Design-construction feature

Precast prestressedclinker storage silosaves time and moneyWilliam E. Pery, P.E.Chief Civil EngineerKilborn Engineering Ltd.Toronto, Ontario

The author, who wasresponsible for the overallstructural design of thisprecast prestressed,conically-shaped storagefacility in St. Constant,Quebec, describes thedesign-constructionhighlights of this veryfunctional and economicallybuilt structure. William E. Pery

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Fig. 1. Clinker storage silo nearing completion.

P recast prestressed concrete wasused very effectively and economi-

cally to build a major portion of a hugecement clinker storage silo (see Fig. 1)for Canada Cement—Lafarge (CanfargeLtd.) in St. Constant, Quebec (about 10miles south of Montreal).

It is estimated that the precast pre-stressed design saved about $225,000 inconstruction and operating costs.

The storage silo was completed inSeptember 1975.

This very functional structure, whichwill be used to store 120,000 tons ofcement clinker at a temperature of120 F, has an inside diameter of 214 ft,a height above ground of about 130 ft,and a depth below grade of 79 ft, andcovers an area of 36,000 sq ft.

The structure, consists essentially offour major elements.

1. A cast-in-place foundation systemmade up of

—A ring beam supported on a con-crete curtain wall. The ringbeam is post-tensioned.

—A series of 64 pile caps inter-connected by a cast-in-placering beam. Each pile cap issupported on a group of threepiles.

—An inverted conical substructuresituated underground, and con-sisting of a cast-in-place rein-forced concrete slab on grade.

2. An inverted conical structurestarting out from Ring No. 1.(precast reinforced concrete).

3. An intermediate circular wall(precast segmental post-tensionedconcrete).

4. A top conical shell roof (precastprestressed concrete).

Fig. 2 shows schematically how thefour major structural elements are inter-connected. Fig. 3 illustrates the statical

PCI JOURNAL/January-February 1976ҟ 51

CONICALSHELL ROOF

CYLINDRICALWALL

INVERTEDCONE BOTTOM

FOUNDATIONSYSTEM

Fig. 2. Schematic diagram showing make up offour structural elements.

force interaction of the major elementsof the structure.

This very heavily loaded structuremakes maximum use of precast con-crete combining mild steel reinforce-ment, prestressing, and post-tensioningtechniques very advantageously. Alto-gether, only five basic types (see Table1) of precast concrete components wereused, each component being repeatedsixty-four times.

The precast elements in the base

structure include radial tie beams,slanted V-shaped columns, bottom conetee-slabs, and wall panels. The units inthe conical roof are prestressed modi-fied single tees, 116 ft long, with flangestapering to the top compression ring.Post-tensioning in ring beams No. 3 and4 tie the components together.

This paper will describe the function-al requirements of the storage facilityand then will discuss the structural de-sign and erection aspects of the project.

52

v INVtKItU cUNt aVI ivmҟv UUNIUAL SKtLL KUUI-

Fig. 3. Statical force system of major interacting structural elemer a.

Functional Requirements

The structure is designed to store 120,-000 tons of cement clinker during thosemonths of the year when production isin excess of sales.

Several other building systems wereinvestigated (including structural steel)for storing the clinker. The final deci-sion to use an essentially precast pre-stressed system was based on economic,functional, and structural considera-tions.

The filling up of the storage facilitywith cement clinker will be done with abelt conveyor coming through a con-veyor gallery at the tip of the uppercone.

The clinker will be dropped into thesilo. When the lower cone and the cyl-indrical portion of the facility is filled,the clinker will freely form the top cone

by depositing according to its angle irepose.

Properties of clinker

Volume weight: 85 to 96 lbs per cu ftTemperature at arrival: 120 FAngle of repose: 31-33 deg.

The cement clinker will be reclaimetat the bottom of the cone where theclinker will enter the reclaiming cham.ber through four large openings viagravity. The material will then be fedby feeders onto a belt conveyor situatedin the reclaiming tunnel.

The advantage of the above systemlies in the very simple and low mainte-nance mechanical installations and theeconomical way of moving the clinker.

We believe that this method of stor-age, also known and applied in themining industry, has not yet been ap-plied for clinker storage in North Amer-ica.

