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Volume 29, Number 2, October25 2018

In order to reduce the deformation and delay the local buckling of concrete filled steel tube (CFST) columns, strengthening the structures with stiffeners is an effective method. In this paper, a new stiffening method with inclined stiffeners was used to investigate the behaviors of short CFST columns under axial compression. Besides, a three-dimensional nonlinear finite element (FE) model was applied to simulate the mechanical performances, including the total deformation, local buckling, and stress-strain relationship. Revised constitutive models of stiffened steel tube and confined concrete are proposed. A good agreement was achieved between the test and FE results. Furthermore, the calculated results of load capacity by using a simplified method also show a good correlation with experimental data.

Key Words
concrete filled steel tube (CFST) columns; self-compacting concrete (SCC); stiffening; axial compression; finite element analysis

(1) Wei Liang, Jiangfeng Dong:
School of Architecture and Environment, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu, 610065, China;
(2) Wei Liang, Qingyuan Wang:
Key Laboratory of Deep Underground Science and Engineering, Ministry of Education, Sichuan University, Chengdu, 610065, China\'
(3) Jiangfeng Dong:
Failure Mechanics and Engineering Disaster Prevention and Mitigation Key Laboratory of Sichuan Province, Sichuan University, Chengdu, 610065, China;
(4) Qingyuan Wang:
State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 610065, China.

Point bending is often used for cambering and curving structural steel girders. An analytical solution, applicable in the elasto-plastic range only, that relates applied loads to the desired curve was recently developed for inducing horizontal curves using four-point bending. This solution does not account for initial residual stresses and geometric imperfections built-in hotrolled sections. This paper presents results from a full-scale test on a hot-rolled steel section curved using four-point bending. In parallel, a numerical analysis, accounting for both initial geometric imperfections and initial residual stresses, was carried out. The models were validated against the experimental results and a good agreement for lateral offset and for strain in the elastoplastic and post-plastic ranges was achieved. The results show that the effect of initial residual stresses on deformation and strain is minimal. Finally, residual stresses due to cold bending calculated from the numerical analysis were assessed and a revised stress value for the service load design of the curved girder is proposed.

Key Words
curving; elasto-plastic; finite element; non-linear; point bending; post-plastic; residual stress; steel

(1) Najib G. Saliba, Antoine N. Gergess:
Department of Civil Engineering, University of Balamand, El-Koura, Lebanon;
(2) Issam Tawk:
Department of Mechanical Engineering, University of Balamand, El-Koura, Lebanon.

vThe plastic hinge method and the plastic zone method are extensively adopted in displacement-based elements and force-based elements respectively for second-order inelastic analysis. The former enhances the computational efficiency with relatively less accurate results while the latter precisely predicts the structural behavior but generally requires more computer time. The displacement-based elements receive criticism mainly on plasticity dominated problems not only in accuracy but also in longer computer time to redistribute the forces due to formation of plastic hinges. The multi-element-per-member model relieves this problem to some extent but will induce a new problem in modeling of member initial imperfections required in design codes for direct analysis. On the contrary, a force-based element with several integration points is sufficient for material yielding. However, use of more integration points or elements associated with fiber section reduces computational efficiency. In this paper, a new force-based element equipped with stress-resultant plasticity model with minimal computational cost is proposed for second-order inelastic analysis. This element is able to take the member initial bowing into account such that one-element-per-member model is adequate and complied with the codified requirements of direct analysis. This innovative solution is new and practical for routine design. Finally, several examples demonstrate the validity and accuracy of the proposed method.

Key Words
second-order inelastic analysis; force-based; steel structures; initial imperfection; stress-resultant plasticity model

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China.

This paper handles the repairing of deficient square Concrete-Filled Steel-Tube (CFST) beams subject to bending through an experimental and numerical program. Eight square-CFST beams were tested. A 5-mm artificial notch was induced at mid-span of seven beams, four of them were repaired by using CFRP sheets and two were repaired by using GFRP sheets. The beam deflection, strain and ultimate moments were recorded. It was found that providing different cut-off points for the different layers of FRP sheets prohibited failure at termination points due to stress concentrations. Using different lengths of FRP sheets around the notch retarded crack propagation and prevented FRP rupture at the crack position. Finite element analysis was then conducted and the proposed FE model was verified against the recorded experimental data. The influence of various parameters as FRP sheet length, tensile modulus and the number of layers were studied. The moment capacity of damaged square-CFST beams was improved up to 77.6% when repaired by using four layers of CFRP, however, this caused a dramatic decrease in beam deflection. U-wrapping of notched-CFST beam with 0.75 of its length provided a comparable behaviour as wrapping the full length of the beam.

