• Steel and Composite Structures
• /
• v.27 no.5
• /
• pp.577-598
• /
• 2018

The imperfect steel-concrete interface bonding is an important deficiency of the concrete-filled double skin tubular (CFDST) columns that led to separating concrete and steel surfaces under lateral loads and triggering buckling failure of the columns. To improve this issue, it is proposed in this study to use longitudinal and transverse steel stiffeners in CFDST columns. CFDST columns with different patterns of stiffeners embedded in the interior or exterior surfaces of the inner or outer tubes were analyzed under constant axial force and reversed cyclic loading. In the finite element modeling, the confinement effects of both inner and outer tubes on the compressive strength of concrete as well as the effect of discrete crack for concrete fracture were incorporated which give a realistic prediction of the seismic behavior of CFDST columns. Lateral strength, stiffness, ductility and energy absorption are evaluated based on the hysteresis loops. The results indicated that the stiffeners had determinant role on improving pinching behavior resulting from the outer tube's local buckling and opening/closing of the major tensile crack of concrete. The lateral strength, initial stiffness and energy absorption capacity of longitudinally stiffened columns with fixed-free end condition were increased by as much as 17%, 20% and 70%, respectively. The energy dissipation was accentuated up to 107% for fixed-guided end condition. The use of transverse stiffeners at the base of columns increased energy dissipation up to 35%. Axial load ratio, hollow ratio and concrete strength affecting the initial stiffness and lateral strength, had negligible effect of the energy dissipation of the columns. It was also found that the longitudinal stiffeners and transverse stiffeners have, respectively, negative and positive effects on ductility of CFDST columns. The conclusions, drawn from this study, can in turn, lead to the suggestion of some guidelines for the design of CFDST columns.

• Journal of the Earthquake Engineering Society of Korea
• /
• v.14 no.4
• /
• pp.37-48
• /
• 2010

The seismic control effect of reinforced concrete structures with low energy dissipating capacity due to stiffness degradation is investigated through nonlinear time history analysis. The primary structure is idealized as a SDOF system of modified Takeda hysteresis rule and an elasto-perfectly-plastic nonlinear spring is added to represent a hysteretic damping device. Based on statistics of the numerical analysis, equivalent linearization techniques are evaluated, and empirical equations for response prediction are proposed. As a result, estimation of the ductility demand with proposed empirical equations is more desirable than the equivalent linearization techniques. The optimal yield strengths based on empirical equations are significantly different from the optimal yield strength of elasto-perfectly-plastic systems. Also, the results indicate that the reduction effect of the ductility demand is more remarkable for smaller natural periods.

• Journal of the Korea Concrete Institute
• /
• v.29 no.2
• /
• pp.179-187
• /
• 2017

The design provisions for the maximum steel ratio in reinforced concrete flexural members is normally provided to ensure sufficient ductility and economy by steel yielding at member failure. In the Concrete Structural Design Code (2012), the maximum steel ratio is expressed in terms of a net strain in tensile steel, and leading to very high steel ratio in the case of using high strength materials. Thereby, this may result in difficulty to satisfy a required workability at concrete placing. On the contrary, in the Korean Highway Bridge Design Code (Limit State Design) the maximum steel ratio is given in terms of the maximum neutral axis depth ratio that is 0.4. From these results, a rational model for the maximum steel ratio is suggested so as to satisfy a ductility as well as a workability.

• Structural Engineering and Mechanics
• /
• v.75 no.1
• /
• pp.49-57
• /
• 2020

This paper presents a full-scale experimental test to investigate the flexural behavior of an innovative dovetail ultra-high performance concrete (UHPC) joint designed for the 5^th Nanjing Yangtze River Bridge. The test specimen had a dimension of 3600 × 1600 × 170 mm, in accordance with the real bridge. The failure mode, crack pattern and structural response were presented. The ductility and stiffness degradation of the tested specimens were explicitly discussed. Test results indicated that different from conventional reinforced concrete slabs, well-distributed cracks with small spacing were observed for UHPC joint slabs at failure. The average nominal flexural cracking strength of the test specimens was 7.7 MPa, signifying good crack resistance of the proposed dovetail UHPC joint. It is recommended that high grade reinforcement be cooperatively used to take full advantage of the superior mechanical property of UHPC. A new ductility index, expressed by dividing the ultimate deflection by flexural cracking deflection, was introduced to evaluate the post-cracking ductility capacity. Finally, a strut-and-tie (STM) model was developed to predict the ultimate strength of the proposed UHPC joint.