PC I JOURNAL/January-February 1976 53

Fig. 4. Plan of structure showing main precast concrete components.

Structural Design

The shape of the structure is composedof two cones with a short cylindricalsegment between them.

The lower cone is partly under-ground, whereas the remaining part ofthis cone, the cylinder and the top coneare above grade.

The main dimensions of the structureare:

Inside diameter: 214 ft.Height above ground: 129 ft 6 in.Depth below grade: 79 ft.Figs. 4 and 5 show a plan and eleva-

tion of the structure and also indicatethe major component parts. Dimension-al details and weights of the five basictypes of precast components are shownin Table 1 and Fig. 6.

Design requirementsThe structure had to satisfy the follow-ing basic design requirements:

54

? \•'ҟLOADINGҟR5CONVEYOR

FLOW

P5

P4

v R3

P2ҟP3

Ip PIҟCRUSHED ROCKҟ GRADE,R 2ҟR.l :.

DISCHARGE CHAMBER

CLAYҟ400 Jco -iROCKҟ PRECAST CONCRETE

TUNNEL CAST-IN-PLACE CONCRETE

DISCHARGE CONVEYOR

LEGEND

P I - RADIAL TIE BEAM ANDҟ R I - RING BEAM NO. IANCHORAGE BLOCK

P2 - SLANTED V-ҟR

COLUMNҟ2 - RING BEAM NO.2

R 3 - RING BEAM NO. 3P3 - LOWER CONE ELEMENTҟ R4-RING BEAM NO . 4P4 -WALL PANELP5 - CONICAL ROOF ELEMENTҟ

R 5 - RING BEAM NO. 5

Fig. 5. Cross section of structure showing location of main precast concretecomponents.

1. Be competitive with other alter-native systems, including the muchused method of storing clinker in along stock pile covered with a steel-framed envelope.

2. Follow the shape of the storedclinker as closely as possible.

3. Have a roof span of 216 ft with aconcentrated equipment load at thecenter.

4. Carry economically the very highvertical and horizontal loads of the

stored material.5. Have minimum maintenance costs

while simultaneously providing aweatherproof enclosure and preventingescape of dust from the inside.

6. Use, preferably, concrete as thebuilding material produced by the own-ers of the plant.

After investigating several alterna-tives, an essentially precast prestressedstructure was found to fulfill best theabove requirements.

PCI JOURNAL/January-February 1976ҟ 55

P5PLITAEDDETAIL P4ҟ CONICAL ROOFWALL PANEL

ELEMENT

3"

2" 1 /2'

R

,'-2„

x

5'-8"ҟ21-5

DETAIL PIRADIAL TIE BEAM AND

ANCHORAGE BLOCK

8 M o

LOWER CONE ELEMENT

Fig. 6. Dimensional details of major precast concrete components.

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Table 1. Summary of dimensionsand weights of precast components

The structure is composed of five basictypes of precast concrete components.Each component is repeated sixty-fourtimes.

P1: Radial tie beams (post-tensioned toRing 1)Length: 27 ft 2 in. Section: 14 x 16 in.Anchorage block: 2 ft 5 in. x 3 ft 5 in. x

5 ft 8 in.Weight: 11.1 kips

P2: Slanted V columns (reinforced)Length: 14 ft 11 in.Width at top: 4 ft 6 in.Section: 1 ft x 2 ftWeight: 9.5 kips

P3: Lower cone element (reinforced)Length: 33 ftWidth:

High end: 10 ft 3 in. Low end: 8 ftSlab thickness: 1 ftRib dimensions (below slab):

Depth: 1 ft and 2 ft. Width: 2 ftWeight: 53.8 kips

P4: Wall panel (reinforced and segment-al post-tensioned)Height: 46 ft 6 in. Width: 10 ft 8 in.Wall thickness: 8 in.Column dimensions:

Width: 2 ftDepth: variable to 2 ft 6 in.

Horizontal ribs: About 4 ft 6 in. x 3 ftWeight: 81.0 kips

P5: Conical roof element (prestressed)Length: 116 ftFlange width:

Bottom: 10 ft 8 in. Top: 1 ft 2 in.Flange thickness:

Generally: 3 in. Bottom: 3 to 6 in.Web dimensions:

Width: 9 in. upDepth: variable from 1 ft 4 in. to 3 ft

Weight: 68.0 kips

Site and soil conditions

The site, which is located within thecement plant, was basically level andhad good access from all sides.

