Vol.35/No.4 (138) (2020)
| Title | Increasing slip coefficient of bolted slip-critical joints using thermal sprayed coating technique |
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| Author | Cheng-Chih Chen, Tsung-Cheng Hsieh, Cheng Chang, Sui-Wei Lee |
| Keywords | Slip-critical joint; thermal sprayed coating; slip coefficient |
| Abstract | The design philosophy of slip-critical joints is to utilize the friction force developed through the clamping force exerted by the pretension of the high-strength bolt. Thus, the slip-critical joint can have resistance in the direction of the bolt shear. This resistance is affected by the bolt clamping force and slip coefficient on the faying surface. The objective of this study is to increase the slip coefficient of bolted slip-critical joints by applying a thermal sprayed coating on faying surface. The specimens were designed to explore the effects of the coating material (aluminum or aluminum-magnesium alloy), coating thickness (150, 300 and 450 µm), and corrosion on the steel plate on the slip coefficient. The test results showed that exist of the rust increased the slip coefficient. In the case of slight corrosion, the blasted-cleaned faying surface resulted in an average slip coefficient of 0.74. The average slip coefficient was 0.88 for either the aluminum or aluminum-magnesium coatings. These slip coefficients are higher than the slip coefficient of 0.33 for unpainted clean mill scale specified in the design code. The coating material and thickness had insignificant effect on the slip coefficient. However, in the case of corrosion, the bolt pretension loss at slip was increased when the coating thickness was increased. The thermal sprayed coating of either aluminum or aluminum-magnesium on the faying surface can enhance the slip coefficient, increase the resistance of the slip-critical joint, and result in a smaller joint size and less high-strength bolts. |
| Title | Study on the sliding shear design for reinforced concrete beams |
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| Author | Yung-Chih Wang, Yu-Ting Kuo |
| Keywords | New RC, plastic hinge, hinge relocation design, sliding shear, diagonal reinforcement. |
| Abstract | This study follow up the previous research on the hinge relocation design by using T-headed bars. The main conclusion is that the T-headed bars as extra reinforcement can successfully relocate the plastic hinge zone from the column face to a distance away from the face, however the final failure mode of sliding shear occurred. The similar sliding shear failure could be also found in the plastic hinge zone occurred at the traditional column face, or in the precast cold connection located in the plastic zone. Therefore, the main purpose of this study is to discuss how to design the diagonal reinforcement of the beam to prevent the occurrence of vertical sliding shear cracks for the general RC beam members subjected to the major earthquake. This article mainly collected the tested beams subjected seismic loading to verify the design of the sliding shear specified in the NZS 3101-2006. It was concluded that the sectional shear stress , 0.25ඥf ୡᇱ(MPa), to check the potential sliding shear failure occurred in the plastic zone of RC beams is suitable for normal-span beams (a/d≧2.5). Therefore, if the potential vertical sliding shear cracks may be occurred in the plastic hinge zone of RC beams, the diagonal reinforcement shall be considered. Meanwhile, the cyclic testing performed comparing two types of beams between with and without diagonal bars repealed that the RC beams with diagonal bars can prevent the sliding shear cracks from occurring, and improve their seismic resistance. The design flow chart for prevention of sliding shear failure occurred in RC beams is finally suggested in the paper. |
| Title | Effect of geometric initial imperfections on seismic collapse capacity of steel special moment frames with deep columns |
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| Author | Ting-Hao Chang ,Tung-Yu Wu |
| Keywords | Geometric initial imperfections; steel special moment frames; seismic collapse capacity |
| Abstract | Although wide-flange (W-shape) steel members are known to have initial geometric imperfections (IGIs) due to fabrication and installation, the effect of IGIs on seismic behavior of steel special moment frames (SMFs) is still not well understood. To address this shortcoming, seismic collapse capacity of 4-story and 8-story prototype SMFs with various types of IGIs is computationally evaluated to quantify the effect of IGIs created by combining buckling shapes. The results show that even though IGIs can affect column buckling behavior and frame collapse mode under certain conditions, their effect on seismic collapse capacity is generally small and inconsistent. Their influence also greatly depends on the directions of applied IGIs and column buckling shapes and may be positive if the directions are misaligned. As a result, it is suggested that initial geometric imperfections need not be incorporated in high fidelity numerical models with high precision, which can generate their own IGIs when loaded. |
| Title | Seismic Performance and Backbone Curve Development of Steel Box Columns Considering Compactness Ratios, Axial Loads and Near-Fault Motions |
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| Author | Chung-Che Chou, Guan-Wei Chen, Te-Hung Lin |
| Keywords | Steel built-up box column, High axial compression force, Lateralcyclic test, Cyclic backbone curve |
