300 Foundation Design and Construction

300 Foundation Design and Construction

AbstractThis section of the manual discusses the design and construction of atmospheric storage tank foundations. Company and industry specifications are discussed and as well as the data required to determine the most appropriate foundation for a tank. It addresses tank foundations preferred for the different types of soil conditions. This section also includes settlement and releveling and provides procedures to address these issues.

Due to the critical nature of tank foundations, a civil engineer should have responsi-bility for the design work.

Contents Page

310 Soils Considerations 300-3

311 Designs to Compensate for Settlement

320 Foundation Design 300-4

321 Environmental Requirements

322 Design Loads and Forces

323 Foundation Types

324 Bottom Support Pad

325 Foundations for Hot Tanks

326 Foundations for Small Tanks

327 Berms and Gutters

328 Catch Basins and Sumps

330 Grounding Considerations 300-45

331 Grounding for Aboveground Metallic Tanks

340 Foundation Construction 300-46

341 Concrete Work

Chevron Corporation 300-1 July 2000

300 Foundation Design and Construction Tank Manual

342 Installing the Secondary Containment and Leak Detection System

343 Bottom-to-Foundation Seal

350 Tank Settlement 300-49

351 Spotting Settlement Problems

352 Kinds of Settling

353 Settlement Criteria

354 Designing for Settlement

355 Releveling Tanks

360 References 300-68

July 2000 300-2 Chevron Corporation

Tank Manual 300 Foundation Design and Construction

310 Soils Considerations

311 Designs to Compensate for SettlementSeveral differential settlement-related tank problems can be minimized at the design stage. Two design solutions are discussed, one for a cone up bottom and the other for a cone down bottom.

Pascagoula Design—Cone Up BottomFigure 300-1 shows a tank bottom configuration designed to compensate for differ-ential settlement. This design has been successfully used at the Pascagoula Refinery.

Tank bottom plates are placed in a cone up configuration to compensate for differential settlement. The tank bottom layout shown in Figure 300-1 was specific for site conditions at the Pascagoula Refinery. This curve is the maximum recommended; steeper slopes may cause the bottom plate to crease.

This design can be applied to other sites where large differential settlement is anticipated. The parabolic portion of the tank bottom layout is defined by considering soil conditions, tank diameter, and tank height. Consult with a soils specialist or the CRTC Fitness for Service, Civil/Structural Team for assistance.

Cone Down Bottom—Center Sump DesignAnother solution is to construct a bottom with a minimum downward slope (1 inch in 10 feet) using a center sump and siphon water draw. While the permissible differential settlement for this configuration is less than that for a cone up bottom, the disadvantages of the cone up bottom are avoided. A cone down bottom assures good drainage to the center sump even if the tank settles.

Fig. 300-1 Cone Up Tank Bottom Configuration—Pascagoula Refinery



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320 Foundation DesignSelecting the appropriate tank foundation depends to a great degree on the type of soil under the specific tank site as discussed earlier and also on the location of the tank field.

For example, special precautions are required for tank foundations in high earthquake zones or frost regions. The dimensions of tanks in high earthquake zones must be proportioned to resist overturning forces or the tanks must be anchored.

In frost regions, extend tank foundations one foot below the frostline to prevent frost heave. A properly designed tank foundation includes leak detection methods and cathodic protection if required.

321 Environmental RequirementsChevron recommends that the following be installed, whenever possible, on tanks handling stock that could contaminate groundwater if spilled:

• secondary containment; • leak detection systems; and • cathodic protection.

These systems can be installed on new tanks or on existing tanks during bottomreplacement. Designs for this type of foundation are discussed in Sections 540Membrane Design and Selection and 650, Cathodic Protection.

When secondary containment and leak detection is not practical (tanks with larsettlement, for example), consider external cathodic protection. Cathodic protecis discussed in Section 650.

Not all of the foundations discussed in Section 323 can accommodate these sys

322 Design Loads and ForcesTank foundations should be designed for the following loads and forces when thexist.

Dead LoadDead load consists of the weight of the metal (shell, roof, bottom plates, accessladders, platforms, nozzles, manways, roof support columns, etc.).

• Design plate thickness to include corrosion allowances.

Product LoadProduct load refers to the weight of the stored product.

• Use maximum product depth and specific gravity when calculating the weig



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Vapor PressureInternal pressure on the roof and surface area of the product is identical; however, the bottom plate (usually ¼-inch thick, lap fillet welded) is not structurally capabof transferring the vapor pressure to the shell to counterbalance the upward prefrom the roof.

• Foundations for tanks subjected to internal pressures must be designed to the uplift forces.

• This topic is discussed in more detail in Section 512, Internal Pressure Tan

Snow LoadFor tanks in Company facilities located in snow regions:

• include the weight of the snow in the design of the foundation • calculate snow load in accordance with ANSI/ASCE 7/98, “Building Code

Requirements for Minimum Design Loads in Buildings and Other Structure

TemperatureTanks that store hot products are subjected to variations in temperature which clead to deformations or movements. In the tank foundation, incorporate details

• allow the tank to move • protect the foundation concrete

WindTank foundations must be designed resist wind pressures.

• This is particularly important for tanks that may sit empty or only partly filled• Calculate wind loads on tank foundations in accordance with Section 530 o

this manual.

EarthquakeEarthquake-induced lateral forces can cause a tank to tip, overturn, or slide.

For additional information regarding seismic design of tank foundations, see Section 530 of this manual.

Tipping

If the tank does tip on edge, the flexible tank bottom diagonally opposite calift only a small amount of contents to resist the seismic overturning force. Tforce of tipping subjects the foundation area under the shell to large verticacompressive forces.

Overturning

The weight of the tank plus its contents and the tank’s H/D (height-to-diameratio affect the tank’s ability to resist overturning.



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– Small diameter tanks are more susceptible to overturning than large diameter tanks, because they usually have greater H/D ratios.

– To verify tank stability, add the foundation weight to the tank’s shell weight, Wt (see API 650 Appendix E) and analyze the tank as unancho

– Unless additional information is available, assume the tank is flexible aringwall designed for the full uplift forces.

– Adjusting the H/D ratio is the preferred method to prevent tipping. (Youcan also anchor the tanks, but this method is not recommended in largtanks.)

a. For flat-bottomed tanks, adjust the H/D ratio rather than add anchors because ringwall or concrete slabs may become exceor require piles to resist the uplift forces. (See Section 140 for recommended H/D ratios.)

b. In some cases, the ringwall weight may be enough to make thtank stable without piles.

Sliding

In seismically active areas, the soil stability must be investigated.

– Analyze the tank site to determine the potential for liquefaction or slidinduring an earthquake (this information should be included in the soils investigation report).

– Calculate earthquake loads in accordance with API Standard 650, Appendix E.

323 Foundation Types

How to Choose a FoundationChoose an appropriate tank foundation design based on the following:

• tank size, • soil type; and • environmental requirements to detect and protect groundwater from leaks.

Tank Size• Large Tanks (50 feet in diameter or greater)

– use concrete ringwall (preferred) or crushed stone ringwall.

• Small Tanks (20 feet in diameter or less)– use concrete slab foundation (preferred) or compacted granular fill

foundation.

• Medium Tanks (20 to 50 feet in diameter) can be classified as either large osmall at the discretion of the foundation designer and tank design engineerthe purpose of choosing the type of foundations only.



Soil TypeIn some instances, large fixed roof tanks can be supported directly on properly prepared good native material. Choose this method only if recommended by the soils consultant.

Pile supported concrete slab foundations are used for tanks on poor soils, regardless of the tank size.

Environmental RequirementsThese are determined by local environmental standards and requirements. Consult with local environmental specialists for recommendations and requirements.

Foundation TypesFigure 300-2 summarizes foundation types, lists the advantages and disadvantages of each type, and makes specific recommendations.


300 Foundation Design and ConstructionTank M

anual

July 2000300-8

Chevron Corporation

Fig. 300-2 Tank Foundation Summary (1 of 4)

Reference Documentation

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Standard Form TAM-EF-364 outlines concrete ringwall design and construction details.

API Standard 650, Appendix B, lists several other advan-tages of foundations with ringwalls over foundations without ringwalls.

ACI 318, “Building Code Requirements for Rein-forced Concrete”

Foundation Type Advantages Disadvantages Recommendations

Concrete Ringwall

circular wall centered continu-ously under shell circumference.

1. Provides level surface for shell construction.

2. Minimizes edge settlement.

3. Easy leveling for tank grade.

4. Minimizes moisture under tank.

5. Retains fill under tank and prevents loss due to erosion.

6. Distributes concen-trated shell load well.

7. Can use cathodic protection.

8. Provides greatest assurance of meeting elevation tolerances around tank circumfer-ence.

9. Better able to transfer shell loads to the supporting soil.

10. Minimizes edge settle-ments and conse-quently shell distortions—very important problems to avoid for trouble-free operation of tanks with floating roofs.

1. Can be expensive, depending on location.

2. May not be suitable for tanks on poor soils. Check with foundation specialist.

3. Ringwall must be reinforced.

4. Anchoring of tanks against earthquake overturning not practical. Requires special design.

The only disadvantage of concrete ringwalls is that they are more expensive than earth foundations without a ringwall.

Preferred foundation typefor tanks larger than 20 ft.diameter. Can also be usefor small diameter tanks when anchorage is not required.

Use on good soils or properly prepared intermediatsoils.

Concrete ringwall is the preferred foundation for alarge tanks, for tanks whethe surface soil is non-cohesive, such as loose sand, for tanks where significant settlement is anticipated, and on all floating roof tanks over 30 feet diameter to protecagainst differential settle-ment-caused problems wiannular space and tank se

Tank Manual


Chevron Corporation300-9

July 2000

I Standard 650, endix B

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ference Documentation

Crushed Stone Ringwall

1. Less expensive than concrete ringwall.

2. Good concentrated shell load distribution to weaker soils below.

3. Construction material usually readily available.

4. Can make use of cathodic protection.

1. Tank cannot be anchored against earth-quake overturning.

2. Greater care required for preparation of tank grade.

3. Foundation material subject to washout.

4. Not suitable for poor soils.

5. May cause increased undertank pitting at points where tank bottom contacts stones.

A drawback with crushed stone is that water and corrosive salts can collect between the stones and cause increased pitting rates. A concrete ringwall will generally cause less bottomside corrosion where it contacts the tank bottom.

Use where concrete for ringwall not readily available or high cost of construction. Use on good soils or properly prepared intermediate soils.

This type of foundation, though not as desirable as a concrete ringwall founda-tion, is a good alternative, especially in areas with good soil and where concrete is either not readily available or is costly.

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ig. 300-2 Tank Foundation Summary (2 of 4)

Foundation Type Advantages Disadvantages Recommendations Re

300 Foundation Design and ConstructionTank M

anual

July 2000300-10

Chevron Corporation

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ACI 318, “Building Code Requirements for Rein-forced Concrete.”

Standard Drawing GF-S1121

Fig. 300-2 Tank Foundation Summary (3 of 4)

Reference Documentation

Concrete Slab 1. Provides level surface for shell and bottom construction.