The profile of the subsoil is as fol-lows:

Grade to —2.0 ft natural overburden—2.0 to —22.0 ft gray soft clay—22.0 to —42.0 ft dense till—42.0 ft on bedrock

Structural components

The structure consists of the followingmain components:

Conical roof structure (above grade)—The components of the roof comprise64 identical precast prestressed pie-shaped units (P5), the compression ringNo. 5, and the circular penthouse slabresting on the compression ring.

The precast prestressed elementswere produced in a regular single teebed with suitable modifications to ac-commodate the variable dimensions ofthe unit.

The unusual shape of the elementscombined with the prestressing pre-sented a formidable design problemwhich was solved with the aid of acomputer.

The stresses in the roof elementswere investigated for the following loadconditions:

1. Stripping, storage, and transpor-tation, with the precast unit in a hori-zontal position (span = 115 ft).

2. Erection and temporary conditionbefore completion of roof, with theprecast unit in an inclined position(span = 94 ft), subject to wind loads.

3. Final position, with the roof act-ing as a conical shell, subject to snow,.wind, and temperature effects.

A computer program was developedto check the stresses occurring in theroof units at the above stages of con-struction.

This program computed the variable

PCI JOURNAL/January-February 1976ҟ 57

section properties along the entirelength of the unit at 1-ft intervals,found moments and shear forces for allloading cases and calculated the stressesdue to prestressing.

Computer output was received for115 sections along the length of theunit, with all relevant information print-ed. Table 2 shows a sample of the out-put fo- one of the sections.

The precast roof units are connectedto each other through the radial jointsby welded connections and grouting ofthe joints.

At the top end, the cast-in-place ringNo. 5 joins the members. At the lowerend, the units rest on the wall paneland a cast-in-place monolithic connec-tion between ring No. 4 and the unitsrestores continuity.

Table 2. Sample computer outputshowing summary of stresses of Sec-

tion 54 of roof slab.

LOADINGSTAGE

VARIABLEPRESTRESS

STRESSES, KSI

TOP BOTTOM

STRIPPG 0.95 -0.824 -1.3131.00 -0.770 -1.5771.05 -0.716 -1.840

STORAGE 0.95 -0.792 -1.3771.00 -0.738 -1.6411.05 -0.684 -1.905

TRANSP. 0.95 -1.273 -0.1771.00 -1.223 -0.4201.05 -1.174 -0.663

ERECTN. 0.95 -0.571 -1.5861.00 -0.521 -1.8291.05 -0.472. -2.071

ERECTN. 0.95 -0.818 -1.090.+ WIND 1.00 -0.768 -1.333

1.05 -0.719 -1.576

ERECTN. 0.95 -0.324 -2.081- WIND 1.00 -0.274 -2.324

1.05 -0.225 -2.567

FINAL 0.95 -0.705 -0.9271.00 -0.662 -1.1351.05 -0.620 -1.344

FINALI 0.95 -1.432 -0.260+MB 1.00 -1.390 -0.052

1.05 -1.347 -0.157

FINAL2 0.95 -0.542 -1.583-MB 1.00 -0.499 -1.791

1.05 -0.457 -1.999

NOTE

A-581.89 D=36.000 YT=11.973I=72499.6 W=0.606 PR=604.20PE=556.37 PF=476.96 EXC=21.152

Weatherproofing was achieved byapplying one coat of epoxy paint onthe surface of the precast units in theplant before shipping the units to thejob site. After erection, the joints be-tween the units were caulked.

Cylindrical wall and columns-Thecylindrical wall is composed of 64 bas-ically identical precast reinforced con-crete elements.

The top part of the units, formingthe vertical cylindrical wall of the silo,has a tee-shaped cross section.

The rib serves as a column and amoment-resisting member and the 8-in.thick flanges form the cylinder.

Below the departure of the lowercone, only the column portion is car-ried down to sit on the tie beams andsupport the wall and roof.

At the intersection of the cones andcylinder, two horizontal ribs are formedwhich, when joined, compose the ten-sion rings (Rings 3 and 4).

