Steel Tanks AISI Publication-1976

8/3/2019 Steel Tanks AISI Publication-1976

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Steel P late Engineering Data - Volume 1

Steel Tanks for Liquid StorageRevised Ed ition -1976

Published byCommittee of Steel Plate Producers. Committee of StainlessSteel ProducersAMERICAN IRON AND STEEL INSTITUTE

In cooperation with and editorial collaboration by.' STEEL PLATE FABRICATORS ASSOCIATION, INC.J . , , " ~_ '_ ' ,_ ~


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cknowledgmentsFor the preparation of the original version of this technicalpublication on carbon steel plate materials and tanks for liquidstorage, the Committee of Steel Plate Producers of AmericanIron and Steel Institute retained Mr. I.E. Boberg as author. Forhis skillful handling of the assignment, the Committee gratefullyacknowledges its appreciation.The Committee also wishes to acknowledge the important and

valuable contribution made by members of the Steel Plate Fabri-cators Association and representatives from the member steelproducing companies of American Iron and Steel Institute in re-viewing, and later revising and updating, the material for publicationin this current edition.The Committee of Stainless Steel Producers of American Iron

and Steel Institute established a Task Force to produce and supplya special section on stainless steel tanks to this publication, andwishes to acknowledge its appreciation to this group for a commen-dable effort.Appreciation is expressed to the American Society for Testing

and Materials, the American Petroleum Institute and the AmericanWater Works Association for their constructive suggestions andreview of this material. Much of the illustrative materials in thismanual appears through their courtesy.

Committee of Steel Plate Producers

It is suggested that inquiries for further information on designsof steel tanks for liquid storage be directed to: Steel PlateFabricators Association, Inc., 15 Spinning Wheel Road, Hinsdale,Illinois 60521.

:.': '" trI,:I~,"

AMER:ICAN IRON AND STEEL INSTITUTE1000 16th Street, N.W., Washington, D.C. 20036

ii5000 (127)


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PrefaceThe purpose of this publication is to provide a design referencefor the usual design of tanks for liquid storage. For unusualapplications, involving materials or liquids not covered withinthese pages, nor referenced herein, designers should consult morecomplete treatments of the subject material.Part I contains general information pertaining to all types of

carbon plate steels. This section may seem elementary to themetallurgist or to one who is thoroughly familiar with steel industryterminology, practice and classification. For others, it should behelpful to an understanding of what follows.Part II deals with the particular carbon steels applicable to tanks

for liquid storage.Part III covers the design and construction of carbon steel tanks

for liquid storage.Part IV covers materials, design, and fabrication of stainless steel

tanks for liquid storage. It has been prepared especially for thispublication by the Committee of Stainless Steel Producers of AmericanIron and Steel Institute.Inquiries for further information on design of steel tanks should

be directed to Steel Plate Fabricators Association, Inc.

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Part IPart IIPart IIIPart IV

ContentsMaterials - General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1Materials - Carbon Steel Tanks for Liquid Storage. . . . . .. 7Carbon Steel Tank Design and Construction 17Stainless Steel Tanks for Liquid Storage. . . . . . . . . . . . . .. 55

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PartIMaterials-General

DesignationMost of the steel specifications referred to in thismanual are contained in the Book of ASTM Stand-ards, Part 4, which can be obtained from the Ameri-can Society for Testing and Materials (ASTM).Each ASTM specification has a number such as

A283, and within each specification there may beone or more grades or qualities. Thus an example ofa proper reference would be "ASTM designationA283 grade C:' In the interest of simplicity, such areference will be abbreviated to "A283-C."ASTM standards are issued periodically to report

new specifications and changes to existing oneshaving a suffix indicating the year of issue such as"A283-C-74." Thus a summary such as is providedhere may gradually become incomplete, and it isimportant that the designer of steel plate structureshave the latest edition of ASTM standards availablefor reference.

DefinitionsAt least a nodding acquaintance with the terminologyof the steel industry is essential to an understandingof steel specifications. This is especially true because,in common with many other industries, a number ofshop and trade terms have become so thoroughly im-planted in the language that they are used instead ofmore precise and descriptive technical terms. Thefollowing discussions may be of assistance.

Steelmaking ProcessesPractically all steel is made by the open hearth fur-nace process, the electric furnace process or the basicoxygen process. ASTM specifications for the differentsteels specify which processes are permissible in eachcase.

Steelmaking PracticeThe steels with which we are concerned are eithercontinuously cast, or cast into ingots which are hotrolled to convenient size for further processing. Inmost steelmaking processes, the principal chemicalreaction is the combination of carbon and oxygento form a gas. If the oxygen available for this reac-tion is not removed, the gaseous products continueto evolve during solidification in the ingot. Coolingand solidification progress from the outer rim ofthe ingot to the center, and during the solidificationof the rim, the concentration of certain elementsincreases in the liquid portion of the ingot. Theresulting product, known as RIMMED STEEL, hasmarked differences in characteristics across thesection and from top to bottom of the ingot.Control of the amount of gas evolved during

solidification is accomplished by the addition ofa deoxidizing agent, silicon being the most com-monly used. If practically no gas evolved, theresult is KILLED STEEL, so called because itlies quietly in the ingot. Killed steel is charac-terized by more uniform chemical compositionand properties than other types. Although killedsteel is a quality item, the end result is often notso specified by name, ,but rather by chemicalanalysis. Other deoxidizing elements are used, butin general, a specified minimum silicon content of0.15% on ladle analysis indicates that a steel is"fu IIy kilied."The term SEM II


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rial from the ingot. As a practical matter, therefore,plate steels are usually furnished as semikilled steelunless a minimum silicon content of 0.15% on ladleanalysis is specified.

Chemical RequirementsA discussion of the effects of the many elementsadded to steels would involve a metallurgical treatisefar beyond the scope of this work. However, certainelements are common to all steels, and it may be ofhelp to briefly outline the effects of carbon, man-ganese, phosphorus, and sulfur on the propertiesof steel.CARBON is the principal hardening element insteel, and as carbon increases, hardness increases.Tensile strength increases, and ductility, notchtoughness and weldability decrease with increasingcarbon content.MANGANESE contributes to strength and

hardness, but to a lesser degree than carbon. In-creasing the manganese content decreases ducti lityand weldability, but to a lesser degree than carbon.Because of the more moderate effects of manganese,carbon steels, which attain part of their strengththrough the addition of manganese, exhibit greaterductility and improved toughness than steels ofsimilar strength achieved through the use of carbonalone.PHOSPHORUS. Phosphorus can result in notice-

ably higher yield strengths and decreases in ductility,toughness, and weldabi lity. In the steels under dis-cussion here, it is generally kept below a limit of0.04% on ladle analysis.SU LFU R decreases ducti lity, toughness, and welda-

bility, and is generally kept below a limit of 0.05% onladle analysis.HEAT ANALYSIS is the term applied to the

chemical analysis representative of a heat of steeland is the analysis reported to the purchaser. It isdetermined by analyzing, for such elements as havebeen specified, a test ingot sample obtained fromthe front or middle part of the heat during thepouring of the steel from the ladle.PRODUCT ANAL YS!S is a supplementary

chemical analysis of the steel in the semifinishedor finished product form. It is not, as the term mightimply, a duplicate determination to confirm a previ-ous result.Carbon SteelSteel is considered to be carbon steel when:

1. No minimum content is specified or requiredfor aluminum, boron, chromium, cobalt, columbium,molybdenum, nickel, titanium, tungsten, vanadium,zirconium, or any other element added to obtaindesired alloying effect;2. When the maximum content specified for any

of the following elements does not exceed the percen-tages noted: manganese 1.65, copper 0.60, silicon 0.63. When the specified minimum for copper does noexceed 0.40%.

Alloy SteelSteel is considered to be alloy when either:1. A definite range or definite minimum quantity

is required for any of the elements listed above in (1)under carbon steels, or2. The maximum of the range for alloying elements

exceeds one or more of the limits listed in (2) undercarbon steels.

High Strength Low Alloy SteelsThese steels; generally with specified yield pointof 50,000 psi and containing small amounts ofalloying elements, are often employed where highstrength or light weight is desired.

MechanicalRequirementsMechanical testing of steel plates includes tensiletests and toughness tests. The test specimens and thetests are described in ASTM specifications A6, A20,andA370.

From the tension tests are determined the TEN-SI LE STRENGTH and YIELD POINT or YIELDSTRENGTH, both of which are factors in selectingan allowable design stress, and the elongation overeither a 2" or 8" gage length. Elongation is ameasure of ducti litv and workabi lity.Toughness is a measure of ability to resist brittle

fracture. Toughness tests are not required unlessspecified, and then usually because of a low servicetemperature and/or a relatively high design stress.Conditions under which impact tests to determinetoughness are required or recommended will bediscussed in connection with specific structures.

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A number of tests have been developed to demon-strate toughness, and each has its ardent proponents.The test most generally accepted currently, however,is the test using the Charpv V Notch specimen. De-tails of the specimen and method of testing can befound in ASTM-A370, "Mechanical Testing of SteelProducts," and in A20 and A673. Briefly described,an impact test is a dynamic test in which a machined,notched specimen is struck and broken by a singleblow in a specially designed testing machine. Theenergy expressed in foot-pounds required to breakthe specimen is a measure of toughness. Toughnessdecreases at lower temperatures. Hence, when im-pact tests are required, they are usually performednear temperatures anticipated in service. The NavalResearch Laboratory's Drop Weight Test, describedin E208, is also used frequently.

