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Information I BAU HAMBURG Central Cone Silos from the structural point of view I BAU HAMBURG
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Central Cone Silos Structural

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Page 1: Central Cone Silos Structural

Information

IBAU HAMBURGCentral Cone Silos

from thestructural point of view

I BAU HAMBURG

Page 2: Central Cone Silos Structural

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I BAU HAMBURG Central Cone Silos from the structural point of view

Thousands of this silo type are in operation for many years

General Information

The civil design of largecapacity silos requiresextensive know-how andexpertise.

This is mainly fulfilledby the involvement ofqualified civil designcompanies, which arefamiliar with the state-of-the-art.

Nevertheless, our experi-ence is, that in somecases, local codes, regu-lations and guidelines areused, that do not takeinto account the com-plexity of large silos, asthis is e.g. stated in theEurocode EN 1991-4, orthe relevant nationalcodes such as the DINEN 1991-4 (2005) andDIN 1055-6.

IBAU HAMBURG worksfor more than 30 yearstogether with the civildesign company Peterund Lochner with regardto the design of the cen-tral cone silo.

This brochure describesthe construction princi-ples according to theabove norms that shouldbe taken into account bythe design company aswell as the plant owner.

This brochure focuseson the critical reinforcedconcrete silo structuralelements, such as thecentral cone, the silo wallin the section adjacent toand below the cone, thesilo walls above the coneand the intermediatewalls for multicompart-ment silos.

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Information

The IBAUHAMBURG CentralCone Silo –The Original

The Storage Silo

IBAU HAMBURG intro-duced the central conesilo to the market in1975, when the companywas established inHamburg, Germany. Thedesign is mainly used forlarge storage silos in thecement industry andother mineral industriesfor cement, raw meal, flyash, ground granulatedblast furnace slag, alumi-na and similar products.Storage silos for theseproducts have diametersof 10 m to 30 m andeven more with storagecapacities up to 40000 tand they require an effi-cient and troublefreeemptying.

The IBAU central conesilo has been proved tobe extraordinarily suc-cessful. Today, more than7000 units are in opera-tion by various customersaround the world, so thatthe central cone silodesign has also beencopied by other suppli-ers. In the original IBAUdesign for large silos, thecentral cone forms a ringspace on the silo bottom.This is divided into indi-vidual aeration sectionsthat are slightly turnedtowards the dischargeopenings in the cone witha small inclination.

The silo bottom isequipped with so-calledfluidslides that have anair-permeable fabric onthe upper side. The aera-tion air is supplied by a IBAU central cone storage silo

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I BAU HAMBURG Central Cone Silos from the structural point of view

View into the central cone silo with ring

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Information

shaped bottom and fluidslide system

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I BAU HAMBURG Central Cone Silos from the structural point of view

Flow channels in an IBAU central cone silo

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Information

The emptying rate of the silo is more than 99 %

Aeration section and silo outlet

blower and is led under-neath the fabric in orderto fluidise the bulk mate-rial above. Each aerationsection has its own flowcontrol gate and airvalve. For the materialdischarge, the ring spacein the silo is aerated sec-tion by section so that ina complete cycle all sec-tions have been aeratedand the entire material inthe silo has been set intomotion.

The discharge conceptrequires only small quan-tities of air and only asmall power consumptionof less than 0.1 kWh/t fordischarge. More than99% material recovery isreported by the users andthe first in-first out prin-ciple still applies for awell designed silo.On the other hand, asonly the material abovethe aerated silo sectionwill get in motion, withthe sectional aerationconcept flow channelsare formed so that thesilo discharge is eccen-tric. In the IBAU con-cept, the eccentric flowwithin the silo is con-trolled by the automatedflow control gates, whichwill discharge only therequired flow rate fromthe silo.

This controlled flow ofmaterial correspondsexactly to the consump-tion of the plant and doesnot generate, as withother silo systems, anuncontrolled materialflow inside the silo,which may damage thesilo body. This leads toour motto: „Safety First“by using IBAU silos.Nevertheless, there are

also other silo designswith central cones andaeration sections avail-able, where the materialflows from the aerationsection through largeopenings directly into thearea under the cone with-out any flow controlgates in-between.Appropriate flow controlgates are only installedfor the discharge fromunderneath the cone.

IBAU Silos - Safety First

IBAU Central Cone silosare designed in such away that the completematerial within the silo isin motion during a fullaeration cycle of the silo,achieving a high empty-ing rate of about 99 %.

Secondly, the aeration isdesigned in such a way,that the flow channelsthat are formed duringdischarge are not or onlyslightly contacting thesilo walls. In the pictureon page 6 an interpreta-tion of the formation offlow channels in largecapacity silos for thecement industry has beengiven. The controlledflow via flow controlgates is the basic conceptof the IBAU „SafetyFirst“ principle.

