International Journal of Advance Engineering and Research Development

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1 Scientific Journal of Impact Factor (SJIF): 4.72 International Journal of Advance Engineering and Research Development Volume 4, Issue 1, January e-issn (O): p-issn (P): PARAMETRIC STUDY OF RCC STAGING (SUPPORT STRUCTURE) FOR OVERHEAD WATER TANKS AS PER 1S: AND IS:3370 Issar Kapadia 1, NileshDholiya 2, Purav Patel 3, Guide Prof. Nikunj Patel 4 1,2,3,4 Civil engineering department, SPCE (Bakrol) Abstract:-Water tanks are important public utility and industrial structure. The design and construction methods in reinforced concrete are influenced by the prevailing construction practices, the physical property of the material and the climatic conditions. For RRC staging of overhead water tanks or towers, should be so constructed against of seismic waves or vibration forces of either naturally or by other ways like due to heavy vehicles or another like as mining operations etc. so that the collapses minimized. Also, the tanks fail due to cracks in the structure of water tower because of the improper designs, improper analyses and estimations, lack of seismic waves resistant design, improper geological selection of support structure. In this paper as per 1S: and IS:3370 recommendation reviews are discussed for the design of RCC overhead water tank. Key words: Support structures, RCC, seismic waves, vibration forces, shaft, bracing, overhead water tanks, damage, analysis, criteria. I. INTRODUCTION Overhead water tanks of various shapes can be used as service reservoirs, as a balancing tank in water supply schemes and for replenishing the tanks for various purposes. Reinforced concrete water towers have distinct advantages as they are not affected by climatic changes, are leak proof, provide greater rigidity and are adoptable for all shapes. A. Components of a water tower or tank consists of: (a) Tank portion with (1) Roof and roof beams (if any) (2) sidewalls (3) Floor or bottom slab (4) floor beams, including circular girder (5) cylindrical portion (b) Staging portion, consisting of (6) Columns (7) Bracings and (8) Foundations B. Types of overhead water Tanks may be: 1. Circular tanks 2. Rectangular tanks 3. Intze tanks 4. Circular tank with conical bottom 5. Spherical tanks. Among these the circular types are proposed for large capacities. Such circular tanks may have flat floors or domical floors and these are supported on circular girder. The most common type of circular tank is the one which is called an Intze Tank. In such cases, a domed cover is provided at top with a cylindrical and conical wall at bottom. A ring beam will be required to support the domed roof.a ring beam is also provided at the junction of the cylindrical and conical walls.the conical wall and the tank floor are supported on a ring girder which is supported on several columns. Usually a domed floor is shown in fig a result of which the ring girder supported on the columns will be relieved from the horizontal thrusts as the horizontal thrusts of the conical wall and the domed floor act in opposite direction. Sometimes, a vertical hollow shaft may be provided which may be supported on the domed floor. C. General & Design Requirement of Liquid retaining: Impervious floor. Minimum strength of cement. Water cement ratio. Tensile stresses. Cracking. D. Permissible stresses in steel (For resistance to cracking): When steel and concrete are assumed to act together for checking the tensile stress in concrete for avoidance of crack, the tensile stress in steel will be limited by the requirement that the permissible tensile stress in the concrete is All rights Reserved 76

2 exceeded so the tensile stress in steel shall be equal to the product of modular ratio of steel and concrete, and the corresponding allowable tensile stress in concrete. Grade of concrete Permissible stress in N/mm 2 tension shear (N/mm 2) (t v ) Direct(σct) Bending(σcbt) M M M M M M Table 1.Basic design requirement for liquid retaining structures to cracking as per IS 3370 are as above. E. Types of staging: 1. Hollow circular shaft is the most popular type of staging to support a tank container. The height of the shaft varied from a minimum of about 10m to a maximum of 20m whereas the shape and size of the tank container largely depended on the storage capacity and required head for the water supply. The affected tanks varied in their storage capacity from 80kL to 1000kL. The diameter of the staging generally increases with increase in the capacity of the tank, however, the thickness of the staging section is usually kept between 150 and 200mm. Fig 1. Column bracing typed water tank or Frame Type. Fig 2. Hollow circular shaft type water All rights Reserved 77