Grain SizeGrain size is affected by both rolling practice anddeoxidizinq practice. For example, the use ofaluminum as a deoxidizer tends to produce finerwains. Unless included in tile ASTM specification,or unless otherwise specified, steels may be furnishedto either coarse grain or fine grain practice at theproducer's option. Fine grain steel is considered tohave {jreater toughness than coarse grain steels. Heat-treated fine grain steels wi II have greater toughnessthan as rolled fine grain steels. The designer is con-cerned only with the question of under what condi-tions is it justifiable to pay the extra cost of specifving fine grain practice with or without heat treatmentin order to obtain improved toughness. Guidelineswill bf! discussed in later sections.

Heat TreatmentPOSTWELD HEAT TREATMENT consists of healing tile steel to a temperature between '11OOF and1250F , furnace cooli ng until the temperature hasreduced to about GOOF and then cooling in air.Residual stresses will be reduced by this procedure.NORMALIZING consists of heating the steel tobetween l GOOF and 1700 F, holdi ng for a su fficie nt

time to allow transformation, and cooling in air,primarily 10 effect grain refinement.QUENCHING consists of rapid cooling in a suit-

able medium from the normal izing temperature.This treatment hardens and strenqthens the steeland is normally followed by tempering.

TEMPERING consists of reheating the steel to arelatively low temperature (which varies with theparticu lar stee I and the properties desired). Th istemperature normally lies between 1OOOFand1250F. Through the quenching and temperingtreatment, many steels can attain excellent tough-ness, and at the same time high strength and goodductility.

To illustrate the effect of heat treatment on tough-ness and strength, refer to Figure 1. The numericalvalues shown apply only to the specific steel described.For other steels, other values would apply, but thetrends would be similar.

Referring to Figure 1, if the designer has selecteda Charpv V Notch value of "x" ft.-Ibs, as desirableunder special service conditions, it will be noted thatthe steel ill ustrated wou Id not be acceptable at tem-peratures lower than about +35F in the as-rolledcondition. In the normalized condition, the samesteel would be acceptable down to about -55F, andif quenched and tempered, to about -80F togetherwith an increase in strength without any increase incarbon, manganese, or other hardeninq elements.Note, however, that heat treatment adds to the costand is indicated only when service conditions indicatethe necessity for increased toughness and/or increasedstrength.

Classification of Steel PlatesPlate steels are defined or classified in two ways. Thefirst classification, which has already been discussed,is based on differences in chemical composition be-tween CARBON STEELS, ALLOY STEELS andHIGH STRENGTH LOW ALLOY STEELS. Thesecond classification is based primarily on the differ-ences in extent of testing between STR UCTU RA LQUALITY STEELS and PRESSURE VESSE LQUALITY STEELS.' It should not be construedthat these terms limit the use of a particular steel.Pressure vessel steels are often used in structuresother than pressure vessels. The distinction betweenstructural and pressure vessel qualities is best under-stood by a comparison of the governing ASTM specifications.

Prnssuro vessel quality steels were previously known iI~FLANGE and FIRE-BOX qualil ius, historically inherit crlterms used to define differences in ihu CXII~1l1 of leslin~J,hUI which have no present-clay si qniticance insofar asthe end lIS(~ of the sll~el i~;onceruorl.

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ASTM designation A6, General Requirements forRolled Steel Plates for Structural Use, covers a groupof common requirements and tolerances for the steelslisted therein, the chemical composition and specialrequirements for wh ich are outlined under separatespecification numbers such as A36, A283, A514, etc.Similarly, ASTM designation A20, General

Requirements for Steel Plates for Pressure Vessels,covers a group of common requirements andtolerances which apply to a list of about 25 steels, thechemical composition and special requirements forwhich are outlined under separate ASTM specificationnumbers.Both A6 and A20define tolerances for thickness,

weight, width, length, and flatness, but for thedesigner the important difference is in the qualityof the finished product as influenced by thedifference in extent of testing. A general com-parison of the two qualities follows:1. Chemical Analysis - no difference in heatanalysis. Both A6 and A20 require one analysis per

heat plus the option of product analysis. Productanalysis tolerances for structural steels are given in A6.2. Testing for mechanical propertiesa) In general, all specifications for structuralquality require two tension tests per heat,size bracket and strength gradation. A6 specifiesthe general location of the specimens.b) In general all specifications for pressure vesselquality require either one or two transverse ten-sion tests, depending on heat treatment, fromeach plate as rolled," (and as heat-treated, ifany). This affords a check on uniformity withina heat. Specification A20 also specifies thelocation from which the specimens are to betaken.

3. Repair of surface defects - the limitations onrepair of surface defects are more restrictive in A20than in'A6.

*The term "Plate as rolled" refers to tile unit plate rolledfrom a slab or directly from an ingot in relation to thenumber and location of specimens, not to its condition.

WeldingInasmuch as practically all plate structures are fabri-cated by welding, a brief discussion of welding pro-cesses follows.Welding consists of joining two pieces of metal by

establishing a metallurgical. bond.between.them. Therare many different types of welding, but we are con-cerned only with arc welding. Arc welding is a fusionprocess in which the bond between the metals is pro-duced by reducing the surfaces to be joined to aliquid state and then allowing the liquid to solidify.The heat required to reduce the metal to liquidstate is produced by an electric arc. The arc is formedbetween the work to be welded and a metal wirewhich is called the electrode. The electrode may beconsumable and add metal to the molten pool, or itmay be non-consumable and of a relatively inertmetal, in which case no metal is added to the work-piece.In the welding of steel 'plate structures, we areconcerned principally with five variations of arc

welding:1. Shielded metal arc process (SMAW)2. Gas metal arc process (GMAW)3. Flux-cored arc process (FCAW)4. Electrogas or Electroslag welding5. Submerged arc process (SAW)

Shielded Metal Arc WeldingIn the early days of arc welding, the consumableelectrode consisted of a bare wire. The pool ofmolten metal was exposed to and adverselyaffected by the gases in the atmosphere. It be-came obvious that to produce welds with adequateductility, the molten metal must be protected orshielded from the atmosphere.This led to the development of the shielded metal

arc process, in which the electrode is coated withmaterials that produce a gas as the electrode is con-sumed which shields the arc from the atmosphere.The coating also performs other functions, includ-ing the possible adding of alloying elements as wellas slag-forming materials which float to the top andprotect the metal during solidification and cooling.In practice, the process is limited primarily to

manual manipulation of the electrode. Not toomany years ago, this process was almost universallyused for practically all welding. It is still widely

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used for position welding, i.e., welding other thanin the down flat position. For the down flat posi-tion some of the later processes described beloware much faster and hence less costly.

Gas Metal Arc WeldingIn the gas-shielded arc welding process, the moltenpool of metal is protected by an externally suppliedgas, or gas mixture, fed through the electrode holderrather than by decomposition of the electrode coat-inQ. The electrode is a continuous filler-metal (con-sumable) bare wire and the gases used include helium,argon, and carbon dioxide. In some cases, a tubularelectrode is used to facilitate the addition of fluxesor addition of alloys and slag-forming materials.Some methods of this process are called MIG orCO2 welding.The gas-shielded process lends itself to high ratesof deposition and high welding speeds. It can beused manually, semi-automatically, or automatically.

Flux-Cored-Arc WeldingThis is an arc-welding process wherein coa lescenceis produced by heating with an arc between acontinuous filler-metal (consumable) electrode andthe work. Shielding is obtained from a flux containedwithin the electrode. Additional shielding mayor maynot be obtained from an externally supplied gas orgas mixture.

Electrogas or Electroslag WeldingThis process is a method of gas metal-arc welding orflux-cored-arc welding wherein molding shoes con-fine the molten weld metal for vertical positionwelding.

Submerged Arc WeldingSubmerged arc welding is essentially an automaticprocess, although semi-automatic applications havebeen used.The arc between a bare electrode and the work is

covered and shielded by a blanket of granular, fusiblematerial deposited on the work ahead of the electrodeas it moves relative to the work. Filler metal is ob-tained either from the electrode or a supplementarywelding rod. The fusible shielding material is knownas melt or flux.In submerged arc welding, there is no visible evi-

dence of the arc. The tip of the electrode and themolten weld pool are completely covered by the fluxthroughout the actual welding operation. High weld-ing speeds are achieved.It will be obvious that the necessity of depositi ng

a granular flux ahead of the electrode lends itselfbest to welding on work in the down flat position.Nevertheless, ingenious devices have been developedfor keeping flux in place, so that the process hasbeen appl ied to almost all positions except overheadwelding.

WeldabilityIt will be observed from the above that all arc weldingprocesses result in rapid heating of the parent metalnear the joint to a very high temperature followedby chi IIing as the relatively large mass of parent plateconducts heat away from the heat-affected zone. Thisrapid cooling of the weld metal and heat-affectedzone causes local shrinkage relative to the parent plateand resultant residual stresses.Depending on the chemical composition of the

steel, plate thickness and external conditions, specialwelding precautions may be indicated. In very coldweather, or in the case of a highly hardenable materialpre-heating a band on either side of the joint will slowdown the cooling rate. In some cases post-heat orstress relief as described earlier in this section is em-ployed to reduce residual stresses to a level approach-ing the yield strength of the material at the post heattemperature.With respect to chemical composition, carbon is

the single most important element because of itscontribution to hardness, but other elements alsocontribute to hardness to a lesser degree.It is beyond our scope to provide a definitive dis-cussion on when special welding precautions areindicated. In general, the necessity is dictated onthe basis of practical experience or test programs.