IBAU HAMBURG hasalso been asked to designsilos with a depressurechamber. IBAU is aware,that comparing thisdesign with the originalIBAU cone silo assumingan identical number ofaeration sections andaeration time, the largeopenings of the conedramatically increase the

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I BAU HAMBURG Central Cone Silos from the structural point of view

max.166 kg/s

2 x bulk loading:2 x 220 t/h

1 x packing plant:1 x 160 t/h

max. 166 kg/s

total discharge capacity 600 t/h

A

0, 03 m2 openingper flow control

Ø from 10 - 30 m

AOriginal I BAU HAMBURG SiloThe original

IBAU HAMBURGcentral cone silois used fordiameters between10 m and 30 m.

Calculation example for a silo diameter of 26 m

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Information

The diameter of26 m is onlyindicated for thecomparisonbetween the originalIBAU HAMBURGcone silo and thesilo with depressurechamber.

IBAU HAMBURGonly uses the siloswith depressurechambersfor silo diametersup to 14 m.

Ø limited to 14 m

BSilo with depressure chamber

B

4000 kg/s

2 x bulk loading:2 x 220 t/h

1 x packing plant:1 x 160 t/h

max. 166 kg/s

total discharge capacity 600 t/h

1 m2 opening each 4 m/s rate ofdedusting

Calculation example for a silo diameter of 26 m

Page 10: Central Cone Silos Structural

slid

ing c

oncre

te

conical shell

silo wall(upper shell)

concrete notreinforced

ring

lower cylindricalshell

ring foundation

weighing pit

pile foundationcircular pile cape.g. one fileof pilesunder silowall

truck opening

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I BAU HAMBURG Central Cone Silos from the structural point of view

specific internal massflows. This internal massflow increases with thecone size, resp. the silodiameter. These are themain reasons, why IBAUHAMBURG has limitedthis design to silo diame-ters of up to 14 m.

The Central Cone Silofrom the structuralpoint of view

CENTRAL CONE

The characteristic featureof the silo structure is thecentral cone, whichforms the bottom of thesilo compartment. Thecentral cone (invertedcone) is spanning over

the complete silo sectionand is supported only bya setback of the outer silowall (lower cylindricalshell as shown above).No intermediate supportsare required for thisstructure. The weight ofthe steel floors for thedischarge equipment andauxiliary equipment suchas intermediate bin andfilters placed below thecone is small comparedto the weight of the bulkmaterial supported by thecentral cone. Therefore itis very economic to sus-pend these floors fromthe central cone, whichmeans no additionalcolumns and foundationsare required, and the use

of steel can be kept to aminimum. A furtheradvantage for the freespanning central conewithout additional sup-porting columns is theclearly defined loadtransfer to the silosubstructure and subsoil.

All loads from the silostructure are transferredto the outer ring wallbelow the cone.Settlements are equallydistributed all over thewall perimeter, due to thesymmetry and stiffnessof the structure, andrestraint forces due todifferent settlements arenormally negligible. Thevertical loads from the

bulk material and thesuspended floors aretransferred to the sup-porting wall by normalcompression forces in thedirection of the meridian.The horizontal pressuresfrom the bulk materialare acting towards thesilo centre, which alsoresults in normal com-pression. Therefore rein-forced concrete is themost advantageous con-struction material forthese silos and theamount of reinforcementcan be kept low. Due tothe shell structure of thecone, bending momentsat the bottom of the coneat the transition to thering beam and at the

conical shell

silo wall(upper shell)

concrete notreinforced

ring

ring foundation

slid

ing c

oncre

te

pile foundationcircular pile cape.g. one fileof pilesunder silowall

Raised bottom version Standard bottom version

slid

ing

conc

rete

slid

ing

conc

rete

conical shell

silo wall(upper shell)

concrete notreinforced

lower cylindricalshell

ring

truck opening

weighing pit

ring foundation

pile foundationcircularpile cape.g. one fileof pilesunder silowall

conical shell

silo wall(upper shell)

concrete notreinforcedring beam

ring foundation

pile foundationcircularpile cape.g. one rowof pilesunder silowall

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Information

SPENNER ZEMENT Multi-compartment silo, Germany

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I BAU HAMBURG Central Cone Silos from the structural point of view

Stresses in the ring beam

upper edge fade outquickly in some distancefrom the edges, whichmeans additional rein-forcement due to therestraint effect, isrequired only locally.

RING BEAM

The ring beam located atthe bottom of the conicalshell transfers the meridi-an compression forces tothe supporting wall,where they are mainlyacting in the verticaldirection. The redirectionof meridian loads alsocauses horizontal loads,which result into hori-zontal tension forces inthe ring beam. For largesilo structures these ten-sion forces can be quitehigh, so that concentratedhoop reinforcement isrequired. But due to thesize of the ring beam theplacement of this hoopreinforcement is quitesimple.

From the structural pointof view it is sufficient toplace the inverted coneon the silo wall withoutany connecting reinforce-ment, which means ahinge in the static systembetween cone and silowall. The horizontal dis-placement due to loadsand temperature is equalfor all members adjacentto the connection.

