissipation makes them a good choice for vibration control of structures. The slip force of friction dampers and also the stiffness of the corresponding bracing system are the major parameters that must be chosen carefully in the design procedure of these dampers. This paper presents an innovative approach to determine these parameters using the data extracted from a series of analyses conducted on three different structures, subjected to five different earthquake records. For this purpose, 900 time-history analyses are conducted. The responses extracted from these analyses are used to compare the effect of different slip forces and to choose the optimum case. Also, a stiffness calibration method is proposed to determine the bracing system stiffness. Finally, two multi-functional optimization methods are introduced to find a single value for optimal slip force. It is shown that between 56 to 74% of the input energy can be dissipated by friction dampers, using this design approach. Additionally, up to 20, 45, 64, and 62% reductions in maximum displacement, velocity, acceleration, and base shear are achieved respectively for the structures studied in this research.

Key Words
energy dissipation; friction damper; optimal slip force; stiffness calibration; stiffness distribution; vibration control

Address
Pourya Sam-Daliri: School of Civil Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
Seyed Mehdi Zahrai: School of Civil Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran; Department of Civil Engineering, University of Ottawa, Ottawa, Canada
Hamid Dahaghin: School of Civil Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

Abstract
In the present research, tensile and flexural mechanical properties of glass epoxy structural composites under simultaneous thermal cycling loading and thermal shocks called hybrid thermal cycling shocks, have been studied. A series of tensile and flexural tests under static and hybrid thermal loading conditions (15 and 30 thermal cycles with -70oC (Degrees Celsius) and +100oC (Degrees Celsius) thermal shocks), were applied on the structural composite specimens and the obtained results are fully compared and investigated. It was found that shocks have a more effective role in changing stiffness in comparison to cycles but the tensile strength of glass/epoxy composites was influenced by the hybrid thermal loads more sensible. For instance, tensile strength was reduced by 12.1% under 30 cycles and thermal shocks. In addition, the flexural bending strength and stiffness were decreased in comparison to static loading conditions. The flexural bending strength and stiffness under hybrid thermal loadings were changed and reduced by 27.64% and 7.2% under 30 cycles under thermal shocks respectively.

Key Words
hybrid thermal loads; mechanical properties; structural composites; thermal cycles; thermal shocks

Address
Abbas Bayat: Department of Mechanical Engineering, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran
Mehrnoosh Damircheli: Department of Mechanical Engineering, Shahr-e-Qods Branch, Islamic Azad University, Tehran, Iran; Department of Mechanical Engineering, Temple University, Philadelphia, Pennsylvania, 19122, USA
Masood Esmkhani: Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, 16846-13114, Tehran, Iran

Abstract
Shear and bending strength of reinforced concrete beams depend on many parameters. It is extremely important to take the necessary precautions in terms of shear in order for the beams to reach their bending capacity. For this reason, it is necessary to determine the effective parameters especially on shear capacity in beams. However, the actual capacity calculation is quite difficult according to regulations that are very conservative in terms of design. Therefore, many experimental studies have been conducted on the shear capacity of the beams. However, this situation is not meaningful in terms of both time and cost, since many experiments will be required to interpret the beam shear behavior, which depends on many parameters. For this reason, the use of advanced software whose verification is performed according to experimental data has become widespread. In this study, a numerical study was carried out on 36 different beam models using the ABAQUS finite element program to examine the effect of the shear span/effective depth (av/d) ratio, stirrup spacing (sw) and the angle of stirrup (a). The results showed that as the av/d increase, the behavior of a shear deficient beam tends to typical bending behavior. Although the effect of stirrup angle on shear capacity is quite high, stirrup angles of 30o and 60o give very similar results. The effect of stirrup spacing is quite limited at relatively high av/d. Stirrups with 90o do not contribute to ductility in beams with high av/d.

