DESIGN OF SUBSTRUCTURE OF HYDRO POWER STATIONS

Abstract

Substructure of a hydro power station accommodates numerous odd shaped and asymmetrical cavities such as scroll case, draft tube, drainage galleries etc., which make the structural 'analysis of the substructure extremely complica-ted. Moreover, because of characteristic features of shape and size of the cavities and the structure, structurally it becomes a three-dimensional problem. Conventional analy- tical methods cannot cater for such complex structures. Therefore, design of substructure is generally tackled by splitting it in different subdivisions and analysing the individual portion by assuming certain behaviour either as a beam or an arch or a cantilever, so as to make it conform to conventional type of structures. Their design is accom-plished by ordinary traditional methods. The lacunas thus inherited by the conventional procedure make the results significantly out. Another p_:eculiarity of the powerhouse substructure is that the superimposed loads are only a small fraction in comparison to self weight of the substructure, generally within about ten percent. Therefore, the stresses due to self weight are dominant. This calls for a procedure which will properly represent the dead load force, both in magnitude and distribution oi~er the structure. The latter assumes significance specially in case of structures, which are continuously supported on foundation all along the base, since the stress pattern in such structures is very much related to the positions of points of application of forces, The conventional analytical method obviously cannot meet this requirement. therefore the only rational methods are model studies or numerical techniques. The model studies also suffer from their own limitations. With the advent of high speed electronic digital computers, number of numerical techniques are coming up. Amongst them 'Finite Element Technique' is comparatively very recent development and is coming more in vogue because of its wide applicability and the ease with which even most complicated structure' can be tackled accurately. As stated earlier, the structural ana-lysis of a powerhouse substructure is essentially a three-dimensional problem and, therefore, needs being analysed by a three-dimensional finite element method. However, the analysis by three-dimensional, finite element method for such complex structures is extremely complicated, time-consuming and needs a very large capacity and fast computer. Therefore, ►n the present dissertation work, the substructure has been ideali sed into a two dimensional problem and has been ana-lysed as a plane strain case in longitudinal direction by considering number of sections at close intervals and as a plane stress case in transiVerse direction considering the whole unit as a plate of variable thickness. Even though some approximations are introduced in the process, the stress pattern at different sections indicated in this dissertation will not be significantly out. As regards magnitudes of the stresses, the powerhouse substructure owing to the character- istic dimensions decided from hydraulic considerations is essentially a low stress, stable structure. Therefore, even though there will be some variation in magnitudes of the stresses obtained from three dimensional and two dimensional finite element methods, the variation. in their absolute values will be insignificant. However, it is felt that at least for one powerhouse substructure the analysis should be carried out both by three dimensional and two dimensional finite element methods to corroborate the results by two dimensional by finite element method. Once confirmed, in practice the analysis of powerhouse substructures can be carried out by two dimensional finite element technique by idealising the structure as indicated in this dissertation. The significant improvement achieved by this method over conventional design practices is discussed in the Chapter 5. Analysis has been carried out on the substructure of Khodri Power Station of Yafruna Stage II, Hydel Scheme (U.P.) .