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2:0i=Coa:0Cf)r I> -C !Ja:w2:w:co XI-02:> -> -Coa::co

Figure 1

Typical Effect of HeatTreatment on Notch Toughnessof a Fine-Grained C-Mn-Si Steel (1 Inch Thickness)

Quenched and Tempered

Normalized

As Rolled

As Rolled

C Mn Si AI0.17 1.26 0.27 0.04

ITensile Strength I Yield Strength

77,400 psi 52,300 psiNormalized 76,500 psi 54,800 psi

63,000 psiuenched 8. Temp'd. 83,100 psi

I75100 -50 --25 502575

TEMPERATURE-DEGREES FAHRENHEIT

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Part IIMaterials-Carbon Steel Tanks for

Liquid StorageIntroductionThe intent of this manual is to provide informationthat may be useful in the design of fiat-bottomed,vertical cylindrical tanks for the storage of liquidsat essentially atmospheric pressure. Major attentionhas been directed to tanks storing oil or water, whichconstitute most of the tanks bu ilt. However, sugges-tions have been included for storage of liquids merit-ing special attention, such as acid tanks.There are two principal construction standards in

general use: the American Petroleum Institute (API )Standard 650 covering "Welded Steel Tanks for OilStorage," and the American Water Works Associa-tion (AWWA) Standard D100 covering "Steel Tanksfor Water Storaqe." The abbreviations API andAWWA will be used for the sake of convenience.Both API and AWWA approve the use of a relative-

Iy large number of different steel plate materials. Inaddition,. API Standard 650 Appendix D and Appen-dixG, and AWWA Standard D100 Appendix C pro-vide alternate design rules for tanks designed at high-er stresses in which the selection of steel is intimatelyrelated to stress level, thickness, and service tempera-ture, as well as the type and degree of inspection. Asa result,.knowledge of available materials and theirlimitations is equally as important as familiarity withdesign principles.Useful information concerning plate steels in generalhas been covered in Part I. It is the purpose of thissection to assist in the selection of the proper steelor steels in the construction of tanks for liquidstorage.It should be emphasized that ASTM, API, and

AWWA periodically issue revisions and additions,with the result that any summary such as provided

herein may gradually become incomplete. Thus thetank designer should have the latest standards avail-able for reference.Tables 1, 2, and 3 summarize the important

properties of thesteels specifically permitted byAPI Standard 650 or AWWA Standard D100. Thetables are not complete for every ASTM designationlisted. Only those grades or thickness ranges permit-ted by API or AWWA are included.Because the high-strength quenched and temperedalloy steels are so different from the others, theyhave been grouped together in Table 3. Table 1 sum-marizesall remaining structural quality steels andTable 2 lists steels of pressure vessel quality.Factors Affecting Selection of Steel.PlateCarbon steels as listed in Tables 1 and 2 comprisethe greatest volume of steel plates used in tanks.Some steels that are rarely used in tank construc-tion today are included also because they arelisted as "permissible" steels by API or AWWAor both.As you will learn in Part III of this manual, both

API and AWWA Standards include basic designs, inwhich the allowable design stresses are the same forany of the permissible materials. Both Standards alsoinclude alternate design criteria, permitting higherdesign stresses iii"return for more restrictive design,inspection, and material requirements.It will be obvious that inasmuch as the basic de-

sign provisions of both standards allow identicaldesign stresses for any of the permissible steeis,economic pressure will lead to the selection of theleast expensive steel that will be satisfactory forthe intended service.

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Referring first to Table 2, A285 is permitted byAWWA and A516 is permitted by both AWWA andAPI basic standards but they are rarely used. Theyare more expensive than the structural carbon steelsof Table 1, which have proven quite satisfactoryfor the purpose.Referring to the carbon steels of Table 1, A283-A

and B are permitted only by AWWA. The fabricatorwho stocks plates for both API and AWWA tanks isnot llkelv to select any of these steels. To do so wouldonly serve to aggravate the problem of maintainingthe identity of different steels in ' inventory in hisplant at any given time.A131-A is only available up to 1/2". The AWWA

standard limits A283-D to 3/4" thickness. The APIbasic standard limits the shell (and reinforcing) platethickness to 1/2" maximum, including corrosionallowance, if any.Of the remaining steels, since A131-B and CSand A36 are more costly than A283-C, it is probablethat any tank built in accordance with either APIor AWWA basic designs will utilize A283-C through-out bottom shell and roof unless the purchaserspecifies otherwise.Steel selection is not so simple and stra ightfor-

ward in the case of tanks built in accordance witheither of the API or AWWA alternate design bases.Unstressed portions of such tanks, including bot-toms and roofs, will probably be furnished as A283-C (or A131-A, which is practically identical toA283-C) unless the purchaser specifies otherwise.The selection of material for the shell demands fur-ther attention.The alternate design bases of both API and AWWA

resulted from a desire to utilize newer and improvedsteels and modern welding and inspection techniquesto bui Id tanks of higher quality. The use of higherstresses demanded attention to other properties ofsteel, primarily toughness. An exhaustive discussionof toughness is beyond the scope of this work, butit can be pointed out that as the stress level increasesand temperature decreases, toughness becomes moreimportant.At the stress level existing in API and AWWA

basic design criteria tanks, history has demonstratedthat the steels used in combination with the specificwelding and inspection rules have been adequate forthe service temperatures involved. Upon venturinginto the field of stress levels higher than the historiclevels, steels having greater toughness have been con-sidered a necessary corollary. Thanks to research in

metals, such steels are available. A number of factorsenter into making a proper selection. For example,for any given steel, toughness generally decreases asthickness increases. The toughness of carbon steelsis improved if part of the hardness and strength isobtained by a higher manganese content and lowercarbon atthe same strength level. Fine-grained steelsexhibit greater toughness than coarse-grai ned steels;this can be accomplished in the deoxidizing process,and in heat treatment.Thus as th ickness increases and service tempera-

ture decreases, more stringent attention must bepaid'to toughness from the standpoint of materialsselection and fabrication.The steels approved by the API Appendix D and

AWWA Appendix C for use at these higher stresslevels have statistically demonstrated that they dohave adequate toughness for the thickness and tem-perature ranges shown. The appendices do not re-quire additional Charpy impact tests to demonstratetheir suitability. API Appendix G, the alternate de-sign rule using high stresses and other materials,usually requires Charpy tests to verify toughness.In the final analysis, the goal is to design the

least expensive but acceptable tank for a given setof conditions. AP I specifies the steels to be usedwithin narrow limits, but AWWA rules permittinghigher design stresses afford a fairly wide selectionof steels and stress levels to choose from, thuspresenting a problem of selection.A definitive treatment of economics is beyond

the scope of this work. Basically, the factorsinvolved are:1. Cost of material2. Weight of material as it affects freight andhandling

3. Fabrication, erection, and welding costs4. Inspection costsNone of these factors is necessarily conclusive in

itself. In any given case, the lightest weight or lowestmaterial cost mayor may not be the least expensiveoverall depending on the relative importance ofthefactors listed above. The tank fabricator is usually inthe best position to judge which steel or combinationof steels will permit construction of the most econo-mical, safe tank.It is generally unwise to specify a more' expensive

steel than can be justified by the application.There are material costs not associated with qual-

ity. The cost of plates will vary with both width andthickness, and from this consideration tank shell

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plates approximately 8' wide will generally be used.Particular situations may dictate the use of wideror narrower plates for all or part of a tank shell.Although both the API and AWWA Standards

permit the ordering of plates for certain parts ofthe tank on a weight rather than thickness basis,there isno longer any economic advantage in doingso.

The FutureTo this point, only those steels permitted by API orAWWA have been discussed. Other steels have beenused to a minor extent by those thoroughly fami liarwith the problems involved. Among these are thematerials referred to in Part I as high strength lowalloy steels, manufactured either as proprietary, tradenamed steels, or to ASTM specifications. Some ofthese steels offer the additional attraction of im-proved atmospheric corrosion resistance, thus elim-inating the necessity for painting outside surfaces.As is the case with all high strength materials, thedesigner and user must assure themselves that factorsother than strength (toughness for example) areproperly allowed for in design and construction.For obvious reasons, all construction codes lag

behind technical progress. The extensive researchfacilities of individual steel producers and AmericanIron and Steel Institute are constantly searching for

ways to better serve the needs of our modern econ-omy. But before any construction standard such asthose of API and AWWA can accept and approve anew material, it must have been established that itis suitable for the structures in which it will be used.Usually, but not always. acceptance by API or

AWWA implies prior acceptance by ASTM. Primarilythis 'is because ASTM specifications clearly delineatethe materials to be furnished, whereas any departurefrom ASTM (as in the case of API Appendix G , Par.G. 10) requires that the standards involved spell outthe requirements in corresponding detail. New ASTMsteels mayor may not eventually find their way intothe construction standards, depending on economicsand the proven properties of the materials.

It should be left to those who have acquired thenecessary experience in tank construction to pioneerin the use of materials not approved by API or AWWA.The designer, the user, and the fabricator assumeadded responsibilities in working outside of recog-nized industry standards. On the other hand, suchpioneering by qualified organizations in the past ledto the progress represented by Appendix C of AWWA0-100 and Appendices 0 and G of API-650.As in the case of steels already approved by API

and AWWA, time and experience will eventually leadto recognition of the steel or combination of steelsthat will yield the highest quality tank at least cost.