2. Minimizes differential settlements.

3. Good concentrated shell and uniform load distribution.

4. Does not requireseparate bottom support pad.

5. Can be designed to allow for tank anchorage against earthquake overturning.

6. Leak detection and containment can be incorporated.

7. Low corrosion rate.

The concrete slab has all the advantages of the ring-wall, plus it can easily incorporate leak detection systems as shown in Stan-dard Drawing GF-S1121.

1. Relatively expensive, especially for large tanks.

2. Shifting and settling on poor soils may cause slab to crack.

3. Cannot use cathodic protection.

The disadvantages of concrete slab foundations are their higher cost and the fact that they do not permit the installation of cathodic protection.

Use for small tanks whereleak detection and containment are required.

Not recommended for tanlarger than 20 ft. in diam-eter because of cost.

Use on good soils or properly prepared intermediatsoils.

Foundation Type Advantages Disadvantages Recommendations

Tank Manual


Chevron Corporation300-11

July 2000

ndard Form TAM-EF-421

ndard Drawing GF-S1121

F

ference Documentation

Compacted Granular Fill

1. Relatively inexpensive.

2. Easy to construct.

3. Construction material readily available.

1. Limited to small tanks on good soils.

2. Tank cannot be anchored against earth-quake overturning.

3. Foundation material subject to washout.

Use on good soils only.

Pile Foundation 1. Minimizes total and differential settlement.

2. No separate bottom pad required.

3. Allows for tank anchorage against earthquake overturning.

4. Leak detection and containment can be incorporated.

1. Most expensive foundation type.

2. More complex design than other types.

3. Good soils information essential.

4. Cathodic protection more difficult to install.

Use for all tank foundations on poor soils where no other foundation type is possible.

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ig. 300-2 Tank Foundation Summary (4 of 4)

Foundation Type Advantages Disadvantages Recommendations Re


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Concrete Ringwall DesignTo minimize differential settlement, the concrete ringwall should be proportioned so that soil pressure under the ringwall equals the soil pressure under the confined earth at the same depth as the bottom of the ringwall.

• Ringwalls need to be a minimum of 12 inches wide and 35 inches deep, wileast 12 inches above grade and 24 inches below grade. A greater depth may be required in loose sand.

• The minimum concrete strength should be 3000 psi at 28 days. Design concrete and reinforcement in accordance with ACI 318, “Building Code Requirements for Reinforced Concrete” and API Standard 650, Appendix B.

• Reinforce the concrete ringwall to reduce shrinkage cracks and to resist hotension. Hoop tension is caused by the lateral earth pressure inside the ringwall duethe product surcharge and applicable tank dead load, such as from the tanbottom plate and roof columns.

• The lateral earth pressure shall be assumed to be at least 50 percent of thevertical pressure due to fluid and soil height, unless substantiated by propegeotechnical analysis. If a granular backfill is used, a lateral earth pressurecoefficient of 30 percent may be used.

• Ringwalls need not be designed to resist active soil pressure inside the ring

• Neglect passive pressure on the outside of the ringwall.

Mobilization of active and passive earth pressures implies substantial movements which are not likely to occur in a circular concrete ringwall.

• The top of the concrete ringwall should be a minimum of three inches abovthe adjacent grade if paved and six inches if unpaved, after predicted settlement.

• Place ½ inch thick maximum, asphalt impregnated board, such as ASTM D1751, on top of the wall directly underneath the shell annular plate, excephot tanks.

An example for designing a ringwall foundation is given at the end of this sectio

Backfill. The space within the ringwall is backfilled with compacted granular fill capable of supporting the tank dead load and the product surcharge load.

• Backfill should be select material of such size and gradation as to be easilycompacted and have good drainage characteristics.

• California standard Class 2 roadway aggregate base, ¾ inch maximum sizsuitable for backfill.

• Material meeting the requirements for roadway base in other localities is alacceptable backfill.



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Crushed Stone Ringwall DesignAPI Standard 650, Appendix B, suggests a general type of crushed stone ringwall foundation. More specifically, the crushed stone ringwall should consist of:

• Crushed gravel or crushed stone one-half to one inch in diameter. Make the crushed stone ringwall base wide enough to distribute the shell loto the underlying soil without exceeding the allowable bearing capacity.

• Base the ringwall base width and depth below the bottom of the tank annulplate on the recommendation of the soils consultant.

• The minimum depth is two feet.

Design all other ringwall dimensions as shown in API Standard 650, Appendix Bexcept that the berm outside the tank should be as discussed in Sections 326 a327.

Backfill. The space within the crushed rock ringwall is backfilled with compactegranular fill of the same quality as that for concrete ringwall foundations.

Concrete Slab Foundation DesignConcrete slab tank foundations can be utilized to support small unanchored or anchored tanks. The concrete slab provides an outstanding level, uniform tank support surface and makes it possible to anchor the tank using conventional anbolts.

• The slab must be thick enough to develop the anchor bolt forces and rigid enough to transfer the tank loads to the soil without cracking.

• Design structural concrete according to ACI 318, “Building Code Requirements for Reinforced Concrete.”

• Reinforce the concrete slab to reduce shrinkage and to resist shear and bemoments produced by soil bearing pressures.

Reinforcement can consist of deformed steel bars or deformed welded wirefabric.

• Make the concrete slab heavy enough to resist overturning forces with a faof safety of 1.5.

For small production tanks, precast concrete slabs transported to site by truck moffer a quick, simple and cheap foundation.

Compacted Granular Fill FoundationsUnanchored small tanks can be supported on compacted granular fill placed dirover native material. The granular fill should be a minimum of one foot deep.

Erosion. Protection against erosion can be accomplished in one of two ways:

• Build a three-foot wide shoulder and berm• Place a steel band around the periphery of the tank.



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The steel band confines the fill and prevents sloughing of loose, non-cohesive surface soil.

Construction details for a tank foundation with a steel band are shown in Figure 300-3.

Drainage. If the native soil does not drain, the fill could stay full of water and cause increased corrosion. It is important that:

• the native soil be sloped for drainage; or • cathodic protection be used to protect the bottom.

Pile Supported Concrete FoundationsIf tank loads and soil conditions do not economically permit any of the previousdiscussed foundation types to be used, then a pile supported foundation may bonly practical alternative.

Standard Form TAM-EF-421 (Pile Supported Concrete Foundation) shows typicdetails for a pile supported mat foundation. Standard Drawing GF-S1121 incorporates the groundwater protection details into the pile supported tank foundation design.

The procedure for the design of pile supported foundations is:

1. It is very important to involve a civil engineer in the design of this type of foundation.

2. Make a soils investigation to determine groundwater levels, allowable pile loads, and required pile lengths.

3. Calculate the loads and estimate the total number of piles.

4. Determine type, capacity, and length of piles. This step is normally done bysoils consultant.

5. Establish pile spacing.

Fig. 300-3 Granular Fill Foundation with Steel Plate Band

3



6. Design the pile cap and concrete slab.

7. Check pile uplift and lateral loads due to wind or earthquake.

Ringwall Foundation Design ExampleBecause of the large compressive forces in the shell, the ringwall design is critical. The following example (Figure 300-4) and accompanying calculations based on API 650 and ACI 318 describe the procedures to be used.

Fig. 300-4 Example—Ringwall Foundation Design (1 of 11)

Legend for this figure is on the next page

Figure 300-4 continues on the next page

ANNULARRING

WEQ

TOP OFBERM

h

2’-

0"e

H

P = P + PT DL EQ

TANK SHELL

R

TANK

PASSIVE SOILPRESSURE(NEGLECT)

TANK BOTTOMPLATE

ACTIVE SOIL PRESSURE(NEGLECT)

SHEAR RESISTANCE (NEGLECT)0.5 W (MIN.)P

12" MIN

RINGWALL & TANK SHELL

CONCRETERINGWALL

b

qrw

q P

WP

Ref. 1) API 6502) ACI 318



Legend:

R = tank radius, ft.

H = tank height, ft.

b = width of ringwall, ft. (should be at least 12 in.)

h = height of ringwall, ft.

γc = unit weight of concrete, pcf

e = distance of top of ringwall from top of berm, ft.

PT = total load on tank shell, lb./ft.

PDL = dead load on tank shell, lb./ft.

PEQ = earthquake load on tank shell, lb./ft.

Wp = product load on tank bottom, psf

WEQ = earthquake load on tank bottom, psf

qp = soil bearing under tank at centerline, psf

qrw = soil bearing under ringwall, psf

qαDL = allowable soil bearing pressure due to tank dead load + product weight

qαEQ = allowable soil bearing pressure due to tank dead load + product weight + earthquake

To = applied uniform torsional moment in the ringwall, kip-ft/ft

x = distance of centerline of tank shell to centerline of ringwall, ft.

MA, MB = internal bending moment in the ringwall, kip-ft

TA, TB = internal torsional moment in the ringwall, kip-ft

fy = yield strength of reinforcing steel, psi

f’c = compressive strength of concrete, psi





1. Determine Ringwall Dimensions

Design objective: For dead load plus product weight, proportion the ringwall so that the average soil bearing pressure under the ringwall is approximately the same as the average soil bearing pressure under the centerline of the tank.

Given

D = 120 ft.

H = 60 ft.

Content = crude oil, s.g. = 0.93

Earthquake zone 4

e = 6 in ⇒ h = 6 " + 2’ - 0" = 2’ - 6"

Soil data:

Allowable soil bearing pressure (from soil report):

Tank dead load (steel) + product weight

Tank dead load + product + earthquake

Projected differential settlement = negligible

Projected uniform settlement: - 2 in. - completed by the end of the hydrotest

Foundation loading (from API 650 Appendix E Evaluation or vendor data):

PDL = 1,800 lb./ft. (including contributing roof dead load)

PEQ = 14,200 lb./ft.

PT = 1800 + 14,200 = 16,000 lb./ft.

Wp = qp = 59 ft. × 62.4 lb./ft.3 × 0.93 s.g. = 3,424 psf

WEQ = 791 psf.

The tank requires no anchorage. (Per API 650 Appendix E Evaluation.)

Case I. DL + Product

qp = Wp = 3424 psf



qDLα

4 500 psf ,=

qEQα

1.33 qDLα

6 000 psf,= =

qDLα 4500 psf. > 3424 psf. Therefore OK=



Case II. DL + Product + EQ

qp = Wp + WEQ

= 3424 + 791 = 4215 psf

Solving for b:

2. Calculate Ringwall Reinforcement

a. Material

Concrete, f’c = 4000 psi

Reinforcing steel, fy = 60,000 psi (ASTM A615, Grade 60)



qrwPDL

b--------- γch( )+ qp= =

Solving for b:

bPDL

qp γch( )–-------------------------≥

b 18003424 150 2.5×( )–------------------------------------------≥

b 0.59 ft. < 1.0 ft. min.≥

qEQα 6000 psf. > 4215 psf. Therefore, OK.=

qrwPTb

------Wp WEQ+( ) b 2⁄( )

b--------------------------------------------- qEQ

α≤+=

bPT

qEQα 1

2-- Wp WEQ+( )–

---------------------------------------------------≥

b 16000

6000 12-- 3424 791+( )–

-------------------------------------------------≥

b 4.1 ft.≥ Use b 4.0 ft. (close enough)=



b. Hoop Tension

where:

D = tank diameter, ft

Th = hoop tension in ringwall, lb.

