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CONTENTS
Volume 71, Number 4, August25 2019
 

Abstract
Assessment of failure probability, especially for a complex structure, requires a considerable number of calls to the numerical model. Reliability methods have been developed to decrease the computational time. In this approach, the original numerical model is replaced by a surrogate model which is usually explicit and much faster to evaluate. The current paper proposed an efficient reliability method based on Monte Carlo simulation (MCS) and multi-gene genetic programming (MGGP) as a robust variant of genetic programming (GP). GP has been applied in different fields; however, its application to structural reliability has not been tested. The current study investigated the performance of MGGP as a surrogate model in structural reliability problems and compares it with other surrogate models. An adaptive Metropolis algorithm is utilized to obtain the training data with which to build the MGGP model. The failure probability is estimated by combining MCS and MGGP. The efficiency and accuracy of the proposed method were investigated with the help of five numerical examples.

Key Words
reliability; genetic programming; surrogate model; metropolis algorithm; failure probability; Monte Carlo

Address
Alireza Garakaninezhad: Iranian Academic Center for Education, Culture and Research, Kerman 7616914111, Iran
Morteza Bastami: International Institute of Earthquake Engineering and Seismology (IIEES), No. 26, Arghavan St., North Dibajee, Farmanieh,
P.O. Box: 19395/3913, Tehran, Iran

Abstract
Exterior beam-column joint is typically the weakest link in a limited-ductile reinforced concrete (RC) frame structure. The use of diagonal haunch element has been considered as a desirable seismic retrofit option for reducing the seismic demand at the joint. Previous research globally has focused on implementing double haunches, while the use of single haunch element as a less-invasive and more architecturally favorable retrofit option has not been investigated. In this paper, the key formulations and a design procedure for the single haunch system for retrofitting RC exterior beam-column joint are developed. An application of the proposed design procedure is then illustrated through a case study.

Key Words
limited-ductile; RC frame; exterior beam-column joint; seismic retrofit; single diagonal haunch

Address
Centre for Sustainable Infrastructure, Swinburne University of Technology, Melbourne, Australia

Abstract
Present article deals with the static stability analysis of compositionally graded nanocomposite beams reinforced with graphene oxide powder (GOP) is undertaken once the beam is subjected to an induced force caused by nonuniform magnetic field. The homogenized material properties of the constituent material are approximated through Halpin-Tsai micromechanical scheme. Three distribution types of GOPs are considered, namely uniform, X and O. Also, a higher-order refined beam model is incorporated with the dynamic form of the virtual work\'s principle to derive the partial differential motion equations of the problem. The governing equations are solved via Galerkin\'s method. The introduced mathematical model is numerically validated presenting a comparison between the results of present work with responses obtained from previous articles. New results for the buckling load of GOP reinforced nanocomposites are presented regarding for different values of magnetic field intensity. Besides, other investigations are performed to show the impacts of other variants, such as slenderness ratio, boundary condition, distribution type and so on, on the critical stability limit of beams made from nanocomposites.

Key Words
buckling; graphene oxide powder; refined higher-order beam theory; non-uniform magnetic field

Address
Farzad Ebrahimi, Mostafa Nouraei: Department of Mechanical Engineering, Faculty of Engineering, Imam Khomeini International University, Qazvin
Ali Dabbagh: School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran
Ömer Civalek: Akdeniz University, Engineering Faculty, Civil Engineering Dept., Division of Mechanics, 07058 Antalya, Turkey

Abstract
A renewable energy storage pile foundation system is being developed through a multi-disciplinary research project. This system intends to use reinforced concrete pile foundations configured with hollowed sections to store renewable energy generated from solar panels attached to building structures in the form of compressed air. However previous research indicates that the compressed air will generate considerable high circumferential tensile stresses in the concrete pile, which requires unrealistic high hoop reinforcement ratio to avoid leakage of the compressed air. One possible solution is to utilize fiber reinforced concrete instead of placing the hoop reinforcement to resist the tensile stress. This paper investigates nonlinear structural responses and post-cracking behavior of the fiber reinforced concrete pile subjected to high air pressure through nonlinear finite element simulations. Concrete damage plasticity models were used in the simulation. Several parameters were considered in the study including concrete grade, fiber content, and thickness of the pile section. The air pressures which the pile can resist at different crack depths along the pile section were identified. Design recommendations were provided for the energy storage pile foundation using the fiber reinforced concrete.