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TABLE 1.(A) CARBON AND ALLOY STEELS OF STRUCTURAL QUALITY (ASTM A6 Unless Noted)

CARBON STEELSSPECIFICATION A131(8) _ . - _ - -DESIGNATION A36 A283

RADE OR THICKNESS CS(13)RANGE 3/4inax >31~11/2 >1-21/2 A B A B Cess Limits, Inches(1)M or eSA(5) 3/4 max >3/4-1 1/2 > 21/2 2 max 1 max 2 max - - -I-650 1/2 0 0 1/2 1/2 1/2 0 0 1/2-650 Appendix D(3) 3[4 1 1/2 0 1/2 1 1/2 0 0 1-650 Appendix G(3) 0 0 0 0 0 0 0 0 03/4 1 1/2 2 1/2 1 0 2 2 2WA-Dl00 Appendix d4) 1/2 0 0 1/2 1 0 0 0 0um Tensile Strength, I


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TABLE 1. (A) (Continued)

CARBON STEELS HIGH STRENGTH LOW ALLOY STEELS

A573 CSAG 40.8(5) A441 API.650G(6) ABS(7)G40.8 G40.8(11 ) A662(11 )

70 Grade A Grade B Grade B 3/4 max 13/4 max 2 max

1/2 max 1 1/2 max 1 1/2 max 1 1/2 max 2 max 3/4 - EH36(13)1/2 1/2 1/2 1/2 1/2 0 0 01/2 1 1/2 1 1 1/2 1 1/2 0 0 01/2 1 1/2 0 0 0 0 13/4 13/4

0 0 0 0 0 0 00 1 1 1/2 1 1/2 1/2 0 0

70 65 65 65 70 70 7190 85 85 85 -- 90 90:

42 36(12) 36(12) 40 50 50 '51

- -- 23 23 23 -- -- 2218 20 20 20 18 18 19.28(9) .21 .19 .19 .22 .23 .18

.85 .85 .85 .85 .85 .80(14) .901.50 1.50 1.150 1.25 1.35(15) 1.601.20

.15 -- .15 .15 -- .15 .105 .30 .35 .35 .30 .30 .50 .50

4 .04 .03 .03 .035 .04 .04 .045 .05 .045 .045 .04 .05 .05 .04

(12) Minimum yield 40 I


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TABLE 1. (A) CARBON AND ALLOY STEELS OF STRUCTURAL QUALITY (ASTM A6 Unless Noted)(Continued)

CARBON STEELS

SPECIFICATIONDESI


18/70

TABLE 2. (A) CARBON STEELS OF PRESSURE VESSEL QUALITY (ASTM A20)

ASTM Designation A285Grade B C

Thickness Limits, InchesASTM 2 max 2 maxAPI650 0 1/2API650 Appendix 0(5) 0 1API650 Appendix G(5) 0 0AWWA0100 3/4 2AWWA0100 Appendix C(6) 0 0

Minimum Tensile Strength, KSI 50 55Maximum Tensile Strength, KSI 70 70Minimum Yield Point, KSI 27 30Minimum Elongation in 2", Percent (4) 28 27Minimum Elongation in 8", Percent(4) 25 23Heat Analysis, PercentCarbon, Maximum .22 .28Manganese, Minimum -- --Manganese, Maximum .90 04 0Silicon, Minimum -- --Silicon, Maximum -- --Phosphorus, Maximum .035 .035Sulfur, Maximum .045 .045

(18) All grades and thicknesses made to fine grainpractice. Over 11/2", normalized.

13


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(19) May be used for insert plates in normalized conditions. Some of the steels listed are available in greaterthicknesses than shown. The tabular values havebeen limited to the thickness ranges applying toAPi ami AwVliA.

TABLE 2(A)(Continued)

A442 A516(18) A53755 60 55 60 65 70 1(13) 2(17)

.(11 (11)1 max >1-1% 1max >1-1% Y o max >%-2 %max >%-2 %max >%-2 %max >%-2 2%max 2%.max% % Y 2 % Y 2 -- % -- % -- % -- % 01 1% 1 1% % 2(19 % 2(19 % 2(19) % 2 (19) 2 (19) 00 0 0 0 0 0 0 0 0 0 0 . .. 13/4 13/41 1% 1 1% % 2 % 2 0 0 0 0 0 00 0 0 0 0 0 % 1% 0 0 % 1% 0 065 55 SO SO 55 55 SO SO 65 65 70 70 70 8085 75 80 80 75 75 80 80 85 85 90 90 90 10030 30 32 32 30 30 32 32 35 35 38 38 50 SO2S 26 23 23 27 27 25 25 23 23 21 21 22 2221 21 20 20 23 23 21 21 19 19 17 17 18 --.22 .24 .24 .27 .18 .20 .21 .23 .24 .2S .27 .28 .24 .24.80 .SO .80 .SO .SO .SO .SO .85 .85 .85 .85 .85 .70 (16) .70(16)1.10 .90 1.10 .90 .90 1.20 .90 1.20 1.20 1.20 1.20 1.20 1.35(16) 1.35 (16)

-- .15 -- .15 .15 .15 .15 .15 .15 .15 .15 .15 .15 .15- .30 -- .30 .30 .30 .30 .30 .30 .30 .30 .30 .50 .50.04 .04 .04 .04 035 .035 .035 .035 .035 .035 .035 .035 .035 .035

.05 .05 .05 .05 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04

(A) This table is intended to provide a general picture ofthe steels permitted by API and AWWA.To attemptcomprehensive notes to coverall permissible variationscould only result in confusion. For such detailedinformation, refer to the pertinent specification.

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TABLE 3 - QUENCHED AND TEMPERED STEEL OF PRESSURE VESSEL QUALITY (ASTM A20)

M Designation A517 (Pressure Vessel Quality)

Type A B C D EN-A Xtra 100Trade Name PX- 1004 "T -1" Type A Jalloy S-100 SSS-100A SSS-100---- ----_.

ickness Limits, Inches:ASTM A517 1 1/4 1 1/4 1 1/4 1 1/4 2 1/2API-650 0 0 0 0 0API-650 Appendix D 0 0 0 0 0API-650 Appendix G 0 0 0 0 0AWWA-Dl00 0 0 0 0 0AWWA-Dl00 Appendix C(3) 1 1/4 1 1/4 (20) 1 1/4 1 1/2

nimum Tensile Strength, KSI 115 115 115 115 115ximum Tensile Strength, KSI 135 135 135 135 135

nimum Yield Strength, KSI 100 100 100 100 100

nimum Elongation in 2", percent:ASTM A517 16 16 16 16 16

at Analysis, Percent:Carbon .15 to .21 _15 to .21 .10 to .20 .13 to .20 .12 to .2Manganese .80 to 1.10 .70 to 1.00 1.10 to 1.50 04 0 to .70 .40 to .7Silicon .40 to .80 .20 to .35 .15 to .30 .20 to .35 .20 to .3Phosphorus, Maximum .035 .035 .035 .035 .035Sultur, Maximum .040 .040 .040 .040 .040Nickel --- --- --- --- ---Chromium .50to .80 AD to .65 --- .85 to 1.20 1.40 to 2.0Molybdenum .18 to .28 .15 to .25 .20 to .30 .15 to .25 .40 to .6Vanadium --- .03 to .08 --- (21) (21 )Titanium --- .01 to .03 --- .04 to .10 .04 to .1Zirconium .05 to .15 --- --- --- ---Copper --- --- --- .20 to AD .20 to ABoron .0025 max. .0005 to .005 .001 to .005 .001 5 to .005 .0015 to .00

15


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TABLE 3(Continued)

A517 (Pressure Vessel Quality)

F G H J K L M P

"T'" FX1004 "T1" Type B RQ100A CHT100 SSS100B RQ100B RQ

21/2 2 2 11/4 2 2 2 2 1/20 0 0 0 0 0 0 00 0 0 0 0 0 0 00 0 0 0 0 0 0 00 0 0 0 0 0 0 0

11/2 (20) (20) 11/4 (20) (20) 1 1/2 (20)

115 115 115 115 115 115 115 115135 135 135 135 135 135 135 135

100 100 100 100 100 100 100 100

16 16 16 16 16 16 16 16

0 to .20 .15 to .21 .12 to .21 .12 to .21 .10 to .20 .13 to .20 .12 to .21 .12to0 to 1.00 .80 to 1.10 .95 to 1.30 045 to .70 1.10 to 1.50 04 0 to .70 04 5 to .70 045 to5 to .35 .50 to .90 .20to .35 .20 to .35 .15 to .30 .20 to .35 .20to .35 .20to.035 .035 .035 .035 .035 .035 .035 .035.040 .040 .040 .040 .040 .040 .040 .0400 to 1.00 --- .30to .70 --- --- --- 1.20to 1.50 1.20to 10 to .65 .50 to .90 .40 to .65 --- --- 1.15 to 1.65 --- .85 to 10to .60 .40 to .60 .20 to .30 .50 to .65 04 5 to .55 .25 to .40 045 to .60 04 5 to3 to .08 --- .03 to .08 --- --- (21) --- ------ --- --- --- --- .04 to .10 --- --5 to .50 --- --- --- --- .20 to 040 --- ---005 to .006 .0025 max .0005 min .001to .005 .001to .005 .0015 to .005 .001to .005 .001to

16

(20) Permissible, provided tank manufacturer determines by teststhat type can bewelded to meet impact requirements.

(21) Vanadium may be substituted for part or all of titaniumcontents on a one for one basis.


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Part IIICarbon Steel TankDesign and Construction

IntroductionParts I and II presented an abbreviated backgroundof carbon plate steels. Part III wi II consider the designof flat-bottomed, vertical cylindrical carbon steeltanks for the storage of liquids at essentially atmos-pheric pressure. Typical examples are illustrated inFigure 2. Tanks of shapes other than cylindrical andsubject to gas pressure in addition to liquid head pre-sent a radically different problem. For further infor-mation, refer to API-650 (Appendix F), and API-620.

