Kyung-Jae Shin

External Posttensioning of Reinforced Concrete Beams Using a V-Shaped Steel Rod System

This study presents the development of an analysis method to predict the moments at first yielding and at ultimate strength of reinforced concrete beams strengthened using a V-shaped posttensioning steel rod system. The analysis requires determination of a total elongation of the external posttensioning rods, which is interdependent upon the neutral axis depth, deflection, and strains and stresses in beam components at both internal steel yielding and ultimate strength stages. The iteration algorithm was computationally programmed. The test results from a total of 14 full-scale beams were used to verify the analysis method that predicts the yield moment, moment capacity, and corresponding unbonded steel rod stresses. Finally, a flexural design method is developed for reinforced concrete beams externally posttensioned using a V-shaped steel rod system.

Experimental Investigation of Composite Moment Connections with External Stiffeners

This paper presents tensile behavior for Concrete-Filled Tubular (CFT) column to H-beam welded connections. These connections were reinforced externally with T-shaped stiffeners at the junction of column and beam. Tensile loading tests and analyses were carried out for the T-stiffener connections. The results demonstrated that the overall behavior and failure modes of all the specimens were governed mainly by the horizontal stiffeners rather than by the vertical stiffeners, while the vertical stiffener only played a role in transferring the load introduced from beam to column. In addition, a simple design method for the T-stiffeneris suggested, with consideration of the effects of the shear strength of the horizontal stiffener and yield line mechanism of the column flange. The prediction and test results are compared and they are in good agreement.

Gust Effect Factor for Wind Load Estimation of Tree Supporting Systems

Strong winds have caused an increasing amount of damage to fruit trees, such as uprooting and fruit drop, and various prop systems or support systems have been introduced to prevent this wind damage. When a prop system is designed against strong winds, it is essential to calculate the wind load acting on each tree in order to accurately assess the wind resistance of the prop system. In particular, the fluctuating effect is significant and cannot be ignored when the natural frequency of a tree is relatively small. In this study, vibration tests were performed to measure the natural frequencies and damping ratios of trees in order to evaluate the wind load acting on the trees. Both ambient and free vibration tests were performed, and the dynamic properties were identified and compared. It was found that the average natural frequency of trees was approximately 1.0 Hz, and therefore the dynamic effect against fluctuating wind load needs to be considered. The analysis results of the gust effect factor indicated that the wind load may be underestimated or overestimated considerably if the flexible nature of trees is neglected, or if the exact values of natural frequency and damping ratio are not available.

Shear Strength of Externally Post-Tensioned Concrete Beams

This paper shows the test results of continuous reinforced concrete beams with external post-tensioning rods. Six three-span beams were prepared and tested to fail. Three beams were designed to have flexure-dominating behavior and the others to have shear-critical behavior. In each group, one beam without external post-tensioning rods was designated as a control beam and two beams had the external post-tensioning rods of 18 mm or 22 mm diameter. External post-tensioning rods were installed within an interior span of 6000 mm. They show V-shaped configuration because two anchorages were located at the top of interior supports and a saddle pin at mid-span was installed at the bottom of the beam. Test results show that the load and shear capacities of strengthened beams were increased when compared with the control beam. Additionally, the measured shear strength was compared with the strength predicted by ACI 318-11 code equations. The detailed ACI 318-11 equation predicted the measured shear strength and failure location of the continuous beam reasonably well.

Flexural Analysis of Full-Scale RC Beam Strengthened with Steel Rods

This paper presents an analytical method of evaluating the flexural behavior of RC beams externally strengthened with unbonded high-tension steel rod. A simple summation method proposed in this paper basically assumes that the total strength of RC beams strengthened with rod is equal to the sum of the strength of RC beam and the strength of rod. The strengthening steel rods connected to beam with U and V shape profile are considered as a truss system for the analysis. This analytical method also includes the effect of compressive force applied to RC beam due to the tension of rod as well as the effect of effective prestress in steel rod. A comparison of the analytical and experimental results leads to the conclusion that the simple summation method can simulate the flexural behavior of RC beam strengthened with rod with a good level of accuracy.

Monitoring of smart concrete beams in flexure using polymer-based composite sensors

Concrete may the economical material available for buildings and civil structures due to various important its properties such as high compressive strength, wear resistance, abrasion resistance and durability. The most disadvantages of concrete structural elements are its cracks in flexure. Visual inspection is difficult and provides little detailed information in crack conditions. Recently, a new trend, called smart concrete or structure, has been emerged using various technologies for monitoring of crack conditions of concrete. A method designed to monitor or characterize the crack conditions in concrete beams in flexure using polymerbased composite sensors is conducted in the present work. The embedded polymer-based composite sensor shows a potential to evaluate the conditions of concretes cracks in beams under flexural loading such as initial and critical crack conditions, using data acquisition system.