1. Minimum wall thickness
2. Circumferential Prestress
3. Vertical Prestress.

**Design Procedure for circular tanks Computations**

1. Minimum wall thickness

2. Circumferential Prestress

3. Vertical Prestress.

Estimate

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, mm^{2}

Wt = average radial Pressure of wires
at transfer at a given section N/mm^{2}

D = Diameter of the tank, mm

S = Spacing of wires at the given
section mm

fs - Stress in wires at transfer,
N/mm^{2}

t - Thickness of the tank wall, mm

fc - compressive stress in concrete,
N/mm^{2}

Hoop compression due toprestressing =
wt . D / 2

Equating

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/mm^{2}. . The
loss ratio is 0.75. Wires of 5mm diameter with an initial stress of 1000N/mm^{2
} are available for circumferential
winding and freyssinet cables made up of 12 wires of 8mm diameter stressed to
1200N/mm^{2} are to be used for vertical prestressing. Design the tank
walls assuming the base as fixed. The cube strength of concrete is 40N/mm^{2}.

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