Within the scope of this Part, practically all tanksin the United States are constructed in accordancewith either the American Petroleum Institute (API)Standard 6501 covering welded steel tanks for oilstorage, or American Water Works Association(AWWA) Standard D1002 covering steel tanks forwater storage.Both of these standards contain detailed minimum

requirements covering inspection. Any attempt tosummarize either standard in entirety would notonly be voluminous but could be dangerously mis-leading. It will be the purpose, therefore, to discussonly those portions necessary to an understandingof the various design bases. Anyone intimately con-cerned with fabrication, erection, or inspectionshould obtain copies of the complete standards.It will be noted that there are basic differences be-

tween the standards of API and AWWA. Viewedobjectively, it is not difficult to understand why thesedifferences exist. AP 1-650 is an industry standardespecially designed to fit the needs of the petroleumindustry. The oil tank is usually located in isolated

1 American Petroleum Institute, Division of Refining,2101 L Street, N.W., Washington, D.C. 20037

2 American Water Works Association,6666 West Quincy Avenue, Denver, Colo. 20235

areas, or in areas zoned for industry where the pro-bable consequences of mishap are limited to theowner's property. The owner is quite conscious ofsafety and potential losses in his operations, and wiIIadjust the minimum requirements to suit more severeservice conditions. Furthermore, no API tank can everbe thinner than determined by the stress existing dur-ing water test, whereas the product stored, as in thecase of gasoline, is often lighter than water,On the other hand, AWWA-D1 00 is a public stand-

ard to be used by anyone without reservation for thestorage of a single product, water. The water storagetank is usually located in the midst of a heavily popu-lated area, often on the highest elevation available.Obviously the consequences of mishap could not betolerated in the public interest.Both basic standards have been in existence for

more than 30 years, and the experience under bothhas been excellent. Before applying them to tanksstoring liquids other than water or oil, however, thedesigner should consider which philosophy best fitshis circumstances. In either case the design standardsprovide minimum requirements for safe oonstructionand should not be construed as a design manual cov-ering all possible service conditions.

General Design Formula for Tank ShellsMembrane theory, as it applies to cylindrical tanksof large diameter, is elementary and needs no ex-p la na tio n h ere . However, it has b ecom e c omm onto publish formulas giving plate thickness in inchesas a function of a coefficient times the diameterand height in feet. The joint factor, the designstress, and the conversion from feet to inches areall hidden in the coefficient.Now that different design stresses are permitted

under varying conditions and materials, it appears

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less confusing to revert to a more basic formula inwhich the designer is aware of the stress being used.Starting with the basic premise that circumferen-

tial load in cylinder equals the pressure times theradius, then expressing Hand D in feet for conven-ience, the circumferential load at any level in a ver-tical cylinder containing water, weighing 62.4#/cu.ft., can be expressed as:

7=2.6HDwhere T = the circumferential load per inch of

shell heightH = depth in feet below maximum liquid

levelC = corrosion allowance factorD .= tank diameter in feet

Then for any design stress S expressed in psi,joint factor E, and specific gravity G, the minimumdesign thickness becomes:

t(inches) = 2.6 HDG + CSEObviously the ideal situation would be to vary

the thickness uniformly from bottom to top, butsince steel plates are rolled to a uniform thickness,any given course of plates is uniform throughoutits width. Thus a course designed for the stress atits lower edge will have excess thickness at the top,which will help carry part of the load in the lowerportion of the course above. API takes advantage ofof this and designs each course of plates for thestress existing one foot above the bottom of thecourse in question. AWWA designs on the basis ofstress existing at the lower edge of each course.

Loads To Be ConsideredAs outlined in the preceding section, the thicknessof the shell is determined by the weight of theproduct stored. However, there are other loads orforces which a tank may have to resist and whichare common to both oil and water tanks.Wind - Wind pressure is assumed to be 30 psf onvertical plane surfaces which, when applying shapefactors of 0.6 and 0.5 respectively, becomes 18 psfon the projected area of a cylindrical surface, and15 psf on the projected area of a cone or surface ofdouble curvature as in the case of tank roofs. Theseloads are considered to be the pressure caused by awind velocity of 100 MPH. For higher or lower windvelocity, these loads are increased or decreased in

proportion to the square of the velocity ratio,(V/1 OO)~ where V is expected wind velocityexpressed in miles per hour.Snow - Snow load is assumed to be 25 psf of hori-zontal projected area. Lighter loads are not recom-mended even in areas where snow does not occurbecause of the live loads that must be resisted bothduring construction and in service.Earthquake - Because of their flexibil ity, flat-bottomed cylindrical steel tanks have had an excel-lent safety record in earthquakes. Steel has the abilityto absorb large amounts of energy without fracture.Prior to the Alaskan earthquake of 1964, oi I tanks

had an almost perfect record of surviving all knownwestern hemisphere earthquakes with essentially noeffects other than broken pipe connections. In theAlaskan quake, the horizontal oscillations of thetank contents caused vertical shell stresses of suffi-cient magnitude to permanently deform the shell ina peripheral accordion-like buckle near the bottom.But again the properties of steel were sufficient toaccommodate this deformation without fracture ofthe shell plates."As a result of this satisfactory experience record,

it is generally considered that earthquake is not animportant consideration in oil tanks where theheight-to-diameter ratio is generally small.The record of water tanks has been correspond-

ingly good, but in the case of a standpipe where theheight-to-diameter ratio is 'high, the problem isobviously aggravated.Unfortunately there is no generally accepted the-

oretical analysis for the effects of earthquake on avertical cylindrical tank full of liquid. If a tank, par-ticularly one whose weight is greater than its diameter,is to be built in a zone of earthquake probability, thedesigner should investigate for existing local regula-tions. Figure 3 indicates earthquake probabilitiesfor the various sections of the United States. In theabsence of overriding local regulations, AWWA re-commends the assumption of a horizontal force equalto a percentage of total mass acting at the center ofmass, the percentage being 2.5% for Zone 1, 5% forZone 2, and 10% for Zone 3.

3See Rinne, John E., "Oil Storage Tanks," The Prince WilliamSound, Alaska, Earthquake of 1964 and Aftershocks, VolumeII, Part A, U.S. Department of Commerce, 1967.

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AP I Standard 650

he followi ng information is based on AP I Standard50. Supplements are issued periodically as indicatedy experience, but it is doubtful that the portionsiscussed here will be seriously outdated for someime to come. Nevertheless, it is suggested that any-ne dealing with tanks regularly should obtain aopy of the complete specification. Figure 2 on page3 may be used as an aid in determining the configura-ion of the tank. The limitations of the basic API 650esign and the alternative design bases of Appendicesand G (as shown on the graph) will affect the choices will the plot and soil conditions available. The in-uiry should specify that the tank is t o be built tohe latest edition of API 650 and the applicable Ap-endices. This will insure compliance with latestdustry practice.

ell Designnasmuch as shell design is the factor affected byhe greatest number of variables, we will deal witht first rather than follow the usual progression fromottom to top.API requires that all joints between shell plateshall be butt welded. Lap joints are permitted onlyn the roof and bottom and in attaching the topngle to the shell.It is apparent from the discussions in Part II thatP 1-650 offers three different shell design bases, thetandard or basic design, and alternately the designases as outlined in Appendix D and Appendix G.he alternate design bases permit higher designtresses in return for a more refined engineering de-ign, more rigorous inspection, and the use of shellplate steels with demonstrably improved toughness.Awareness of the importance of better toughness

s stress level increases, thickness increases, and tem-perature decreases has already been discussed in PartII. However, it may be interesting to comment brieflyon the choice of A131 as the principle material se-lected for Appendix D tanks.A131 is patterned after the American Bureau of

Shipping steel developed to eliminate the brittlecracking of Liberty Ships during World War II. Thedifferences between grades A, B, and CS reveal a mostlogical approach toward the attainment of improvedtoughness. From Table 1, you will note that grade

A (up to 1/2") requires no chemistry other than theusual phosphorus and sulfur limits. Grade B (upto 1") attains increased toughness by imposing amaximum for carbon and a range for manganese.Grade CS further increases toughness by requiringfully-killed, fine-grain practice with 0.16% maximumcarbon and higher 1.00 - 1.35% manganese. In addi-tion, normalizing is required for all thicknesses. Also,API permits use of higher strength grades for Appen-dixG - High Stress Design. Included are A573, A537Class I and ABS 'H' grades. As originally devised and asdescribed in ASTM, grades A, B, and CS denotedthese specific thickness ranges, but as a practical mat-ter each grade can be obtained in all thicknesses upto the specified maximum for the grade; e.g., gradeCS is specified for all thicknesses in the lowest tem-perature range.The effect of differences in welding and shell

penetrations need no further comment. The proba-bility of detrimental notches is higher at disconti-nuities such as shell penetrations. The added re-quirements of Appendices 0 and G pertaining towelding, stress relief, and inspection minimize theeffects of stress concentration at disconti nuities.The differences in inspection are important. The

salutary effect of the greater number of spots to betested will be obvious, but the prohibition of sec-tioning by Appendices 0 and G should be noted.Briefly stated, sectioning consists of trepanning

plugs or cutting sections containing the completeweld cross section, so that after etching, the crosssection of the weld can be examined for defects.WhiIe such a test is not as revealing as a radiograph,it affords a reasonably satisfactory spot check thathas been used for more than 40 years.The principal disadvantage lies in the repair of

the holes left where the sections are removed. Thewelding up of these small holes inevitably resultsin local residual stresses and the possibi lity ofnotches. Inasmuch as a crack across a horizontalweld is perpendicular to the direction of principalstress, it is one of the most serious defects that canexist and cannot be tolerated at the higher stresslevels permitted in Appendices 0 and G.Of course, joints with partial penetration (Figure

5) cannot be inspected satisfactorily by radio-graphic methods, and it is worth noting that someregular users of oi I tanks specify complete fusionhorizontal joints to be inspected by radiograph even

