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Design Procedure for circular tanks Computations
1. Minimum wall thickness
2. Circumferential Prestress
3. Vertical Prestress.
1) Maximum, ring tension Nd
2) Bending Moment Mw
3) Minimum wall thickness = Nd/?fct - fmin.w
- Minimum cover 35mm
4) Circumferential Prestress
5) Spacing of wires
As = Cross sectional area of wire coinding, mm2
Wt = average radial Pressure of wires at transfer at a given section N/mm2
D = Diameter of the tank, mm
S = Spacing of wires at the given section mm
fs - Stress in wires at transfer, N/mm2
t - Thickness of the tank wall, mm
fc - compressive stress in concrete, N/mm2
Hoop compression due toprestressing = wt . D / 2
wt. D/2 = fs As/S1
Wt = 2 fs As / sD
Nd - hoop tension due to hydrostatic working pressure, Ww
Nt - hoop compression due to radial pressure of wires, wt
Then Nt = Nd (wt/Ww)
Also Nt = t fc
Spacing of the wire winding
S = (2 Nd/ Ww) . ( fs./fc ) .( As/Dt) mm
Mt = Mw (wt /Ww)
Where Mt = Vertical moment due to the prestress at transfer.
Mw = Vertical moment due to hydrostatic pressure.
The compressive prestress required
Fc =( fmin. W / ? ) + ( Mw/ ?z)
When the tank is empty
Fc =( fmin. W / ? ) + ( Mw/Z)
Vertical prestressing force is required
P = fc. Ac
(Note: Vertical Prestressing force = 30% of hoop compression.]
1. A cylindrical prestressed concrete water tank of internal diameter 30m is required to store water over a depth of 7.5m. The permissible compressive stress in concrete at transfer is 13 and the minimum compressive stress under working presuure is 1 N/mm2. . The loss ratio is 0.75. Wires of 5mm diameter with an initial stress of 1000N/mm2 are available for circumferential winding and freyssinet cables made up of 12 wires of 8mm diameter stressed to 1200N/mm2 are to be used for vertical prestressing. Design the tank walls assuming the base as fixed. The cube strength of concrete is 40N/mm2.
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