Key Words
fiber reinforced concrete, pile foundation, nonlinear structural responses, compressed air, renewable energy storage

Address
Department of Civil and Environmental Engineering, Nazarbayev University, 53 Kabanbay Batyr Ave, Nur-Sultan, Republic of Kazakhstan

Abstract
A method for estimating the floor response spectra (FRS) of elastic structures under earthquake excitations is proposed. The method is established based on a previously proposed direct estimation method for single degree of freedom systems, which generally overestimates the FRS of a structure, particularly in the resonance period range. A modification factor is introduced to modify the original method; the modification factor is expressed as a function of the period ratio and is determined through regression analysis on time history analysis results. Both real and artificial ground motions are considered in the analysis, and it is found that the modification factors obtained from the real and artificial ground motions are significantly different. This suggests that the effect of ground motion should be considered in the estimation of FRS. The modified FRS estimation method is further applied to a 10-story building structure, and it is verified that the proposed method can lead to a good estimation of FRS of multi-story buildings.

Key Words
floor response spectra; modal superposition; nonstructural component; artificial ground motion; higher mode

Address
School of Civil Engineering and Architecture, Wuhan University of Technology,
Luoshi Road 122, Wuhan, 430070 China

Abstract
Compared to the ambient vibration test mainly identifying the structural modal parameters, such as frequency, damping and mode shapes, the impact testing, which benefits from measuring both impacting forces and structural responses, has the merit to identify not only the structural modal parameters but also more detailed structural parameters, in particular flexibility. However, in traditional impact tests, an impacting hammer or artificial excitation device is employed, which restricts the efficiency of tests on various bridge structures. To resolve this problem, we propose a new method whereby a moving vehicle is taken as a continuous exciter and develop a corresponding flexibility identification theory, in which the continuous wheel forces induced by the moving vehicle is considered as structural input and the acceleration response of the bridge as the output, thus a structural flexibility matrix can be identified and then structural deflections of the bridge under arbitrary static loads can be predicted. The proposed method is more convenient, time-saving and cost-effective compared with traditional impact tests. However, because the proposed test produces a spatially continuous force while classical impact forces are spatially discrete, a new flexibility identification theory is required, and a novel structural identification method involving with equivalent load distribution, the enhanced Frequency Response Function (eFRFs) construction and modal scaling factor identification is proposed to make use of the continuous excitation force to identify the basic modal parameters as well as the structural flexibility. Laboratory and numerical examples are given, which validate the effectiveness of the proposed method. Furthermore, parametric analysis including road roughness, vehicle speed, vehicle weight, vehicle\'s stiffness and damping are conducted and the results obtained demonstrate that the developed method has strong robustness except that the relative error increases with the increase of measurement noise.

Key Words
dynamic test; vehicle-bridge-interaction; continuous excitation; modal scaling factor; flexibility identification

Address
1 Jiangsu Key Laboratory of Engineering Mechanics, Southeast University, Nanjing 210096, China
2 School of Civil Engineering, Southeast University, Nanjing 210096, China

Abstract
Novel steel fibre reinforced ultra-lightweight cement composite (ULCC) with compressive strength of 87.3MPa and density of 1649kg/m3 was developed for the flat slabs in civil buildings. This paper investigated structural behaviours of ULCC flat slabs according to a 4-specimen test program under concentrated loading and some reported test results. The investigated governing parameters on the structural behaviours of the ULCC slabs include volume fraction of the steel fibre and the patch loading area. The test results revealed that ULCC flat slabs with and without flexure reinforcement failed in different failure mode, and an increase in volume fraction of the steel fibre and loading area led to an increase in flexural resistance for the ULCC slabs without flexural reinforcement. Based on the experiment results, the analytical models were developed and also validated. The validations showed that the analytical models developed in this paper could predict the ultimate strength of the ULCC flat slabs with and without flexure reinforcement reasonably well.

Key Words
ULCC; flexural resistance; fibre reinforced concrete; flat slab; analytical model

Address
Jun-Yan Wang, Xiao-Long Gao: Key Laboratory of Advanced Civil Engineering Materials, Tongji University, Ministry of Education, Shanghai 201804, P.R. China
Jia-Bao Yan: School of Civil Engineering, Tianjin University, Tianjin 300350, P.R. China

Abstract
Seismic performance analysis of steel-brace reinforced concrete (RC) frame using topology optimization in highly seismic region was discussed in this research. Topology optimization based on truss-like material model was used, which was to minimum volume in full-stress method. Optimized bracing systems of low-rise, mid-rise and high-rise RC frames were established, and optimized bracing systems of substructure were also gained under different constraint conditions. Thereafter, different structure models based on optimized bracing systems were proposed and applied. Last, structural strength, structural stiffness, structural ductility, collapse resistant capacity, collapse probability and demolition probability were studied. Moreover, the brace buckling was discussed. The results show that bracing system of RC frame could be derived using topology optimization, and bracing system based on truss-like model could help to resolve numerical instabilities. Bracing system of topology optimization was more effective to enhance structural stiffness and strength, especially in mid-rise and high-rise frames. Moreover, bracing system of topology optimization contributes to increase collapse resistant capacity, as well as reduces collapse probability and accumulated demolition probability. However, brace buckling might weaken beneficial effects.