Th = 0.5 Wp D (d/2)

c. Torsion and Bending Moment

Since the tank shell will be placed nomi-nally at centerline of ringwall, therefore, x = 0.

Note: The procedure presented above for calculating the internal shear, bending moment and torsion is applicable for footing on soil with negligible differential settlement. Beam-on-elastic foundation analysis or other similar analysis should be employed if significant differential settlement is anticipated.



0.5 3424 120 2.5 2⁄×××1000

--------------------------------------------------------- 257 kips==

To PTx WP WEQ+( ) b4--- x

2--+

+=

Tob4--- WEQ( WP )+=∴

44-- 3424( 791 ) 1

1000----------×+=

4.22 kip-ft/ft=

MA MB To D 2⁄( )= =

4.22 120 2⁄( )× 253 kip-ft==

TA TB– 4.22 120 2⁄( ) 253 kip-ft=×==



d. Longitudinal Reinforcement for Hoop Tension

Factored Load, Thu = 1.7 Th = 1.7 × 257 = 437 kips (per ACI 318)

Strength Reduction Factor, φ = 0.9

Thu/φ = ultimate design hoop tension, kips

AsH = Required Hoop Steel Reinforcement, in2

e. Longitudinal Reinforcement for Flexure

Factored Load, Mu = 1.4 MA or 1.4 MB.

Mu= 1.4 × 253 = 354 kip-ft [per UBC for Seismic Zone 4 U = 1.4 (D + L)]

Strength Reduction Factor, φ = 0.9

where:

d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement



Thuφ

-------- 4370.9------- 485 kips= =

Thuφfy-------- 485

60------- 8.1 in2

= ==

Muφ

------- ultimate design moment, kip-ft=

Muφ

------- 3540.9------- 393 kip-ft= =

ρ A A2 2MuA

φfybd2----------------––=

A 0.85f′cfy

---------------- 0.85 4000×60000

-------------------------- 0.057= = =

d 30″ 3″ 0.5″ 0.5″ – 26″=––≅

ρ 0.057 0.0572 2 393 12 0.057×××

0.9 60 4× 12 262×××----------------------------------------------------––=

ρ 0.0028 ρmin< 200fy

------- 20060000------------- 0.0033 Govern⇐= = = =

ρmin per ACI 318



AsM = Required steel reinforcement for flexure, in2

= ρbd =

= 0.0033 × (4 × 12) × 26

= 4.12 in2

f. Reinforcement for torsion and shear

x1 = 48" - 2(1.5 + .25) = 44.5"

y1 = 30" - 3 - 1.5 - 2(.25) =25

b = 48"

h = 30"

Factored torsional moment, Tu = 1.4TA (or) 1.4TB

= 1.4 × 253 kip-ft = 354 kip-ft

Factored shear force, Vu = 1.4 V= 0

Note: The shear force in the ringwall is not always equal to zero. When large differential settlement exist, shear force in the ringwall could be significant.

Strength Reduction factor, φ = 0.85

The effect of torsion may be neglected if

Tu = 354 kip-ft > 59.5 kip-ft, therefore the effect of torsion must be considered.

where:Acp = Area of concrete cross section, in2

= b x h

Pcp = Outside perimeter of concrete cross section, in

= 2 (b + h)

AL = Required longitudinal reinforcement for torsion, in2

At = Required ties reinforcement for torsion, in2



;�E ��

\� K ��

TnTuφ----- 354

0.85--------- 416 kip-ft= ==

Tu φ f′c A2cp

Pcp-----------

<

φ f′c A2cp

Pcp-----------

0.85 4000 48 30×( )2

2 48 30+( )------------------------ 1

12 1000×----------------------×=

59.5 kip-ft=



Av = Required ties reinforcement for shear, in2

Aoh = Area enclosed by the centerline of the outermost closed hoops (ties), in2

= x1y1

Ao = 0.85 Aoh

Ph = Perimeter of centerline of outermost closed hoops (ties), in

= 2 (x1 + y1)

fyv = Yield strength of tie reinforcement, psi

fyL = Yield strength of longitudinal reinforcement, psi

θ = 45°

s = tie spacing, in

Design of Ties Reinforcement

For Torsion:

Tn = Tu/φ = 416 kip

Ao = 0.85 x 44.5 x 25 = 945.6 in2

Solving for :

= 0.044 in2/in (per one leg of ties)

For Shear:

(per two legs of ties, in2/in)

since no shear in footing, ,



Tn2AoAtfyvCo gentθtan

s-----------------------------------------------------=

Ats-----

Ats-----

Tu φ⁄2AofyvCo gentθtan-----------------------------------------------=

416 12× 1000×2( ) 945.6( ) 60000( ) Cotangent45( )

------------------------------------------------------------------------------=

Avs

------Vsfyd-------

Vuφ

------ Vc–

fyd----------------------- = =

Vuφ

----- 0=Avs

----- 0=∴



Min Required Ties,

or

For #4 ties, At = 0.2 in2; Required

For #5 ties, At = 0.31 in2; Required

Select #5 @ 7 in. o.c. closed ties

Design of Longitudinal Reinforcement



2At Av50b s×

fy-----------------=+

Av 0=

At

s-----

min

50 4× 12×2 60000×

------------------------- 0.02in2 in⁄ 0.044in2 in⁄<= =

Govern

smaxx1 y1+

4---------------- 44.5 25+

4-------------------- 17.4 in= = =

smaxd2--- 26

2----- 13 in Govern←= = =

s 0.20.044----------- 4.55 in. 13 in.<= =

s 0.310.044----------- 7.05 in. 13 in.<= =

ALAts-----

PhfyvfyL------

Co genttan 2 θ=

0.044( ) 2( ) 44.5 25+( ) 6000060000-------------

Co genttan 245°=

6.12 in2=



Therefore, provide AL = 6.12 in2

g. Select Longitudinal Reinforcement

For Hoop Tension,

For Torsion, AL = 6.12 in2

--- Distributed on all sides ---

Bar Size # of Bars As

#6 32 14.1 in2

#7 24 14.4 in2

#8 18 14.22 in2 SELECT 6 bars on top and bottom

3 bars on each side

For Flexure, ASM = 4.12 --- On top and bottom

Bar Size # of Bars AS

#6 10 4.40 in2

#7 7 4.20 in2

#8 6 4.74 in2 SELECT 6 bars on top and bottom

See the following figure for reinforcement detail

Concrete Ringwall Reinforcement Detail



AL( )min 5 f′c( )AcpAts-----

fyvfyL------

Ph–=

5 4000( ) 48 30×( ) 0.044( )–6000060000-------------

2( ) 44.5 25+( )=

7.59 6.16–=

1.43 in2 6.12in2<=

Ash 8.1 in2=

Ash AL+ 8.1 6.12+ 14.22 in2= =

2

2

4

1 - #8 ON TOP AND BOTTOM3 - #8 ON EACH SIDE(TOTAL )STAGGER SPLICES

@ 7" o.c.5

2

30



r wer

d

324 Bottom Support PadDepending on the choice of corrosion protection and leak detection method, the area within the ringwall and above the aggregate backfill can be covered with:

• reinforced concrete slab• sand pad• asphaltic concrete pavement, or• penetration macadam. Tank pad settling due to compression, particularly on asphaltic concrete or penetration macadam pads, makes a groove at the edge of the tank shell. Wateaccumulating in the groove causes the tank to corrode. To prevent corrosion, lothe shoulder around the tank and properly drain away water from the tank.

Figure 300-5 summarizes the bottom pad types and makes specific recommendations regarding leak detection and containment, and corrosion protection. Further discussion on this topic can be found in Sections 250, 260, an650.

Reinforced Concrete SlabMake this pad:

• a minimum of five inches thick • over a four-inch sand or compacted fill cushion

Refer to Standard Form TAM-EF-364 and Standard Drawing GF-S1121.

h. Temperature Requirement

Per API 650, Appendix B,

Ast = (0.002) (Arw)

where:

Ast = total area of temperature rebar above grade, in2

Arw = cross-sectional area of ringwall above grade, in2

Arw = b × e = 4 ft. × .5 ft. = 288 in2

Ast = .002 × 288 = 0.576 in2

From Step 2g above, and the accompanying figure, actual amount of rebar above grade = 12 bars × 0.79 in2/bar = 9.48in2.

Therefore, 30 #8 bars as shown are okay.

This is the end of Figure 300-4




Fig. 300-5 Summary of Tank Bottom Support Pads (1 of 2)

Incorporation of

Bottom SupportPad Type

Leak Detectionand Containment

External Cathodic

Protection CommentsReference Documents

Reinforced Concrete Slab

Can be incorporated.

It can easily accom-modate leak detection

Not required.

will not permit cathodic protection.

Must be reinforced.

Use where leak detection and containment are required.

Do not use where cathodic protec-tion is required.

Outstanding support surface for the bottom plate.

Do not use this pad where the anticipated differential settlement is more than one inch in ten feet.

EF 364

GFS1121

Plain Sand Pad Can be incorporated. Easiest to incorporate.

Sand subject to shifting and voids can be created under bottom. Easy to construct; difficult to maintain while installing bottom.

Use where leak detection and containment and/or cathodic protection are required.

it can accommodate both cathodic protection and leak detection.

Disadvantages:

The sand can shift causing voids and low spots.

Laying of the bottom can disrupt the contour of the sand.

While shifting sand is a concern, however, the problems caused by shifting sand are generally less than those caused by a concrete pad on shifting ground, since cracking and break-up of the concrete is a serious problem.

Any oil added to the sand can represent pollution and potential groundwater contamination.



e e

Plain Sand PadMake this pad:• at least four inches thick • out of clean, salt free sand (causes much less corrosion than either gravel or

crushed stone).• Some localities allow the use of oil in sand as a corrosion inhibitor. We hav

found that oil does not increase corrosion resistance much, and that in somcases has actually increased corrosion rates. Any oil added to the sand canrepresent pollution and potential groundwater contamination.

• Rather than using oil as a corrosion inhibitor, consider installing cathodic protection in the sand pad.

Asphaltic Concrete

Leak detection possible but not usually used.

Difficult to incorporate.

Has been widely used in the past. Use for tanks not requiring leak detection or long-term cathodic protection.

We now recommend against asphaltic concrete pavement since it does not deter corrosion in the long run, and limits the future instal-lation of cathodic protection unless the pavement is removed.

EF-364

Section 650

Penetration Macadam

Leak detection possible but not usually used.

Difficult to incorporate.

Use for tanks in remote sites where other material not readily available. Do not use if leak detection and containment and/or cathodic protection are required.

Generally less costly than other pavement types for locations remote from a mixing plant.

It is believed to provide an adequate water barrier for most tank foundation conditions, but is not likely to be as good in inhibiting corrosion as a clean sand or concrete pad.

Do not use where corrosion protec-tion of tank bottom plates is required.

EF-364

Granular Fill Leak detectionvery difficult or impractical.

Can beincorporated.

See “Tank Foundation Summary” Figure 300-2.

Fig. 300-5 Summary of Tank Bottom Support Pads (2 of 2)

Incorporation of

Bottom SupportPad Type

Leak Detectionand Containment

External Cathodic

Protection CommentsReference Documents



essive lling,

r

fill as

ction

F to

t or ation

Asphaltic Concrete PavementThis type of pad has been used in the past to support tank bottoms.