Due to the orientation ofthe horizontal loads thedisplacement is orientat-ed outwards, whichresults in circumferentialtensile stresses. Becauseof an equal displacementand equal circumferentialstresses, tension forceswill occur in the ring

Filling pressures in a central cone silo

patch load

(distributed uniformly)

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Information

beam as well as in thesilo wall adjacent to theconnection. The relationof tension forces is corre-sponding to the relationof concrete sections.

SILO WALL ABOVETHE CONE

The main loads on thesilo wall are the loadsfrom the bulk material,which will be applied ashorizontal pressures (ori-entated outwards) andwall friction loads (orien-tated downwards). Thereare several codes all overthe world, which specifyloads from the bulkmaterial.

All common silo codesincluding the newEurocode EN 1991-4predict the same fill pres-sures from concentricfilling and use the Jansenformula, in which thehorizontal pressuresincrease with the heightfrom the silo top to thebottom, based on ane-function and with thesilo diameter, the wallfriction coefficient, thematerial specific weightand the horizontal pres-sure ratio as the mainparameters.

Much more difficult tocalculate and to predictare the silo dischargepressures, especiallywhen flow channels areformed above the aeratedsection in the centralcone silo during dis-charge. Practical calcula-tion methods are given inthe Eurocode EN 1991-4as well as the latest revi-sion of the DIN 1055-6,which is mandatory inGermany for the calcula- IBAU central cone silo 25.000 m3 at the Mehrum Power Plant

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I BAU HAMBURG Central Cone Silos from the structural point of view

tion of silo loads andwhich was released in2005 in Germany togeth-er with the Eurocode.

Generally, the stresslevel of the materialwithin the flow channelis much below the bulkmaterial at rest.Accordingly, there is apressure reduction in thearea, where the flowchannel touches the walland on the other handthere is a pressureincrease beside this areaover a correspondinglength of the perimeter.

The pressure on theremaining wall perimeteris specified as the loaddue to filling. An illustra-tion of wall loads duringdischarge is given in thefigure on the oppositepage. The parameter rcdescribes the radius of aflow channel, ec theeccentricity of the flowchannel from the silocentre and lc the lengthof the flow channel at thesilo wall. According toEN 1991-4 and the Ger-man code DIN 1055-6,for silos with largeeccentricity (ec = 0.5 rc)and assessment class 3(silo capacity larger than1000 t) calculationsshould be performed forno less than three valuesof the radius of the flowchannel, when the geom-etry of the flow channelcannot be directlydeduced from the dis-charge arrangements andsilo geometry. All threesub-load cases have to beanalysed for design andthe maximum horizontalreinforcement requiredfor these three cases hasto be provided.

For the analysis of acylindrical silo wall for aload case with variablepressures the finite ele-ment method has to beadopted, which takes intoaccount the 3-dimension-al performance of thewall structure. Thismeans it is not longerpossible to perform thestructural analysis of thesilo wall of a centralcone silo with simpleequations as it was possi-ble according to the 1987edition of the DIN code.

Nevertheless, DIN1055-6, edition 2005 stilluses the patch load con-cept, which shall beapplied on the silo wall,at several heights of thewall, resulting in a vari-able increment over theheight. The analysis anddesign due to a patchload applied on a cylin-drical wall has beenfocused on the bendingmoments and normalforces, which are causedby the patch load. Shearforces have to be consid-ered in a separate calcu-lation.

The German DIN codeDIN 1045-1 for thedesign of reinforced andpost-tensioned concretespecifies the ultimateshear force, which can betaken by a concrete sec-tion without shear rein-forcement depending onthe concrete strength,ratio of reinforcementand stress due to a nor-mal force.

Compression on the sec-tion means an increase ofthe ultimate shear forcewhereas tension means areduction.

The shear design of largecentral cone silos forshear and tension forcescaused by the loads dueto a flow channel leads tothe conclusion, that forusual concrete strengthand a wall thickness of30 – 35 cm there is alimit for these shearforces for a silo diameterof approx. 14-16 m. Thismeans due to this designequation a wall of such asilo without shear rein-forcement would not bepermissible for diameters> 14 – 16 m whenassuming large dischargeeccentricity. One solutionwould be the installationof shear reinforcement,which is not a preferableway for such a large wallarea. Another moreadvantageous solution ispost-tensioning of thewall. As described above,compression, which canbe gained by post-ten-sioning, increases theshear capacity of a con-crete section.

Simultaneously the hori-zontal wall reinforcementcan be reduced by aremarkable amount,because the post-tension-ing tendons or strandshave a much higher ten-sile strength thandeformed rebars, whichmeans a double positiveeffect.

The remaining questionis the proper ratio ofpost-tensioning. Becausea ring shaped tendon orstrand in a cylindricalwall will cause compres-sion only, it is not eco-nomic to counteractbending moments due toloading or temperaturedrop in the wall by post-

tensioning, because thiswould need very highpost-tensioning forces.

