huangyhg

forces acting on a circular tank walls-fixed bottom
in circular tanks (fixed at the bottom-in this case d=16m, h=5m), the main ring forces come from the water load acting on the walls (calculating the tension forces along the height of the wall using portland cement association tables).....
a friend of mine (dr. in engineering) says that the major ring forces comes from the hydration heat of the concrete in the first 3 days and the stresses due to shrinkage. he claims that according to the european codes, these forces are greater than the tension forces results from the water load; and the tank walls' reinforcement will be determined upon these forces. as a result the amount of ring rebar is 2 times the amount of reinforcement needed to take the tension forces.
i read the aci code #350 and 318 , a book issued by pca (design of concrete circular tanks)and another book called handbook of concrete engineering/fintel- all these books and codes do not mention what the dr said.
according to the handbook of concrete engineering, the shrinkage reinforcement shall be 0.65% when control joints are eliminated...but using a ratio of 0.45% as minimum is acceptable (in our case ; using 0.45% will be the same amount of the required rebar and will be 50% less of his results- according to his design the amount of reinforcement is 0.88% of ac)
according to aci code #350 , the maximum stress in reinforcement for elements in direct tension shall not exceed 20,000psi (steel grade 60).
according to pca (portland cement assosiation) book- low steel stresses tend to make concrete crack because the lower the allowable steel stress, the greater the area of steel provided to carry the tensile loads; it is desirable to use higher allowable steel stress so that less steel is used , resulting is less restraint shrinkage and smaller tensile stresses in concrete.
do you have an idead about these huge stresses?
in my experience with the design rc tanks it is nearly always the limitation on crack width which determines the required reinforcement. the crack width needs to be assessed under two conditions, (1) flexural cracking as a result of applied loading, (2) thermal cracking as a result of the concrete hydrating as it cures.
the current (but soon to be superseded) british standard is bs8007 design of concrete structures for retaining aqueous liquids the minimum reinforcement requirement as 0.35%. bs 8110, structural design code states a minimum of 0.45% for members subject to pure tension.
however, the actual requirement for reinforcement to bs8007, is dependent on type of formwork used, type of aggregate, type of cement (opc or an opc/ggbs/pfa mix), thickness of section, type of member (wall slab etc), time of year of placing, and level of restraint offered by adjacnet

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