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RISS 인기검색어
- 검색키워드
- 저자 : Jijun Miao (검색결과 4 건)
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Calculation of Thermal Fields of Cracked Concrete at Elevated Temperatures
Yanchun Liu,Jijun Miao,Jialiang Liu,Guangzhong Ba,Caiwei Liu 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.1
This paper analyzes the effects of cracking on the thermal field of concrete at elevated temperatures. In four concrete specimens with pre-made cracks, fire tests were performed. Then finite element analysis and theoretical analysis were conducted, which have indicated that heat conduction was the primary source of heat transfer in cracked concrete. At last mathematical software MATLAB was used to calculate the temperatures distributions of a simplified model based on theoretical analysis, which were compared with the test results. The results show that as the thermal diffusion of air is far greater than that of concrete, there is an increase in the thermal propagation through the cracked regions in comparison with undamaged regions, and the temperatures increase with the increase of cracks’ widths. The temperature of concrete farther away from a crack was less affected because concrete has a lower thermal diffusion coefficient. The water content of concrete has an impact on the temperature distributions, which should be considered at the early stage of fire when determining the concrete temperature distributions.
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Effect of Stirrup Corrosion and Fire on Shear Behavior of Reinforced Concrete Beams
Jialiang Liu,Jijun Miao,Guangzhong Ba,Jiangzhuang Xiao,Dongshuai Hou 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.9
This article presents an investigation on corrosion damage effects on the shear bearing capacity of reinforced concrete (RC) beams after a fire. Accelerated corrosion tests were conducted using four RC beams designed with corrosion crack widths ranging from 0.1 mm to 0.3 mm to simulate an aggressive corrosion-prone environment. One control beam (B1) did not undergo accelerated corrosion. The fire test was conducted separately on each beam for two hours to explore how the different widths of corrosive cracks affect heat propagation at elevated fire-induced temperatures. A residual capacity test determined the effects of corrosion on the beams’ residual shear strength after a fire. The experimental results showed that corrosion cracks accelerate the heat propagation in concrete during a fire; moreover, the post-fire residual shear strength for corroded RC beams slightly decreased with an increasing degree of corrosion on the stirrups. The authors also developed a corresponding simplified calculating method to determine residual shear strength, which showed shear design provisions that could improve some existing codes.
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Caiwei Liu,Xiuliang Lu,Guangzhong Ba,Hao Liu,Jijun Miao 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.12
To explore the influence of the loading conditions and fire exposure time on the residual mechanical properties of concrete T-beams, 10 concrete T-shaped beams were designed and manufactured. Three levels of loading were applied to the concrete T-beams during fire tests, namely, unloaded, loaded without concrete cracking and normal service conditions. The fire test durations were 60 min, 90 min, and 120 min. After the fire tests, static loading tests were carried out on the fire-damaged T-beams to determine the residual flexural capacity. The test results showed that with an increase in load ratio and fire time, the maximum temperature experienced by the concrete and steel bar increased. For example, for the specimens that underwent 120 minutes of fire exposure, the temperature of the specimen with a load ratio of 0.44 was larger than that with load ratios of 0.12 and 0, and the difference was 60°C and 80°C, respectively. During the fire tests, the mid-span deflection also increased significantly with an increase in load ratio and fire duration. In addition, the flexural load-bearing capacity after fire exposure decreased with an increase in load ratio and fire time. Compared with the bearing capacity, the degradation of the flexural rigidity was more obvious. For example, when the fire exposure time was 120 minutes, the bending capacity of the beam with a load ratio of 0.44 was approximately 4.5% lower and 5.1% lower than that with a load ratio of 0.12 and 0, respectively, and the reduction in flexural rigidity became 6.8% and 15%, respectively. Considering the effect of cracking, ANSYS was used to analyze the temperature field, deflection, and bearing capacity of the T-beams. It was determined that the calculation model that considers the effects of cracks was more accurate than the calculation model that disregards the appearance of cracks. This study can provide a basis for the assessment of fire damage and the repair of concrete structures.
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Fire Damage Identification in RC Beams based on Support Vector Machines considering Vibration Test
Chaofeng Liu,Caiwei Liu,Chengxin Liu,Xuhong Huang,Jijun Miao,Wenlong Xu 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.10
In order to obtain the degree of damage in reinforced concrete (RC) beams exposed to fire, using the equivalent fire exposure time as the damage index, a new method of damage identification based on the support vector machine technology was proposed. Firstly, the feasibility analysis was conducted based on finite element models of simply supported beams. Thereafter, four RC simply supported beams were designed for fire test and vibration test, which were used to amend the finite element model and the SVM-based identification method. Fire tests were carried out on 4 beams for 60, 90, 120, and 150 min, respectively. During and after the fire tests, structural modal information were recorded. The first two order modal information, as SVM input paraments, was used to predict the equivalent fire exposure time based on SVM. The predicted results were very close to the actual fire exposure time. The residual bearing capacities of the beams after fire were calculated according to the predicted fire exposure time, which were close to experimental results. It indicated that the equivalent fire exposure time as the output parameter for damage identification was reliable. Finally, on the basis of damage identification method for simply supported beams, a new three-step positioning method was established for identifing the degree of damage in continuous beams. The method was applied to a thress-span continuous beam. The numercial situlation results revealed that the three-step positioning method was accurate.
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