The most economic wayis an amount of post-ten-sioning forces whichcompensates the tensionforces from the bulkmaterial and an amountof inner and outerdeformed rebars, whichcan counteract the bend-ing moments due toloads. With this combi-nation the control ofcrack width due to tem-perature restraint stressesin the wall is very eco-nomic and the amount ofpost-tensioning steel anddeformed rebars is well-balanced.

LOWER PART OF THESILO WALL

The lower part of thecompartment wall, whichis adjacent to the ringbeam and the plain con-crete on the ring beam,where the fluidslides arelocated, is not loadeddirectly with the horizon-tal pressures from thebulk material. But due tothe compatibility of thestructure this wall part isalso stressed by the mate-rial stored above, whichcauses horizontal tensiondecreasing from the topof the plain concretedownwards. As alreadydescribed before, thearea, where the invertedcone is supported, willalso be tensioned, whichmeans the tension forcesare increasing againwhen approaching to thisarea. These changing ten-sion forces are caused bychanging horizontaldeformations of the silowall, which in turn

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Information

Flow channel and pressure distribution (EN 1991-4)

Internal forces in lower part of the silo wall

nϕ nϑ mϕvertical forces horizontal forces bending moments

means bending momentsand shear forces in thevertical direction of thesilo wall. Combined withvertical compressionforces due to dead loadand wall friction loadsmainly this has to beconsidered for the walldesign.

Equally distributed pres-sures from the bulk mate-rial and high tempera-tures inside the silo dueto hot material stored inthe silo will cause hori-zontal deformations tothe outside. The bendingmoments and the shearforces in the verticaldirection will be obvious-ly smaller, if the wall canmove without restraint.

This means it is prudentnot to provide any con-nection between the silowall and the ring beam(including the plain con-crete). As describedabove there are alsounequally distributedhorizontal pressures onthe silo wall, which caus-es unequal horizontaldeformations accordinglyand thus an oval shape ofthe wall mainly withchanging deformations tothe inside and outsidealong the wall perimeter.This effect has beenobserved during measure-ments at several silo walls.

A horizontal inwardsmovement of the silowall will be restricted bythe ring beam and theplain concrete on top ofthe ring beam, which willcause high restraintstresses in this area. Inorder to reduce theserestraint stresses it is rec-ommended to install a

Staticsolid

Static solid

Elevation with flow channel Cross-section with pressure pattern

Flowchannel

Staticpressures

Local highpressure

Flowchannel

Channel edgepressures

Staticpressures

Flow channelpressures

Pressure values varywith depth in the silo

pressureschannel geometry assumed for analysis

Flowpressures

vertical forces horizontal forces bending moments

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I BAU HAMBURG Central Cone Silos from the structural point of view

soft board between silowall and the plain con-crete with a minimumheight of at least 1m and2 cm thickness. Alsopost-tensioning of thewall causes inward defor-mations, which is a fur-ther reason for the instal-lation of such a softboard. There have beensome severe damages ofsilo walls in this area inthe past, because thesemovements were neglect-ed and the restraintstresses from a rigid hori-zontal support were notconsidered for design.

SILO WALL BELOWCENTRAL CONE

The wall below the cen-tral cone is loaded withthe complete verticalloads of the structureabove. This is one advan-tage of the central conesilo: All main verticalloads are equally distrib-uted on the silo perimeterand transferred to thefoundation. There is nodoubt about the distribu-tion of the loads and nodifference for loadingand discharging. If thereare large truck openingsbelow the cone the loadsare concentrated besideand between the open-ings. These areas arecomparable to columnsand the reinforcementshould be provided corre-spondingly with verticalrebars and envelopinghorizontal stirrups.

The wall above the open-ings is working as a deepcontinuous beam, whichmeans additional hori-zontal reinforcement inthis area. As alreadydescribed near the sup-

port of the cone high hor-izontal tension forceswill occur, which requireconcentrated ring rein-forcement. This ringreinforcement togetherwith the ring reinforce-ment of the ring beamhas to carry the horizon-tal part of the thrust fromthe conical shell. Due tothe concentrated horizon-tal reinforcement open-ings shall be avoided inthis area, at least for 1 mheight below the supportof the inverted cone. Atthe support of the coneloads from the upper walland from the cone areapplied on the lowerwall. Different loads willcause transverse splittingforces below the conesupport, which requiretransverse ties or stirrupsin this area.

LOADS ON THECENTRAL CONE

A part of the bulk materi-al weight is transferred tothe silo wall by wall fric-tion, the remaining partis transferred to the cen-tral cone as the bottom ofthe silo compartment.The inclined surface ofthe cone is loaded withnormal pressures andmeridian friction forcesfrom the bulk materialdue to the vertical andhorizontal pressures inthe silo compartment.