Note Asphaltic concrete pavement is not recommended for new construction. The pad can initially prevent water from migrating up through the foundation and corroding the tank bottom. Experience shows, however, that water eventually migrates up through the pavement and corrodes the tank bottom.

Design and construction details for this pad are shown on Standard Form TAM–EF–364.

Note We now recommend against the use of an asphaltic concrete pavement since it does not deter corrosion in the long run, and limits the future installation of cathodic protection unless the pavement is removed.

For more information on Cathodic Protection, see Section 650.

Penetration Macadam PavementPenetration macadam is used extensively as a tank pavement. It contains succlayers of progressively smaller angular stones. Each layer is consolidated by roafter which it is sprayed with bituminous binder.

Penetration macadam is generally less costly than other pavement types for locations remote from a mixing plant. It is believed to provide an adequate watebarrier for most tank foundation conditions, but is not likely to be as good in inhibiting corrosion as a clean sand or concrete pad.

Tanks can be supported directly on penetration macadam over compacted backshown in Standard Form TAM-EF-364.

Like asphaltic concrete, do not use penetration macadam where corrosion proteof tank bottom plates is required.

325 Foundations for Hot TanksThis section provides guidelines for

• hot tank bottom foundation design • leak detection • leak containment

Although the principles are applicable to any hot tank, the designs have been tailored for tanks storing hot asphalt products in the temperature range of 200°600°F.

These guidelines do not address tanks exposed to a large temperature gradienfrequent heating and cooling cycles. For these conditions, give special considerto fatigue, thermal expansion, and creep.

The recommendations made here have the following goals in mind:

• Minimize the costs for design, installation, and maintenance.



.

ions

ral e-

the h

ome

could

lab its

state e for

e

ed

ch a ank

rs

• Provide a high quality installation that is safe, reliable, and easy to maintain

• Provide standardized designs which have the flexibility to meet local conditand requirements.

• Include tank bottom retrofits in the design standards.

Under-tank TemperaturesIn a temperature distribution study, high temperatures were found to exist sevefeet below the bottom of a hot tank. Initial temperature profiles will vary from sitto-site due to factors such as presence of moisture or different soil thermal conductivity. Once a tank is put into hot service it may take months or years forground temperatures to reach steady-state conditions. However, eventually higtemperatures will extend several feet below the tank’s foundation.

Field tests also confirm high under-tank temperatures:

• One company found temperatures of 160°F at a depth of 30 inches below stanks after a relatively short period of service. If moisture is present or the steady-state temperature condition has not been reached, this temperaturebe even higher.

• In another instance, an asphalt tank resting on a refinery tank, wood-piled sfoundation (wood piles are not recommended for hot tank foundations) hadpiles charred to a depth of several feet below the tank’s concrete slab.

Under-tank Insulation. To counter the effects of high under-tank temperatures, some designers have suggested using under-tank insulation. However, our temperature distribution study indicated that insulation does not reduce steady-temperatures because the thermal gradient across the insulation has to be largthe insulation to be effective.

• Unless the insulation’s thermal conductivity is much lower than the soil’s, thinsulation will not work.

• Also, soils’ thermal conductivity vary and may be even lower than those usin our temperature study.

• Therefore, although adding insulation may increase the time required to reasteady-state condition, eventually it will not ease the effects of high under-ttemperatures.

• Insulation can also generate other problems such as increased settlement,moisture entrapment, tank bottom corrosion, and maintenance difficulties.

Note Do not use under-tank insulation.

Environmental ConsiderationsMany regulatory agencies now require release-prevention barriers and leak-detection devices for tanks, including hot tanks. Release-prevention barrietypically consist of under-tank liners.

Materials that are solid at ambient temperatures. Materials such as asphalt, typically stored in a temperature range of 350°F - 500°F, or molten sulfur stored



se n rom

ank irect

below ibed

n

to

y

. ock-

above its melting point of 115°C, are solid at ambient temperature. Because thematerials would solidify if leaked and because both asphalt and sulfur have beeused to pave highways, it is unlikely that any environmental harm would occur funder-tank leaks. For these substances, it is recommended that tank owners negotiate a leak containment solution on a case-by-case basis.

Materials that are liquid at ambient temperatures. MUse liners for hot substances that are liquid at ambient temperature or are toxic if leaked.

Leak Detection and ContainmentFor leak detection, API 650 requires tank-bottom leakage be redirected to the tperimeter where the leakage can be observed. An undertank liner can both redthe flow for leak detection and also act as a release-prevention barrier or liner.

Note If leak containment is required, the preferred method is a double steel bottom. See Figure 300-6.

Under-Tank Liners. For ambient-temperature tanks, plastic liners provide leak detection and containment. However, high temperatures can exist several feet a hot tank. Use either a double bottom (metallic liner) or a concrete liner (descrbelow) for temperatures exceeding 250°F.

Note Concrete is the liner of choice because it can be designed to resist the high under-tank temperatures.

Clay, concrete and steel liners have been used for hot tanks. Base the choice oeconomics, maintenance concerns, and local regulations.

• Clay liners

– Can withstand temperatures over 200°F without melting, but they are susceptible to drying and cracking unless kept continuously moist.

– High under-tank temperatures drive moisture away causing clay liners crack.

– Place it close to the water table to keep the clay moist and prevent cracking.

– Lay the clay liner inside the ring-wall and covered with chloride-free, drsand prior to tank construction (Figure 300-6).

Note Do not use clay liners unless required by law because they degrade when subjected to the high under-tank temperatures.

• Polymer-based liners — including HDPE

– Will melt or stretch and tear apart from the tank’s weight or shifting soilTherefore, do not use plastic liners for hot tanks unless designed for stside temperature.

– Design all liners (including plastic liners) for stock-side temperatures.

Note Do not use plastic liners unless required by law because they degrade when subjected to the high under-tank temperatures.



-

, by igh

t ble

he are

rature, ete th the ction.

of

si

• Elastomeric liners

– Although most are only reliable to approximately 250°F, Teflon can withstand 450°F temperatures.

– Heat-seamable PFA teflon (available in 60 to 90 mil sheets in 4' widths100 or more feet long) could be used but has not been tried due to its hcost.

• Concrete as a Liner– Concrete may be an undertank liner or a release-prevention barrier if it

meets certain requirements. – American Concrete Institute publication ACI 350R-89, “Environmental

Engineering Concrete Structures” lists these requirements and recommendations for structural design, materials and construction of concrete tanks and other reservoirs.

– Although permeability is not addressed, water tightness is. A water-tighconcrete liner should prevent an environmental release; however, localregulators have the final say as to what actually constitutes an acceptarelease-prevention barrier.

– In order to be water tight, the concrete cracking must be controlled by tuse of temperature and shrinkage reinforcement. These specifications given in ACI 350R-89.

Foundation Design for Hot Tanks

Designing with Concrete at High Temperatures. Concrete compressive strength decreases as temperatures increase. Reduction in strength results from tempemoisture content, loading history, and the type of aggregate used. As the concrheats up, the aggregate and cement expand at different rates. This, coupled widifferent stiffnesses for the aggregate and the cement, creates a complex intera

• For concretes with limestone or gravel aggregate up to 600°F, the strengthreduction is very small.

• However, concrete with other aggregates may have up to a 40% strength reduction at 600°F.

• At temperatures greater than 600°F, the cement starts to dehydrate and itsstrength drops off more dramatically. – Therefore, for temperatures higher than 600°F, consider special types

cement (such as alumina cement)– Using alumina cement concrete for tank foundations with tank

temperatures below 600°F is very costly and probably not necessary.

• For temperatures required to tolerate under 600°F– Regular concrete with an appropriate strength-reduction factor may be

used for foundations. – For tanks with temperatures in the range of 200°F to 400°F, use 4000 p

concrete.



000

rete ld be e

.

hot

ete

re ocal

rete

the

the

– For tanks in the range of 400°F to 600°F, use 5000 psi concrete. – In both cases, design the foundation using a reduced strength of only 3

psi to provide the required safety factor.

Concrete Mix. Use high quality concrete with a low water/cement ratio for hot tanks. The following design mixture is recommended:

• 0.4 water-to-concrete ratio• a minimum of 490 lbs per cubic yard cement• a maximum of 5% entrained air• no accelerators (especially accelerators with chlorides)

Proper curing practice is essential and consists mainly of keeping the new concsurface damp for at least the first seven days. Locally available aggregate shouacceptable because the design already takes into account the reduced concretstrength at high temperatures.

Selecting Foundation Type. Figure 300-6 simplifies selecting a hot tank foundation, taking into consideration the line, leak detection and other variablesThe selection chart refers to figures occurring later in this section.

Single Bottom Designs with Concrete Liners. Single-bottom designs with slabs under the tank are shown in Figures 300-8 and 300-10.

Single bottom concrete slabs and/or ringwall foundations are recommended fortanks because the slab:

1. provides a release-prevention barrier or liner under the tank. As such, it is imperative that the concrete be properly reinforced. Using reinforced concrreduces the chances of differential settlement and failure.

2. reduces the possibility of moisture collecting under the tank bottom. Moistucan accelerate corrosion or cause temperature variations that create high lstresses on the shell-to-bottom welds and the bottom plates.

3. provides the opportunity to install leak-detection grooves that meet the requirements of API 650. See Figure 300-9.

Install the concrete slab to cover the entire bottom of the tank. The concrete foundation acts as a liner, creating a barrier which prevents groundwater contamination. The foundation also includes leak-detection grooves which will guide the leaking product towards the tank’s periphery for easy detection.

• Reinforce the concrete so that cracks cannot propagate and undermine theconcrete’s integrity.

• As with any other design, include temperature steel in the ringwall and concslab.

• However, because of thermal gradients, place additional reinforcing steel incircumferential (hoop) direction near the outside edge.

If the tank is under 30 feet in diameter, it is less costly and more effective to useintegral ringwall-slab design shown in Figure 300-8. Instead of a ringwall, a slab



to

with thickened edges is used. The required reinforcing, leak detection, and thermal considerations are the same as those for larger tank foundations.

A double steel bottom is the preferred method for leak detection/containment.

The design of Figure 300-10 includes an expansion joint to accommodate the thermal growth of the slab relative to the ringwall. The temperature range for this design is from 200°F to 600°F.

Note In Figure 300-10 a leak will not be contained, but will run out into the secondary containment area. However, this is probably not a critical factor in the protection of the environment because the leak is detected soon and can be stopped and cleaned up quickly.

Figure 300-11 is an alternative to a slab under the tank. This design uses a curbprovide more leak containment. However, it is probably no more effective than other designs and probably more costly. Its use may be governed by local authorities.

Fig. 300-6 How to Select a Hot Tank with Leak Detection/Leak Containment



ld

t-

d se

r of er

ot ot

nsion

s

s.

or

isk shown

may

Designs for Tank Bottom Replacement or Retrofitting. When upgrading or replacing the bottom of existing tanks for high temperature service, Figure 300-12 shows an economic and reliable method for providing a liner and leak detection.

• A new concrete spacer, at least four to six inches thick, is poured over the otank bottom.

• Reinforce the concrete liner according to ACI 350R-84 to provide water tighness and to prevent excessive cracking.