Key Words
CFST; deficient; damaged; composite; FRP; beam; experimental; finite element

Department of Structural Engineering, Faculty of Engineering, Zagazig University, Egypt.

In this study, the effects of magnesium sulfate on the mechanical performance and the durability of confined and unconfined geopolymer concrete (GPC) specimens were investigated. The carbon and basalt fiber reinforced polymer (FRP) fabrics with 1-layer and 3-layers were used to evaluate the performances of the specimens under static and cyclic loading in the ambient and magnesium sulfate environments. In addition, the use of FRP materials as a rehabilitation technique was also studied. For the geopolymerization process of GPC specimens, the alkaline activator has selected a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide solution (NaOH) with a ratio (Na2SiO3/NaOH) of 2.5. In addition to GPC specimens, an ordinary concrete (NC) specimens were also produced as a reference specimens and some of the GPC and NC specimens were immersed in 5% magnesium sulfate solutions. The mechanical performance and the durability of the specimens were evaluated by visual appearance, weight change, static and cyclic loading, and failure modes of the specimens under magnesium sulfate and ambient environments. In addition, the microscopic changes of the specimens due to sulfate attack were also assessed by scanning electron microscopy (SEM) to understand the macroscale behavior of the specimens. Results indicated that geopolymer specimens produced with nano-silica and fly ash showed superior performance than the NC specimens in the sulfate environment. In addition, confined specimens with FRP fabrics significantly improved the compressive strength, ductility and durability resistance of the specimens and the improvement was found higher with the increased number of FRP layers. Specimens wrapped with carbon FRP fabrics showed better mechanical performance and durability properties than the specimens wrapped with basalt FRP fabrics. Both FRP materials can be used as a rehabilitation material in the sulfate environment.

Key Words
Geopolymer Concrete (GPC); Fiber Reinforced Polymer (FRP); static and cyclic loading; magnesium sulfate environment; nano-silica

(1) Mehmet Eren Gülşan, Abdulkadir Çevik:
Department of Civil Engineering, Gaziantep University, Gaziantep, Turkey;
(2) Radhwan Alzeebaree, Alaa Mohammedameen:
Department of Civil Engineering, Duhok Polytechnic University, Duhok, Iraq;
(3) An

This paper presents an analytical model to analyze the mapping relationship between bridge lateral deformation and track geometry of high-speed railway. Based on the rail deformation mechanisms, the deformation of track slab and rail at the locations of fasteners are analyzed. Formulae of rail lateral deformation are derived and validated against a finite element model. Based on the analytical model, a rail deformation extension coefficient is presented, and effects of different lateral deformations on track geometry are evaluated. Parametric studies are conducted to evaluate the effects of the deformation amplitude, fastener stiffness and mortar layer stiffness on the rail deformation. The rail deformation increases with the deformation of the girder, and is dependent on the spacing of the fasteners, the elastic modulus of the rail's material, and the moment of inertia of the rail's section.

Key Words
analytical model; bridge lateral deformation; high-speed railway; mapping relationship; track geometry

(1) Hongye Gou, Longcheng Yang, Dan Leng, Qianhui Pu:
Department of Bridge Engineering, School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China;
(2) Hongye Gou:
Key Laboratory of High-Speed Railway Engineering, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China;
(3) Yi Bao:
Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109, USA.

As a lateral load resisting component, buckling restrained steel plate shear walls (BRW) have excellent energy dissipating capacity. Similar to thin steel plate shear walls, the mechanical behavior of BRWs depends on the boundary elements (adjacent beams and columns) which need adequate strength and stiffness to ensure the complete yielding of BRWs and the emergence of expected plastic collapse mechanism of frame. This paper presents a theoretical approach to estimate the design forces for boundary elements of beam-connected BRW (i.e., The BRW is only connected to beams at its top and bottom, without connections to columns) using a fundamental plastic collapse mechanism of frame, a force transferring model of beamconnected BRW and linear beam and column analysis. Furthermore, the design method of boundary beams and columns is presented. The proposed approach does not involve nonlinear analyses, which can be easily and efficiently used to estimate the design forces of beams and columns in a frame with BRWs. The predicted design forces of boundary elements are compared with those from nonlinear finite element analyses, and a good agreement is achieved.

Key Words
buckling restrained steel plate shear wall; beam-connected BRW; design forces; boundary elements; plastic collapse mechanism

(1) Wen-Yang Liu:
College of Engineering, Heilongjiang Bayi Agricultural University, Daqing, China;
(2) Guo-Qiang Li:
State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai, China;
(3) Jian Jiang:
Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, USA.