This causes horizontaland meridian compres-sion stresses in the conewall. In principle themaximum vertical pres-sures calculated from thebulk material on the silobottom result from theload case “filling”.During discharge the

wall friction loads will beincreased and due to theequilibrium of the totalloads the vertical pres-sures on the silo bottomwill be reduced. Theresulting loads on aninverted cone are diffi-cult to measure andtherefore not exactlyknown, but correspond-ing to the loading on aflat silo bottom the loadcase “filling” can beassumed as governing forthe vertical loading aswell.

Due to the 2005 editionof DIN 1055-6 the verti-cal load from the loadcase “filling” on a hori-zontal silo bottom hadnot to be increased anylonger with a factor cbfor cement, raw meal, flyash etc. in order to con-sider pressure increasedue to dynamic effects.But as the uncertainty forthe loading on the cone isremaining, it is recom-mended to increasecb = 1.0 to 1.33.

The discussion still refersto uniformly distributedloads, but as describedfor the silo wall no uni-formly distributed loadswill occur in a centralcone silo. This was notconsidered when the firstinverted cone silos weredeveloped and for manyyears the inverted coneswere designed with theassumption of uniformlydistributed, but increasedloads. Nevertheless, therecommended incremen-tal factor can be seen as afactor for consideringunequal loads on IBAUcentral cones and thusleads to an adequatedesign result.

ConstructionGuidelines

SILO WALL

Today, the silo wall of areinforced concrete orpost-tensioned silo isnormally performed as aslipform concrete struc-ture. This means that aformwork of about 1.2 mheight has to be movedupwards continuouslywith an average hourlyrate of about 10 – 20 cm.The rate of concretingand installing of rein-forcement has to beadapted to this speed aswell as the concrete mix,because hardening of theconcrete governs theprogress of the slipformprocedure. The shape andarrangement of rebars orbuilt-in parts must beadequate for this method,because the place forinstalling the reinforce-ment and built-in parts isrestricted by the so-calledyokes of the slipform.

Slipform concrete needsboth – skilled plannersand skilled personnel onsite, otherwise there willbe severe quality prob-lems. During the slip-forming procedure a per-manent supervision isstrongly recommendedbecause there is no possi-bility for amendmentslater. If all is done prop-erly the concrete strengthis corresponding to acast-in situ concrete.

There can be minor defi-ciencies of the concretecover, which is in contactwith the moving form-work. Therefore the bondquality between rein-forcement and the

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Information

Principal stresses in the central cone

Pv

Pn

Pt

loads acting upon the cone

nϕ nϑ

n

ϕ

ϕm

m

ϑn ϑn

ϕ

- -

+

+

-

normal forces

meridian

normal forces

hoop direction

bending moments

enveloping concretecould be reduced. Inorder to get an adequatesafety level the anchor-age length as well as thelength of lap splices ofthe rebars has to beincreased compared to acast-in situ concrete.

Due to the normal pres-sures on the silo wallfrom the bulk materialthe horizontal rebars atthe inside and outsideface of the silo wall areunder high tensile stress-es, which means the ten-sion lap splices of theserebars are essential forthe structural integrity ofthe silo wall. From testsof such tension lapsplices it is a well-knownphenomenon, that thecapacity of lap splices forlarge bar diameters isreduced due to splittingstresses in the envelopingconcrete. Therefore onemeasure is proper stag-gering of horizontal ten-sion lap splices from ringto ring.

As an additional measureenveloping ties or stir-rups have to be providedfor tension lap splices ofrebars with 16 mm diam-eter or more. Withoutsuch ties brittle failurescan occur, which couldpossibly cause the failureof a complete verticalline of tension lap splicesin a wall. This phenome-non is known as “zip”effect and has beenexperienced from severalcollapses of silo walls inthe past.

The inverted cone isplaced on the silo wall ona setback of the wall,which means the lower

part of the wall is thickerthan the upper. In spite ofthis change the wall canbe executed continuouslyby slipforming. Withproper preparation andskilled personnel it is pos-sible to change the slip-form without interruption.Of course all connectingor starter bars for othermembers will hinder theprogress and should beavoided when possible.From the structural pointof view there is no con-nection required betweenthe silo wall and theinverted cone and there-fore no connecting rebarsshould be provided.

For large silos the amountof deformed rebarsrequired for the horizontaltension combined withbending moments will bevery high. Due to theslipforming process,which is usually providedfor the construction ofthe silo wall, the amountof rebars, which canbe installed per meter ofheight, is limited.Furthermore, a congestionof reinforcing steel willaffect the proper installa-tion and compaction ofthe concrete. Therefore,it is prudent to providehorizontal post-tensioningfor the silo wall of largediameter silos in orderto reduce the amount ofdeformed rebars.