Key Words
ABAQUS; numerical study; reinforced concrete beams; shear; shear span/effective depth (av/d)

Address
Yasin Onuralp Özkiliç: Department of Civil Engineering, Faculty of Engineering, Necmettin Erbakan University, Konya 42000, Turkey
Ceyhun Aksoylu, Musa Hakan Arslan: Department of Civil Engineering, Faculty of Engineering and Natural Sciences, Konya Technical University, Konya 42130, Turkey

Abstract
Most of the existing output-only damage detection methods require the number of sensors should be larger than the number of unknown excitation force, and the force location should be available. This paper presents a novel output-only damage detection method without these requirements. The proposed method is based on the correlation function of acceleration responses. When the structure is under white noise excitations (or ambient excitations), the correlation function of acceleration responses can be treated as free vibration responses with unknown initial conditions. The unknown structural parameters and initial conditions can be simultaneously identified by minimizing the difference between the measured and calculated correlation functions. The unknown initial conditions are identified with state space method and the unknown structural parameters are updated with sensitivity method. Numerical studies of a 2D truss and a five-bay 3D frame structure are conducted to demonstrate the accuracy, effectiveness, and robustness of the proposed method. Experimental studies on an eight-floor steel frame are further carried out. Results show that the proposed method is not only insensitive to environmental noise but also applicable when the number of sensors is less than that of unknown excitations. Also, the proposed method can be used for damage detection when the force location is unknown.

Key Words
condition assessment; damage detection; output-only; structural health monitoring; system identification

Address
Pinghe Ni, Xiaojuan Wang and Hongyuan Zhou: Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China

Abstract
The contribution of shear deformations to total deflections in slender concrete beams is often ignored in design practices. The negligence of shear deformations could lead to the underestimation of total deflections in slender concrete beams, which will result in unsafe designs of slender concrete beams at service conditions. The shear deformations could be more critical in concrete beams reinforced with FRP bars than reinforced concrete beams because of the lower elastic modulus of FRP bars as comparing to steel ones. To address this problem, this paper attempts to determine shear deformations of slender concrete beams reinforced with FRP bars based on the variable angle truss model. The shear span of the slender concrete beams is divided into the truss units. The inclinations of struts in these truss units are first calculated by using the principle of virtual work. The shear deformations of the beams are then evaluated by summing all the shear deformations in the truss units. The calculated shear deformations are incorporated into ACI 440.1R-15's deflection approach (2015). Comparisons are made between the analytical and available experimental results of slender concrete beams reinforced with FRP bars with respect to the deflections at 50% and 75% of the failure loads. Better results as comparing to the existing design guidelines in calculating the deflections of the slender concrete beams reinforced with FRP bars are obtained by incorporating the proposed shear deformations.

Key Words
concrete beams; FRP bars; inclination angle; shear deformations; truss

Address
Ngoc C.T. Tran: Department of Civil Engineering, International University, Quarter 6, Linh Trung Ward, Thu Duc City, Ho Chi Minh City, Viet Nam; Vietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc City, Ho Chi Minh City, Viet Nam
Son N. Vu: Department of Structural Mechanics at National University of Civil Engineering, Vietnam

Abstract
The dimensional analysis method is used to study the pounding response of two inelastic MDOF (multi-degree-offreedom) structures under simplified earthquake excitation. The improved Kelvin pounding model is adopted to simulate the force and deformation of the collisions during the contact process. The bilinear interstory resistance model is used to describe the inelastic characteristics of the MDOF structures. The expression of dimensionless pounding force and the equation of dimensionless motion during the collision process are derived. Based on the above theoretical derivation, the accuracy of the improved Kelvin model is verified by comparing the pounding responses in the form of spectra between the improved Kelvin model and Kelvin model. The effects of the pounding on the response of the left structure (with a smaller mass and stiffness) are analyzed in different trend (amplification region, suppression region and unaffected region), and the self-similarity of the pounding response for the two inelastic MDOF structures is revealed. The effects of the story mass ratio, post-yield stiffness ratio, yield displacement and structure spacing on the pounding response are studied. The peak displacement response of the left side structure increases with the increasing of story mass ratio and decreases with the increasing of yield displacement and postyield stiffness ratio. With the increasing of structure spacing, the peak displacement decreases in the first spectrum region, and in the second spectrum region, the peak displacement increases. Moreover, the change of the parameters has little effect on the response of the right structure (with a larger mass and stiffness).