3 2. Brace column staging This typed staging is laid when the capacity of water quantity in the container is not large. The cost of staging for this is not more compare to the hollow circular shaft. If the frame members and the brace column joints are not designed and detailed for inelastic deformations, a collapse of the staging may occur under seismic overloads. II. CLASSIFICATION AND LAYOUT OF ELEVATED TANKS Based on the capacities of the tank, the possible classification for types of elevated tanks may be as followed as given for general guidance. 1. For tank, up to 50 m 3 capacity may be square or circular in shape and supported on staging three or four columns. 2. Tanks of capacity above 50m 3 and up to 200 m 3 may be square or circular in plan and supported on minimum four columns. 3. For same criteria, (as above in point 1 and 2) depending upon the discretion of the designer, tanks of unusual shapes, such as spherical, conical or multicell may also be adopted. 4. For capacity above 200 m 3 and up to 800 m 3 the tank may be square, rectangular, circular or intze type tank. The number of columns to be adopted shall be decided based on the column spacing which normally lies between 3.6 and 4.5 m. For circular, intze or conical tanks, a shaft supporting structures may be provided. 5. Different shapes of water towers with certain arrangements of bottom construction are shown in figure as Fig. 3 Arrangement for tank with 8 columns and 12 columns III. OVERVIEW, DESIGN CRITERIA AND ANALYSIS OF WATER 1. Column Foundations: Separate footings may be provided for column staging and designed.for all towers situated in seismic zones where seismic co-efficient is above 0.05, at base of the tower all columns shall be tied together above foundation level and well within ground by a structural member such as braces or ring beam. Alternately continuous strip (or annular strip) foundation, mat or raft foundations shall be All rights Reserved 78

4 The foundation shall be so proportioned that under vertical loads of tower (with tank full as well as empty) and effects of horizontal forces, the pressure on the soil is within the safe bearing capacity and in the critical direction the footing does not lift up at any point. Loss of contact in the soil under footing should not be allowed. In locations where the soil bearing capacity is high, loss of contact may be allowed provided it is safe against overturning and such other conditions that are to be fulfilled. 2. Shaft type staging: The tower may be in the form of single shaft circular or polygonal in plan and may be tapering. The area enclosed with the shafts may be used for providing the pipes, stairs, electrical control panels, etc. The minimum thickness of concrete shell for staging shall be 150 mm. When internal diameter exceeds 6 m, the minimum thickness in mm shall be [150 +( D ) / 120]. Where, D is the internal diameter of concrete shell in mm. 3. Bracing and columns type staging: For staging of height above foundation greater than 6 m, the column shall be rigidly connected by horizontal bracings suitably spaced vertically at distances not exceeding 6 m. 4. Forces and moments on columns: The entire load of the tanks shall be transferred to the columns in the way the floor of the tank contributes to each column. The effects of continuity of the beam at the top of columns, if any, shall be accounted for in calculating the reactions on columns. In addition to tank load, axial forces, forces and bending moments due to wind, earthquake or vibration shall be considered. Fig 4. bending moment, axial stresses, bending stress, centre of mass position in shaft type water tank IV. CONCLUSION 1. For basic staging overturning moment is highest as compare to the other staging pattern. 2. The current designs of RC shaft type circular staging (supporting structure) for elevated water tanks are extremely vulnerable to lateral loads caused by earthquakes. It is evident from the damages sustained to staging as far as 125km away from the epicentral tract of the Bhuj earthquake. 3. for same capacity of both frame and shaft typed tank, In hollow circular shaft type staging displacement value increase with increase in Height level of staging. 4. The horizontal displacement value will be more in frame type staging water tank than shaft type water tank. 5. Due to system frame type staging base shear value is more than shaft type staging. 6. In places where continuous vibration is likely to occur like near rail- way tracks, steel staging may be All rights Reserved 79

5 V. REFERENCES 1. Durgesh, C. R. (2001) Performance of Elevated Tanks in Mw 7.7 Bhuj Earthquake. Department of Civil Engineering, Indian Institute of Technology, India. 2. Pavan.S. Ekbote, Seismic Behaviour of RC Elevated Water Tank under Different Types of Staging Pattern Journal of Engineering, Computers & Applied Sciences (JEC&AS) ISSN No: Volume 2, No.8, August Reinforce concrete structures (Dr B.C PUNMIA). 4. Tehrani Zade. M, Haj Najafi. L, Assessing seismic behaviour of eccentrically braced frames (EBFs) due to near-field ground motions. The 14th World Conference on Earthquake Engineering, china, BIS 2000 IS: Indian standard for plain and reinforcedconcrete Codeofpractice,BureauofIndianStandards, New Delhi. 6. BIS 1985 IS: Criteria for design of RCC staging for overhead water tanks, Bureau of Indian Standards, New Delhi. 7. Bis code of practice for concrete structures for the storage of liquids part iii prestressed concrete structures,bureau of Indian standards,new Delhi The Architectural Institute of Japan: Design Recommendation for Storage Tanks and Their Supports, All rights Reserved 80