• Earthquakes and Structures
• /
• v.19 no.4
• /
• pp.303-313
• /
• 2020

Spacious experimental and numerical investigation has been conducted by researchers to increase the ductility and energy dissipation of concentrically braced frames. One of the most widely used strategies for increasing ductility and energy dissiption, is the use of energy-absorbing systems. In this regard, the cyclic behavior of a chevron bracing frame system equipped with multi-pipe dampers (CBF-MPD) was investigated through finite element method. The purpose of this study was to evaluate and improve the behavior of the CBF using MPDs. Three-dimensional models of the chevron brace frame were developed via nonlinear finite element method using ABAQUS software. Finite element models included the chevron brace frame and the chevron brace frame equipped with multi-pipe dampers. The chevron brace frame model was selected as the base model for comparing and evaluating the effects of multi-tube dampers. Finite element models were then analyzed under cyclic loading and nonlinear static methods. Validation of the results of the finite element method was performed against the test results. In parametric studies, the influence of the diameter parameter to the thickness (D/t) ratio of the pipe dampers was investigated. The results indicated that the shear capacity of the pipe damper has a significant influence on determining the bracing behavior. Also, the results show that the corresponding displacement with the maximum force in the CBF-MPD compared to the CBF, increased by an average of 2.72 equal. Also, the proper choice for the dimensions of the pipe dampers increased the ductility and energy absorption of the chevron brace frame.

• Earthquakes and Structures
• /
• v.9 no.3
• /
• pp.657-671
• /
• 2015

The level of ductility is determined by depending on the intended use of the building, the region's seismic characteristics and the type of structural system when buildings are planned by engineers. Major portion of seismic energy is intended to be consumed in the plastic zone in structural systems of high ductility, so the occurrence of damages in load bearing and non-load bearing structural elements is accepted in planning stage under severe earthquakes. However, these damages must be limited among specific values in order not to endanger buildings in terms of the bearing capacity. Isolators placed between the basement and upper structure make buildings behave elastically by reducing the effects of seismic loads and improving seismic performance of building significantly. Thus, damages can be limited among desired values. In this study, the effectiveness of seismic isolation is investigated on both fixed based and seismic isolated models of a hospital building with high ductility level with regard to lateral displacements, internal forces, structural periods and cost of the building. Layered rubber bearings are interposed between the base of the structure and foundation. Earthquake analysis of the building are performed using earthquake records in time domain (Kocaeli, Loma Prieta and Landers). Results obtained from three-dimensional finite element models are presented by graphs and tables in detail. That seismic isolation reduces significantly the destructive effects of earthquakes on structures is seen from the results obtained by seismic analysis.

• Structural Engineering and Mechanics
• /
• v.75 no.6
• /
• pp.643-657
• /
• 2020

This study presents experimental and numerical investigations on circular steel tube confined ultra high performance concrete (UHPC) columns under axial compression. The plain UHPC without fibers was designed to achieve a compressive strength ranged between 150 MPa and 200 MPa. Test results revealed that loading on only the UHPC core can generate a significant confinement effect for the UHPC core, thus leading to an increase in both strength and ductility of columns, and restricting the inherent brittleness of unconfined UHPC. All tested columns failed by shear plane failure of the UHPC core, this causes a softening stage in the axial load versus axial strain curves. In addition, an increase in the steel tube thickness or the confinement index was found to increase the strength and ductility enhancement and to reduce the magnitude of the loss of load capacity. Besides, steel tube with higher yield strength can improve the post-peak behavior. Based on the test results, the load contribution of the steel tube and the concrete core to the total load was examined. It was found that no significant confinement effect can be developed before the peak load, while the ductility of post-peak stage is mainly affected by the degree of the confinement effect. A finite element model (FEM) was also constructed in ABAQUS software to validate the test results. The effect of bond strength between the steel tube and the UHPC core was also investigated through the change of friction coefficient in FEM. Furthermore, the mechanism of circular steel tube confined UHPC columns was examined using the established FEM. Based on the results of FEM, the confining pressures along the height of each modeled column were shown. Furthermore, the interaction between the steel tube and the UHPC core was displayed through the slip length and shear stresses between two surfaces of two materials.