CENTRAL CONE

Since the development ofthe inverted cone somealternative methods havebeen used for the con-struction of the cone. Inthe beginning severalcones were performed asa cast-in-situ structure

loads acting upon the cone

normal forcesmeridian

normal forceshoop direction

bending moments

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I BAU HAMBURG Central Cone Silos from the structural point of view

Lifting of cone segments

Precast cone segments

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Information

Central cone made of precast cone segments

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I BAU HAMBURG Central Cone Silos from the structural point of view

Prefabricated cone for multi-compartment silo

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Information

Multi-compartment cone silo

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I BAU HAMBURG Central Cone Silos from the structural point of view

IBAU Ring silo design

with scaffolding and bot-tom formwork as well asa top formwork. This is atime consuming con-struction and thereforeother solutions weredeveloped. The most suc-cessful method, which isused in the meantimenearly without exceptionare precast segments withtrapezoidal shape in com-bination with cast-in situconcrete for the remain-ing joints.

The precast segmentscover the complete bot-tom side of the cone andare placed on the setbackof the silo wall. Due totransport and erectionreasons the maximumwidth of such segmentsis limited to approx.3.2 m for usual condi-tions. Most of the seg-ment area has the thick-ness of the final cone, thebottom part adjacentto the ring beam and themeridian sides have areduced thickness withrough surface and stir-rups protruding from thebottom concrete. Thebottom part of the precastsegments is the innerformwork of the ringbeam, the silo wall is theouter formwork.

The ring beam is a cast-in situ structure, which isjoined to the cone seg-ments by connecting stir-rups. The meridian sidesof the cone with reducedthickness form meridianjoints, which are alsofilled with cast-in situconcrete. Since there ishorizontal compressionmainly in these joints thelap length of the connect-ing stirrups can be small.The top side of the

meridian joints can bemade with formworkpanels, which areclamped to the precastsegments or expandedmetal attached to the stir-rups can be used as form-work when using a stiffconcrete mix.

Precast segments, cast-insitu ring beam andmeridian joints as well asa top slab form a com-plete composite rein-forced concrete structure,which can be constructedwithin much shorter timethan a complete cast-insitu member. Because thesegments are produced ina flat horizontal form, theresulting cone structurehas a polygon shape inplan, which must be con-sidered for the geometryof the steel floors, whichare suspended from thecone, that does not affectthe bearing capacity ofthe structure.

For the first invertedcones made with precastsegments scaffoldingtowers placed in the silocentre were used, sup-porting the segments nearthe top. This solutionwas followed by suspen-sion members fixed atthe silo wall and support-ing the segments near thetop. Though the installa-tion of rebars is hinderedby the suspensionmembers this solutionis preferred by mostcontractors nowadays.For large truncatedcones, where a formworkis needed for the topslab, the use of a scaf-folding tower stillremains a useful option. Depending on the weightof segments, height of

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Information

the silo wall and avail-ability of heavy cranesthe cone segments can belifted over the top of theof the silo wall. Alter-natively the slipform pro-cedure of the wall can bestopped approximately atthe level of the cone top,which makes the con-struction of the conemuch easier, but needstwo slipform phases.

Another solution, whichhas been used by somecontractors, is a bottomscaffolding and form-work, where the concreteis installed as a so-calledshotcrete or sprayed con-crete. Due to severalconstruction deficienciesthis method was notreally successful and isnot recommended.

MULTI-COMPART-MENT SILOS

The central cone silowith a single circularcompartment was verysuccessful from thebeginning of its develop-ment. One of the reasonswas the very robust per-formance of the conestructure as a silo bottomwith a very high bearingcapacity. Therefore, thetransformation of a circu-lar single cell section intomulti-compartment silosections was the nextlogical step.

The conical shell is ableto carry ring loads inplan as well as meridianloads with compressionforces mainly combinedwith some bendingmoments, which do notaffect the cone structure.Therefore, the invertedcone will not restrict the IBAU Ring silo design

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I BAU HAMBURG Central Cone Silos from the structural point of view

patch load; the size of thepatch load should refer tothe outer silo diameterand the inner silo wallshould be neglected; thepatch pressures can bededucted from the uni-formly distributed loads.A combination of theseload assumptions hasbeen used successfullyfor many ring silos in thepast. Further comparisonsto calculations with aninclined surface of the fillin the silo along the wallperimeter, which wereintended to represent thedischarge from one open-ing for a long time, haveshown good congruence.

According to the flowchannels in the case oflarge eccentric discharge,it is prudent to assumesimilar flow channels ina ring silo as well.As a recommendation thediameter of the flowchannel should be adapt-ed to the width of theannular gap. The pressurepattern should be gainedaccording to a silo cellwith the diameter of theouter wall and withoutconsidering an inner cell.

Another possibility fora multi-compartmentdivision is separatinga circular cell by radialdiaphragm walls. Forsmall diameters 3 cellsare possible, for largediameters a minimum of4 diaphragm walls shouldbe used. For an economicdesign the followingrecommendations shouldbe considered:

- The cross section ofa single cell should beas close to a circle aspossible

Multi-compartment silos evolvedfrom the circular-cylindrical single cell silo

Multi-compartment silosevolved from the ring silo

division into severalcompartments. The silowalls are the governingcriterion for an economicand reasonable division.One of the first requestsfrom the clients were twocompartments. A divi-sion of a circle by astraight wall through thecentre of the circle needsan enormous amount ofconcrete and reinforcingsteel for usual silo diam-eters. Only for silo diam-eters < 10 m such a solu-tion would make sense.

