| The spiral case embedded with a membrane has a broad application prospect owing to convenient construction, short construction period and low cost. Among the large-scale hydropower stations with unit installed capacity exceeding 700MW, this type of spiral case has been used in 9 units of the Three Gorges Hydropower Station and all units of Laxiwa and Longtan Hydropower Stations. And another combined type with partial cushion layer embedded between spiral case and concrete was or will be used in Xiangjiaba, Xiluodu, Wudongde, Baihetan Hydropower Stations.The mechanical characteristics of spiral case is affected by several factors, such as contact behavior between the steel spiral case and the membrane or concrete, spatial properties and material properties of membrane and so on. It is necessary to study the influence mechanism of structural characteristics to provide the instruction for the large-scale projects. On the other hand, underground powerhouse is used more and more widely. The dynamic interaction between powerhouse structure and surrounding rock, which-has an effect on natural vibration characteristics and dynamic responses of powerhouse structure, needs focused research. Combined with Ludila Hydropower Station, the primary research contents in this paper are as follows.(1) To fully reveal the influence factors and extent of the structural characteristics of spiral case with a membrane, the friction coefficient, plane covering range of membrane, covering range of membrane on the meridian cross section, stiffness coefficient of membrane, expansion joint and anti-thrust ring were researched meticulously. The results show that friction coefficient, plane covering range and stiffness coefficient of membrane are important factors of shear-resisting ability of stay ring. In detail, the plane covering range is benefit for stay ring when less than 45° or more than 270°, and friction coefficient, covering range of membrane on meridian cross section and stiffness coefficient of membrane are critical factors affectting the percentage of internal water pressure transmitting outward. Otherwise, the plane covering range of membrane is benefit for the non-uniform deformation of generator pedestal if it ranges from 90° to 180°. The non-uniform deformation of generator pedestal is sensitive to stiffness coefficient of membrane rather than covering range of membrane on cross section and friction coefficient. In addtition, expansion joint is not good for shear-resisting of stay ring, which can be improved by anti-thrust ring.(2) To further demonstrate that, how the method decreasing traditional covering range of membrane works on harmonizing principal contradictions of spiral case, based on concrete plastic-damage model, combined with Ludila Hydropower Station, the 3-D nonlinear finite analysis was conducted on traditional spiral case with a membrane, complete bearing spiral case and a combination of above two schemes called combination scheme. The results show that, combination scheme achieves the same effect on limiting crack damage of concrete and non-uniform deformation of generator pedestal as traditional spiral case with a membrane, and further more combination scheme can do benefits to shear-resisting of stay ring, so it can be considered preferentially for similar projects.(3) The structural form of underground powerhouse and interaction with surrounding rock are determining factors of dynamic characteristics. Equivalent elastic linkages were adopted to simulate the constraints of surrounding rock when discussing natural vibration characteristics. The results show that,The effect of different elastic boundary exceeds 10% for powerhouse's whole-vibration frequency, but the effect is less than 5% for structural component in the first 20 orders. The effect of thickening the floors with 10-20cm is less than 5% for structural component's vibration frequency while the effect of 40cm is more than 10% for some orders. But the frequency range of the first 20 orders is changed slightly. The effect of adding columns exceeds 10% for structural component's vibration frequency in some orders. But as powerhouse's natural vibration frequency is very intensity, from the view of resonance check, above measures are unefficient for avoiding the inside exciting frequency in hydropower house.(4) Internal oscillation and measures to reduce vibration were researched.3-D finite dynamic method was used to discuss dynamic response of powerhouse under vibration loads of hydro-turbine generator units and pulsating pressure in flow channel. The results show that, the vibration amplitudeof generator pedestal is low caused by unit vibration during rated operation, and the dynamic stress is large only in loading area. Usually the vortex strip with low frequency in draft tube is a main kind of vibrating source, but if the excitation frequency is a multiple of turbine wheel blades, the vertical amplitude of combination area between lifting hole and stair hole will be obvious both on generator floor and generatrix floor, and adding columns or visible beams can reduce the amplitude.(5) The seismic analysis of underground powerhouse has been paidless attention to. Interaction between powerhouse structure and surrounding rock considered, visco elastic artificial boundary and seismic wave input were accomplished by extended-developmen tprogram using ANSYS Parametric Design Language (APDL). The results show that, the program mentioned above is feasible and efficient. The seismic responses of structure above generatrix floor are more notable, but the peak values of tension and compression stresses for floors are less than 0.6MPa, which indicates thatunderground powerhouse has good seismic performances. |
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