Comparative Study of Design of water Tank With Reference to IS: 3370

Transcription

1 Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) Comparative Study of Design of water Tank With Reference to : 337 M Bhandari 1, Karan Deep Singh 1 Dr B R Ambedkar National Institute of Technology, Jalandhar, India Graduate Civil Engineer, Jalandhar, India Abstract-- Limit state method which is widely used has been adopted in the new version of Code of practice for concrete structures for storage of liquids. As per the provisions of the earlier version of the code ( ), the designing of water tanks was permitted by working stress method only.this study was conducted in order to compare the design provisionsof 337 (1965) and 337 (9).In this study, a comparison of design of water tanks using the Working Stress and Limit State methods was carried out. To accomplish the comparative study, design of three types of water tanks that is an elevated circular tank of 5 kl capacity, an elevated square tank of 5 kl capacity and an underground rectangular water tank of 5 kl capacity were taken up. The quantities of materials were calculated for each problem. The results have been presented in the form of graphs and tables, and it has been observed that Design of water tank by Limit State Method is most economical as the quantity of material required is less as compared toworking stress method. Keywords-- working stress method, limit state method, effective cost. I. INTRODUCTION Water is considered as the source of every creation and is thus a very crucial ele for humans to live a healthy life. High demand of Clean and safe drinking water is rising day by day as one can not live without water. It becomes necessary to store water. Water is stored generally in concrete water tanks and later on it is pumped to different areas to serve the counity. Water tanks can be classified as overhead, resting on ground or underground depending on their location. The tanks can be made of steel or concrete. Tanks resting on ground are normally circular or rectangular in shape and are used where large quantities of water need to be stored. Overhead water tanks are used to distribute water directly through gravity flow and are normally of smaller capacity. As the overhead water tanks are open to public view, their shape is influenced by the aesthetic view in the surroundings. Water storage tanks are designed as per the provisions of 337. This code has been revised in 9. In the pre revised version, the tanks were designed using working stress method and on the philosophy of no cracking. As per 337:9, use of limit state method has been permitted. Hence this study was undertaken to compare the provisions of 337: 1965 and 337: 9 and to analyze the cost effectiveness in the terms of amount of steel reinforce and concrete by comparing the design results of different types of water tanks by limit state and working stress design methods. II. GENERAL REQUIREMENTS ACCORDING TO : 337 Concrete - In the construction of concrete structures for the storage of liquids, the imperviousness of concrete is an important basic require. Aggregates and ce are to be proportioned to yield a high quality concrete. The permeability of any uniform and thoroughly compacted concrete of given mix proportion is largely dependent on the water ce ratio.while an increase in the water ce ratio leads to an increase in inherent permeability, a very much reduced water ce ratio of a mix with a given ce content may cause compaction difficulties and thus may prove equally harmful. The mix should be designed in such a way that the resultant concrete has a high degree of imperviousness. Honeycombing and segregation of aggregates are to be minimized as these lead to defects which are responsible for leakage in water storage structures. For a given mix made with particular materials, there is lower limit to water-ce ratio which can be used economically on any job. It is essential to select a rich mix compatible with available aggregates, whose particle shape and grading have an important bearing on workability which must be suited to the means of compaction selected. The minimum ce content, maximum water ce ratio and minimum grade of concrete are tabulated below. 31

2 Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) Table.1 Minimum Ce Content, Maximum water- Ce Ratio and Maximum Grade of Concrete SI No. Concrete Minimum ce content Maximum free water ce Ratio Minimum Grade of Ce 3.1 Comparison in Minimum A comparison of provisions regarding minimum reinforce is shown below in Table 3.1. Table 3.1 Comparison of Minimum Provisions : : Plain concrete 5.5 M Minimum Minimum 3 d ce concrete Prestrese d concrete 3.45 M M4 For small capacity tanks up to 5 m 3 at locations where there is difficulty in providing M3 grade concrete, the minimum grade of concrete may be taken as M5 in areas other than the coastal areas. III. COMPARON OF & In this section, a comparison of different provisions in the pre revised and revised versions of 337 has been made. The revision incorporates a number of important modifications and s, the most important of them being are- Scope has been clarified further by ioning exclusion of dams, pipes, pipelines, lined structures & damp proofing of bases. A clause on exposure condition has been added. A new sub clause on loads has been added under the clause of design. Regarding method of design, it has been specified that one of the two alternative methods of design ie LSD or WSD may be used. A clause on durability has been added giving due reference to 456 in place of earlier clause on protection against corrosion. Provision of crack width calculations due to temperature and moisture and crack width in mature concrete has been incorporated in limit state design. A ) The minimum reinforce in walls, floors and roofs in each of two directions at right angles shall have an area of 1..3 % of cross sectional area of sections thickness < 1. Linearly varying from.3 % to.% for thickness 1 to % for section of thickness > In concrete sections of thickness >5, two layers of reinforce be placed one near each face. B) The minimum reinforce specified above may be decreased by % in case of HYSD bars. A) The minimum reinforce in walls, floors and roofs in each of two directions at right angles, within each surface zone shall not be less than % of surface zone as shown in for HYSD bars...64 % of surface zone for mild steel bars. B ) The minimum reinforce can be further reduced to 1..4 % for HYSD bars. 3. Comparison in Permissible Stresses..4 % for mild steel bars. For tanks having any dimension not more than15 m. C) In wall slabs less than in thickness, the reinforce may be placed in one face. Provisions regarding permissible stresses in steel reinforce different conditions have been tabulated for the two codes. 3