• Journal of the Korea Institute of Building Construction
• /
• v.3 no.3
• /
• pp.83-90
• /
• 2003

This study examines the material characteristics of fibers and their influences on reinforced concrete through the tests of reinforced concrete by the types of fibers including non-reinforced, steel, polypropylene and cellulose fibers and the test of compressive strength and reinforced concrete beam without shear reinforcement and consequently it obtains the following conclusions. As a result of conducting compressive strength by the types of specimens, fiber reinforced specimen with the highest compressive strength value at 28 days of age was cellulose fiber reinforced specimen as 280.4kgf/$\textrm{cm}^2$ and steel fiber specimen had the highest compressive strength of 250.7kgf/$\textrm{cm}^2$ at 180 days of age. In case of non-reinforced specimen, its compressive strength was 277.4kgf/$\textrm{cm}^2$ at 28 days of age and 273.1kgf/$\textrm{cm}^2$ at 180 days of age. Comparing the compressive strength of non-reinforced specimen to that fiber reinforced specimen showed that the compressive strength of fiber reinforced specimen was lower in the passage of age and the results of this experiment showed no effects of fiber reinforcement. As a result of testing reinforced concrete beam without shear reinforcement, ductility factors of specimens were 4.67 for non-reinforced specimen, 8.18 for steel fiber reinforced specimen, 6.20 for polypropylene fiber reinforced specimen and 5.49 for cellulose reinforced specimen, and it is found that steel fiber reinforced specimen was highest. When non-reinforced specimen and steel fiber reinforced specimen were compared, steel fiber reinforced specimen had higher ductility factor of about 75.2% than that of non-reinforced specimen.

• Computers and Concrete
• /
• v.20 no.4
• /
• pp.391-407
• /
• 2017

Fiber reinforced polymer (FRP) bars have been recently used to reinforce concrete members in flexure due to their high tensile strength and especially in corrosive environments to improve the durability of concrete structures. However, FRPs have a low modulus of elasticity and a linear elastic behavior up to rupture, thus reinforced concrete (RC) components with such materials would exhibit a less ductility in comparison with steel reinforcement at the similar members. There were several studies showed the behavior of concrete beams with the hybrid combination of steel and FRP longitudinal reinforcement by adopting the experimental and numerical programs. The current study presents a numerical and analytical investigation based on the data of previous researches. Three-dimensional (3D) finite element (FE) models of beams by using ANSYS are built and investigated. In addition, this study also discusses on the design methods for hybrid FRP-steel beams in terms of ultimate moment capacity, load-deflection response, crack width, and ductility. The effects of the reinforcement ratio, concrete compressive strength, arrangement of reinforcement, and the length of FRP bars on the mechanical performance of hybrid beams are considered as a parametric study by means of FE method. The results obtained from this study are compared and verified with the experimental and numerical data of the literature. This study provides insight into the mechanical performances of hybrid FRP-steel RC beams, builds the reliable FE models which can be used to predict the structural behavior of hybrid RC beams, offers a rational design method together with an useful database to evaluate the ductility for concrete beams with the combination of FRP and steel reinforcement, and motivates the further development in the future research by applying parametric study.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.21 no.3
• /
• pp.69-75
• /
• 2017

Ultra-high performance concrete and high ductile cementless composite are considered as promising construction materials because those exhibits higher performance in terms of high strength and high ductility. The purpose of this study is to investigate experimentally the compressive strength and tensile behavior of ultra-high performance concrete and high ductile cementless composite. A series of experiments including density, compressive strength, and uniaxial tension tests were performed. Test results showed that the compressive strength and tensile strength of alkali-activated slag based high ductile cementless composite were lower than those of ultra-high performance concrete. However, the tensile strain capacity and toughness of alkali-activated slag based high ductile cementless composite were higher than those of ultra-high performance concrete. And it was exhibited that a high ductility up to 7.89% can be attainable by incorporating polyethylene fiber into the alkali-activated slag based cementless paste.