Key Words
topology optimization; collapse resistant capacity; steel brace; seismic performance assessment; incremental dynamic analysis; residual deformation

Address
Shengfang Qiao, Huqing Liang, Mengxiong Tang, Wanying Wang and Hesong Hu
Shengfang Qiao, Mengxiong Tang and Hesong Hu: Guangzhou Institute of Building Science Co., Ltd., Baiyun, Guangzhou 510440, China
Shengfang Qiao: School of Civil Engineering and Transportation, South China University of Technology, Tianhe, Guangzhou 510641, China
Huqing Liang: Guangzhou Municipal Construction Group Co., Ltd., Yuexiu, Guangzhou 510030, China
Wanying Wang: School of Civil Engineering and Transportation, Guangdong University of Technology,Panyu, Guangzhou, 510006, China

Abstract
Shear studs are often used to connect steel girders and concrete deck to form a composite bridge system. The application of precast concrete deck to steel-concrete composite bridges can improve the strength of decks and reduce the shrinkage and creep effect on the long-term behavior of structures. How to ensure the connection between steel girders and concrete deck directly influences the composite behavior between steel girder and precast concrete deck as well as the behavior of the structure system. Compared with traditional multi-I girder systems, a twin-I girder composite bridge system is more simplified but may lead to additional requirements on the shear studs connecting steel girders and decks due to the larger girder spacing. Up to date, only very limited quantity of researches has been conducted regarding the behavior of shear studs on twin-I girder bridge systems. One convenient way for steel composite bridge system is to cast concrete deck in place with shear studs uniformly-distributed along the span direction. For steel composite bridge system using precast concrete deck, voids are included in the precast concrete deck segments, and they are casted with cast-in-place concrete after the concrete segments are erected. In this paper, several sets of push-out tests are conducted, which are used to investigate the heavier of shear studs within the voids in the precast concrete deck. The test data are analyzed and compared with those from finite element models. A simplified shear stud model is proposed using a beam element instead of solid elements. It is used in the finite element model analyses of the twin-I girder composite bridge system to relieve the computational efforts of the shear studs. Additionally, a parametric study is developed to find the effects of void size, void spacing, and shear stud diameter and spacing. Finally, the recommendations are given for the design of precast deck using void for twin I-girder bridge systems.

Key Words
shear stud; twin-I girder; finite element; load-slip; void

Address
Ye Xia, Limu Chen, Haiying Ma: Department of Bridge Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
Dan Su: Embry Riddle Aeronautical University, Daytona Beach, FL, USA

Abstract
This study mainly aims to assess the performance of soil-structure systems designed by direct displacement-based method coupled with strong column-weak beam design concept through various system identification techniques under strong ground motions. To this end, various system identification methods are employed to evaluate the dynamic characteristics of a structure (i.e., modal frequency, system damping, mode shapes, and plastic hinge formation pattern) under a strong seismic excitation considering soil-structure interaction for different site conditions as specified by ASCE 7-10. The scope of the study narrowed down to the code-complying low- to high-rise steel moment resisting frames with various heights (4, 8, 12, 16-story). The comparison of the result of soil-structure systems with fix-based support condition indicates that the modal frequencies of these systems are highly influenced by the structure heights, specifically for the softer soils. This trend is more significant for higher modes of the system which can considerably dominate the response of structures in which the higher modes have more contribution in dynamic response. Amongst all studied modes of the vibration, the damping ratio estimated for the first mode is relatively the closet to the initial assumed damping ratios. Moreover, it was found that fewer plastic hinges are developed in the structure of soil-structure systems with a softer soil which contradicts the general expectation of higher damageability of such structural systems.

Key Words
direct displacement design; dynamic structural identification; soil-structure system; steel moment resisting structures

Address
Vahidreza Mahmoudabadi: Glenn Department of Civil Engineering, Clemson University, Clemson SC 29634, USA
Omid Bahar: International Institute of Earthquake Engineering and Seismology (IEES), Tehran, Iran
Mohammad Kazem Jafari: International Institute of Earthquake Engineering and Seismology (IEES), Tehran, Iran
Amir Safiey: Glenn Department of Civil Engineering, Clemson University, Clemson SC 29634, USA


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