EFFECT OF CODE REVISION ON COST OF WATER TANK by Harkomal Kaur
In present scenario structural designer are not using working stress method but limit state method is extensively used comparison to working stress method. Indian standard does not added limit state design for a very long time IS: 3370 for water tank, accepted limit state design after considering the cracking width check. In the new design provision the code has three design procedures for designing the concrete structure
which store water.
i) Working Stress Method.
ii) Limit State Design Method – Crack width check.
iii) Limit State Design Method – Deemed to satisfy.
The objectives of the present investigation were
1. Investigating, effect on materials used at component level due to revision of code.
2. Investigating, effect on cost at component level due to revision of code.
3. Proposing guidelines for designer, to obtain cost effective solution, based on IS: 3370 (2009).
In the investigation two types of tank were taken for a comparative study Circular tank, Intze tank. Circular tank is used for less capacity where as Intze tank is used for large capacity. The tanks are designed as elevated reservoir both tanks are designed by same dimensions and studied with following method.
1) Working stress method IS: 3370 (1965)
2) Working stress method IS: 3370 (2009)
3) Limit State Method of Design and checking cracking width by limit state of Serviceability IS: 3370 (2009)
4) Limit state design process by limiting steel stresses in accordance IS: 3370 (2009)
(Deemed to be satisfied) The tanks were designed by above method with different Mix design; steel used in the design is high yield strength deformed bars. All tanks were taken as 1000 m3 capacities with same dimensions for particular elements of tank, which helped to study the elements in depth and considering the change in the various parameters.
Following are the points observed in this study:-
1. Permissible stress in steel in tension according to working stress method IS: 3370 (1965) and (2009) the value of is 150 N/mm2 and 130 N/mm2 where as in limit state method IS: 3370 (2009) the value of is 130 N/mm2 . These changes the reinforcement quantity for both tanks considered in the study.
2. It has been seen that the area of reinforcement increases in intze tank and decreases in circular tank
because of less structural element. So the circular tank is considered to be the cost effecting water tank design then intze tank for the present calculated capacity.
3. The size of different member changed in the limit state design method IS: 3370 (2009) (crack width) and remained same for working stress method by IS: 3370 (1965) and IS: 3370 (2009). The requirement of area of steel increased in the working stress method IS: 3370 (2009) for intze type water tanks as the allowable stresses in steel were lower. In working stress method the stresses in members is calculated and taken from the working loads which increases the thickness and quantity of steel where as in the limit state design the load is taken from the design strength of the structure