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on API-650 Basic Design tanks in order to avoidsectioning. With the advent of automatic welding,complete penetration of horizontal joints is nolonger prohibitively expensive.Tank shells designed in accordance with Appen-

dices D and G will be thinner than the basic AP 1-650 tank, and thus will have reduced resistance tobuckling under wind load when empty. The shellmayor may not need to be stiffened, but must bechecked. This is discussed in the section on WindGirders.Whereas the foregoing discussion deals with thedifferences between basic and alternate designs, thefollowing comments pertain to those portions ofAPI-650 which apply equallv to both.Bottoms - Tank bottoms are usually lap welded ofplates having a minimum nominal thickness of 1/4".After trimming, bottom plates shall extend a mini-mum of 1 inch beyond the outside edge of the weldattaching the bottom to the shell plates. The attach-ment weld shall be a continuous fillet inside andout as shown in Figure 6 and the following table ofsizes:

Maximum t ofShell PlateInches3/16over 3/16 to 3/4over 3/4 to 1-1/4over 1-1/4 to 1-1/2

Maximum Size ofFillet Weld*Inches3/161/45/163/8

'Maximum size Fi Ilet 1/2"Butt-welded bottoms are per missible, but becausethey add appreciably to cost, are seldom used ex-cept in special services.Top Angle - Except for open-top tanks and thespecial requirements applying to self-supportingroofs, tank shells shall be provided with top anglesof not less than the following sizes:

Tank Diameter35 feet and less

MinimumSize of Top Angle2-1 /2 )( 2-1 /2 x 1/42-1 /2 x 2 -1/2 x 5/163 x 3 x 3/8

over 35 to 60 ft. incl.over 60 feet

Roofs - The selection of roof type depends onmany factors. In the oi I industry, many roofs areselected to minimize evaporation losses. Inasmuchas the ordinary oil tank is designed to withstandpressures only slightly above atmospheric, it must

be vented both against pressure and vacuum. Thespace above the liquid is filled with an air-vapormixture. When a nearly empty tank is filled withliquid this air-vapor mixture is expelled. Even with-out filling, the air-vapor mixture expands in theheat of the day and the resulting increase in pres-sure causes venting. During the cool of the night,the remaining air-vapor mixture contracts, morefresh air is drawn in, more vapor evaporates to sat-urate the air-vapor mixture, and the next day thecycle is repeated. Either the loss of valuable "lightends" to the atmosphere from fi l Iing, or the breath-ing loss due to the expansion-contraction cycle, isa very substantial loss and has led to the develop-ment of many roof types designed to minimizesuch losses. Roofs that rise whenever the pressureexceeds the weight of the roof are designed to pro-vide more vapor space before the roof reaches itslimit of travel and must vent. The many other de-vices available for providing a variable vapor spacedesigned to accommodate normal breathing aretoo numerous to mention here.However, the floating roof is probably the most

popular of all conservation devices and as such hasbeen included as Appendices C and H to API Stand-ard 650. The principle of the floating roof is simple.It floats on the liquid surface, therefore there is novapor space either to be expelled on filling or toexpand or contract from day to night.Inasmuch as all such conservation devices are

represented by proprietary and often patented de-signs, they are beyond the scope of this discussion,which will be limited to the fixed roofs covered byAPI Standards.The most common fixed roof is the co lumn sup-

ported cone roof, except in relatively small dia-meters where the added cost of a self-supportingroof is more than offset by savi ng the cost of astructural framing. The dividing line cannot be ac-curately defined because different practices andavai lable equipment may affect the decision in anygiven case. If economy is the only consideration,however, the purchaser would be well advised tospecify the size of tank and let the manufacturerdecide whether or not to use a self-supporting roof.Of course, a seif-supporting roof is sometimes de-

sirable for special service conditions such as an inter-nal floating roof, or where cleanliness and ease ofcleaning are especially important.

2 0


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API-650 provides rules for the design of a num-ber of types of fixed roofs as summarized in thefollowing:General - All roofs and supporting structuresshall be designed to support dead load plus a liveload of not less than 25 psf of projected area.Roof plates shall have a minimum nominal

thickness of 3/16 inch. A greater thickness maybe required for self-supporting roofs.Structural members shall have a minimum thick-ness of 0.17".Roof plates shall be attached to the top angle

with a continuous fillet weld on the top side only.

Supported Cone RoofsSupported cone roofs are lap welded from the topside only with continuous full fillet welds (Fig. 5).Plates shall not be attached to supporting members,and shall be attached to the top angle by a continu-ous fillet weld not larger than 3/16" on the top sideonly.The usual slope of supported cone roofs is 3/4"

in 12". Increased slopes should be used with caution.The columns transmit their loads directly to the sup-porti ng soi I through bases resting on but not attachedto the bottom plates. Some differential settlement canbe expected. A relatively flat roof will follow suchvariations without difficulty. As pitch increases, acone acquires stiffness, and instead of smoothly fol-lowing a revised contour, unsightly local bucklesmay develop. In general, slopes exceeding 1-1/2" in12" may be undesirable.Rafters in direct contact with the roof plates

may be considered to receive adequate lateral sup-port from friction, but this does not apply to trusschord members, rafters deeper than 15", nor roofslopes greater than 2" in 12".Rafters are spaced so that, in the outer ring, their

centers are not more than 6.28' apart at the shell.Spacing on inner rings does not exceed 5-1/2'.

Allowable Stresses for Roof and SupportsAll parts of the structure shall be so proportionedthat the sum of the maximum static stresses shallnot exceed the following:Tension:Rolled steel, on net section in psi. . . . . .. 20,000Complete penetration groove weldson thinner plate area, in psi 18,000

Compression:Rolled steel, where lateral deflectionis prevented, in psi. . . . . . . . . . . . . . .. 20,000

Complete penetration groove weldson thinner plate area, in psi 20,000

Columns, on cross sectional area in psi:

L .For - not over 120 ..r [ 1 - ( + ) l r 3 3 , 0 0 0Y ]34,700 t FS (NoLFor - over 120 to 131.7, incl ....

r 1 _ ~ l r 3 3 ' 0 0 0 Y ][34,700 t FS

L

LFor - over 131.7 ...r

1.6--- 200r(149,000,000 Y)

(No

(No

Notes:1. The allowable stresses, not including the factor Y,

have been tabulated in AISC S310-311: Specification forthe Design, Fabrication, and Erection of Structural Steelfor Buildings (1969), see Table 1-33 under column heading"Main and Secondary Members."

2. The allowable stresses, not including the factor Y,have been tabulated in AISC S310-317, see Table 1-33under column heading "Secondary Members."

"From API Standard 650 Fifth Edition, July, 1973. Thesestress values and limiting ratios are based on ASTM-A7 steeland according to API-650 are applicable to A283-C, A131and A-36.

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where:L unbraced length of column, in inches.r ::: least radius of gyration of column, in inches.

5 - + - e ) 3FS ::: factor of safety = - + -- - -----3 350 18,300,000Y ::: 1.0 (for structural sections or tubu lar sections

having tlR values equal to or exceeding 0.0150.015).

U J ] (for tubular2003sections having tlR values less than 0.015).R = outside radius of tubular section, in inches.

t = thickness of tubular section, in inches; 1/4in. minimum for main compression mem-bers, 3/16 in. minimum for bracing andother secondary members.

For main compression members, the ratio Llr shallnot exceed 180. For bracing and other secondarymembers, the ratio Llr shall not exceed 200.Bending:Tension and compression on extreme fibers ofrolled shapes and built-up members with an axis ofsymmetry in the plane of loading, where the laterallyunsupported length of compression flange is nogreater than 13 times its width, the compressionflange width-thickness ratio does not exceed 17,andthe web depth-thickness ratio does not exceed 70,in psi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22,000Tension and compression on extreme fibers of un-symmetrical members, where the member is sup-ported laterally at intervals no greater than 13 timesits compression flange width, in psi 20,000Tension on extreme fibers of other rolled shapes,built-up members, and plate girders, in psi .. 20,000Compression on extreme fibers of other rolledshapes, plate girders, and built-up members havingan axis of symmetry in the plane loading, the largervalue computed by the following, in psi:17\220,000 - 0.571 ~~)Or: 12,000,000 ~20,000

.ls:Af

where:unbraced length of compression flange.

r = radius of gyration of section about an axisin the plane of loading.

d ::: depth of section.Af ::: area of compression flange.

Compression on extreme fibers of other unsym-metrica I sections, in psi:

12,000,000ldAf

~20,000

Shearing: /Fillet, plug, slot and partial penetration groovewelds of throat area, in psi. . . . . . . . . . .. 13,600On the gross area of the webs of beams and girders,where h (the clear distance between web flanges, ininches) is not more than 60 times t (the thicknessof the web, in inches) or when the web is adequatelystiffened, in psi. . . . . . . . . . . . . . . . . . . .. 13,000On the gross area of the webs of beams and girders,if the web is not stiffened so that h is more than 60times t, the greatest average shear, VIA, shall notexceed, in psi:

19500h21+----(7,200) t2

where V is the total shear, and A is the gross area ofthe web, in square inches.

Self-Supporting Cone Roofs*Self-supporting cone roofs shall conform to the fol-lowing requirements:Maximum e = 37 deg (tangent = 9:12)Minimum sin e = 0.165 (slope 2 in. in 12 in.).The following formulas applying to self-supportingroofs provide for a live load of 25 lb. per sq. ft.

DMinimum t = but not less than 3/16 in.400 sineMaximum t = 1/2 inch.

*API-650Fifth Edition, July ,1973.

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The cross-sectional area of the top angle, in squareinches, plus the cross-sectional areas of the shell androof plates within a distance of 16 times their thick-nesses, measured from their most remote point ofattachment to the top angle, shall equal or exceed

D23,000 sin e

where:De nominal diameter of tank shell, in feet.angle of cone elements with the horizontalin degrees.t = nominal thickness of roof plates, in inches.

