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CONTENTS
Volume 38, Number 6, March25 2021
 


Abstract
This paper numerically investigated the behavior of built-up square concrete-filled steel tubular (CFST) columns under combined preload and axial compression. The finite element (FE) models of target columns were verified in terms of failure mode, axial load-deformation curve and ultimate strength. A full-range analysis on the axial load-deformation response as well as the interaction behavior was conducted to reveal the composite mechanism. The parametric study was performed to investigate the influences of material strengths and geometric sizes. Subsequently, influence of construction preload on the full-range behavior and confinement effect was investigated. Numerical results indicate that the axial load-deformation curve can be divided into four working stages where the contact pressure of curling rib arc gradually disappears as the steel tube buckles; increasing width-to-thickness (B/t) ratio can enhance the strength enhancement index (e.g., an increment of 1.88% from B/t=40 to B/t=100), though ultimate strength and ductility are decreased; stiffener length and lip inclination angle display a slight influence on strength enhancement index and ductility; construction preload can degrade the plastic deformation capacity and postpone the origin appearance of contact pressure, thus making a decrease of 14.81%~27.23% in ductility. Finally, a revised equation for determining strain e corresponding to ultimate strength was proposed to evaluate the plastic deformation capacity of built-up square CFST columns.

Key Words
built-up square CFST columns; compressive behavior; FE modelling; full-range analysis; construction preload

Address
Jian-Tao Wang and Fa-Cheng Wang: Department of Civil Engineering, Tsinghua University, Beijing 100084, P. R. China

Abstract
The component-based method is widely used to analyze the initial stiffness of joint in steel structures. In this study, an analytical component model for determining the column face stiffness of square or rectangular hollow section (SHS/RHS) subjected to tension was established, focusing on endplate connections. Equations for calculating the stiffness of the SHS/RHS column face in bending were derived through regression analysis using numerical results obtained from a finite element model database. Because the presence of bolt holes decreased the bending stiffness of the column face, this effect was calculated using a novel plate-spring-based model through numerical analysis. The developed component model was first applied to predict the bending stiffness of the SHS column face determined through tests. Furthermore, this model was incorporated into the component-based method with other effective components, e.g., bolts under tension, to determine the tensile stiffness of the T-stub connections, which connects the SHS column, and the initial rotational stiffness of the joints. A comparison between the model predictions, test data, and numerical results confirms that the proposed model shows satisfactory accuracy in evaluating the bending stiffness of SHS column faces.

Key Words
bolted endplate joints; component-based method; initial stiffness; hollow section column; regression analysis

Address
Dongchen Ye: Department of Structural Engineering, Tongji University, Shanghai, China
Ke Ke: Key Laboratory of New Technology for Construction of Cities in Mountain Area, School of Civil Engineering,
Chongqing University, Chongqing, China
Yiyi Chen: Department of Structural Engineering, Tongji University, Shanghai, China;
Department of Construction Engineering, Sanda University, Shanghai, China

Abstract
This paper presents a designing method for enhancing fire resistance of steel box bridge girders (closed steel box bridge girder supporting a thin concrete slab) through taking into account such parameters namely; fire severity, type of longitudinal stiffeners (I, L, and T shaped), and number of longitudinal stiffeners. A validated 3-D finite element model, developed through the computer program ANSYS, is utilized to go over the fire response of a typical steel box bridge girder using the transient thermo-structural analysis method. Results from the numerical analysis show that fire severity and type of longitudinal stiffeners welded on bottom flange have significant influence on fire resistance of steel box bridge girders. T shaped longitudinal stiffeners applied on bottom flange can highly prevent collapse of steel box bridge girders towards the end of fire exposure. Increase of longitudinal stiffeners on bottom flange and web can slightly enhance fire resistance of steel box bridge girders. Rate of deflection-based criterion can be reliable to evaluate fire resistance of steel box bridge girders in most fire exposure cases. Thus, T shaped longitudinal stiffeners on bottom flange incorporated into bridge fire-resistance design can significantly enhance fire resistance of steel box bridge girders.

Key Words
steel box bridge girders; bridge fires; finite element model; fire performance; fire-resistance design

Address
Xuyang Li, Gang Zhang and Shuanhai He: School of Highway, Chang'an University, Xi'an, Shaanxi 710064, China
Venkatesh Kodur: Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI 48864, USA
Qiao Huang: School of Transportation, Southeast University, Nanjing, Jiangsu 210096, China

Abstract
Seismic responses of RC core wall with two outriggers are investigated in this study. In the models analyzed here, one of the outriggers is fixed at the top of the building and the second is placed at different levels along the height of the system. Each of the systems resulting from the placement of the outrigger at different locations is designed according to the prescriptive codes. The location of the outrigger changes along the height. Linear design of all the structures is accomplished by using prescriptive codes. Buckling restrained braces (BRBs) are used in the outriggers and forward directivity near fault and far fault earthquake record sets are used at maximum considered earthquake (MCE) level. Results from nonlinear time history analysis demonstrate that BRB outriggers can change the seismic responses like force distribution and deformation demand of the RC core-walls over the height and lead to the new plastic hinge arrangement over the core-wall height. Plasticity extension in the RC core wall occurs at the base as well as adjacent to the outrigger levels. Considering the maximum inter-story drift ratio (IDR) demand as an engineering parameter, the best location for the second outrigger is at 0.75H, in which the maximum IDR at the region upper the second outrigger level is approximately equal to the corresponding value in the lower region.