Key Words
dimensional analysis; improved Kelvin pounding model; inelastic MDOF structures; parametric analysis; pounding; self-similarity

Address
Xuyong Chen, Xuehao Xiao, Xixuan Bai: School of Civil Engineering and Architecture, Wuhan Institute of Technology, Wuhan, Hubei, China
Qiaoyun Wu: School of Civil Engineering and Architecture, Wuhan Institute of Technology, Wuhan, Hubei, China; Institute of Engineering Mechanics, China Earthquake Administration, Key Laboratory of Earthquake Engineering and Engineering Vibration of China Earthquake Administration, Harbin, Heilongjiang, China

Abstract
Hygro-thermal buckling of the porous FG nanobeam incorporating the surface effect is investigated. The even distribution of porosities is assumed in this paper. Various porous FG nanobeam models including classical beam theory (CBT), Timoshenko beam theory (TBT), Reddy beam theory (RBT), sinusoidal beam theory (SBT), hyperbolic beam theory (HBT) and exponential beam theory (EBT) are developed in this paper. The nonlocal strain gradient theory with material length scale and nonlocal parameters is adopted to examine the buckling behavior. The governing equations of the porous FG nanobeam are derived from principle of minimum potential energy. In the numerical examples, the effect of the nonlocal parameter, material length scale parameter, the temperature rise, the moisture concentration, surface effect, material gradient index, and porosity volume fraction on the buckling temperature and moisture are analyzed and discussed in detail. The results show that the stiffness of the beam depends on the relation of size between nonlocal parameter and length scale parameter. The paper will be helpful for the design and manufacture of the FG nanobeam under complex environments.

Key Words
buckling; hygro-thermal environment; nonlocal strain gradient theory; porous functionally graded beams; surface effect

Address
Y.S. Li: College of Architecture and Civil Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, PR China; College of Mechanical and Equipment Engineering, Hebei University of Engineering, Handan, 056038, PR China
B.L. Liu: College of Architecture and Civil Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, PR China
J.J. Zhang: College of Mechanical and Equipment Engineering, Hebei University of Engineering, Handan, 056038, PR China

Abstract
This paper presents a shaking table test carried out on a 1:5 reduced-scale five-story masonry-infilled reinforced concrete (RC) frame model. Multi-level simulated earthquake motions with increasing shaking severity were used as input to deform the model structure from an elastic to a near-collapse state. The dynamic characteristics, acceleration response, displacement response, damage state, energy dissipation behavior and stiffness degradation of each story were summarized for each stage. The tests indicate that cracks developed at the masonry-frame interface during minor shaking that caused infill to separate from the frame; however, its in-plane load bearing capacity was maintained. Moreover, the infill was able to resist infrequent earthquakes without causing instability or collapse of the structure. Thus, it is rational to consider masonry infill as a structural element in the seismic design of structures. Moreover, the story drift ratio of 1/400 can be regarded as the performance criterion for controlling frame structure cracking, and the story drift ratio of 1/100 can be regarded as the performance criterion for the peak bearing capacity of a frame structure. The test results could provide a reference not only for the seismic appraisal of existing buildings, but also for the seismic design of new buildings.

Key Words
capacity spectrum; demanded spectrum; existing buildings; seismic performance; shaking table test

Address
Shao-Ge Cheng: China Academy of Building Research, Institute of Earthquake Engineering, 30 East North Third Ring Road, Beijing 100013, China
Yi-Xiu Zhu: College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
Kui Sun: China Academy of Building Research, Institute of Earthquake Engineering, 30 East North Third Ring Road, Beijing 100013, China
Wei-Ping Zhang: College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

Abstract
In this study, new development was debated as a logical method for increasing the road safety under regional classification of AL-Zahir neighbourhood. Owing this, during this experimental study; some measurements have been accomplished and introduced as Project I and II under AASHTO standards. The main purpose of these two levels of projects would be introducing the three parts of stages for each project. Reports of project I and II describe the numerous stages of experimental project. The first stage was done for Project I that included selection of the study area, dividing the study area to four Zones, collection the data required for evaluation the present status of asphalt pavement condition depending on the type, severity, and extent of distress, first strategy; Application of current maintenance requirements. Furthermore, a maintenance strategy after 10 years would be best way for maintenancing after 10 years next to the previous strategy and at last, a maintenance strategy after 20 years direct.

Key Words
AASHTO standards; AL-Zahir neighbourhood; development; measurement; pavement; project; road safety; strategy

Address
Abdulmajeed S. Alsultan: Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, Alkharj, 16273, Saudi Arabia


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