Self-Supporting Dome and Umbrella RoofsSelf-supporting dome and umbrella roofs shall con-form to the following requirements:R = D (unless otherwise specified by the purchaser).Minimum R = 0.80DMaximum R = 1.2D

RMinimum t = 200 but not less than 3/16 inch.Maximum t = 1/2 inch.The cross-sectional area of the top angle, in square

inches, plus the cross-sectional areas of the shell androof plates within a distance of 16 times their thick-nesses, measured from their most remote point ofattachment to the top angle, shall equal or exceed

DR1,500

where:D nominal diameter of tank shell, in feet.R = radius of curvature of roof, in feet.

nominal thickness of roof plates, in inches.

Top-Angle Attachment for Self-Supporting RoofsThe top-angle sections for self-supporting roofs shallbe joined by butt welds having complete penetrationand fusion. Joint efficiency factors need not be ap-plied.

AccessoriesAPI Standard 650 contains specific designs for ap-proved accessories which include all dimensions,

thicknesses, and welding details. For all cases.OSHA requirements must be satisfied. Figure 7shows welding symbols employed.

No details are shown, but specifications are in-cluded for stairways, walkways and platforms. Allsuch structures are designed to support a movingconcentrated load of 1000 Ibs. and the handrailshall be capable of withstanding a load of 200 Ibs.applied in any direction at any point on the toprail.Normally all pipe connections enter the tankthrough the lower part of the shell. Historicallytank diameters and design stress levels have beensuch that the elastic movement of the tank shellunder load has not been difficult to accommodate.With the trend to larger tanks and higher stresses,

the elastic movement of the shell can become an im-portant factor. Appendix G recognizes this effect.Steel being an elastic material, the tank shell in-

creases in diameter when subjected to internal pres-sure. The flat bottom acts as a diaphragm and re-strains outward movement of the shell. As a result,the shell is greater in diameter several feet above thebottom than at the bottom. Since the pipe connec-tions are normally in this lower portion of the shell,it will be obvious that the pipe must rotate verticallyor the shell will be subjected to external loads andlocal bending. Appendix G requires that such areasof the shell be reinforced for the externally appliedloads, but preferably that the external loads be elim-inated in the design of the piping, or the shell connec-tion be relocated outside of the rotation area. Thesituation must be recognized in tanks other than thoseconstructed in accordance with Appendix G if thediameter is unusually large.

Wind Girders for Open-Top TanksOpen-top tanks must have stiffening rings at or nearthe top of the shell to resist distortion or collapseunder wind load.The minimum required section modulus of the

stiffening ring is determined by the equation:z = 0.0001 DL H where

Z = section modulus in inches cubed.D = nominal tank diameter in feet.H = height of tank shell in feet.

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The section modulus, Z, may be based on a crosssection including that portion of the shell consideredto be effective. Figure 15 gives section moduli valuesfor typical wind girders and illustrates that portion ofthe shell considered to act with the girder.The outer periphery of the wind girder may be

circular or polygonal.If the wind girder is more than 2 feet below thetop of the shell, a top angle must be provided.When a stair opening is installed through a stiffen-ing ring, the required section modulus must be main-tained at all cross sections and the shell stiffened ad-jacent to the opening as required by API-650.When the width of the horizontal leg or web ex-

ceeds 16times its thickness, supports shall be pro-vided at a spacing not to exceed 24 times the widthof the outside compression flange.

Intermediate Wind GirdersTank shells designed in accordance with AppendicesD and G must be checked for stability against windpressure whether they are fixed-roof tanks or open-top tanks with a top wind girder designed in accord-ance with the preceding paragraphs. Inasmuch asanticipated wind velocities vary with location, andfailure can occur only when tank is empty or nearlyempty, intermediate wind girders are not mandatoryunder API standards.API-650 Appendix D provides two solutions fordetermining the spacing of intermediate stiffeners(if any). Only one of the two solutions will be de-scribed here.The maximum height of unstiffened shell shall notexceed:*

Hl =6(100t) (1~Ot) ~ where,H 1 vertical distance in feet between the inter-

mediate wind girder and the top angle of theshell or the wind girder of an open-top tank.

t = average shell thickness in inches withinheight H 1 o 2R = nominal tank diameter in feet.

"Based on wind velocity of 100 MPH. For other velocities,f . l 0 0 )multiply H by\vwhere V = specified velocity MPH.

An initial calculation for Hl shall be made usingthe thickness of the top shell course. If Hl is greaterthan the width of the top shell course, make addi-tional calculations based on the average t obtainedby including part or all of the next lower course, orcourses, until the calculated Hl is equal to or smallethan the height of shell used in determining the average t. If H 1 continues to calculate greater than theheight of shell used in determining the average t, nointermediate girder is needed.After establishing the location of the first inter-

mediate girder, if any, a similar check of the lowershell shall be made, assuming the first intermediategirder to be the top of the tank.The required section modulus in inches cubed

shall be determined by the equation:*Z = 0.0001 02 H 1

The section modulus may include a portion ofthe tank shell for a distance of 0.6pbove andbelow the attachment to the shell.Compatibility of the material used in wind stiffen

ers with that in the tank shell is important. This posno problem in the case of tanks constructed of themore common carbon steels. When high strengthmaterials are used in the tank shell it is considereddesirable to use a stiffener material that will not bestressed to more than 2/3 of its yield strength whensubjected to the shell strain existing in the shell atthe point of attachment under full liquid load.

AWWA Standard 0100General - The following information is based onthe AWWA Standard D100 issued in 1973. It is sug-gested that anyone dealing regularly with tanks ob-tain a copy of the full specification.Shell Design - AWWA D100 offers two differentdesign bases, the standard or basic design and thealternate design basis as outlined in Appendix C.The alternate design basis permits higher design stresses, in return for a more refined engineering design,more rigorous inspection, and the use of shell platesteels with improved toughness. A comparison ofthe principal requirements of the two bases is shownin Table 4."Based on wind velocity of 100 MPH. For other velocities,

1 _ 1 0 0 ) 2multiply Z bY~ where V = specified velocity MPH.

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It should be pointed out that API-650 AppendixD did not introduce any steels of significantly higherstrength levels than had been used previously. Onlya modest increase in design stress resulted.On the other hand, AWWA D100 Appendix C in-

cludes steels of significantly higher strength levelsand correspondingly higher design stress levels. Thisintroduces new design problems. For example, forA517 steels, the permissable design stress of 38333psi will result in reaching the minimum requirednominal thickness several courses below the ta.iktop. It would be uneconomical to continue the re-latively expensive steel into courses of plates notdetermined by stress. The obvious answer is to useless expensive steels in the upper rings. To governhis transition, Appendix C adds the following re-uirements:"In the interest of economy, upper courses maye of weaker material than used in the lower coursesf shell plates, but in no instance shall the calculatedtress at the bottom of any course be greater thanermitted for the material in that course nor shallny plate course be thinner than the course above it. "Compliance with this requirement will probably

result in the course or courses immediately belowhe transition point being somewhat heavier than re-uired by stress. Using a steel of intermediate strengthlevel as a transition between the A514 or A517 steeland carbon steel may help the situation. In any eventhe use of two or more steels will result in plates ofhe same thickness made of several different steels.Careful attention to plain marking for positive identi-fication becomes very important. Consideration mightbe given to varying plate widths for different materialsof the same thickness to aid in identification in theevent markings are lost.With the exception of roofs and accessories, the

comments made in connection with API tanks alsoapply to AWWA tanks and will. not be repeated herein detail.Bottoms may be either lap or butt welded and not

less than 1/4" thick.AWWA does not specify top angle sizes, but the

rules of API represent good practice.AWWA rules for wind girders at the top of open-top tanks are identical with those of AP 1.For intermediate wind girders (Appendix C tanks)

AWWA uses the same basic approach, but expressesthe equation for spacing stiffeners in a different form.Reduced to the same form, AWWA values for stiffener

spacing are slightly less than those obtained from theAPI formula. The AWWA form for stiffener spacing is:

10.625 (10)6 (t)h = ---,-----,:-----p ( ~ r 5

where: h stiffener spacing in feet.D = diameter in feet = 2R.t shell thickness in inches.

P (wind pressure psf) =18 (specified wind velocity in MPH ) 2

100AWWA uses the same equation as API for section

modulus of intermediate wind girders:Z = 0.0001 D 2 H

except that in calculating Z, AWWA permits the in-clusion of shell plate for a distance of 16t above orbelow the point of attachment, whereas API permitsshell participation above or below the point of attach-ment, a distance 0.6.jR-i in the case of intermediatestiffeners.Intermediate wind girders are mandatory under

AWWA Appendix C where indicated by the aboverules.Compatibility of stiffener material with that in the

tank shell is discussed under the section on API-650.

RoofsWhereas oil tanks are strictly utilitarian, a pleasingappearance is generally an important considerationin the case of water tanks. Since the roof line has animportant effect on appearance, this striving forbeauty has led to a wide variety of roof designs, large-ly developed by the fabricators as an aid in sellingtheir particular design.Often a self-supporting roof, such as an ellipsoid,

will extend a considerable distance above the cylin-drical portion of the shell, and the high water levelwill extend up into the roof itself. The resultant up-ward pressure on the roof is resisted by the combina-tion of the roof dead weight and the welded connec-tion between the roof and shell.

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AWWA requires that for all surfaces in contactwith water, the minimum metal thickness shall be1/4". Surfaces not in contact with water may be3/16" .As applied to rolled shapes for roof framings, theforegoing minimum thicknesses shall apply to themean thickness of the flanges regardless of webthickness.Roof plates not subject to hydrostatic pressurefrom tank contents may be welded from the topside only with either a continuous full fillet or buttjoint weld. Where roof plates are subjected to hydro-static pressure, consideration must be given tostrength requirements.