3 Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) Table 3. Comparison of Provisions for Permissible Stresses in Steel Type of Stress Tensile stress in members under direct tension Tensile stress in members under direct tension a)on liquid retaining face b)on face away from liquid for members less than 5 c)on face away from liquid for members more than 5 Compressive stress in columns subjected to direct load Permissible Stresses in N/ Plain round mild steel bars High strengt h defor med bars Permissible Stresses in N/ Plain round mild steel bars High strengt h deform ed bars Design on Basis of Crack Width Working stress method : From no cracking criteria ( ) m = where, m = Modular Ratio T = Design Tensile force Ac = Area of concrete As = Area of steel = Allowable tensile stress in concrete = Allowable tensile stress in concrete 3.3. Limit state method: The permissible limit of crack width is.. The crack widths due to temperature and moisture effects shall be calculated as given below: To be effective in distributing cracking, the amount of reinforce provided needs to be at least as great as given below: ρ crit Where,ρ crit = critical steel ratio, the minimum steel ratio, of steel area to the gross of the whole concrete section, required to distribute the cracking. = direct tensile strength of the iature concrete is given in Table 3.3. f y = characteristic strength of the reinforce. Table 3.3: Comparison of Provisions for Permissible Stresses in Steel Grade of concrete M5 M3 M35 M4 M45 M5 f ct N/ Maximum spacing of crack Smax = = ratio of the tensile strength of the concrete to the average bond strength between concrete and steel which can be taken as /3 for iature concrete. = size of each reinforcing bar Width of fully developed crack, Wmax = Smax α Where α = (Coefficient of thermal expansion of concrete) IV. PROBLEM FORMULATION In order to carry out the comparative study, three types of problems of water tank design were taken into consideration. 1. Overhead circular water tank - 5 kl. Overhead square water tank - 5 kl 3. Underground rectangular water tank - 5 kl 33

4 Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) 4.1. Design Methods A thorough study through both the versions of : 337 reveals the following four methods of designs: 1. Working stress method in accordance 337 (1965).. Working stress method in accordance 337 (9). 3. Limit State method and then checking cracking width by limit state of serviceability 337 (9). 4. Limit state design method by limiting steel stresses in accordance 337 ( 9 ) (Deemed to be satisfied). The tank portion of tanks of different capacities was designed by the above ioned four methods as per the provisions of 337:1965 and 337:9. The quantities of steel and concrete for different tanks by various design methods have been calculated and presented in Tabular as well as graphical form. The quantities of materials for different components of a circular overhead tank for different design theories have been tabulated in Table 4.1. Tables 4. and 4.3 present the quantities of materials for Overhead square Tank and an underground tank, respectively. A comparison of design by working stress method as per and shows that the amount of concrete remains und. The quantity of steel slightly increases as per as the permissible stresses are slightly lower than those adopted in It is further observed that the steel quantity decreases significantly as per in those members where the designed amount of reinforce is less than the minimum reinforce. This is because, in 337-9, the minimum reinforce is calculated on the basis of the area of cross section of the surface zones and not on the basis of entire cross section as in The steel and concrete quantities for Limit State design Method as per decrease significantly as compared to working stress methods of and because higher permissible stresses are permitted and crack width is checked. The quantities of steel for all tanks considered in this study were observed to be the maximum for design as per Limit State Method based on Deemed to Satisfy Criteria. 34 PARAME TER Table 4.1 Comparative Results of Overhead Circular Tank Working Stress Method Limit State Design Method Crack Theory TOP DOME x-sec Area Top thickness Bottom Crack Width Deemed To Satisfy Nil Nil Nil m m TOP RING BEAM 44m m 3m m Top R/F 4 Bottom R/F CYILINDRICAL WALL