This study conducts an optimization and sensitivity analysis on rectangular reinforced concrete (RC) beam using Lagrangian Multiplier Method (LMM) as programming optimization computer soft ware. The analysis is conducted to obtain the minimum design cost for both singly and doubly RC beams according to the specifications of three regulations of American concrete institute (ACI), British regulation (BS), and Iranian concrete regulation (ICS). Moreover, a sensitivity analysis on cost is performed with respect to the effective parameters such as length, width, and depth of beam, and area of reinforcement. Accordingly, various curves are developed to be feasibly utilized in design of RC beams. Numerical examples are also represented to better illustrate the design steps. The results indicate that instead of complex optimization relationships, the LMM can be used to minimize the cost of singly and doubly reinforced beams with different boundary conditions. The results of the sensitivity analysis on LMM indicate that each regulation can provide the most optimal values at specific situations. Therefore, using the graphs proposed for different design conditions can effectively help the designer (without necessity of primary optimization knowledge) choose the best regulation and values of design parameters.

Key Words
optimum design; Lagrangian Multiplier Method; sensitivity analysis; reinforced concrete beam

(1) Mehran Shariat, Amirhossein Madadi:
Department of Civil Engineering, Hakim Sabzevari University, Sabzevar, Iran;
(2) Mahdi Shariati:
Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran;
(3) Karzan Wakil:
Information Technology Department, Technical College of Informatics, Sulaimani Polytechnic University, Iraq;
(4) Karzan Wakil:
University of Human Development, Iraq.

The research of shear connectors composed from mechanical couplers with rebar anchors, embedded in concrete, and steel bolts, as a mean of shear transfer in composite connections is presented in the paper. Specific issues related to this type of connections are local concrete pressure in the connector vicinity as well as the shear flow along the connector axis. The experimental research included 18 specimens, arranged in 5 series. Nonlinear numerical analyses using Abaqus software was conducted on corresponding FE models. Different failure modes were analysed, with emphasis on concrete edge failure and bolt shear failure. The influence of key parameters on the behaviour of shear connector was examined: (1) concrete compression strength, (2) bolt tensile strength and diameter and (3) concrete edge distance. It is concluded that bolted shear connectors with mechanical couplers have sufficient capacity to be used as shear connectors in composite structures and that their behaviour is similar to the behaviour of post installed anchors as well as other types of connectors anchored without the head.

Key Words
steel-concrete composite structures; shear connectors; mechanical couplers; push-out tests, concrete edge breakout, finite element analysis

(1) Branko Milosavljević, Ivan Milićević, Milan Spremić:
Faculty of Civil Enginering, Univercity of Belgrade, Bulevar kralja Aleksandra 73, 11000 Belgrade, Serbia;
(2) Marko Pavlović:
Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, Delft, CD, Netherlands.

In recent years, significant progress has been made in developing design rules for stainless steel members, while the investigation on bolted connections is relatively limited, in particular at elevated temperatures. In this paper, experimental and numerical investigations on stainless steel bolted connections at ambient and elevated temperatures from the literature were reviewed. Firstly, the research program that focused on structural behavior of cold-formed stainless steel (CFSS) bolted connections at elevated temperatures carried out by the authors were summarized. Over 400 CFSS single shear and double shear bolted connection specimens were tested. The tests were conducted in the temperature ranged from 22 to 950°C using both steady state and transient state test methods. It is shown that the connection strengths decrease as the temperature increases in the similar manner for the steady state test results and the transient state test results. Generally, the deterioration of the connection strengths showed a similar tendency of reduction to those of the material properties for the same type of stainless steel regardless of different connection types and different configurations. It is also found that the austenitic stainless steel EN 1.4571 generally has better resistance than the stainless steel EN 1.4301 and EN 1.4162 for bolted connections at elevated temperatures. Secondly, extensive parametric studies that included 450 specimens were performed using the verified finite element models. Based on both the experimental and numerical results, bearing factors are proposed for bearing resistances of CFSS single shear and double shear bolted connections that subjected to bearing failure in the temperature ranged from 22 to 950°C. The bearing resistances of bolted connections obtained from the tests and numerical analyses were compared with the nominal strengths calculated from the current international stainless steel specifications, and also compared with the predicted strengths calculated using the proposed design equations. It is shown that the proposed design equations are generally more accurate and reliable than the current design rules in predicting the bearing resistances of CFSS (EN 1.4301, EN 1.4571 and EN 1.4162) bolted connections at elevated temperatures. Lastly, the proposed design rules were further assessed by the available 58 results of stainless steel bolted connections subjected to bearing failure in the literature. It is found that the proposed design rules are also applicable to the bearing resistance design of other stainless steel grades, including austenitic stainless steel (EN 1.4306), ferritic stainless steel (EN 1.4016) and duplex stainless steel (EN 1.4462).

Key Words
bearing strength; bolted connection; cold-formed stainless steel; elevated temperatures; steady state tests; transient state tests

Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China.

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