• Radial grooves are added for leak detection. • For substances that may not be considered hazardous, such as asphalt an

sulfur, welded wire mesh is adequate reinforcement in lieu of rebars becaucracking would not create environmental problems.

Designs Using Double Steel Bottoms. Figure 300-13 can be used for new tanks ofor replacing a tank’s bottom plate. This design provides containment in the forma double steel bottom, with the tank bottom closest to the ground forming the linor release-prevention barrier. The system is built on compacted fill soil.

Hot Tank AnchoringIn general, tanks should be designed with a low H/D ratio so that anchoring is nrequired for the seismic loadings specified by API 650, Appendix E. When it is npossible to keep the tank’s H/D ratio low enough (approximately 0.4 to 0.5 in seismic Zone 4), anchors may be required.

The anchorage must be designed to accommodate the differential thermal expain the radial direction between the tank and the slab.

Use the detail of Figure 300-14 when a hot tank requires seismic anchorage. It allows for the different radial expansions that will occur between the tank and itfoundation without generating significant bending stresses in the anchor bolts.

Hot Tank SumpsEmptying a hot tank for cleaning, inspection, maintenance, and repair, can be difficult if the contents solidify or become hard to handle at ambient temperatureTherefore, tank owners often wish to install bottom sumps.

However, in hot tanks, the indiscriminate use and design of tank-bottom sumpsappurtenances have led to failures due to the thermal expansion of the tank’s bottom. Presently, for sumps or appurtenances to perform reliably and without rof failure they must be designed on a case-by-case basis. One such concept is in Figure 300-15.

Hot Tank CorrosionCorrosion in hot tanks can occur anywhere water is in contact with the tank’s bottom plate. Most of the time, the high under-tank temperatures drive away existing moisture, especially near the tank’s center. However, in a location withfrequent rains, a high water table, or an area subject to frequent flooding, waterbe in contact with the tank’s bottom.



nly

nown

ture g

the

rmal om

ide ture sion.

Generally, any corrosion is limited to the tank’s periphery, because that is the oarea where water can have lasting contact with the tank’s shell and bottom.

The tank’s edge may never become completely dry because of a phenomena kas moisture pumping: as the water under the tank is heated, it rises, pushing the water above it out of the way and drawing more water in to take its place. Moispumping can be minimized by placing a tank well above the water table. Also, aconcrete pad or ringwall foundation should create an effective barrier, minimizinmoisture pumping.

For tanks in the temperature ranges being discussed, any water in contact withbottom plate will probably turn to steam. Although steam is less corrosive than liquid water, its corrosive effects should not be discounted.

In existing tanks where the chime (the external part of the annular ring) sits in apuddle of water, severe corrosion can be expected. With the combination of thestresses and corrosion, there is a potential for failure at this critical shell-to-bottjoint. Excavate the tank perimeter and drain it to assure that no standing water collects around the tank’s base.

Note The best way to reduce under-tank corrosion is to keep the tank’s undersdry. Raising the tank four to six inches above the adjacent grade — including fufoundation settlement — should reduce moisture contact and bottom-side corro

Note Cathodic protection under hot tanks is not recommended because the anode’s life is greatly reduced at elevated temperatures.



Fig. 300-7 High-Temperature Tank Foundation with Leak Detection & Containment Using Clay Liner

Fig. 300-8 High-Temperature Tank Foundation with Leak Detection for Small Tanks – Tanks < 30’ in Diameter

Tank Shell

Insulation

Tank Foundation

Chamfer

Finished Grade.Slope AwayFrom Tank

Clean Dry Sand

Tank Bottom

Clay Liner(Claymax or Equal)

Note: Clay Liner Shallbe Chloride Free

Compacted Fill

(See Note 5)

Tank Shell

Leak Detection Groove(See Foundation Plan)

Tank Bottom

Insulation

Tank Foundation

Chamfer

Finished GradeSlope AwayFrom Tank

Slope

(See Note 6)

(See Note 5)



Fig. 300-9 Hot Tank Leak Detection Foundations

4’–0” Minimum

Sawcut 3/4” Deep× 1½” Wide Grooves

1'–0” Typ

8'–0” MaximumSpacing Typ

Repeating Pattern

Slop

e

¼ R 5'–

0”

Single Slope Configuration

Grooving is Typical for

Concrete SlabTypical

Inside Face ofExist Tank Shell

Slope

4'–0” MinimumSpacing Typ

Sawcut3/4” Deep × 1½” WideGrooves

8'–0” MaximumSpacing Typ

Repeating Pattern

Length of Groovesto be Determined byMinimum Spacing

Cone Up Configuration

Grooving is Typical for

(See Note 8)

Figures 300-8, 300-10, 300-12, 300-13

Figures 300-8, 300-10, 300-12, 300-13

(See Note 8)



Fig. 300-10 High-Temperature Tank Foundation with Leak Detection – Tanks >20’ in Diameter

Tank Shell

Insulation

Tank Foundation

Chamfer

Finished GradeSlope AwayFrom Tank

Leak Detection Grooves(See Foundation Plan)Tank Bottom

Slope

Compacted FillConcrete Slab

Smooth Ringwall Top Surface& Use Paper Joint

Ties

Ringwall

(See Note 6)

(See Note 5)

Fig. 300-11 High-Temperature Tank Foundation with Leak Detection and Leak Containment

Tank Shell

Insulation

Tank Foundation

Chamfer

Finished Grade

Leak DetectionDrain Pipe Cap May Be

Used in Lieu of Valve

Tank Bottom

Gravel atDrain

Pea Gravel orSand Fill

Slope

Ties

Concrete Slab

(See Note 5)



Fig. 300-12 Retrofit Existing with New Bottom to Include Leak Detection

Fig. 300-13 High-Temperature Tank Foundation with Leak Detection and Leak Containment

Insulation

Install New TankBottom by SlottedShell Method

Leak Detection Slope

Concrete Spacer

Leak Detection Grooves(See Foundation Plan)

(See Note 4)

(See Note 7)

Tank Shell

Insulation

Leak Containment

Chamfer

Tank Foundation

Tank BottomsConcrete Liner

Leak Detection Grooves(See Foundation Plans)

FinishedGrade

Slope

Compacted Fill

Ties

Ringwall

(See Note 6)

(See Note 9) (See Note 4)



Fig. 300-14 High-Temperature Tank Anchor Detail

A. B.

Tank Shell

Anchor Bolt & Tank

Anchor Bolt

Slotted Hole

Teflon Washer

Anchor Chair

Tank Shell

Tank Bottom



Fig. 300-15 Hot Tank Sumps

“B” ∅

in Fdtn Mat’l “d” + 1”

Metal Sump

1”±

Blockout

Tank ShellTo Centerof Tank

Plan

100% Penetration &Fusion Welds at These Locations

Bottom WeldedInsert PL

Tank Shell

RadialGrowth“d”

Bottom PL

Foundation

MetalSump

“A” O.D. Sump

“B” I.D. of Block-out

B = A + d + 2” Insulation

(“d” + 1”)

(6” Max)

1” ±

“d” = Expected Radial Growth of Tankat Shell



psi

o in -77.

ire

nfil-

10'.

.

- for

on

an be sing

es to

Figure Notes1. Concrete strength shall be Fc′ = 5000 psi for tank temperatures ranging from

400°F to 600°F, and shall be Fc′ = 4000 psi for temperatures from 200°F to 400°F. The design strength for concrete in all cases is assumed to be 3000due to the high temperatures affect on concrete strength.

2. Chloride salts shall not be added to the concrete to accelerate hardening. Tprevent corrosion, concrete shall not exceed 0.15 percent soluble chloridesaccordance with the recommendation of American Concrete Institute 201.2R

3. Reinforced concrete design shall follow ACI 318 requirements and ringwalldesign guidelines as specified in this manual.

4. Spacer reinforcement shall be ASTM A-185 6 x 6 - W1.4 x W1.4 welded wreinforcement. Splices shall have a 6" minimum lap.

5. Edge of concrete surface shall slope away from the tank to prevent water itration under tank bottom.

6. Foundation should be up or single slope. Slope shall not be less than 2" in

7. Where grooves come to edge of tank notch existing steel for leak detection

8. For small tanks, the 4' - 0" minimum spacing between grooves should be reduced.

9. Where grooves come to edge of tank, a coupling shall be installed.

326 Foundations for Small Tanks

Small, Shop-welded TanksThe size of shop-welded tanks is limited by what can be transported over publichighways or railroads. A concrete pad is the most desirable foundation for shopwelded tanks. The pad provides a level surface for placing the tank, and allowsanchoring the tank when required, and can be used for leak detection.

In good soil locations, unanchored small shop-welded tanks can be supported compacted granular fill foundations. A gravel pad does not provide as level a surface as a concrete pad, but it is structurally adequate. Gravel or sand pads csubject to surface irregularities during tank placement. They can also shift, cauvoids underneath the bottom during operation.

Tanks With Design Pressures to 2.5 psigAPI Standard 650, Appendix F, discusses the use of tanks with internal pressur2.5 psig. Such tanks must be anchored to resist the uplift forces induced by thepressure.



As an example, consider a 100-foot diameter tank with 2.5 psig internal pressure.

Pu = Pi (πD2 /4) (144) - TDL

(Eq. 300-1)

where:Pu = uplift force, lb.

Pi = tank internal pressure = 2.5 psig

D = tank diameter = 100 ft.

TDL = tank dead load (shell + roof) = 827,000 lb.

Pu = 2.5 (π1002) (144/4) - TDL

= 2,827,000 lb. - 827,000 lb.

= 2,000,000 lb.

With a factor of safety against uplift of 1.5, then 2,000,000 (1.5)/4000 pounds per cubic yard = 750 cubic yards of concrete needed to resist the uplift.

The above example, though oversimplified, points out the special foundation requirements for this type of tank.

API Standard 650, Appendix F sets forth the appropriate safety factors to be used in calculating the resisting force.

Small Tanks on Elevated SupportsWhen tanks are required to have a prompt leak detection system, positive leak detection can be achieved by supporting the tanks on steel beams over a concrete pad. This arrangement provides a clear area where leaks can be seen. This type of foundation can generally be used for small tanks up to 20 feet in diameter. Elevated tank foundations are more expensive than other types of small tank foundations. Therefore, justify the decision to support the tank on an elevated foundation by an economic comparison with other methods of secondary containment and leak detec-tion.

Example —Design Calculation for Determining Beam SpacingTank size: 20 ft. diameter × 14 ft. high

Specific gravity of contents, s.g. = 1.0

Bottom plate:

Plate thickness, t = 0.25 in (ASTM A283)

Corrosion allowance, tc = 0.125 in

Allowable bending stress, Fb = 20,000 psi (API 650)

L = steel beam spacing, ft.



et.

h

W = uniform load on bottom plate, psf

M = bending moment in bottom plate, lb.-ft.

S = section modulus of bottom plate, in3

W = H × unit wt. water × s.g. + × unit wt. steel

(Eq. 300-2)

= 890 psf

(Eq. 300-3)

S = 2t2 = 2 × 0.252 = 0.125 in3

(Eq. 300-4)

(Eq. 300-5)

Therefore, the beams must be spaced at 1.5 feet on center.

