| In recent years, electric Power Projects of our country has developed quickly. With regard to electric voltage, its class has risen from 110kV、220kV to extra high voltage of 330kV、500kv. Nowadays, our country’s electric network line is launching the plan and research work of ultra high voltage of 1000kV. Compared with that of today, weight of one tower in this transmission line will increase about 4-5 times. Then it will call for higher requirement on steel intensity and structural design of electric transmission towers. In order to make building cost of towers more economic, steel intensity must be strengthened on a large scale, which includes intensity class of Q390、Q420、Q460.Steel tubular structure has lots of advantages and is widely used for large span public structures and medium-to-high-rise buildings. Although significant progress has been made in the study of static and fatigue behavior of tubular joints, many deficiencies still exist if compared with the demands of engineering practices.The study on ultimate strength of steel tubular joints is very important to the design of steel tubular structures. Because of the influence of the behavior of joints and various complicated geometric shape, not only material and geometrical nonlinear but also the inelastic property of joints should be included in the analysis regarding the ultimate strength of tubular joints.The thesis is based on the reviewing state-of-the-art of tubular joints, Combining the steel tubular tower joints in river crossing transmission power from Shiyan to Xiangfan, K-joints, T-joints and h-joints are studied by full scale model experiment research and non-linear finite element analysis, in which steel tubular Q420 is main material of the tower at first in China. It’s revealed whether it’s feasible to the utilization of steel tubular tower with high-strength steel in 500kV double-circuit transmission line.The main contents include:1. Test study is the one of most important research method of the stress, the displacement and the ultimate bearing capacity of tubular joints. Five groups and ten full size are designed to be tested,3-d strain rosette set in the joint region and dial indicator on the chord and branches, applying the one-way loading methods to test. The stress value in the joint region and the chord and branches deflection can be got through step stress test, the stress distribution regularity in the joint region and ultimate bearing capacity also analyzed.And a complex of testing equipment is constructied, which can change stress to pressure.It provides valuable suggestion for the design of test facility.2. An ideal elastic-plastic material is assumed, along with the material and geometry being considered nonlinear, a six-nodes shell element in ANSYS is employed to simulate the K1-joint, K2-joint, K3-joint, T-joint, h-joint in the analysis. Non-linear finite element analysis was also been done. By comparing the experimental data with the results of the analysis, it’s shown that the joint t strength, the failure mechanism, the stress and strain are very close to each other of joints. It is found that the numerical simulation is a effective method to make sure of the working performance and ultimate bearing capacity. Then by comparing with the tests result and results of finite-element analysis, the difference of them is founded that manufacturing error of specimen size, error of material characteristic, the effect of welding residual stresses, the treatment of boundary conditions, location of observation point and so on.3. Full scale tests on K-joint samples are carried out in this dissertation, which some experimental date is obtained about the failure mechanism, the stress and strain, ultimate strength.The test results indicate that under high axial force, local plastic deformation of the chord may become the main failure mode of K-joints. It shows through the deformation analysis in joint region that large plastic deformation may occur under the repetitive load, becoming convex or sunken in this position, so the deformation of chord in region responds a lot to the failure. Installation of stiffened plate in K3-joints can transferr the load form branch to chord effectively, which plays an important role in delaying the process of chord deformation and increasing stiffness of joints, so as to improve the mechanics performance of structure in joint region. It is reason that the ulimate capacity of K3-joint is higner than that of K2-joint by 12.4%.4. The bearing capacity has increase with the inhance of strength of steel in chord of K-joints, but the increase speed of the bearing capacity was slower than those of strength of steel. When other parameter is similar, the influence of stiffed plate thickness on the bearing capacity of K3-joints is weak, and it’s feasible that thickness of stiffed plate is equated with that of the chord or the branch in practice project. Moreover, these results are calculated according to GB50017-2003 code for design of steel structures of Chinese code in line with our code, which finds out the existing formula cannot be applied mechanically in K-type high strength steel joint.5. The experimental study of T-joint directly welded with ring plate is conducted, attach and the experimental are compared with that acquired by the numerical analysis. The results show that subjected to the specific load, the T-joint perform well, and the ultimate bearing capacity can meet the design requirement. This kind of joint has larger rigidity. The ring plate might play a positive role in the restriction of chord deformation and delaying development of plastic deformation.In addition, finite element method is used in this research to study the effect of the steel strength and the geometrical parameters on the ultimate capacity of tubular T-joints. The results indicate that the material intensity of chord is the determining factor of the ultimate capacity to T-joints. However, the bearing capacities of joints increase slightly with a improving of the material intensity or thickness of ring plate. For this project, it is fit and economical that the thickness of ring plate is about two times thickness of chord.6. The tube and ear-plate joint is widely applied in tall steel tubular tower of transmission line, but rarely at home and abroad to research. By experimental study on typical h-joints with full scale model and analysis based on FEM calculation, this research finds out mechanics behavior and distribution of stress on the kind of joint. Furthermore, based on the stress features of the connections, the bending stress model can be established.The results show that the first point on steel tube nearing ear-plate is a stress control point on the measure section during this experimental loading process; bending moment transferred by ear plate beyond nallow region leads to local buckling on the chord, so local strength of chord is one of the most important control factors in the design of this typical joint; the bearing capacity might be improve with inhancing the strength of steel or increasing the thickness, but it increases or decreases no more than 10% when the thickness increases or decreases by 25%.Combining the achievements on dangerous section of chord in touch of ear-plate through test and infinite element analysis, improvement measures of the connects is put forward as stress concentration is easy to emerge, such as adding outer half-ring stiffening plate, adding outer-ring plate, adding inner-ring plate, and ear-plate revising as the very smooth nook and so on.In summary, based on the results of experiment and numerical analysis, the distribution and changing trends of stress state is found out in steel tubular joints, and it is proven that the design of steel tubular tower is reliable and safe. Some useful suggestions for the optimization-design of steel tubular tower with high-strength steel Q420 are presented. Generally speaking, Q420 tubular-type joint can be applied and extended in design of tall steel tubular tower in transmission line. |
PDF Full Text Request