Thin-Shell Concrete Dome Built Economically With Rotating Forming and Shoring System_tcm45-342154

7/27/2019 Thin-Shell Concrete Dome Built Economically With Rotating Forming and Shoring System_tcm45-342154

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Spanning 270 feet, the thin-shell concrete dometopping the Sundome sta-dium in Yakima, Wa s h i n g-

ton, has 24 wedge-shaped seg-ments arranged in a radial patternlike the pieces of a pie. But only six

wood forms we re nee ded to cas tthe 24 segments because of an in-n ova t i ve rota ting forming ands h o ring system.

The Su n d o m e, which opened tothe public in Ja n u a ry 1990, is a mul-tipurpose exhibition and sport facil-ity thats part of the Ce n t ral Wa s h-ington State Fa i rg ro u n d s. The entire9 0 , 0 0 0 - s q u a re-foot stru c t u re is re i n-f o rced concrete including the exte-rior walls, which are made of insu-lated concrete masonry units.

The stadium was designed bys t ru c t u ral engineer Jack Chri s-tiansen, who also designed Se a t t l esKingdome sports stadium, complet-ed in 1976. The thin-shell concre t edome capping this stadium is morethan twice the size of the Su n d o m eroof and is the largest concre t edome in the world. Its 670 feet in di-ameter and has forty 5-inch-thickc o n c rete segments.

Co m p a red with monolithic con-c rete domes, segmental t hin-shellc o n s t ruction offers greater form i n ge c o n o m y. The segments are cast in-dividually so forms can be re u s e d .The Sundome roof was cast usingsix site-built wood forms support e don a movable aluminum shori n gsystem. This system allowed use ofthe forms four times to cast thed o m es 24 segments (see dra w i n g ) .

Thin-shell concrete dom e builteconomically with r otatingfor ming and shor ing systemTwenty-four-segment dome uses only six forms

BY FRANK RANDALL AND ANNE SMITH

Drawings showpositioning of thesix forms duringeach of the fourcasting stages.The coloredsegments instages two, t hree,and four indicat ethe new formpositions.

Wood saddles resting on aluminum st rongback beams are ready for joists andcurved plywood deck.


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Structural designThe Sundome roof has a rise of 40

feet and a maximum clear heighta b ove the floor of 8 0 feet. Its 24identical wedge-shaped segmentsa rch to a compression ring at thec rown of the roof and their bases ares t a b i l i zed by a post-tensioned con-

c rete tension ring supported on 24re i n f o rced concrete columns. Thesegments are 4 12 inches thick in thel ower 18 feet and 3 inches thick therest of their length. Each segment isdoubly curved like a saddle. Thisc u rva t u re increases the bucklingstability of the dome. To further pre-

vent buckling and increase thed o m es load capacity, 12-inch-thick,30-inch-deep concrete ribs re i n-f o rce the segment edges.

Forming and shoring details

The six forms used to cast thedome segments each consisted ofwood saddles supporting a curve d

deck. Wo rkers built the decks o fs t raight 2x12-inch wood joistssheathed with 34-inch-thick ply-wood. To produce the saddle-shaped curva t u re re q u i red, they an-gled the joists according to detaileddesign dra w i n g s.

Su p p o rting the forms we re alu-minum shoring towers consisting of6-foot truss sections with 4 12- f o o textension legs topped by 5- and 6-foot shoring frames with tubularb racing. Wo rkers assembled the

s h o ring frames on the ground thenstacked them on the trusses usingc ra n e s. St rongback beams on theu p p e rmost shoring frames support-ed the wood saddles.

Shell segments we re cast at inter-vals of about 60 around the roof toe q u a l i ze thrusting at the tensionand compression ri n g s. Cast beforesegment erection, the 10-foot-di-ameter compression ring was sup-p o rted by a stationary shoring tow-e r. The tension ring was

post-tensioned after casting andc u ring of the first six segments.

Wo rkers pumped concrete foreach shell segment in a continuouspour starting at the tension ring andending at the compression ring. It

took 4 to 5 hours to place concre t efor one segment. The concrete ri b swe re cast the following day.

