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CONTENTS
Volume 39, Number 4, May25 2021
 


Abstract
Cored moment resisting stub column (CMSC) was previously developed by the features of adopting a core segment which remains mostly elastic and reduced column section (RCS) details around the ends to from a stable hysteretic behavior with large post-yield stiffness and considerable ductility. Several full-scale CMSC components with various length proportions of the RCSs with respect to overall lengths have been experimentally investigated through both far-field and near-fault cyclic loadings followed by fatigue tests. Test results verified that the proposed CMSC provided very ductile hysteretic responses with no strength degradation even beyond the occurrence of the local buckling at the side-segments. The effect of RCS lengths on the seismic performance of the CMSC was verified to relate with the levels of the deformation concentration at the member ends, the local buckling behavior and overall ductility. Estimation equations were established to notionally calculate the first-yield and ultimate strengths of the CMSC and validated by the measured responses. A numerical model of the CMSC was developed to accurately capture the hysteretic performance of the specimens, and was adopted to clarify the effect of the surrounding frame and to perform a parametric study to develop the estimation of the elastic stiffness.

Key Words
buildings; energy dissipation; high-strength steel; moment/moment resistance; near-fault ground motion; steel/steel structure

Address
Po-Chien Hsiao: Department of Civil and Construction Engineering, National Taiwan University of Science and Technology,
43 Keelung Rd., Sec.4, Taipei City 106, Taiwan
Kun-Sian Lin: Department of Civil Engineering, National Chung Hsing University,
145 Xingda Rd., South Dist., Taichung City 402, Taiwan


Abstract
Reinforced concrete (RC) columns can be strengthened by direct fastening of steel plates around a column, forming composite actions. This method can increase both the total load bearing area and the concrete confinement stress. To predict the axial load resistance of strengthened RC columns, the equivalent passive confinement stress of the stirrups and the steel jacket should be accurately quantified, which requires the stress in the stirrups and shear force in the connections to be first obtained. In this paper, parameters, i.e., the stress ratio of the stirrups and shear force ratio of steel plate connectors are utilized to quantify the stress of the stirrups and shear force in the connections. A mechanical model for determining the stress ratio of the stirrups and shear force ratio of steel plate connectors is proposed and validated using the experimental results in a previous study. The model is found to be robust. Subsequently, a parametric study is conducted and the optimum stress ratios of the stirrups and the optimum shear force ratios of connectors are proposed for engineering designs.

Key Words
strengthened RC column; steel jacketing; direct fastening; passive confinement; stress ratio; shear force ratio

Address
Z.W. Shan: Key Laboratory of Concrete and Prestressed Concrete Structures of the Ministry of Education,
Southeast University, Nanjing 210096, China
D.T.W. Looi: School of Engineering, Swinburne University of Technology, Sarawak Campus, Sarawak, Malaysia
R.K.L. Su: Department of Civil Engineering, The University of Hong Kong, Hong Kong, China


Abstract
In the study, three 1:3-scale unequal span steel–concrete composite substructures with top-seat angle and double web angle connection were designed and identified as specimens GTSDWA-0.6, GTSDWA-1.0, and GTSDWA-1.4. Pseudo-static tests and refined numerical model analysis were conducted to examine the anti-progressive collapse performance of a semi-rigid steel–concrete composite substructure. The results indicated that the failure modes of the three specimens revealed that the fracture occurred in the root of the long leg of the top/seat angle in tension at the connection. With increases in the span ratio of the left and right composite beams, the bearing capacities of the composite substructures decreased, and the corresponding displacement increased. With respect to GTSDWA-0.6 and GTSDWA-1.4, the resistance due to the short composite beam corresponded to 62% and 60%, respectively, and the total resistance provided by the short composite beam exceeded that of the long composite beam. With respect to GTSDWA-1.0, the resistance due to the left and right composite beams was similar. All three specimens underwent the flexure mechanism and flexure–axial mixed mechanism stages. They resisted the external load mainly via the flexure mechanism. Moreover, the addition of stiffeners on both sides of the top and seat angles is advantageous in terms of improving the collapse resistance and ductility of unequal span composite substructures.