Therefore, as an alterna-tive the so-called ringsilo was developed withtwo concentric circularwalls, which form aninner circle and an outerannular cell. Because thepressures in the outerannular cell = ringdepend on the width ofthe gap, the horizontalwall pressures on theouter wall are muchsmaller than for a singlecell, where the pressuresdepend on the radius.

As the DIN 1055-6excludes specificationsfor ring silos, modifica-tions in the formulas ofthe code should be adopt-ed with the followingrecommendations:- Uniformly distributed

loads from the bulkmaterial can be calcu-lated with the ratioA / u = b / 2,

- A = cross section areaof the cell

- u = internal perimeterof the cross section

- b = width of the annu-lar gap

None uniformly distrib-uted loads can be calcu-lated with a modified

D = 14,0 mD = 14,9 m

D = 14,0 m

D = 18,0 m

D = 19,5 mD = 16,0 m

D

D/d = 18,0 / 10,0 m D/d = 24,0 / 15,9 m

D/d = 14,0 / 5,2 m D/d = 20,0 / 9,5 m D/d = 22,0 / 7,0 m

D d

D/d = 27,0 / 17,0 m (2x)D/d = 27,0 / 18,5 m (2x)

Page 25: Central Cone Silos Structural

25

Information

Two examples of multi-compartment silo designs

- The diaphragm wallscause restraintmoments at the inter-section with the circu-lar wall. Therefore thecircular wall should bestrengthened in thisarea by increasing thewall thickness with achord-like section. Theend of the diaphragmwall should bestrengthened by trian-gular fillets (haunches).

Combining the ring siloconcept with the radialcell design, the annularcell of a ring silo can bedivided by diaphragmwalls into several cells aswell as the inner circularcell. This can be done forthe inner and outer cellsimultaneously or for oneof these cells only. Incase diaphragm walls willbe provided inside andoutside simultaneously,the pattern of the wallsshould be adapted prefer-ably in this way, thatinner and outer diaphragmwalls are in one line.This will reduce bendingmoments and restraintstresses in the walls andallow a better installationof rebars during the slip-forming process.

The construction of amulti-compartment silowith inverted cone can inprinciple be done accord-ing to a single cell silo.But there are some modi-fications required due tothe more complicatedconstruction sequence:

- The outer wall and theinverted cone of a ringsilo can be performedin the same way as fora single cell silo andthe inner ring wall can

Page 26: Central Cone Silos Structural

26

Multi-compartment silos evolved from single cell and ring silo references dated Jan. 2008

Dyckerhoff Mark IIZementwerk Lauffen

CBR HarmigniesHolcim (Espana)

Carboneras

Johann SchäferKalkwerke

Natal Portland CementAtlacim, Maroc

RohrbachDotternhausen

LafargeKarsdorf

Spenner ZementGermany

Andre Büechl

ZementwerkBerlin

TCEC/USCC TaiwanDyckerhoff Mark II

Hornos Ibéricos Alba, S.A.Unicem Vernasca

Diamond Cement DamohRKW Wülfrath Schwelgern

Italcementi SalernoPolysius AG Cape Portland

Bateman LTD. LethaboItalcementi Vibo Valencia

Schencking KG LienenPanUnited Singapore

Ciments Français Bussac

Perlmooser MannersdorfTaiwan Cement NankangKedah Cement Langkawi

Yura S.A.Tasek Cement

Asia Cement SingaporeKedah Cement Johor Port2 x Dyckerhoff LengerichUnited Cement PTE LTDAlsen Breitenburg, Hansa

Bremen

Unicem VernascaItalcementi Vibo Valencia

Cementeria Adriatico PescaraPerlmooser Mannersdorf

Ciments de la Loire,Villiers au Bouin

KRC Umwelttechnik, WalheimItalcementi Salerno

Ciments Français BeaucaireTaiwan Cement Nankang

Uniland Los Monjos

Castle Cement ClitheroeKedah Cement Port Prai

Cementos LemonaHOLCIM (France) S.A.,

DunkerqueCham Pha Cement Plant

Usine de MarakechCementos Occidentales

Cementval S.L.United Cement Pte. Ltd.

Heidelberg SchelklingenAnneliese Zementwerke

Cimento ItambeCimento Rio Branco

DyckerhoffAmöneburg

Alpha Cement

2 x Ciments Vicat MontalieuDeuna Cement

Readymix DortmundENCI MaastrichtCCB Ramecroix

CBR Lixhe

Central Cone Ring Silo

Groupe Obourg OrignyUsine Lumbres

DyckerhoffLengerich

CBR Cement, GentDyckerhoff Neuwied

Cimento MinasVotorantim Cimentos

Juan Minetti S.A.