5 Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) BOTTOM RING BEAM x-sec Area Support midsection Tension Ast Nil BOTTOM SPHERICAL DOME PARAMETER Table 4. Comparative results of overhead square tank WORKING STRESS METHOD LIMIT STATE DESIGN METHOD CRACK THEORY CRACK WIDTH DEEMED TO SATFY SIDE WALL Top Bottom VERTICAL R/F At Base At Mid Height HORIZONTAL R/F Top at corner At middle of wall BOTTOM SLAB mid section End section

6 STEEL (KG) STEEL (KG) International Journal of Emerging Technology and Advanced Engineering Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) Table 4.3: Comparative results of underground rectangular tank PARAME TER support center TOP SLAB Top thickness edge center WORKING STRESS METHOD LIMIT STATE DESIGN METHOD CRACK THEORY CRACK WIDTH DEEMED TO SATFY LONG WALL SHORT WALLS TOP PORTION BOTTOM PORTION m m BOTTOM SLAB Total Quantity Of Steel Fig 1: Quantity of steel for overhead circular water tank OVERHEAD CIRCULAR WATER TANK Fig : Quantity of steel for overhead square water tank 138 WSM-1965 WMS-9 LSD DTS Quantity OVERHEAD SQUARE WATER TANK Quantityof steel

7 STEEL (KG) International Journal of Emerging Technology and Advanced Engineering Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) 18 UNDERGROUND RECTANGULAR WATER TANK 7 OVERHEAD SQUARE WATER TANK Quantity of steel CONCRETE m Volume of WSM-1965 WMS-9 LSD DTS Fig 5: Concrete volume for overhead square water tank Fig 3: Quantity of steel for underground rectangular water tank 4.3 Quantity Of Concrete CONCRETE m OVERHEAD CIRCULAR WATER TANK Volume of Concrete CONCRETE m UNDERGROUND RECTANGULAR WATER TANK WSM-1965 WMS-9 LSD DTS Volume of Concrete WSM-1965 WMS-9 LSD DTS Fig 4: Concrete volume for overhead circular water tank Fig 6: Concrete volume for underground rectangular water tank V. CONCLUSIONS Limit State Method was found to be most economical for design of water tanks as the quantity of steel and concrete needed is less as compared to working stress method. 37

8 Website: (SN 5-459, O 91:8 Certified Journal, Volume 4, Issue 11, November 14) There was no in size of members for working stress method by : 337 (1965) and : 337 (9). However, steel require increased in : 337 (9) for overhead circular type, overhead square type and for underground rectangular water tanks, as the allowable stresses in steel were lower. The size of members remained same for limit state design methods by : 337 (9) in as well in deemed to satisfy criteria for all the three tank designs. However, the require of area of steel decreased in :337 (9) in limit state design method and increased in deemed to satisfy criteria for all the three tank designs as the allowable stresses in steel were lower. It was found that the provisions of reinforce through the surface zones in : 337(9) provides economical and more effective reinforce by limit state method. REFERENCES [1] Cusson D., Hoogeveen T., 7. Test method for determining coefficient of thermal expansion of high-performance concrete at early stages, National Research Council Canda, Ottawa, Canada. [] Dayaratnam P., Design of d Concrete Structures, 3rd edition, Oxford &IBH Publishing Co. Pvt. Ltd. [3] Durgesh C. Rai, Review of code Designing Forces for Shaft Supports of Elevated Water Tanks. [4] Durgesh, C. R. (1) Performance of Elevated Tanks in Mw 7.7 BhujEarthquake.Depart of Civil Engineering, Indian Institute of Technology, India [5] : 456, Plain and d Concrete Code of Practice [6] : 337 ( Part I )- 1965, Code of Practice for Concrete Structures for the Storage of Liquids [7] : 337 ( Part II )- 1965, Code of Practice for Concrete Structures for the Storage of Liquids [8] : 337 ( Part IV )- 1965, Code of Practice for Concrete Structures for the Storage of Liquids [9] : 337 ( Part )- 9, Concrete Structures for the Storage of Liquids Code of Practice [1] : , Specification for High Strength Deformed Steel Bars and Wired for Concrete [11] 11. : , Specification for High Strength Deformed Steel Bars and Wired for Concrete [1] Jain Ashok K.,. d Concrete Limit State Design, 6thedition, Nem Chand &Bros, Roorkee 38