Key Words
steel–concrete composite substructure; semi-rigid connection; progressive collapse; performance analysis; unequal span

Address
Bao Meng: School of Civil Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China;
Key Lab of Structural Engineering and Earthquake Resistance, Ministry of Education (XAUAT), Xi'an 710055, China
Liangde Li, Weihui Zhong, Zheng Tan and Yuhui Zheng: School of Civil Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China

Abstract
This study experimentally reveals the influence of welding on grade S690Q high strength steel (HSS) butt joints from both micro and macro levels. Total eight butt joints, taking plate thickness and welding heat input as principal factors, were welded by shielded metal arc welding. In micro level, the microstructure transformations of the coarse grain heat affected zone (CGHAZ), the fine grain heat affected zone (FGHAZ) and the tempering zone occurred during welding were observed under light optical microscopy, and the corresponding mechanical performance of those areas were explored by micro-hardness tests. In macro level, standard tensile tests were conducted to investigate the impacts of welding on tensile behaviour of S690Q HSS butt joints. The test results showed that the main microstructure of S690Q HSS before welding was tempered martensite. After welding, the original microstructure was transformed to granular bainite in the CGHAZ, and to ferrite and cementite in the FGHAZ. For the tempering zone, some temper martensite decomposed to ferrite. The performed micro-hardness tests revealed that an obvious "soft layer" occurred in HAZ, and the HAZ size increased as the heat input increased. However, under the same level of heat input, the HAZ size decreased as the plate thickness increased. Subsequent coupon tensile tests found that all joints eventually failed within the HAZ with reduced tensile strength when compared with the base material. Similar to the size of the HAZ, the reduction of tensile strength increased as the welding heat input increased but decreased as the thickness of the plate increased.

Key Words
welded high strength steel; heat-affected zone; light optical microscopy; micro-hardness test; microstructure; tensile strength

Address
Cheng Chen: School of Civil Engineering and Geomatics, Southwest Petroleum University, Xindu Street No.8, China
Sing Ping Chiew and Mingshan Zhao: Singapore Institute of Technology, 10 Dover Drive, Singapore
Chi King Lee: School of Engineering and Information Technology, University of New South Wales Canberra, Canberra 2600, Australia
Tat Ching Fung: School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Ave, Block N1, Singapore

Abstract
In composite structures, shear connectors are crucial components to resist the relative slip between the steel and concrete, and thereby to achieve the composite actions. In the service stage, composite structures are usually in elastic state, so the elastic stiffness of the shear connection is a quite important parameter in the structural analysis of composite structures. Nevertheless, the existing studies mainly focus on the load-slip relationship rather than the tangent stiffness at the initial elastic stage. Furthermore, when composite beams subjected to torque or local load, shear connections are affected by both tensile force and shear force. However, the stiffness of shear connections under combined effects appears not to have been discussed hitherto. This paper investigates the initial elastic stiffness of stud connections under combined effects of biaxial forces. The initial expression and the relevant parameters are obtained by establishing a simplified analytical model of the stud connection. Afterwards, parametric finite element analysis is performed to investigate the effects of the relevant factors, including the stud length, stud diameter, elastic modulus of concrete, elastic modulus of steel and volume ratio of reinforcement. The feasibility of the proposed modelling has been proved by comparing with sufficient experimental tests. Based on the analytical analysis and the extensive numerical simulations, design equations for predicting the initial elastic stiffness of stud connections are proposed. The comparison between the equations and the data of finite element models demonstrates that the equations are accurate enough to serve for engineering communities.

Key Words
composite beam; stud connection; elastic stiffness; combined load; FEA; design equation

Address
Xi Qin and Guotao Yang: School of Civil Engineering, Qingdao University of Technology, Qingdao 266033, China