On-site post-tensioning was used totension the ring beams of this structure.

Connecting Rings 3 and 1-The cone-shaped bottom is composed of 64 iden-tical precast reinforced concrete ele-ments.

These tee-shaped members have a12-in, thick flange forming the cone and24-in, wide ribs with a 24 and 36-in.depth.

The precast units are supported attheir ends on the rings and in betweenby the V-shaped precast concrete col-umns. The columns in turn rest on theslanted face of the tie beams.

Radial tie l-eam system-To carry thehorizontal thrust of the tilted V-col-umns, there are 64 precast post-ten-sioned t'e beams radiating outwardfrom Ring 1.

The tie beams are supported at theirouter end by the pile caps situated inRing 2. Between the tie beam and thepile caps, there is a low-friction bear-ing device to allow unrestrained radialmovements for the structure.

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Pile caps and cast-in-place founda-tion below grade—Each of the 64 tiebeams is supported on pile caps restingon three 20-in. diameter Franki piles.The piles are built into the very denseglacial till and their onion-shaped footis only a few feet above the rock level.

Pile caps are connected with the cast-in-place tie ring No. 2 (108 ft radius)which also carries the small horizontalforces due to friction of the bearingpads.

Bentonite wall and tension ring (Ring1)—A 24-in, thick concrete curtain wall,with an 81 ft 3 in. radius, was builtusing the bentonite slurry method.

The purpose of the wall is to:1. Support Ring 1 and its vertical

reaction.2. Provide the cofferdam for the ex-

cavation.3. Serve as a cut-off wall to stop in-

fljw of ground water.The bentonite wall is anchored 5 ft

into the bedrock. On the top of the wallthere is a 5 ft x 5 ft 2 in. cast-in-placepost-tensioned ring beam (Ring 1).

This ring beam receives the Iateralforce of the tie beams and converts theload into a large ring tension force of7500 kips.

Lower cone on ground—An approxi-mate 20-ft thick layer of soft and weakclay was removed and replaced by com-pacted granular backfill.

Between the rock and/or till and thecast-in-place 1-ft thick concrete apron,a 2 to 3-ft layer of compacted crushedstone was placed.

This aggregate is used as a base forthe slab and for draining any groundwater inside the bentonite wall.

The concrete slab on ground is rein-forced with mild steel and subdividedwith control joints.

Reclaiming chamber and tunnel—These substructures were built usingcast-in-place reinforced concrete. Twopoints are worth mentioning:

1. The top cone of the 32-ft diame-ter reclaiming structure supports theweight of the clinker pile, totaling 13,-500 kips.

2. The tunnel was built using openexcavation at the higher end, steel sheetpiling and cofferdam protection at themidsection, and rock tunnelling com-bined with shotcrete solidification ofthe rock faces at the lower end.

Connections

Three main types of connections wereused in the structure:

1. Welded connections between pre-cast elements to secure erection stabil-ity and transfer of forces acting at thisloading stage.

2. Cast-in-place concrete connec-tions with overlapping reinforcing steeldowels to insure continuity and mono-lithic behavior.

3. Post-tensioning to join the seg-ments of Rings 3 and 4.

Design Schedule andConstruction Sequence

Preliminary design alternatives and es-timating were conducted from Januaryto March 1973. Final design was ac-complished between August 1973 andFebruary 1974.

The foundations and undergroundwork were done from October 1973through May 1974. The precast ele-ments were fabricated between Novem-ber 1973 and June 1974. Erection ofthe precast elements started in May1974 and concluded in September 1974.

Due to construction strikes, the jobsite was shut down during August-Sep-tember, 1974, and erection of the roofhad to be postponed until June, 1975.

Erection sequence

Figs. 7 through 20 show in pictorialform the various construction stages of

PCI JOURNAL/January-February 1976ҟ 59

Fig. 7. Completed foundation showing pile caps and inner ring beam.

Fig. 8. Precast post-tensioned tie beams connected to anchorage blocks andinner ring beam.

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Fig. 9. Close-up of precast tie beams and anchorage blocks.

Fig. 10. Slanted precast V-columns being connected to anchorage block.

