A Few Notes on the Design of Reinforced Concrete Tanks

A Few Notes on the Design of Reinforced Concrete Tanks

CVEN 4830/4434 University of Colorado, Boulder

Spring Semester 2008

Prepared by Ben Blackard

For Exterior Wall: external earth pressure only internal fluid pressure only

Load Cases ACI 350.4R-04 section 4.1.1

For Interior Wall: Fluid pressure on one side of wall only

Tank Flotation: 1.25 Uplift Dead Load [ACI 350.4R-04 section 3.1.2]

Load Factors/Combinations:

1.4(D + F) [ACI 350-06 section 9.2] D = dead load F = fluid load

Flexural Analysis

c d

As bw

fc

c = 0

1c

N.A.

As fy

s fy/Es

assumed stress assumed strain

distribution distribution

compression = tension

1 c bw 0.85 fc As fy

c

As fy

1 bw 0.85 fc

"nominal" moment capacity Mn

As fy

d 1 c 2

multiply Mn by a safety factor = 0.9

"design" moment capacity

Mn = 0.9 As f y

d

1 c 2

ACI 350-06 section 10.2.7.3:

0.85

if fc 4,000 psi

1

1.05 fc 20,000

if 4,000 psi fc 8,000 psi

0.65

if 8,000 psi fc

1

0.85 0.65

fc

4,000 psi

8,000 psi

Balanced Section

c d

As bw

fc

c = 0

1c

N.A.

As fy

s = fy/Es

assumed stress strain

distribution distribution

similar triangles from the strain distribution: 0.003 fy Es c dc

0.003 d c

0.003 fy Es

equilibrium: compression = tension

1 c bw 0.85 fc As,b fy

1

0.003 0.003

d fy

bw

0.85 fc

Es

As,b f y

A s,b

0.003 0.85 0.003

1 bw d fc

fy Es

fy

Shear

The shear strength provided by concrete is Vc 2 fc bwd as per ACI 350-06 section 11.3.1.1. Walls, slabs and footings should be designed so that the concrete is capable of resisting the ultimate shear load at the same section that the ultimate moment is calculated:

Vu Vc , where = 0.75

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