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Your efficient partner for modern and effective bulk material
handling
PLANT DESIGN - ENGINEERING - EPC-CONTRACTING
� + 49 (0)40 36 13 090 [email protected]
I BAU HAMBURG
CEMENT - THERMAL POWER - MINERALS
Single silos. Ring silos.
Multicompart-ment silos. From 2 to
22 chambers,diameters:
14 to 27 m.
Advanced technology for
self-discharging Cement Carriers
including the Midship tunnel.
Marine Cement Terminals
Central Cone Silos EPC-Contracting
Silo Conversions
Components
Spare Parts
Cement Carriers
Ship Unloaders
Floating terminals.
Mini terminals.Silo systems.
Dome systems.Flat storage
terminals.
Stationary ormobile types:
From the5,000 class
up to the60,000 class.
Piling. Civil works. Steel structure, supply/erection.
Electrical/mechanical supply and erection.
Economic modifications with advanced cutting-edge
technology.
The key for a well functioning plant: Components, all made to
measure.
High stock availability:Just-in-time supply of spare
parts.After-sales Service.
I BAU HAMBURG
I BAU HAMBURGCentral Cone Silos
from thestructural point of view
A HAVER & BOECKER Company
Z/1
2/1
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ilos f
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2 3
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
The I BAUHAMBURG Centralcone silo –THE ORIGINAL
The storage silo
I BAU 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 I BAU 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
I BAUdesign 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 aTHE ORIGINAL: Thousands of this silo type are in
operation for many years The I BAU Central cone storage silo
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.
I BAU HAMBURG worksfor more than 30 yearstogether with the
civildesign company Peterund Lochner with regardto the design of
thecentral 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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4 5
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
shaped bottom and fluidslide systemView into the central cone
silo with ring
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76
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
Flow channels in an I BAU Central cone silo
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
theother hand, as only thematerial above the aerat-ed silo section
will get inmotion, with the section-al aeration concept
flowchannels are formedso that the silo dischargeis eccentric. In
the I BAUconcept, the eccentricflow within the silo iscontrolled by
the auto-mated flow control gates,which will dischargeonly the
requiredflow rate from the 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: „SAFETYFIRST“ by using
I BAUsilos. Nevertheless, thereare also other silo designs
with central cones andaeration sections avail-able, where the
materialflows from the aerationsection through largeopenings
directly intothe area under the conewithout any flow controlgates
in-between.Appropriate flow controlgates are only installedfor the
discharge fromunderneath the cone.
I BAU Silos - SAFETYFIRST
I BAU 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 I BAU „SAFETYFIRST“
principle.
I BAU HAMBURG hasalso been asked to designsilos with a
depressurechamber. I BAU is aware,that comparing thisdesign with
the originalI BAU Cone silo assum-ing an identical numberof
aeration sections andaeration time, the largeopenings of the
conedramatically increase the
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98
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
The diameter of26 m is onlyindicated for thecomparisonbetween
the originalI BAU HAMBURGCentral cone siloand the silo
withdepressure chamber.
I BAU HAMBURGonly uses the siloswith depressurechambersfor silo
diametersup to 14 m.
Ø limited to 14 m
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
BSilo with depressure chamber
BA
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 each0,03 m2 opening perflow control gate 4 m/s rate
of flowdedusting
Ø from 10 - 30 m
AOriginal I BAU HAMBURG SiloThe original
I BAU HAMBURGCentral cone silois used fordiameters between10 m
and 30 m.
Calculation example for a silo diameter of 26 m Calculation
example for a silo diameter of 26 m
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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
1110
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
specific internal massflows. This internal massflow increases
with thecone size, resp. the silodiameter. These are themain
reasons, why I BAUHAMBURG 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
SPENNER ZEMENT Multi-compartment silo, Germany
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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12 13
InformationThe original I BAU HAMBURG Central cone silofrom 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
Nϕ
Nϕ
Filling pressures in a central cone silo
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- The I BAU Central cone silo 25.000 m3 at
the Mehrum Power Plant
patch load
(distributed uniformly)
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1514
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
Flow channel and pressure distribution (EN 1991-4)
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-tensioning, 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 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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1716
InformationThe original I BAU HAMBURG Central cone silofrom 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- Principal stresses in the central
cone
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 I BAUCentral 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
Pv
Pn
Pt
loads acting upon the cone
nϕ nϑ
n
ϕ
ϕm
m
ϑnϑn
ϕ
nϕ
mϕ
- -
+
+
-
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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1918
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
Lifting of cone segments
Precast cone segments
Central cone made of precast cone segments
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2120
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
Prefabricated cone for multi-compartment silo Multi-compartment
cone silo
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2322
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
The I BAU 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
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-COMPARTMENT 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 The I BAU Ring silo
design
-
2524
InformationThe original I BAU HAMBURG Central cone silofrom the
structural point of view
Two examples of multi-compartment silo designs
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
- 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
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)
-
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
26
Multi-compartment silos evolved from single cell and ring silo,
references dated Jan. 2008
Dyckerhoff Mark IIZementwerk Lauffen
CBR HarmigniesHolcim (España)
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
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.
The original I BAU HAMBURG Central cone silofrom the structural
point of view
Although the informationis intended to be up-todate at the time
of print-ing, I BAU 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 soI BAU HAMBURGcannot be held responsi-ble for
any claims fromusing these 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