Shoring rolls on c asters

After casting the first six shell seg-

ments and allowing them to attainthe re q u i red strength, work e r ss t ripped the form w o rk and ro l l e dthe entire forming and shoring as-sembly to the next casting position.To lower the shoring, workers usedten 54-inch hyd raulic ra m s.Clamped to truss extension legs, therams we re activated with a singlepump until they supported shori n g

Although fabric domes have be-come popular in recent years fortopping large stadiums, concretewas chosen for the Sundome roofbecause it offers the following ad-vantages:

Economy. Fabric domes aremore expensive than thin-shell

concrete domes. Thin-shell con-crete construction uses minimumquantities of concrete and rebar.

The Sundome roof cost only $14per square foot, including materi-

als and labor (see table). The av-erage cost of a fabric dome, ei-ther air-supported or cable truss,is more than $30 per square foot(Ref. 1).

Fire resistance. Concretedomes are fire resistant. This re-duces fire protection system re-quirements and lowers insurance

costs. Noncombustible mem-brane materials are available forfabric domes but these materialsare expensive.

Durability. Concrete domes aredurable, weather resistant, andrequire little maintenance. Fabricdomes, on the other hand, can

fail if the membrane materialused is inadequate for the roofsperformance needs. This is espe-cially true for long-span roofs. Thecost of replacing a failed mem-

brane can exceed the total costof the original roof including thesupport structure (Ref. 1).

Can be built with local materi-als. Most of the materials re-quired to build a concrete domeare readily available locally. Andlocal labor usually can handle theconstruction.

CONCRETEVERSUS FABRIC DOMES

This constr ucti on cost summary for the Sundome includes materials, labor,and equipment for t he roof shell, ribs, and tension ring (Ref. 3). It does notinclude costs for insulation and roofing, columns, foundations, or walls. Totalconstruction cost of the Sundome was $6.5 million.


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loads and re l i e ved jacks on truss ex-tension legs.

After raising the jacks, work e r sl owe red the shoring onto doublealuminum strongback beams sup-p o rted on heavy-duty swivel casters.

The casters rode in 8-inch-widesteel channel sections that had beenshop-bent to the re q u i red ra d i u s.Wo rkers bolted together the 12-foot-long sections to achieve thelength needed to roll the shoring to

the next position.While two forklifts pulled the

c a s t e r-mounted shoring, work e r sused steering levers attached to thecasters to help guide caster move-ment. Once the shoring was in po-sition, the hyd raulic rams raised thes h o ring to the proper eleva t i o n .Wo rkers then re m oved the channel

sections and caster assemblies. Thisrotation process was repeated twom o re t imes to complete casting ofthe 24 dome segments.

References

1. Dr. David H. Geiger, A Cost Com-parison o f Roof Sys tems for SportsHalls.

2. Ja ck Christiansen, Hyperbolic P a-rabo loid Thin-shell Conc rete S truc-tures for Sports Buildings.

(The ab ove pa pers appe ar in S pac eStructures for Spo rts Buildings: P ro-

ceedings of the International Colloqui-um on Spa ce S tructures for SportsBuildings , Oc tob er 27-30, 1987, B ei-jing, C hina, e dited by Tien T. Lan a ndYuan Zhilian, Science Press, 1987)

3. Ja ck Christiansen, Eco nomy of Hy-perbolic P arab oloid C oncrete S hells,Co ncrete International, Augus t 1990,American Concrete Institute, P.O. Box19150, De troit, Ml 48219.

Cre d i t sSt ru c t u ral engineer: Jack Chri s-

tiansen, Se a t t l eA rchitect: Loofburrow Arc h i t e c t s,

Yakima, WAGe n e ral contra c t o r: Gi l b e rt H.

Moen Co., Yakima, WAFo rm w o rk / s h o ring subcontra c-

t o r: The Bu rke Co., Kent, WA

After workers lowershoring onto swivel

casters, like the oneshown here, t hey can

roll it t o the nextposition. The caster

rides in a curvedsteel channel. A

worker guides thecaster using an

attached steeringlever.

Radiating from a c entral compression ring, the first six shell segments have justbeen cast. After stripping formwork, workers will rotate the entire forming andshoring assembly to the next casting position.

Publication # C910490Co py right 1991, The Ab e rd e e n

Gro u p. All rights re s e rve d

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Thin-Shell Concrete Dome Built Economically With Rotating Forming and Shoring System_tcm45-342154