Static Analysis Of Prestressed Concrete Spiral Case Structure For Large Scale Hydropower Station

Spiral case is a concernful diversion-component of hydroelectric generating set in hydropower house,and the choice of its structural form is very important.Metal volute is widely used in hydropower station with a head above 40 m.However,for medium head large hydropower station with head between 40 m and 60 m,the dimension of volute is large,steel spiral case is of poor economy,concrete spiral case is still the preferred structural form.Concrete volute generally uses trapezoidal section,and it is prone to stress concentration under complex stress conditions,so crack control and anti-seepage have been the difficulty of design.Prestressed anchor cable can be used to reinforce the concrete volute,it can play role in balancing tensile stress and improving the stress state of structure,and it can also optimize reinforcement,so it is the preferred solution to crack control and anti-seepage of concrete spiral case.The reinforcement effect of anchor is determined by cable arrangement scheme.In this paper,a large-scale concrete spiral case structure is taken as an example,a cost-effective prestressed anchor scheme was proposed according to the distribution range of high tensile stress area in each direction and mechanics characteristics of weak parts for volute concrete,which can provide a beneficial reference for the anchor cable arrangement of future similar projects.The main work of this article is as follows:(1)Based on the finite element analysis software,a volute monomer calculation model was established,after comparing concrete stress characteristics of the whole structure and each typical cross-section under 7 working conditions,control load case of load-bearing capacity was found,and the distribution location of volute concrete high tensile stress area was preliminary understood.(2)The stress distribution rule of high tensile stress in concrete was investigated in whole flow passage,the results show that radial tensile stress and principle tensile stress in the connection inner corner area between inlet sidewall and roof(or floor)is largest,and vertical tensile stress in the outside midspan of left sidewall is large,stress in flow direction is little.Therefore,it is considered that radial(longitudinal)anchor cable in inlet roof and floor near runner is very necessary,anchor cable in flow direction can be cancelled.(3)After comparing the reinforcement effect of 4 anchor cable arrangement schemes,a best arrangement scheme was recommended.In this scheme,radial anchor cable was arranged in roof and floor of volute inlet,and the arrangement scope of radial anchor cover the vicinity of 0°section in downstream,the vertical anchor should be arranged on both the inside and outside of sidewall.Finally,the maximum principle tensile stress of concrete in the connection area between left sidewall and roof(or floor)decreased from 4.92 MPa to 3.81 MPa,so it can greatly reduce the technical difficulty of limiting crack and anti-seepage requirements for concrete in the area,at the same time,the maximum value of the principal tensile stress of the concrete in the outside midspan of the left sidewall is lower than the design value of the tensile strength,which can achieve the crack-resisting requirement.(4)A 3-D nonlinear finite element model of volute monomer model was established,the concrete stress of volute and cracks distribution of different reinforcement scheme were examined,steel stress and crack width were checked,and optimum design of reinforced bar was carried out.The results show that reinforcement of concrete spiral case according to the optimal anchor scheme enables structure to meet safety requirements at lower reinforcement ratio.The increase of rebar spacing ensures the quality of concrete poured,and saves engineering cost.