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CONTENTS
Volume 46, Number 2, January25 2023
 

Abstract
Rectangular concrete-filled steel tubular (RCFST) column, a type of concrete-filled steel tubular (CFST), is widely used in compression members of structures because of its advantages. This paper proposes a robust machine learning-based framework for predicting the ultimate compressive strength of RCFST columns under both concentric and eccentric loading. The gradient boosting neural network (GBNN), an efficient and up-to-date ML algorithm, is utilized for developing a predictive model in the proposed framework. A total of 890 experimental data of RCFST columns, which is categorized into two datasets of concentric and eccentric compression, is carefully collected to serve as training and testing purposes. The accuracy of the proposed model is demonstrated by comparing its performance with seven state-of-the-art machine learning methods including decision tree (DT), random forest (RF), support vector machines (SVM), deep learning (DL), adaptive boosting (AdaBoost), extreme gradient boosting (XGBoost), and categorical gradient boosting (CatBoost). Four available design codes, including the European (EC4), American concrete institute (ACI), American institute of steel construction (AISC), and Australian/New Zealand (AS/NZS) are refereed in another comparison. The results demonstrate that the proposed GBNN method is a robust and powerful approach to obtain the ultimate strength of RCFST columns.

Key Words
concrete-filled steel tube; composite structure; gradient boosting neural networks; machine learning; predictive model; ultimate compression strength

Address
Van-Thanh Pham and Seung-Eock Kim:Department of Civil and Environmental Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 05006, South Korea

Abstract
Stainless steel-concrete composite beam has become an attractive structural form for offshore bridges and iconic high-rise buildings, owing to the superior corrosion resistance and excellent ductility of stainless steel material. In a composite beam, stainless steel shear connectors play an important role by establishing the interconnection between stainless steel beam and concrete slab. To enable the best use of high strength stainless steel shear connectors in composite beams, high strength concrete is recommended. To date, the application of stainless steel shear connectors in composite beams is still very limited due to the lack of research and proper design recommendations. In this paper, a total of seven pushout specimens were tested to investigate the load-slip behaviour of stainless steel shear connectors. A thorough discussion has been made on the differences between stainless steel bolted connectors and welded studs, in terms of the failure modes, load-slip behaviour and ultimate shear resistance. In parallel with the experimental programme, a finite element model was developed in ABAQUS to simulate the behaviour of stainless steel shear connectors, with which the effects of shear connector strength, concrete strength and embedded connector height to diameter ratio (h/d) were evaluated. The obtained experimental and numerical results were analysed and compared with existing codes of practice, including AS/NZS 2327, EN 1994-1-1 and ANSI/AISC 360-16. The comparison results indicated that the current codes need to be improved for the design of high strength stainless steel shear connectors. On this basis, modified design approaches were proposed to predict the shear capacity of stainless steel bolted connectors and welded studs in the composite beams.

Key Words
bolted connector; pushout test; shear connector; stainless steel; welded stud

Address
Yifan Zhou, Brian Uy, Jia Wang, Dongxu Li and Xinpei Liu: School of Civil Engineering, The University of Sydney, Sydney, NSW, 2006, Australia

Abstract
Dynamic deflection analysis of sandwich beams with cellular core under thermal and pulse loads has been performed in the present article. The cellular core sandwich beam has two layers fortified by graphene oxide powder (GOP) which are micromechanically modeled by Halpin-Tsai formulation. The pulse load has blast type and is applied on the top side of sandwich beam. The system of equations has been developed based on higher-order beam theory and Ritz method. Then, they are solved in Laplace domain to derive the dynamic deflections. The dependency of beam deflection on temperature variation, GOP content, pulse load duration/location and core relative density has been studied in detail.

Key Words
composite; design; dynamic response; heat; thermal effect; sandwich beam; pulse load

Address
Mamoon A.A. Al-Jaafari, Haider Ali Hussein, Abdulaziz Saud Khider, Raad M. Fenjan and Nadhim M. Faleh: Al-Mustansiriah University, Engineering Collage P.O. Box 46049, Bab-Muadum, Baghdad 10001, Iraq

Abstract
In the present research, dynamic deflections of a sandwich beam having functionally graded (FG) porous core have been investigated assuming that the sandwich beam is exposed to a pulse load of blast type. The two layers of sandwich beam have been made of a polymeric matrix reinforced by graphene oxide powder (GOP). The micromechanical formulation of the layers has been done via Halpin-Tsai model. The solution method is chosen to be Ritz method which is an efficient method to solve the system of equations of beams modeled based on a higher-order theory. To derive the time history of sandwich beam under pulse load, Laplace method has been used. The porosity content of the core, the GOP content of the layers, thickness of the layer and also duration of the applied load have great influences of the responses of sandwich beam.