| Abstract | This paper presents the seismic evaluation of high-strength steel columns in a seven-story buckling-restrained braced frame under two sets of 11 far-field motions and 11 near-fault motions, representative of maximum considered earthquake (MCE) level. The proposed near-fault displacement protocol contains a large displacement pulse from -2% to +4% drift with several small displacement cycles and a residual drift of 2.5%.The AISC 341 (2016) has a more stringent width-to-thickness (b/t) limit for highly ductile hollow box columns (HBCs) than the AIJ (2010) or Taiwan Code (2010), resulting in significant thickness difference in design. For example, the b/t limits for a highly ductile box column member with a nominal yield strength, Fyn=420 MPa, and an over-strength factor, Ry=1.2, are 12.9 and 21 based on AISC 341 (2016) and Taiwan Code (2010), respectively.Moreover, the cyclic backbone curves based on ASCE 41 (2013) and NIST (2017) underestimate the post-buckling flexural strength of HBCs, particularlyin high axial compression force.The authors conducted cyclic tests of six full-scale, built-up HBCs using SM 570M steel with the actual yield strength of 460-530 MPa using standard and proposed loading protocols.The gathered test data, supported by more test data in this work, are analyzed by a multiple regression method to obtain empirical formulations for the backbone curves of box columns that can predict the maximum column moment, plastic rotation and post-yield hardening stiffness. The proposed formulation reasonably predicts the first-cycle envelope curves of built-up HBCs, significantly improving prediction results based on both ASCE 41 (2013) and NIST (2017). |
| Title | Theoretical and experimental study on vibration mitigation of off-shore wind-turbine using TMD |
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| Author | Ging-Long Lin, Lyan-Ywan Lu, Kai-Ting Lei, Kuang-Yen Liu |
| Keywords | offshore wind turbine, tuned mass damper, jacket-type structure, shaking table test, supporting structure, seismic vibration reduction, simplified model. |
| Abstract | Wind energy is clean and sustainable. Taiwan is establishing offshore wind farms using wind turbines in the Taiwan Strait. Since Taiwan is located in an earthquake active zone, in order to ensure the safety and reliability of offshore wind turbines under waves, wind and earthquakes, this study aims to investigate the suitability of using a tuned mass damper (TMD) to reduce the vibration of offshore wind turbine supporting structures. The TMD can be integrated as a part of the wind turbine structure, so it has less influence on the supporting structure of a wind turbine. In this study, based on the specifications of a 5-MW jacket-type offshore wind turbine suggested by the National Renewable Energy Research Center (NREL, USA), a 1/25 scaled-down test model and its corresponding TMD were fabricated and tested by a shaking table. Additionally, for numerical simulation, a simplified theoretical model for a jacket-type offshore wind turbine structure is proposed and its equation of motion was derived in this study. The proposed theoretical model was verified both in time domain and frequency domain using the result of the shaking table test. The experimental seismic responses of the offshore wind turbine model before and after the installation of TMD were compared, and the control performance of the TMD system for vibration mitigation of the wind turbine structure was evaluated in this study. |
| Title | Seismic Behavior of Bridge Columns with Partially Unbonded and Non-prestressed Steel Strands |
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| Author | Yu-Chen Ou, Jhen-Wei Wu, Ade Yuniati Pratiwi |
| Keywords | Columns, near-fault ground motions, residual displacement, self-centering, post-yielding stiffness, cover concrete |
| Abstract | The impulse vibrations generated by near-fault earthquakes are likely to cause large residual displacements of RC bridge columns after the earthquake, which seriously endangers the safety and the serviceability of the bridge. A new type self-centering bridge column is developed in this research. The new column uses high-strength steel strands as the elastic element, which can reduce the large residual displacement of the bridge after the near-fault ground motions. In this study, four large-scale columns were tested using single-curvature cyclic loading, including a conventional column and three new self-centering bridge columns. The test parameters were the use of steel strands and different cover concrete thicknesses. According to the post-yielding stiffness ratio and the actual thickness of the cover concrete from the test results, a linear regression formula for the ratio between the depth of the column to the thickness of the cover concrete and the post-yielding stiffness ratio was established. For the same specimen, the lower the thickness of the cover concrete on the compression side, the higher the post-yielding stiffness. The use of steel strands in the tension zone is important to maintain the post-yield stiffness of the bridge column. The maintenance of the strength in the compression zone is also an important factor. Because the steel strands in the tension zone remain elastic, the tension in the tension zone continues to increase after the conventional longitudinal steel bars yield. Due to force equilibrium of the section, the compression force has to increase accordingly. Test results show that the aforementioned increased compression force is likely to cause early crushing of the cover concrete of the compressive zone, which leads to the loss of the compressive zone and decrease the distance between the tension and compression resultant forces of the section. The test results of CSC3 show that when the ratio between the depth of the bridge column to the thickness of the cover concrete is 30, the average post-yield stiffness ratio can reach 5.7%. |