Roof supports or stiffeners, if used, shall be inaccordance with current specifications of the Amer-ican Institute of Steel Construction covering structuralsteel for buildings, with the following exceptions:1. Roof plates are considered to provide the

necessary lateral support by friction betweenroof plates and rafters to el iminate reductionin the basic allowable compressive stress. fb

2. The roof purlin depth may be less than ----times the span length in inches 600,000where fb is the maximum bending stress in psi,providing slope of the roof is 3/4" to 12" orgreater.

3. The maximum slenderness ratio (Llr) for roofsupport columns shall be 175.

4. Roof support columns shall be designed assecondary members.

5. Roof trusses, if any, shall be placed above themaximum water level in climates where ice mayform.

6. Roof rafters shall preferably be placed abovemaximum water level, although their lower ends,where connected to the tank shell, may projectbelow the water level.

AccessoriesAWWA does not provide detailed designs of tank fit-tings, but specifies the followinq:1. Two manholes shall be provided in the first ring

of the tank shell. One must be of a configurationto accept an exhauster and the other may be

either a 24" diameter or at least an 18" x 22"elliptical manhole.

2. The purchaser shall specify pipe connections,sizes, and locations. Due to freezing hazardthese connections are normally made throughthe tank bottom and as near to the shell aspractical. A concrete valve box may be pro-vided to permit access to piping. This valvebox must be designed as a part of the ringwall.

3. If a removable silt stop is required, it shall beat least 4" high. If not required, then the con-necting pipe shall extend at least 4" above thetank bottom.

4. The purchaser shall specify the overflow sizeand type. A stub overflow is recommended incold climates. If an overflow-to ground is re-quired, it should be brought down the outsideof the tank. Inside overflows are not recom-mended. They can be easily damaged by ice,and regardless of cause of damage or deterior-ation/a failure in the overflow will empty thetank to the level of the break.

b . An outside vertical ladder shall begin 8 feet(or as specified) above the tank bottom andafford access to the roof. Note that AP I tankstairs and platforms are designed to give easyaccess for daily gaging activities, etc. On theother hand, need for access to AWWA tanksis infrequent and a conscious effort is made torender access difficult for unauthorized personnel

6. Roofs flat enough to walk on safely need noroof ladder. Where necessary, a fixed roof lad-der is required for access to the center of theroof.

7. A roof door or hatch whose least dimension is24", with a curb 4" high, provided with ahinged door and clasp for locking, shall beplaced near the outside tank ladder. A secondopening must be provided near the center ofthe tank to accept an exhauster.

8. Adequate venting shall be provided to accom-modate the maximum filling or emptying rate.9. Safety devices shall be provided on ladders asrequired by federal or local regulations, or aspurchaser so specifies.

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Anchor BoltsThe normal proportions of oil tanks are such (diam-eter greater than height) that anchor bolts are rarelyneeded. It is quite common, however, for the heightof water tanks to be considerably greater than thediameter. There is a limit beyond which there is dan-ger than any empty tank will overturn when subjectedto the maximum wind velocity. As a good rule ofthumb, if C in the following formula exceeds 0.66,anchor bolts are required:

2MC = wheredwM = overturning moment due to wind, ft. lb.d diameter of shell in feet.w weight of shell and portion of roof

supported by shell, lb.Maximum desirable spacing of bolts is 10 feet. Thediameter of the anchor bolt circle must be largerthan the diameter of the shell. W., 4MDesign tension load per bolt = - - -ND Nwhere M and Ware as above andN = number of anchor boltsD = diameter of anchor bolt circle, feet.The diameter of the anchor bolts shall be deter-

mined by an allowable stress of 15000 psi on the netsection at the root of the thread.Because of proportionately large loss of section by

corrosion on small areas, it is recommended that noanchor bolt be less than 1-1/4" in diameter.

FOUNDATIONSIf anchor bolts are required, a foundation is necessaryto provide the necessary resistance to uplift, but con-sideration should be given to foundation conditionswhether or not anchor bolts are used.The flat-bottom storage tank is one of the simplest

of structures and has sufficient flexibility to accom-modate appreciable changes in shape. The thin steelbottom can easily adjust to local variations in soilbearing whereas more rigid materials may not.Nevertheless, the tank user will be rewarded by

paying ample attention to foundation conditions.Uneven settlement at the periphery, if not a plane,will inevitably cause some distortion that will be

unsightly and, if carried to extremes, could be dan-gerous. Maintenance of a circular cross-section isparticularly important in the case of open-toppedoil tanks containing floating roofs.The initial consideration should be to determine

the bearing capacity of the soil at the site. This is amatter of soil mechanics, which is beyond our scope.The services of qualified foundation consultants areinvaluable for this purpose.The proportions of the tank are an important fac-tor. An oil tank 40 feet high, storing a liquid weigh-

ing less than water, can be, and frequently is, placedon a well-prepared earth grade. On the other hand, astandpipe 100 feet high, producing a uniform bearingload of 6250 psf plus a concentrated line load at theshell, will require a foundation structure except un-der most unusual circumstances.The roof of a tank is often supported by a system

of rafters, girders and columns. For such a tank, thecolumns are supported by a base structure which,through the bottom plate, bears directly on the earth.The roof load must be taken into consideration to-gether with the liquid load to avoid overloading thesoil.It will be observed that a ring wall foundation dis-

tributes the load over an area essentially equal to thecross section of the tank. In the case of high stand-pipes, the soil at any reasonable depth may not beable to resist the resultant load. In such cases, theload must be spread over a greater area by means ofa reinforced concrete slab type foundation capableof distributing the load from the tank to the greaterarea.Where soil conditions are such that none of thesimpler foundation types are adaptable, piled found-ations or other special types will require individualconsideration for each case.Assuming that bearing conditions have been deter-

mined to be adequate, the simplest form of founda-tion is a sandpad laid directly on the earth. See Fig-ure 14. All loam or organic material should be re-moved and replaced with suitable material wellcompacted, usually topped by sand.Drainage away from the tank grade is importantand sufficient berm should be provided to prevent

washing and weathering under the shell. SeeFigure 14.The sand should be clean and free from corrosive

elements. Care should be taken to exclude clay orlumps of earth from coming into contact with thebottom. Frequently the difference in potential be-

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tween two types of earth will set up an electrolyticcell with resultant pitting.Sometimes crushed rock is substituted for sand,but sand is easier to grade and usually more available.If the sand cushion is placed on top of crushed

rock fill, the rock should be carefully graded fromcoarse at the bottom to fine at the top. If this is notdone, the sand will percolate down through the voidsin the coarser rock.An excellent base can also be obtained by substi-tuting about 1 1/2 in. of asphalt road paving mix forthe sand cushion. This material is available from readymix plants in many sections of the country. There isone precaution that must be carefully watched. Afterthe material has set up, it is difficult if not impossiblefor the tank builder to' correct inaccuracies by takingdown the high and filling in the low spots. It is, there-fore, most important that a paved tank grade beleveled with extreme accuracy under the shell.Drainage is important both from the standpoint of

soil stability and bottom corrosion. Good drainageshould be provided not only under the tank itself,but the general area should preferably be well drained.Where the terra in does not afford natural drainage,proper ditching around a group of tanks may help tocorrect the deficiency.Where suitable bearing soil is not available at the

surface, but is avai lable a reasonable distance belowthe surface, a ring wall foundation is indicated. Thepurpose of the ring is to confine the soil and preventlateral movement. The ring wall is founded in thefirm stratum and confines the weaker materials. To-tally inadequate material should be removed andreplaced with well-compacted fill.AWWA tanks must be supported on a concrete

ring wall (or slab foundation), and many tank own-ers use ring wall construction as standard even whensoi I conditions do not indicate its use. There are anumber of advantages in this practice that may wellcompensate for the added cost. The incidental advan-tages of the ring wall are neat appearance, an excellentfoundation for the tank shell, the elimination of wash-ing and weathering of berms, and the exclusion of sur-

face water running into the grade. Ring walls can beso proportioned that the unit soil bearing at the levelof the bottom of the wall is the same under the con-crete as under the confined soil. This can be accom-plished and will encourage uniform settlement of thefoundation as a whole.The top of the wall should be smooth and level

within : t . 1/8 inch in any 30 foot circumferentiallength. No point in the circumference of the wallshould vary more than.. 1 /4 inch from the establishedelevation.Assuming the tank shell to be centered on the wall,

and earth to weigh 100 Ibs. and concrete 140 Ibs. percubic foot, the thickness of the concrete wall may bedetermined from the following formula:

24WT= qH - 80hwhere T thickness of wall (inches) to give uniform

soil pressure at the bottom of the ringwall.W = weight of metal in shell and roof suppor-

ted on the ring wall in Ibs. per ft. ofcircumference.

H = height of tank shell in feet.h = height of ring wall in feet.q = weight of stored product Ibs. per cu. ft.

In no case should the ring wall be less than 10 inchesth ideWhere a ring wall is used, it should be reinforced

circumferentially to resist the hoop stress resultingfrom lateral pressure of the confined earth. Becausesoil conditions are rarely known in advance, practiceis to design such walls on the basis of a lateral pres-sure equal to 0.3 of the combined liquid and earthvertical pressure. For shallow rings the vertical loadcontributed by the earth is small, but on deep wallsit can become important.If there are openings in the wall, the reinforcing

must be carried around such openings to preservethe continuity of the hoop action. Nominal verticalsteel is normally provided primarily for conveniencein placing hoop- steel.

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Steel Tanks AISI Publication-1976

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Transcript of Steel Tanks AISI Publication-1976