Key Words
composite; dynamic response; porous; pulse load; sandwich beam; stability

Address
Weijia Yu:School of Architecture, Southeast University, Nanjing 210096, Jiangsu, China

Linyun Zhou: School of Transportation, Southeast University, Nanjing 210096, Jiangsu, China

Abstract
Welded hollow spherical joints are commonly used joints in space grid structures. An internal stiffener is generally adopted to strengthen the joints when large hollow spheres are used. To further strengthen it, external stiffeners can be used at the same time. In this study, axial compression tests are conducted on four full-scale 550 mm spherical joints. The failure modes and strengths of the tested joints are investigated. It shows that the external stiffeners are able to increase the strength of the joint up to 25%. A numerical model for large spherical joints with stiffeners is established and verified against the experimental results. Parametric studies are executed considering six main design factors using the verified model. It is found that the strength of the spherical joint increases as the thickness, height and number of the external stiffeners increase, and the hollow sphere's diameter has a neglectable effect on the enhancement caused by the external stiffeners. Based on the experimental and numerical results, a practical formula for the compressive bearing capacity of large welded hollow spherical joints with both internal and external stiffeners is proposed. The proposed formula gives a conservative prediction on the compressive capacity of large welded hollow spherical joints with both internal and external stiffeners.

Key Words
compressive behavior; external stiffener; failure mode; large diameter; numerical simulation; welded hollow sphere

Address
Tingting Shu:Department of Civil Engineering, Zhejiang University, Hangzhou, China

Xian Xu:1)Department of Civil Engineering, Zhejiang University, Hangzhou, China
2)Key Laboratory of Space Structures of Zhejiang Province
3)Center for Balance Architecture, Zhejiang University, Hangzhou, China

Yaozhi Luo:1)Department of Civil Engineering, Zhejiang University, Hangzhou, China
2)Key Laboratory of Space Structures of Zhejiang Province

Abstract
In order to study the seismic response of the main plant of steel reinforced concrete (SRC) structure of the CAP1400 nuclear power plant under the influence of different high-mode vibration, the 1/7 model structure was manufactured and its dynamic characteristics was tested. Secondly, the finite element model of SRC frame-bent structure was established, the seismic response was analyzed by mode-superposition response spectrum method. Taking the combination result of the 500 vibration modes as the standard, the error of the base reactions, inter-story drift, bending moment and shear of different modes were calculated. Then, based on the results, the influence of high-mode vibration on the seismic response of the SRC frame-bent structure of the main plant was analyzed. The results show that when the 34 vibration modes were intercepted, the mass participation coefficient of the vertical and horizontal vibration mode was above 90%, which can meet the requirements of design code. There is a large error between the seismic response calculated by the 34 and 500 vibration modes, and the error decreases as the number of modes increases. When 60 modes were selected, the error can be reduced to about 1%. The error of the maximum bottom moment of the bottom column appeared in the position of the bent column. Finally, according to the characteristics of the seismic influence coefficient aj of each mode, the mode contribution coefficient yj.Xji was defined to reflect the contribution of each mode to the seismic action.

Key Words
high-mode vibration; nuclear power plant; response spectrum method; seismic action; SRC frame-bent structure

Address
Biao Liu:1)College of Water Resources and Architectural Engineering, Northwest A&F University,
No. 22 Xinong Road, Yangling, Shaanxi Province, P.R. China
2)Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University,
No. 22 Xinong Road, Yangling, Shaanxi Province, P.R. China

Zhengzhong Wang, Bo Zhang and Mingxia Gao:College of Water Resources and Architectural Engineering, Northwest A&F University,
No. 22 Xinong Road, Yangling, Shaanxi Province, P.R. China
School of Civil Engineering and Architecture, Xi'an University of Technology, No. 5 Jinhua South Road, Xi'an, Shaanxi Province, P.R. China

Guoliang Bai:School of Civil Engineering, Xi'an University of Architecture & Technology, No. 13 Yanta Road, Xi'an, Shaanxi Province, P.R. China

Abstract
The article describes the development of a novel dissipative bracing connection device (identified by the acronym DRBrC) for concentrically braced frames in steel and composite structures. The origins of the device trace back to the seminal work of Kelly, Skinner and Heine (1972), and, more directly related, to the PIN-INERD device, overcoming some of its limitations and greatly improving the replaceability characteristics. The connection device is composed of a rigid housing, connected to both the brace and the beam-column connection (or just the column), in which the axial force transfer is achieved by four-point bending of a dissipative pin. The experimental validation stages, presented in detail, consisted of a preliminary testing campaign, resulting in successive improvements of the original device design, followed by a systematic parametric testing campaign. That final campaign was devised to study the influence of the constituent materials (S235 and Stainless Steel, for the pin, and S355 and High Strength Steel, for the housing), of the geometry (four-point bending intermediate spans) and of the loading history (constant amplitude or increasing cyclic alternate). The main conclusions point to the most promising DRBrC device configurations, also presenting some suggestions in terms of the replaceability requirements.