Abstract
Shear lag effect was a significant mechanical behavior of steel-concrete composite beams, and the effective flange width was needed to consider this effect. However, the effective flange width is mostly determined by static load test. The cyclic vehicle loading cases, which is more practical, was not well considered. This paper focuses on the study of shear lag effect of the concrete slab in the negative moment region under fatigue cyclic load. Two specimens of two-span steel-concrete composite beams were tested under fatigue load and static load respectively to compare the differences in the negative moment region. The reinforcement strain in the negative moment region was measured and the stress was also analyzed under different loads. Based on the OpenSees framework, finite element analysis model of steel-concrete composite beam is established, which is used to simulate transverse reinforcement stress distribution as well as the variation trends under fatigue cycles. With the established model, effects of fatigue stress amplitude, flange width to span ratio, concrete slab thickness and shear connector stiffness on the shear lag effect of concrete slab in negative moment area are analyzed, and the effective flange width ratio of concrete slab under different working conditions is calculated. The simulated results of effective flange width are compared with calculated results of the commonly used specifications, and it is found that the methods in the specifications can better estimate the shear lag effect in concrete slab under static load, but the effective flange width in the negative moment zone under fatigue load has a large deviation.

Key Words
steel-concrete composite beam; fatigue; shear lag; negative moment region; effective flange width

Address
Jinquan Zhang: Research Institute of Highway Ministry of Transport, Beijing, China
Bing Han, Huibing Xie, Wutong Yan and Jiaping Yu: School of Civil Engineering, Beijing Jiaotong University, Beijing, China
Wangwang Li: China Academy of Railway Sciences Corporation Limited, Beijing, China

Abstract
The cracking at the transverse diaphragm cutout is one of the most severe fatigue failures threatening orthotropic steel decks (OSDs), whose mechanisms and crack treatment techniques have not been fully studied. In this paper, full-scale experiments were first performed to investigate the fatigue performance of polished cutouts involving the effect of an artificial geometrical defect. Following this, comparative experimental testing for defective cutouts strengthened with carbon fiber-reinforced polymer (CFRP) was carried out. Numerical finite element analysis was also performed to verify and explain the experimental observations. Results show that the combinative effect of the wheel load and thermal residual stress constitutes the external driving force for the fatigue cracking of the cutout. Initial geometrical defects are confirmed as a critical factor affecting the fatigue cracking. The principal stress 6 mm away from the free edge of the cutout can be adopted as the nominal stress of the cutout during fatigue evaluation, and the fatigue resistance of polished cutouts is higher than Grade A in AASHTO specification. The bonded CFRP system is highly effective in extending the fatigue life of the defective cutouts. The present study provides some new insights into the fatigue evaluation and repair of OSDs.

Key Words
fatigue; orthotropic steel deck; carbon fiber reinforced polymer (CFRP); strengthening; cracking

Address
Lu Ke: Key Laboratory of Disaster Prevention and Structural Safety of China Ministry of Education, School of Civil Engineering and Architecture, Guangxi University, Nanning 530004, China
Chuanxi Li and Jun He: School of Civil Engineering, Changsha University of Science and Technology, Changsha 410114, China
Yongjun Lu: Department of Engineering Mechanics, Northwestern Polytechnical University, Xi'an 710129, China
Yang Jiao and Yongming Liu: School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe AZ 85287, USA


Abstract
In this study, we estimate the ultimate load of rectangular concrete-filled steel tubes (CFST) by developing a novel hybrid predictive model (ANN-BCMO) which is a combination of balancing composite motion optimization (BCMO) - a very new optimization technique and artificial neural network (ANN). For this aim, an experimental database consisting of 422 datasets is used for the development and validation of the ANN-BCMO model. Variables in the database are related with the geometrical characteristics of the structural members, and the mechanical properties of the constituent materials (steel and concrete). Validation of the hybrid ANN-BCMO model is carried out by applying standard statistical criteria such as root mean square error (RMSE), coefficient of determination (R2), and mean absolute error (MAE). In addition, the selection of appropriate values for parameters of the hybrid ANN-BCMO is conducted and its robustness is evaluated and compared with the conventional ANN techniques. The results reveal that the new hybrid ANN-BCMO model is a promising tool for prediction of the ultimate load of rectangular CFST, and prove the effective role of BCMO as a powerful algorithm in optimizing and improving the capability of the ANN predictor.

Key Words
CFST column; artificial neural network; ultimate axial load; balancing composite motion optimization

Address
Panagiotis G. Asteris and Minas E. Lemonis: Computational Mechanics Laboratory, School of Pedagogical and Technological Education, 14121 Athens, Greece
Thuy-Anh Nguyen and Binh Thai Pham: University of Transport Technology, Hanoi 100000, Vietnam
Hiep Van Le: Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam


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