parsonsparsonsEngineering Department Design Guide

Design Guide Number

DSG-SE-02

App

SAL

Date

Jan 2000

Sheet of

1 5

SubjectTANK FOUNDATION (CONCRETE RING WALL)

1.0 SCOPE

This design guide provides a method for designing a ring foundation for a cylindrical steel, liquid storage tank.

2.0 GUIDE

Tank foundation consists of a reinforced concrete ring wall designed for static, hydrodynamic, wind, and seismicloads. Listed below are "general guidelines" for design. Engineers should consult their project-specific criteria, aswell as API-650.

1) The depth of the ring wall should be determined based on the soils reports and bolt embedmentlength requirements. However, the bottom of the ring wall must be below the frost line.

2) For shallow walls, the hoop tension is assumed uniform from the top to the bottom of the wall.Because lateral pressure increases with the depth, high walls will require closer steel spacing atthe bottom than at the top.

3) Minimum width of ring wall shall be 12 inches.

4) To maintain continuity of the hoop tension, reinforcing must be continued around wall openings.

5) Top of wall should be smooth and level within plus-or-minus 1/8-inch in any 30-footcircumferential length. No point in the circumference should vary more than 1/4 inch from theestablished elevation.

6) The ring wall should be sufficiently reinforced to take into account the forces produced by theapplied loads.

7) Check the foundation for overturning stability, sliding, and settlement.

8) For small tasks, octagonal, rigid concrete mats are often more economical than ring wallfoundations.

9) The geometrical characteristics of the ring wall and anchorage of tank should determine whetherthe foundation should be analyzed as flexible or rigid. For most instances, rigid foundation designmethodology will be adequate, and is discussed in the following paragraph. However, if flexiblefoundation analysis is envisaged, a finite element model (using plate elements) needs to beprepared using programs like GTSTRUDL, SAFE, etc. Effect of soil volume underneath thefoundation should be modeled using stiffness derived from the modules of subgrade reaction.

A typical sketch showing tank foundation is shown in Figure 3 at the end of this document.

3.0 RIGID FOUNDATION ANALYSIS

Static analysis should be performed to ensure that the ring wall foundation meets the strength and stabilityrequirements for the following load conditions:

1) Operating

2) Operating plus seismic

3) Operating plus wind

It is preferred that the ring wall thickness be such that the average unit soil bearing under the wall isapproximately the same as under the tank at the same depth. Occasionally, it is not practical to meet thisrequirement. Also, the tank shell should be centered with the ring wall. Therefore, the thickness of the ring wall isbased on:

t / ]ht tqH 1/2 [w h qH cs γ++=γ+

parsonsparsonsEngineering Department Design Guide

Design Guide Number

DSG-SE-02

Date

Jan 2000

Sheet of

2 5

therefore:

where:

t = Thickness of ring wall, ft

q = Unit weight of tank liquid, pcf

H = Height of tank liquid, ft

γs = Unit weight of soil, pcf

γc = Unit weight of concrete, pcf

h = Height of ring wall, ft

w = Weight of tank shell including the tributary weight of a cone roof per foot ofcircumference, plf. A floating roof weight does not add to the shell weight.

Figure 1

min ft 1 )] - ( 2h - [qH2w / t sc >γγ=

parsonsparsonsEngineering Department Design Guide

Design Guide Number

DSG-SE-02

Date

Jan 2000

Sheet of

3 5

3.1 Operating Condition

Under operating condition, the applied loads to the ring wall are as shown in Figure 1. Passive pressure isusually neglected for the design of the ring wall.

where:

Fr = Radial force on the ring wall, plf

ko = At rest earth pressure coefficient

= (1 - Sin φ)

φ = Angle of internal friction of soil, degrees

The radial force thus produced will cause hoop tension in the ring wall.

T = FrD/2000

where:

T = Axial tension in ring wall, kips

D = Tank shell diameter, ft

As = Area of hoop steel (in2) = T/fs

where:

fs = allowable stress in reinforcing steel, (ksi)

3.2 Operating Plus Seismic Condition

The hydrodynamic loads at the base of the tank should be provided by the tank manufacturer because a detaileddynamic analysis is performed for the tank shell design. This will maintain a design consistency for the tank shelland foundation. As an alternative, the hydrodynamic loads for the preliminary design loads can be determinedusing Reference 1 or 3 (see Paragraph 4.0).

The following method should be used to determine the soil pressure due to the overturning moment.

O.T.M. = Overturning moment caused by the seismic loads.

Pt = Total vertical load

= Pftg + Pshell + Pliquid

where:

Pftg = weight of ring wall footing

= πD (0.15 ht) kips

Pshell = weight of tank shell

= πDw/1000 kips

Pliquid = weight of liquid in tank

= π(D-t/2) (t/2)qH/1000 kips

Eccentricity, e = O.T.M. / Pt

Obtain soil pressure using 'Concrete Design Handbook' by M. Lintel (page 118), see Paragraph 4.0. Factor ofsafety against overturning and sliding shall be calculated and checked against allowable limits, usually 1.5.

Pressure SurchargePressure Active

o2

sor

qHh) k )h k ( 1/2 F

↑↑

+γ=

parsonsparsonsEngineering Department Design Guide

Design Guide Number

DSG-SE-02

Date

Jan 2000

Sheet of

4 5

Figure 2

3.3 Operating Plus Wind Condition

Tank operating load and wind load shall be used to obtain overturning moment and sliding force. Soil pressureand factor of safety against overturning and sliding shall be calculated.

4.0 REFERENCES

1) API - Standard 650, Welded Steel Tanks for Oil Storage Appendix - B

2) Concrete Design Handbook by Mark Fintel.

3) Seismic Design for Buildings, Technical Manual, TM-5-809-10, NAVFAC P-355, AFM 88-3.

parsonsparsonsEngineering Department Design Guide

Design Guide Number

DSG-SE-02

Date

Jan 2000

Sheet of

5 5

R = Radius of tank or CL of ring wall

H = Height of shell, ft

h = Height of ring wall

t = Thickness of wall

Figure 3 - Concrete Ring Foundation for Vertical Storage Tanks