Key Words
concentrically braced frames; dissipative connections; experimental tests; reparability; replaceability; seismic design; stainless steel

Address
Jorge M. Proença:CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001, Lisboa, Portugal

Luís Calado: CERIS, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001, Lisboa, Portugal

Alper Kanyilmaz:Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, Milan, Italy

Abstract
The goal of this paper is to investigate dynamic responses of simply-supported laminated micro beams under moving load. In the considered micro-scale problem, the modified coupled stress theory which includes the length scale parameter is used. The governing equations of problem are derived by using the Lagrange procedure. In the solution of the problem the Ritz method is used and algebraic polynomials are used with the trivial functions for the Ritz method. In the solution of the moving load problem, the Newmark average acceleration method is used in the time history. In the numerical examples, the effects of stacking sequence of laminas, fibre orientation angles and the length scale parameter on the dynamic responses of laminated micro beams are examined and discussed.

Key Words
dynamic analysis; laminated composite micro beams; modified coupled stress theory; moving load; size dependent

Address
Ş.D. Akbaş:Department of Civil Engineering, Bursa Technical University, 16330, Bursa, Turkey


Abstract
Free vibration analysis of multi-directional porous functionally graded (FG) sandwich plate has been performed for two cases namely: FG skin with homogeneous core and FG core with homogeneous skin. Hamilton's principle was employed and the solution was obtained using Navier's technique. This theory imposes traction-free boundary conditions on the surfaces and does not require shear correction factors. The results obtained are validated with those available in the literature. The composition of metal-ceramic-based functionally graded material (FGM) changes in longitudinal and transverse directions according to the power law. Imperfections in the functionally graded material introduced during the fabrication process were modeled with different porosity laws such as evenly, unevenly distributed, and logarithmic uneven distributions. The effect of porosity laws and geometry parameters on the natural frequency was investigated. On comparing the natural frequency of two cases for perfect and imperfect sandwich plates a reverse trend in natural frequency result was seen. The finding shows a multidirectional functionally graded structures perform better compared to uni-directional gradation. Hence, critical grading parameters and imperfection types have been identified which will guide experimentalists and researchers in selecting fabrication routes for improving the performance of such structures.

Key Words
FGM sandwich plate; free vibration; multi-directional FGM; Navier's solution; porosity effects

Address
Guermit Mohamed Bilal Chami, Amar Kahil:Mouloud Mammeri University, Department of Civil Engineering, Tizi Ouzou 15000, Algeria

Lazreg Hadji:1)Department of Civil Engineering, University of Tiaret, BP 78 Zaaroura, Tiaret,14000, Algeria
2)Laboratory of Geomatics and Sustainable Development, University of Tiaret, Algeria

Royal Madan:Department of Mechanical Engineering, G H Raisoni Institute of Engineering & Technologie, Nagpur, Maharashtra, India

Abdelouahed Tounsi:1)YFL (Yonsei Frontier Lab), Yonsei University, Seoul, Korea
2)Department of Civil and Environmental Engineering, King Fahd University of Petroleum & Minerals,
31261 Dhahran, Eastern Province, Saudi Arabia

Abstract
Structural design against the progressive collapse has been a vital necessity for decades due to occasional tragic events. The question of whether designed structures against the progressive collapse are still robust if subjected to multi-hazard scenarios containing column removal and successive localized fires is ad-dressed in the current study. Two seven-story steel structures with an identical area but different structural configurations of 4- and 5-bays are designed against the progressive collapse; the structural components are also fireproofed for a 60 min fire resistance. The structures are then subjected to different column re-moval scenarios over different stories followed immediately by localized fires. Results indicate that the structures are not able to keep their stability under all of the considered scenarios; the 4-bay structure is more vulnerable than the 5-bay structure. It is also indicated that upper stories are more sensitive toward the considered scenarios than lower stories. To advance structural safety, two strategies are adopted: in-creasing the thickness of the insulation materials to reduce the thermal effects, or, increasing the safety fac-tor (ΩN) of the structures when designing against the progressive collapse. As for the first strategy, provid-ing a 35% and a 25% increase in the insulation thicknesses of the structural components of the 4-bay and 5-bay structures, respectively, can prevent a progressive collapse to trigger. As for the second strategy, in-creasing ΩN by 10% can enhance the structural integrity to where no collapse occurs under all of the sce-narios.

Key Words
column removal; localized fire; progressive collapse; steel structures; structural stability

Address
Behrouz Behnam: School of Civil and Environmental Engineering, Amirkabir University of Technology, 424 Hafez Street, Tehran, Iran


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