327 Berms and GuttersDesign requirements for the area outside the perimeter of large tanks include:

• A minimum eight-foot wide sloped berm to drain liquids away from the tankand to facilitate maintenance and painting.For tanks 20 feet in diameter or less, berm width needs to be at least six fe

• The slope should be 2% minimum.

• Dress the berm to protect it from erosion. Use either a spray coating of suitable asphaltic binder material or a two-incminimum asphaltic concrete or other permanent paving material.

t tc+

12------------

14 62.4 1.00.25 0.125+

12------------------------------ 490×+××=

MWL

2

10------------ 890L

2

10--------------- 89.0L

2= = =

FbMS-----=

20 000, 89.0L2

12×0.125

-----------------------------=

L20 000 0.125×,

89.0 12×-------------------------------------

1 2/1.53ft on center= =



y

ost.

r l. asins

y

on

he

um

ion

c,

Tank grades that are properly constructed require little maintenance except occasional oiling and clearing of gutters and drains.Selecting a good berm dressing is particularly important from the maintenance point of view.

• Plant mix asphaltic concrete dressings are the most durable types, but mantypes of dressing using well-graded soils mixed with road oils have been successfully used.

• The type of dressing to use will depend on the availability of material and cAsphaltic concrete is more expensive than oil-coated soils.

Refer to Section 700 of the Civil and Structural Manual for more information on paving and grading.

328 Catch Basins and SumpsDesign catch basins for tank water draw-off, sumps, and tank bottom outlets fotanks on concrete ringwalls to resist the hoop tension of the interrupted ringwalDesign and construction details for concrete sumps, bottom outlets, and catch bare shown in Standard Drawings GC-Q1075, GC-Q78677 and GB-S78986, respectively.

330 Grounding Considerations

331 Grounding for Aboveground Metallic TanksChapter 6 of NFPA 78, “Lightning Protection Code,” provides guidance on lightning protection of aboveground tanks. Metallic tanks should be grounded bone of the following methods:

1. Set the tanks directly on earth, concrete or pavement. NFPA 78 specificallystates that vertical, cylindrical tanks at least 20 feet in diameter and restingearth or concrete, and tanks at least 50 feet in diameter and resting on bituminous pavement, are properly grounded. Tanks with a secondary containment membrane beneath the bottom are sufficiently insulated from tground to require bonding in accordance with Method 3 below.

2. Connect the tank to a grounded metallic piping system without insulating joints or flanges. Piping will normally be insulated from tanks where impressed-current cathodic protection systems are applied. See the Corrosion Prevention and Metallurgy Manual for more details.

3. Bond the tank through a minimum of two ground terminals (rods) at maxim100-foot intervals along the tank's perimeter.

For more information on lightning protection, grounding, and bonding, see Sect900 of the Electrical Manual, NFPA 78, “Lightning Protection Code”, or API Recommended Practice 2003, “Protection Against Ignitions Arising Out of StatiLightning, and Stray Currents.”



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340 Foundation ConstructionThe Company has installed many types of tank foundations over the years: oiled sand or dirt pads, plank, crushed rock, rock, brick and concrete ringwalls, etc. More recently it has used the ringwall design with secondary containment and leak detection. This section will discuss what to watch out for during the construction of the latter design, although most of the critical factors and checks will apply to other designs as well. The remarks below apply both to new foundations and, during bottom replacement, to the spacer installed between the old and the new bottom.

The foundation consists of a concrete ring on which the tank shell will rest. Inside the ring is a layer of compacted fill. An HDPE membrane liner is stretched over the fill and impaled on the reinforcing bars that stick up from the ring about 1½ inchFor bottom replacement, the membrane is placed on top of the old bottom (seeSection 342).

A concrete pad (or spacer, for bottom replacements) is poured on top of the membrane liner. If the pad is to be reinforced with polypropylene fiber or wire mesh, this material is placed on the membrane before the concrete is poured. Athe pour, grooves in a pie shape arrangement are cut in the pad to drain any liqleaking from the tank to the outside where it can be seen.

Standard Drawings GD-D1120 and GF-S1121 provide excellent illustrations of requirements for new leak detection bottoms and foundations.

341 Concrete Work

Dimension ChecksDuring construction of the foundation, critical dimensions such as diameters, depths, levels, ringwall depth, fill depth, waterdraw basin dimensions, telltale linlocation, etc., must be checked for accuracy against the drawings.

Excavation and Fill

Before Concrete is Poured. Any backfilling of the excavation made for the foundation should be well tamped into place. The bottom of the excavation shobe checked for adequate compacting. Forming for the vertical walls of the foundation should extend below the grade specified.

After Concrete is Poured. Backfill around the ringwall and waterdraw basin afterremoval of forms should be well compacted.

Concrete for FoundationBefore ordering the concrete, check mix proportions and mix timing with concresubcontractor. Chloride salts should not be added to the mix to accelerate hardeand soluble chlorides should not exceed 0.15%, as recommended by the AmerConcrete Institute’s publication 201.2R-77 “Guide to Durable Concrete.” Also check proportion of concrete to polypropylene fiber reinforcement material, wheused for the pad.



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Before Pouring

Slump Test. Be sure you have cylinders on hand to perform slump tests.

Ringwall Forms. Before concrete is poured, the top of the ringwall forms should be checked for level by survey: the elevation of the top of the concrete must be within ½ inch of the specified elevation at all points. In addition, elevations should not by more than 1/8 inch in any 30-foot circumferential length, nor more than ¼ inabout the entire circumference.

Reinforcing Bars. Before concrete is poured, check that the bars are the correctand dimensions and that they are placed according to the drawings and specifications. The bars must be at least 1½ inches away from the foundation ffor adequate coverage when the concrete is poured.

Concrete Pad. If wire mesh is used as a concrete pad reinforcement instead of trecommended polypropylene fiber, check that there are sufficient “chairs” to hothe wire the proper distance above the fill or old bottom. Before pouring, check slope to ensure there will be sufficient concrete over the wire reinforcement.

During Pouring

Mix Consistency. Perform slump test and check that concrete is worked into all areas so there are no voids or trapped bubbles of air.

Coverage, Concrete Pad. The minimum concrete coverage depth should be checked against the specification.

After Pouring

Concrete Ringwall. Immediately after the ringwall is poured, elevations and tolerances should be checked by survey. Swelling of the formed area usually rein a slight lowering of the top edge of the form. A slight variation in the ringwall diameter is not critical but any variation in the top of the ringwall and pad elevais. The height (top elevation) of the pad edge form should be checked for elevaby survey, not by measuring from the top of the ringwall pour.

Check that drain pipes through the concrete ringwall are clear, not plugged.

Exposed Edges. All exposed edges of final pours should be chamfered. Minimumthicknesses should be checked immediately following the pour.

Concrete Pad. After the forms are removed and needed patching completed on outside edge of the pad, check that the concrete patches or grout adhere prope

When to Cut the Leak Detection Grooves. Saw-cutting of the grooves in the concrete pad should be done as soon as the concrete is cured enough for foot Usually this is 24 to 48 hours after the pour. This is the optimum time for ease ocutting and to avoid broken edges. See Section 342 below for the proper methosaw-cutting the grooves.



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342 Installing the Secondary Containment and Leak Detection SystemTogether, the membrane liner and the grooves cut into the concrete pad are the secondary containment and leak detection system. This section tells you what to watch for during membrane liner installation. Also refer to the following additional sources of information in the Tank Manual: Specification TAM-MS-1, “Tank Bottom Replacement and Membrane Placement”; Specification TAM-MS-4763,“Membrane Liner for New Tanks”; and Tank Manual, Section 250, “Leak Detec-tion and Containment.”

When to Install the Membrane Liner

New foundations. The membrane is placed after completion of the concrete ringwall, removal of the internal ring forms, and backfilling and compacting (to tproper slope) of the area inside the ringwall.

Cone up bottom foundations. The membrane is installed under the waterdraw basin prior to its pour.

Cone down bottom foundations. The center sump and sump liner along with thetelltale line from the sump liner to the standpipe outside the tank are placed priomembrane installation.

How the Membrane Liner is Attached

New foundations. The membrane liner is impaled over the concrete ring foundatreinforcing bars extending vertically from the foundation (see Standard DrawingGF-S1121).

Replacement bottoms. The membrane is attached to the old bottom at the shell adhesive/sealant and by impaling (see Standard Drawing GD-D1120). The old center sump is cut out and replaced with a new sump and sump liner, and telltaline run to a standpipe outside the tank for cone down bottoms.

Forming the Membrane Liner. The membrane liner should be level, smooth andfree of wrinkles as practical before the sheets are extrusion welded (or bonded)together. Check extrusion welds (or lap joint adhesion) for bond and leakage. Bcan be checked with a dulled ice pick, and leakage by vacuum test similar to thused for welded steel plate seams.

On replacement bottoms, the membrane at the “rat holes” should be well sealewith adhesive/sealant. (On bottom replacement jobs, rat holes are the cutouts iold shell that allow leaks to drain from the grooves in the concrete pad and out gutter.)

Telltale Pipes. These pipes carry the liquid from leaks away from the tank to whean operator can see it. On cone down bottoms, telltale pipes should be checkedlevel and tested for leakage. The backfill should be tamped. On replacement boinstallation of the telltale line, the area under the concrete ringwall (or area undthe shell) should be back filled with concrete to avoid local settlement.



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Sump. Center sump elevation should be exactly to specification. The sump should rest fully on well compacted soil. If the base under the center sump has any tendency to shift or settle, an unformed, polypropylene fiber reinforced 4-inch thick pad should be installed and checked for elevation before the basin is installed.

Leak Detection Grooves. Follow the rules below for grooves:

• Grooves in the concrete pad are best made by saw-cutting. As an option, “floating” the grooves while concrete is fresh has also given good results.

• Grooves should line up and extend to the “rat holes” cut in the existing shecone up bottom replacements. The last 12 to 15 inches will have to be chisOn cone down bottoms, grooves shall stop 12 to 15 inches from the shell, which will not have “ratholes.”

• Grooves should extend to the distance from the shell that the concrete sawcut on replacement cone down bottoms.

• The groove layout should be checked against proper drawing detail. Note tdifference between the cone up and cone down groove pattern.

343 Bottom-to-Foundation SealBefore placing the new bottom plates (or annular ring), a band of sealant is placthe edge of the foundation or pad. This sealant prevents groundwater from enteunder the tank.

350 Tank SettlementTanks are relatively flexible structures which tolerate a large amount of settlemewithout signs of distress. However, tank settlement has caused failures such asinoperative floating roofs, shell and roof buckling damage, leaks, and loss of tacontents. Foundation design, soil conditions, tank geometry and loading, as wedrainage, all have a significant effect on settlement.

Large petroleum tanks are generally constructed on compacted soil foundationgranular material, while smaller tanks are often built on concrete slabs. The settlement covered in this discussion pertain to large tanks (over 50 feet in diambecause most large tanks are built on foundations where the thickness, elasticicompressibility of the foundation and subsoil layers can vary enough to producnon-planar distortions when uniformly loaded. However, the basic principles apto all tanks, especially uniform settling and planar tilt.

When filled, tank contents will uniformly load the foundation beneath the tank athe result of hydrostatic pressure in a disk pattern. However, the tank edge:

• carries an increased load from the shell and roof weight.• can suffer loading effects such as twisting of the plates under the shall due

shell rotation.