PCI JOURNAL/January-February 1976ҟ 61

Fig. 11. Lower cone elements showing slanted precast V-columns and precasttee slabs.

Fig. 12. Precast wall panel erection.

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Fig. 13. Lifting of precast wall panel.

Fig. 14. Inside view of lower cone and partial wall panel erection.

PCI JOURNAL/January-February 1976ҟ 63.

Fig. 15. Partially completed cone and wall.

Fig. 16. Structure ready for roof erection.

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Fig. 17. First two roof units being lifted into place. Note that the segmentswere erected in pairs to keep the structural loads balanced.

Fig. 18. Close-up of temporary erection tower.

PCI JOURNAL/January-February 1976 65

Fig. 19. Partially erected roof.

the storage silo from casting of thefoundation to roof erection.

The first precast units to be erectedwere the radial tie beams, connectingthe inner at-grade tension ring with theperimeter ring. After these were post-tensioned, erection of the V-shaped in-termediate columns got underway. Theerection of the above-grade sloped floorslabs followed, as soon as several of theV columns had been erected andbraced.

After concreting the floor slab, ex-terior column/wall panels were erect-ed, all site concrete joints were cast,and the lower tension ring was seg-mentally post-tensioned with tendonsanchored at quarter-points around theperimeter of the silo.

Initially, the upper tension ring wasonly 50 percent post-tensioned, withthe balance of the prestressing force tobe applied after the dead load of theroof slabs had been added, and the roofhad become self-supporting.

To provide temporary support forthe ends of the roof slabs at the centerof the silo, a 150-ft high square scaffoldwas erected, extending from the top of

the reclaiming chamber at the base ofthe silo, to the peak of the roof. Builtup of a series of 14 x 14 x 14-ft cubes,the scaffold rested on sand jacks, andwas cable guyed at three levels witheight tendons per level, each tensionedto 10,000 lbs.

Erection of the silo superstructure,including the perimeter column/wallpanels, and the center bearing scaffoldtower, was finished by the end of July,1974. However, a series of strikes,coupled with winter weather, stallederection of the roof slabs until June1975.

Using two 200-ton cranes, the con-tractor erected the 64 roof slabs in onlynine working days, simultaneouslyerecting two slabs at a time to keep thestructure balanced. The units were tem-porarily supported at the perimeter onthe wall and at the center of the scaf-fold.

After the required concrete strengthwas attained in the concrete compres-sion ring at the peak of the roof, thescaffold jacks were lowered, and thescaffold cubes were withdrawn bycrane, through the opening in the roof.

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Fig. 20. Storage silo nearing completion.

The final 50 percent of the post-ten-s?oning was applied to the upper ten-sion ring before decentering. The pre-cast closure slab was then placed atopthe s'lo peak compression ring.

The structure was completed in Sep-tember, 1975,

Concluding Remarks

Comparative cost figures worked out tobe as follows:

Structural steel bid ..................... $33.40 per ton of clinker

Precast prestressed alternate ............. $31.60 per ton of clinker

The precast prestressed design re-sulted in a total savings in constructionand operating costs of $225,000.

The structure has been in operationsince October, 1975. Because of thevery unusual loads and temperature ef-fects imposed on the structure, an ex-tensive observation and surveying pro-gram extending for about 2 years wasspecified and is now in progress. Re-sults of this study will be reported uponin the future.

This structure was honored in the1975 PCI Awards Program.

In conclusicn, it is felt that a verylarge and heavily loaded structure wasdes*gned very efficiently and economi-cally using only five different types ofprecast concrete components in com-bination with mild steel reinforcement,prestressing, and post-tensioning tech-niques.

Finally, it is believed that the abovedesign and construction method can beapplied advantageously to similar silostorage structures.

Credits

Structural Engineer: William E. Pery,P. E., Kilborn Engineering, Ltd.,Toronto, Ontario.

General Contractor and Supplier ofPrecast Prestressed Concrete: Franc-on, Division of Canfarge Ltd., Mon-treal, Quebec.

Construction Manager: Cipriano DaRe.Post-Tensioning: Potenco, Inc., Mon-

treal, Quebec.Owner: Canada Cement LaFarge Ltd.,

Montreal, Quebec.

PCI JOURNAL/January-February 1976ҟ 67