AZE Mühendislik,Müsavirlik ve Ticaret

BelapatfalvaDenizli Cimento

Karcim S.A.Italcementi Collefero

2 x Rajashree Cement2 x Grasim Cement

ENCI Maastricht

Ciments LuxembourgeoisCiments d’Origny

AltkirchCBR Lixhe

Phoenix CementCementos Chihuahua

Heidelberg Schelklingen3 x Alamo Cement

Rüdersdorfer CementHeidelberg Moldan

2 x La Cemento Nacional2 x Ciments LuxembourgeoisDyckerhoff Mark II / Geseke

Dyckerhoff AmöneburgUnicem Vernasca

Cementownia NowinyCementos Hispania SA. Yeles

Many Thousands of IBAU Silos

CBR Gent

Central Cone Silo

I BAU HAMBURG Central Cone Silos from the structural point of view

Page 27: Central Cone Silos Structural

In case of a silo withdiaphragm walls, whichare connected to theouter circular silo wallthe following adaptationis required:

- The part of thediaphragm wall, whichis connected to thecone, cannot be builtwith a slipform, butmust be constructed ascast-in situ concrete. This means, in a firststep the outer wallmust be approximatelybuilt up to the top ofthe cone. The shape ofthe wall in plan abovethe ring beam must be

adapted to the finalshape of the wallincluding thickening atthe intersections withthe diaphragm walls.

Starting from the ringbeam up to the top ofthe cone horizontalstarter bars with cou-plers for the diaphragmwalls have to be pro-vided.

- The construction of theinverted cone isaccording to the coneof the single cell silo,but as an additionalmeasure starter bars forthe diaphragm walls

27

Information

be added by a separateslipforming process.The main advantage isthe protection of thework inside from envi-ronmental influences,but a disadvantage isthe lifting of all build-ing materials over thetop of the outer wall,which has been fin-ished before.

- Therefore preferablythe outer wall is con-structed up to the topof the cone. Thenthe cone is made asdescribed for the singlecell silo. Following thisthe base ring of theinner wall at the inter-section with the conecan be constructed.Now slipforming of theinner wall can follow.If there are enoughskilled workers andenough crane capacity,inner and outer wall canbe built simultaneously.

have to be provided.Depending on the divi-sion of cone segmentsand diaphragm wallsthese starter bars areplaced in the segmentsor in the meridianjoints in between thesegments. Afterfinishing the cone thelower part of thediaphragm walls has tobe added as a cast-insitu member.

Now the outer wall,diaphragm walls andcone are ending at thesame level with thevertical starter bars forthe section aboveprotruding from theconstruction joint.

- Following this the slip-form for the diaphragmwalls has to be con-nected to the slipformof the outer wall andthe complete multi-compartment sectioncan be performedsimultaneously.

SummaryWith regard to the silo body there is world-wide no silo system which is more economi-cal than the IBAU HAMBURG central conesilo. A proof for that are more than 7,000 siloswith diameters ranging from 10 to 30 m.

Although the informationis intended to be up-todate at the time of print-ing, IBAU HAMBURGdoes not warrant thecompleteness and/orcorrectness of the contentand is not responsiblefor any changes in theguidelines or to give noteon such changes.

Accordingly, thisbrochure is not intendedas a replacement oflocal guidelines or thereplacement of theexpertise of the designcompany and soIBAU HAMBURG cannotbe held responsible forany claims from usingthese documents.

For more than thirty years Peter und Lochner are taking partin development and improvement of the I BAU Central Cone Silo.

We are one of the international leading engineering companiesfor planning and design of largereinforced concrete and post-tensioned storage and blending silos.

Silos for cement, clinker, raw meal, ... Various structures for kiln lines, ...

Our secret is ... EXPERIENCE

Peter und LochnerBeratende Ingenieurefür Bauwesen GmbHConsulting Civil Engineers

Haußmannstr. 78, D-70188 StuttgartPhone +49 711 / 92377-0Telefax +49 711 / 92377-28e-mail [email protected]

I BAU HAMBURGrecommends as civil engineers for I BAU SILOS:www.PuL.ingenieure.de

Page 28: Central Cone Silos Structural

07/0

8

I BAU HAMBURG

Marine Terminals, Shiploaders and -unloaders,Cement Carriers, Equipment for Power Plants,

Special Applications

I BAU HAMBURG · Rödingsmarkt 35 · D-20459 Hamburg · PHONE +49 (0) 40 36 13 090FAX +49 (0) 40 36 39 83 · Email: [email protected] · Internet: www.ibauhamburg.de

Mixing plant for RÜDERSDORFERZEMENT, Eisenhüttenstadt, Germany

High-Capacity mixing plant for HOLCIM,Antwerp, Belgium

Shiploading station for CIVIL AND MARINESLAG CEMENT, Port Talbot, UK

Mobile Shipunloader alonga river in Germany