Note The tank edge is defined as that area of the tank which is comprised of the tank shell, the roof supported by the shell, and the foundation directly beneath.



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For these reasons, most settlement problems occur in the foundation that is under the outside edge of the tank. Settlement problems are assessed by taking elevation readings at the base of the tank. Nonetheless, failures have occurred from interior settling that went undetected in elevation readings.

Settlement failure poses serious consequences to safety and surrounding property. Until the mid 1950s, tanks were limited to about 200,000 bbls capacity. Since then, capacity has increased to 800,000 and 1,000,000 bbl. Considering these tank sizes, criteria must be available to ascertain the extent of settlement and correction procedures.

351 Spotting Settlement ProblemsTank settling can be indicated by any of the following:

• Roof binding on floating roof tanks.• Damage or early wear-out of floating roof seals.• Shell buckling in fixed or floating roof tanks.• Roof buckling in fixed roof tanks.• Loss of support in fixed-tank, roof support columns.• Cracking of welds.• Loss of acceptable appearance.• Over stressed piping connections• Accelerated corrosion due to drainage pattern changes on the outside of

the tank.• Inoperative or less effective drainage on the interior of the tank, especially

where cone-up, cone-down, or single sloped bottoms are used.• Increased susceptibility to seismic damage as a result of distorted, over str

or deformed bottoms.• Leaks in the bottom of shell.The most serious failure results in leakage or loss of contents. The presence ofa small crack in the tank bottom can be a serious threat to the integrity of the taSeveral notable settlement failures have followed this sequence:

1. Development of an initial leak caused by a crack in the tank bottom.

2. Washed out foundation support immediately near the initial leak location, causing the crack to grow from lack of support.

3. Increased leakage and undermining of the support under the tank. The botplates separate from themselves or from the shell where the foundation hawashed away.

Prior to several incidents [1] leakage was seen emanating at the chime, but thecontents could not be pumped out before a major failure occurred.



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352 Kinds of SettlingTank settlement occurs in the following categories:

• Uniform Settlement• Planar Tilt• Differential Shell Settlement• Global Dishing• Local Interior Settling• Sloped Bottoms• Edge Settlement

Uniform SettlingIn this type of settling the soil conditions are relatively uniform, soft or compressible, and a storage tank will slowly, but uniformly sink downward as shown in Figure 300-16. Uniform settling poses no significant problems; howevthere are two important side effects.

Water Ingress. Water Ingress occurs when a depression or water trap is formedaround the tank's periphery where it meets the soil. When it rains, moisture accumulates under the tank bottom near the shell or chime region and corrodebottom.

Piping. Piping connected to the tank will eventually become over stressed by thtank movement unless sufficient flexibility is designed into the piping system.

To assess the degree of uniform settlement, simply monitor elevations at the bathe tank.

Fig. 300-16 Uniform Settlement



Planar TiltIn this mode the tank tips as a rigid structure. (See Figure 300-17). Often planar tilt accompanies uniform settlement. Planar tilt can be assessed from an external tank inspection conducted by taking elevation readings at the base of the tank. The following may occur as the tilt becomes severe.

Appearance. The human eye is sensitive to vertical lines. With a relatively small angle of tilt the appearance of a tank begins to look strange. The public or employees may begin to question the safety of the tank and the operating and maintenance practices of the owner. Planar tilt limited to D/50 is a reasonable plumbness specification that provides an acceptable tank appearance.

Hydrostatic Increase. The tilt of the tank results in an increase in hydrostatic head as shown in Figure 300-17.

If the increased stress causes the shell to exceed the design-allowable stress, there are several solutions:

• Lower the liquid level.• Operate the tank slightly above allowable stresses.

Fig. 300-17 Planar Tilt Settlement



Reduced Storage Capacity. Because the maximum liquid level is often just beneath the roof or overflow, the allowable liquid level may have to be reduced to accommodate the planar tilt.

Ovalizing. If a tank tilts, the plan view will be an ellipse, shown in Figure 300-17. Because floating roof tanks have specific clearances and out-of-round tolerances for their rim seals to work properly, planar tilt can cause a seal problem. However, the amount of planar tilt would have to be extreme for ovalizing to become a problem.

Differential Shell SettlementDifferential settlement, alone or in combination with uniform settlement and planar tilt, results in a tank bottom which is no longer a planar structure. This type of settlement problem can be assessed by taking elevation readings around the circumference of the tank shell, where the bottom projects beyond the shell.

The readings can then be plotted as shown in the Figure 300-18. If the bottom of the tank is planar, then a cosine curve may be fitted through the measured points. However, if there is differential edge settlement, then a best-fit cosine curve can be fitted to these points.

Differential shell settlement is more serious than uniform or planar tilt settlement because deflection of the structure on a local scale is involved which produces high local stresses. Differential edge settlement results in two main problems.

Ovalizing. As shown in Figure 300-19, differential settlement occurring in the tank bottom near the shell produces an out-of-round condition at the top of tanks which are not restricted in movement (e.g., a floating roof tank). One of the most serious problems with bottom differential-edge settlement in floating roof tanks is the operation of the floating roof. Because floating roof seals have specific tolerance limits between the edge of the roof and the tank shell, ovalizing can interfere with the operation or destroy the seal itself.

If the bending stiffness of the tank is much less than the extensional stiffness (thin wall structure), then the theory of extensionless deformations may be used to compute the relationship between differential settlement and radial deformation at the top of the tank.

It has been found that with specific readings of settlement, the following finite difference equation may be used to estimate ovaling:

(Eq. 300-6)

rDH

2---------

N2

π2-------∆Si=



Fig. 300-18 Differential Tank Settlement



where:i = station number of elevation reading taken at base of tank

r = radial shell displacement at top of tank

N = number of stations or readings

H = shell height at which radial displacements are calculated

D = tank diameter

∆S = measured settlement at ith location

x = circumferential shell coordinate

Shell Stresses. Non-planar, differential settlement may generate shell stress near the top of the tank and may result in buckling of the upper shell courses. In the past, the amount of differential settlement allowed was determined by arbitrarily limiting the differential settlement to a constant, which represented a ratio of the settlement to the span between consecutive settlement measurements. Figure 300-20 shows how various structures, particularly buildings, are damaged when the slope represented by the deflection-to-span ratio exceeds some value.

One commonly used limit [2] is

(Eq. 300-7)

Fig. 300-19 Problems Resulting from Shell Out-of-Roundness Due to Nonuniform Settlement Derived from Hydrocarbon Processing, August, 1980. Pg. 102. Used with permis-sion.

∆S1

450---------=



ng.

where:l = length between settlement readings, feet

∆S = allowable settlement

Local slopes limited to approximately l/450 to l/350 applied to tanks have proven conservative, and result in tanks being releveled when further settlement could have been tolerated.

The API 653 formula uses a factor of safety of two times:

(Eq. 300-8)

Global DishingThe entire tank bottom settles relative to the shell. This may occur singly or in combination with other forms of settlement. There is no one form of global settling, however, the majority of tank bottoms do tend to form a dished shape as shown in Figure 300-21. There are several other common global settling patterns and investi-gators have recommended criteria for each type as shown in Figure 300-22. [3]

The problems associated with general global settling are:

• High stresses generated in the bottom plates and fillet welds.

• Tensile stresses near the shell-to-bottom welds that may cause shell buckli

Fig. 300-20 Limiting Angular Distortion Adapted from Berrum, 1963

:+(5(

∆S .011σy1

2

2EH------------=



• Change in calibrated tank volumes (strapping charts and gauges).

• Change in the drainage of the tank bottom profile and puddling when attempting to empty tank.

The literature suggests maximum global dishing values that range from D/50 toD/100 depending on foundation type, safety factor or empirical data. The value

Fig. 300-21 Dish Settling

Fig. 300-22 Normalized Settlement of Tank Bottom



stated in the 1st edition, of API 653 is D/64. For global dishing these values appear to be reasonable. A 100 foot diameter tank using the provisions of Appendix B of API 653 would have a total dish settlement of B=.37R where B is in inches and R is in feet of 18.5 inches. However, for values of R less than 3 - 5 feet these limitations are not really applicable to local settling as explained later.

The methods presented above are based upon the large deflection theory of circular flat plates with edges that are not free to move radially. However, when the difference in settlement between the center and the periphery of the tank is large, there are indications that the bottom membrane does move inward radially or the shell will be pulled in as shown in Figure 300-21. From theoretical considerations, the difference in membrane stresses generated between a circular plate simply supported with a fixed edge and an edge that is free to move radially is a factor of about 3. [4] This means that the stresses will be 1/3 as high for bottom plates that are free to slide as for those that are not. When the tank is loaded with liquid, the bottom plates are probably held in place more securely; therefore, it may not be a valid assumption to use the free edge condition.

For other modes of global settling it has been suggested [5] that different allowable settlements be provided for the different configurations. This is shown in Figure 300-22.

Local Interior SettlingLocal settling that occurs in the interior of tanks often takes the form of depressions as shown in Figure 300-23. Local interior settling poses similar problems to Global Dishing and the proposed methods of assigning a tolerance are again based upon the theory of large deflection. Some of the methods include a relaxation, when the settling occurs near the tank wall, to take into account the freedom of the plate near the shell to slide radially inward as the depression increases.

Fig. 300-23 Bottom Settlement From API 653, Figure B-7 & B-8. Courtesy of American Petro-leum Institute.



t to

Note that the tank fabrication process leads to buckles and bulges in the bottom plates. When the tank is filled with liquid, these tend to level out, but often reappear when the liquid is removed. Most of the models currently proposed for developing settlement criteria do not take into account the initial waviness of the bottom.

This type of settling is inevitable in compacted earth foundations because soil composition and thickness varies under the tank. Deformations are usually formed gradually, without sharp changes in slope, so that the bottom plates are adequately supported. Risk of failure from this type of settlement is minimal unless there are serious problems with the welding integrity.

When large voids form under the tank bottom, the bottom plates may lift off the soil completely as shown in Figure 300-23. Although this is not usually a problem, a large void can lead to localized rippling effects. The tank releveling section covers the problems associated with filling these voids with grout.

Sloped BottomsThe previous settling discussions apply to flat bottom tanks; however, many tanks have slopes intentionally built into the bottom. They fall into three categories:

1. Single slope

2. Cone up

3. Cone down

Because the design slope of these bottoms averages about one inch in ten feet, they can still be considered flat bottoms and the previous sections apply.

However, one special situation arises when the bottom is sloped: Cone up bottoms, subject to general dish settlement, can tolerate more total settlement than either flat bottom, cone-down, or single-slope bottoms. As settling occurs, the bottom compresses and becomes flat. As the soil settles below the tank, the compressive stresses that were generated become relieved until the shell base becomes cone down, approximately equal to the magnitude of the original cone up condition. See Figure 300-24.

However, if the initial cone-up slope is significant, the settling relatively uniform, and the bottom constructed with lap welded joints, a phenomenon known as rippling can occur, usually during the hydrostatic test on newly constructed tanks. Because of the linear layout of bottom plates and the use of fillet welds, a crease or a fold can form, covering large parts of the diameter, as shown in Figure 300-24. The ripples are typically unidirectional and occur in the long direction of the bottom plates. The crease may be very severe (a radius curvature of approximately one foot is not uncommon) and indicates that yield stresses have been exceeded. The ripple can act as a stiffening beam and cause increased differential settlement and bottom failure.