Key Words
near fault earthquakes; forward directivity; outrigger; reinforced concrete; core wall; BRB

Address
Hamid Beiraghi and Mansooreh Hedayati: Department of Civil Engineering, Mahdishahr Branch, Islamic Azad University, Mahdishahr, Iran

Abstract
In this study, experiments were conducted to evaluate the flexural performance of prestressed hybrid wide flange (PHWF) beams with hollowed steel webs. A total of four PHWF beams were fabricated, where the width and spacing of the steel webs and the presence of cast-in-place (CIP) concrete were set as the main test parameters, and their flexural behavior and crack patterns, and the longitudinal strain distribution in a section with respect to the width and spacing of the steel webs were analyzed in detail. The experiment results showed that, as the ratio of the width to the spacing of the steel webs decreased, the flexural stiffness and strength of the PHWF beams without CIP concrete decreased. In addition, in the case of composite PHWF beam with CIP concrete, fully composite behavior between the precast concrete and the CIP concrete was achieved through the embedded steel member. Finite element analyses were performed for the PHWF beams considering the bond properties between the hollowed steel webs and concrete, and nonlinear flexural analyses were also conducted reflecting the pre-compressive strains introduced only into the bottom flange. From the comparison of the test and analysis results, it was confirmed that the analysis models proposed in this study well evaluated the flexural behavior of PHWF beams with and without CIP concrete.

Key Words
composite beam; prestress; precast concrete; finite element analysis; nonlinear flexural analysis

Address
Sun-Jin Han,Seung-Ho Cho and Inwook Heo: Department of Architectural Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Soeoul 02504, Republic of Korea
Hyo-Eun Joo: Department of Civil Engineering, The University of Tokyo, 7 Chome-3-1 Hongo, Bunkyo City, Tokyo 113-8654, Japan
Kang Su Kim: Department of Architectural Engineering and Smart City Interdisciplinary Major Program, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Soeoul 02504, Republic of Korea

Abstract
An innovative partially precast partially encased composite beam (PPECB) is put forward based on the existing research. In order to study the flexural performance of the new composite beam which has precast part and cast-in-place part, six prefabricated specimens and one cast-in-place specimen are designed with considering the influence of the production method, the steel flange thickness, the concrete strength grade and the stirrup process on the behavior of the composite beam. Through four points loading and test data collection and analysis, the behavior of partially prefabricated specimen is similar to that of cast-in-place specimen, and the casting method, the thickness of the steel flange, the concrete strength grade and the stirrup process have different influence on the crack, yield and peak load bearing capacity of the component. Finally, the calculation theory of plastic bending of partially precast partially encased concrete composite beams is given. The calculation results are in good agreement with the experimental results, which can be used for practical engineering theory guidance. This paper can provide reference value for further research and engineering application.

Key Words
precast; partially encased concrete; beam; flexural

Address
Jiong-feng Liang: College of Civil and Architecture Engineering, East China University of Technology, 330013, Nanchang, China;
Guangxi Key Laboratory of Disaster Prevention and Engineering Safety, Guangxi University, Nanning, China
Liu-feng Zhang adn Ying-hua Yang: College of Civil Engineering, Xi'an University of Architecture & Technology, 710055, Xi'an, China
Li Wei: College of Civil and Architecture Engineering, Wenzhou University, Wenzhou, China

Abstract
A theoretical energy-based model to capture the mechanical response of thick woven composite laminates, which are used in such applications as maritime or aerospace, to high-velocity impact was developed. The dependences of the impact phenomenon on material and geometrical parameters were analysed making use of the Vaschy-Buckingham Theorem to provide a non-dimensional framework. The model was divided in three different stages splitting the physical interpretation of the perforation process: a first where different dissipative mechanisms such as compression or shear plugging were considered, a second where a transference of linear momentum was assumed and a third where only friction took place. The model was validated against experimental data along with a 3D finite element model. The numerical simulations were used to validate some of the new hypotheses assumed in the theoretical model to provide a more accurate explanation of the phenomena taking place during a high-velocity impact.

Key Words
energy-absorption; impact behavior; analytical modelling; numerical modelling; FRP

Address
L. Alonso: Department of Chemical Technology, Energy and Mechanics, Rey Juan Carlos University, C/Tulipán s.n., 28933
Garcia-Gonzalez, C. Navarro and S.K. García-Castillo: Department of Continuum Mechanics and Structural Analysis, University Carlos III of Madrid, Avda. de la Universidad 30, 28911

Abstract
This paper introduces an improved design equation to evaluate the resisting capacity of circular reinforced concrete (RC) columns partially strengthened with outer steel tube. When RC column members are required to be strengthened according to the change in the loadings considered and/or the deterioration progress in columns, wrapping up RC column with steel circular tube, which takes the form of concrete filled steel tube (CFST), has been popularly considered because of its structural advantage induced from the confinement effect. However, the relatively high construction cost of steel tube is restricting its use to the required region, while deriving the shape of a partial CFST column. To evaluate the resisting capacity of a partial CFST column, numerical analyses need to be performed, and a numerical model proposed in the previous study for the numerical analysis of full CFST columns is used to conduct parametric studies for the introduction of a design equation. The bond-slip effect developed along the interface between the in-filled concrete and the exterior steel tube is taken into consideration and the validity of the numerical model has been established through correlation studies between experimental data and numerical results for partial CFST circular columns. Moreover, parametric studies make it possible to introduce a design equation for determining the optimum length of outer steel tube which produces partial CFST circular columns.

Key Words
concrete filled steel tube; Partial strengthening; P-M interaction diagram; design equation; circular RC column

Address
Ju-young Hwang: Department of Civil Engineering, Dong-Eui University, 176 Eomgwangno, Busanjin-gu, Busan, Republic of Korea
Hyo-Gyoung Kwak: Department of Civil and Environmental Engineering, Korea Advanced Institute for Science and Technology,
291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea



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