The allowable settlement of cone up should be more than twice that of a flat or otherwise sloped-bottom tank. The maximum slope should be ¾ inch per 10 feeavoid rippling.



Edge SettlementEdge settlement occurs in the bottom plates near the shell as shown in Figure 300-25. It is difficult to determine this condition from the exterior of the tank; however, seen from inside the tank, this is one of the most obvious forms of settling.

Edge settlement occurs frequently in tanks that have been built on grades or compressible soils. If the soil has not been compacted sufficiently or becomes soft when wet, the probability of edge settlement increases. Edge settlement is mainly due to increased loading on the foundation at the periphery from the weight of the steel. Usually the foundation has not been extended far enough beyond the tank radius to prevent lateral squeezing of the foundation (see Figure 300-25).

Edge settling can occur locally in soft spots around the edge of the foundation; however, it usually involves a rather substantial portion of the tank. Edge settlement is rarely seen in tanks that are constructed on reinforced concrete ringwall foundations. It is more common where the tank is built on a crushed stone ringwall foundation.

The two fillet welds between the annular plate, shell, and the bottom plates induce stresses into the annular plate that cause upward bulges. Not strictly edge settlement, these bulges may contribute to it by creating an initial slope in the annular plate which in turn sets up residual stresses that cause the tank bottom under the shell to apply greater downward pressure on the soil. The initial slope may be attributed to edge settlement when it was caused by the welding. Proper weld procedures, careful selection of the welding sequence for all welds in the bottom annular plate, and careful fitup should minimize this problem.

Fig. 300-24 Tank Bottom Ripples



353 Settlement CriteriaTo date there is no appropriate method for estimating tolerable edge settlement. There are, however, numerous tanks in service showing edge settlement with magnitudes of 6 to 18 inches over a span of 1 to 2 feet and functioning without leaks or failures.

Edge settlement is unlike other kinds of settling. API 653 and other proposals are based upon a model that is similar to the dishing models described above. Because this type of settlement involves substantial yielding of the bottom plates (apparent from the large deflections over short spans), any model that uses an allowable stress basis for limiting settlement is probably extremely conservative. A strain-limiting approach may be more appropriate.

The following two figures (Figure 300-26 and Figure 300-27 can be used to deter-mine the maximum allowable edge settlement for areas with bottom lap welds approximately parallel to the shell (Figure 300-26) as well as perpendicular to the shell (Figure 300-27). The maximum allowable edge settlement for areas with a lap weld at an arbitrary angle to the shell can be determined by using the equation:

Bα = Be − (Be − Bew) x sinα (Eq. 300-9)

Fig. 300-25 Edge Settlement From API 653, Figure B-5. Courtesy of American Petroleum Institute.



where Bew and Be are from Figures 300-26 and 300-27 respectively, and α is the angle of the weld to a tank centerline (see Figure 300-28).

Fig. 300-26 Maximum Allowable Edge Settlement for Areas with Bottom Lap Welds Approxi-mately Parallel to the Shell From API 653, Figure B-10. Courtesy of American Petroleum Institute.



Fig. 300-27 Maximum Allowable Edge Settlement for Areas with Bottom Lap Welds Approxi-mately Perpendicular to the Shell From API 653, Figure B-11. Courtesy of Amer-ican Petroleum Institute.



354 Designing for SettlementDepending on the degree and type of settlement expected (determined from similar installations in the area or from soil surveys), there are several means of designing for expected settlement with increasing effectiveness:

1. Standard lap-welded bottom

2. Annular plates with lap-welded bottom

3. Butt-welded bottoms

These construction methods increase in effectiveness (1-3), and they also increase in price. Unless needed for reasons high settlement, the butt-welded tank bottom is generally ruled out on a cost/benefit basis. Because the standard lap welded tank bottom is the most economic, there is a tendency to use this design for locations even where significant settlement is expected.

Additional construction measures can be more effective, such as deeper levels of soil compaction, crushed stone ringwalls, reinforced concrete ringwalls or slabs on ringwall foundations.

The use of annular plates reduces edge settlement. The use of concrete ringwalls virtually eliminates edge settlement.

Fig. 300-28 Edge Settlement with a Lap Weld at an Arbitrary Angle to the Shell From API 653, Figure B-12. Courtesy of American Petroleum Institute.



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355 Releveling TanksReleveling tanks is a common procedure for correcting excessive settlement problems such as buckling shell plates, leakage in the bottom plates, excessive out-of-round and high stresses. When floating roof tank bases have experienced differential settlement, the roofs can bind and seals may be damaged or ineffective. Frequently, releveling causes the tank to reassume a round shape. Tanks that have been buckled due to settlement or tanks that have been constructed with initial out-of-round are usually not improved by releveling.

Releveling MethodsSome companies specialize in tank releveling. Deal only with reputable contractors who have carefully planned a shell-releveling procedure which has proven effective.

All releveling procedures should include these factors:

• For floating roof tanks, the roof should be supported from the shell to preveexcessive stresses and the possibility of cracks occurring from differential movement. Figure 300-29 shows one way of supporting the roof.

Fig. 300-29 Floating Roof Support



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• When tank jacking methods are used, it is possible to jack tanks up approximately 10 feet high, allowing for bottom inspection, cleaning, removcontaminated soil where leakage has occurred, rebuilding of the foundationnecessary, or coating from the underside.

• Support must be supplied for fixed-roof supports so that roof buckling and damage does not occur.

• The amount of differential jacking must be controlled so that shell buckling weld damage in the corner welds, or in the bottom plates, does not occur.

• In all tank releveling procedures large groups are involved and mistakes cocause injuries or unanticipated costs. Any work of this nature should be carfully reviewed for safety, environmental concerns, and good practices. The owner should also be convinced that those performing the work have direcexperience using the proposed methods.

• A releveled tank should be hydrostatically tested. Testing may not be necesin a few cases such as small tanks where the shell stresses are low or thervery limited jacking.

• Corrected piping should be disconnected if releveling will produce excessivstresses causing equipment damage. Underground piping connections to ttank should be exposed for monitoring.

Shell Jacking. Shell Jacking is a common releveling method where lugs are welto the shell near the base as shown in Figure 300-30. Typical spacing is about feet. Once the lugs are in place and a suitable jacking pad set up, jacking procearound the tank circumference in small increments. Jacking in small incrementsprevents warping the bottom excessively out of plane. Shims are installed as thjacks are moved around and the tank can be raised to any desired elevation. Ttank bottom will sag down somewhat, but will not cause structural problems witthe bottom welds if the welds are sound.

Typical specified tolerances average about ¼ inch of level for any measured poon the tank perimeter at the base.

Contractor responsibilities include:

• Furnish, design, install, and remove lugs.

• Remove any weld arc strikes and ground out remaining slag.

• Recommend the prior loading under each shimmed area to prevent foundadamage and settling. (Recommended shim spacing is 3 feet.)

• Propose if and how sand or grout should be applied to low points under thebottom.

• Monitor radial tolerances when correcting an out-of-round tank.

• Provide complete written procedures for all work to be undertaken.



If the jacking exposes a large area under the tank, applying a flowable grout or sand layer will provide a planar foundation for the tank to rest on. However, miscellaneous injection of grout through holes cut into the bottom plates is usually ineffective or makes the situation worse.

If the work is meant to correct out-of-round, require frequent monitoring of the radial tolerances as well as the effect of releveling on these tolerances. At least eight equally spaced points at the top of the shell should used for monitoring. Elevations as well as radial measurements should be made before and after the work.

A hydrostatic test should be conducted after the tank is releveled.

Under-the-Shell Releveling Method. The Under-the-Shell releveling method uses jacking under the bottom of the shell. Small pits are excavated to hold the jack under the tank shell. Figure 300-31 shows a typical jack arrangement for this method. The principle objection to this method is that pits must be excavated beneath the tank shell. In soil foundations, this may cause a loss of compaction in the order of 40 -50%. [6] Another problem is that the spacing for shims and for jack points must be greater than the shell-jacking method and therefore would provide higher soil stresses while the work is in progress.

The same procedures, specifications, precautions and testing as covered under shell jacking should be observed.

Tank Leveling by Pressure Grouting. This method, often called sand pumping, is used to force low spots or settled areas upward. This method can be used to raise small or large areas where tank bottoms are low. The contractor forces sand or grout under pressure into the area to stabilize the bottom plates. Where the involved areas are small and numerous, this method is usually ineffective because the mixture will

Fig. 300-30 Jacking Lugs Used on Large Tanks



flow through the areas of least resistance and lift the plates even further. It also causes the tank to rest on points rather than uniformly.

However, there are some cases where grout can be used effectively: pressure grouting has been effectively used to level areas under fixed roof supports, for example.

A tank owner considering this method should examine a step-by-step proposal from the contractor to assure that good practices are involved and that all safety and envi-ronmental regulations are considered. Before cutting the bottom to inject grout, precautions must be taken to handle the possible existence of flammable liquids or toxic substances that could have been stored or leaked in the past.

360 References1. Cummiskey, B. J., Impoundment Liner Testing - Western Producing Oil

Cleaning Plant, June 30, 1983, Materials and Equipment Engineering Unit File 25.6.

2. Klein, L. J., Storage Tank Containment Membrane Tests—El Segundo, December 7, 1983, Materials and Equipment Engineering Unit File 6.85.

Fig. 300-31 Jacking Pit Dimensions



7

3. Stofanak, R. J., Storage Tank Containment Membrane Tests—El Segundo, April 3, 1985, Materials and Equipment Engineering Unit File 6.85.

4. Rippel, T. E., Permeability Testing Flexible Membrane Liners, February 28, 1986, Materials and Equipment Engineering Unit File 25.06.01.

5. Rippel, T. E., Immersion Testing of Flexible Membrane Liners for SecondaryContainment, May 30, 1986, Materials and Equipment Engineering Unit File 6.85.

6. Kmetz, J. H., Adhesives Testing for Secondary Containment Membrane Systems, December 3, 1987, Materials and Equipment Engineering Unit File 56.1.

7. Environmental Protection Agency, Document EPA-600/2-88/052, Lining of Waste Containment and Other Impoundment Facilities, Appendix K

8. James S. Clarke, Recent Tank Bottom and Foundation Problems, Esso Research and Engineering Co., Florham Park, NJ 1971.

9. DeBeer, E. Foundation problems of petroleum tanks, Annal. l’Inst. Belge Petrole 1969 6 25-40.

10. D’Orazio and Duncan, Differential Settlements in Steel Tanks Journal of Beotechnical Engineering Vol 113, No 9, 12/4/86.

11. Timeshenko, Theory of Plates and Shells, 2nd edition, Table 82.

12. Timothy B. D’Orazio and James M. Duncan, Differential Settlements in Steel Tanks Journal of Geotechnical Engineering, Vol 113, No. 9, September, 198ISSN 0733-9410/87/0009-0967/$01.00.

13. James S. Clarke, Recent Tank bottom and Foundation Problems, Esso Research and Engineering Co., Florham Park, NJ 1971.


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