spiral ramp

Spiral Ramps forParking Structures —The

Prefabricated SolutionThe New Parking Facility at

Fornebu Airport, Oslo, Norway

Sven AlexanderChief Structural Engineer

' Ostlandske Spennbetong A.S.Honefoss, Norway

As air traffic keeps increasing, thedemand for parking spaces at air-

ports continually grows, In order tosolve the parking problem at FornehuAirport, Oslo, it was necessary to createa parking area of about 54,000 m2(580,000 sq ft).

To meet this challenge, it was decidedto build a main parking structure (seeFigs. la and 1b) with overall dimensionsof approximately 81.5 x 96.0 in (270 x315 ft) comprising six levels of parking.In addition, there is a wing with threelevels of parking, roughly 48 x 48 m (155x 155 ft).

Experience has shown that it is ad-

vantageous to move the transportationramps outside the parking structurewhen the size exceeds a certain numberof parking spaces. Otherwise, the trafficbecomes too congested in the parkingareas, Consequently, it was decided touse spiral ramps located outside themain structure to bring the traffic in andout of the parking facilities.

For the detailing of the structure, ourfirm used the recommendations in thePCI publication "Survey of Precast Pre-stressed Concrete Parking Structures,"Research Project No. 7. This publicationprovided answers to many questionsthat arose and is highly recommended.

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The author presents the design, production anderection features of a precast prestressed concreteparking structure recently completed at FornebuAirport in Oslo, Norway. A key element in the designof the facility was the use of prefabricated spiral ramps.

Fig. 1 a. Spiral ramp at night with main parking structure in background.

Fig. 1 b. Overall view of parking structure on a winter day.

PCI JOURNAL/March-April 1988 53

1 in=25.4 mm

600 ;. 1200

125

Fig. 2. Typical cross sections of ledger beams and double tees.

The Main Parking StructureThe basic grid for the main structure

was chosen to be 16 x 7.2 m (52 ft 6 in. x23 ft 7 in.). To obtain the necessaryslopes for drainage, the foundations forthe columns were lowered in every sec-ond tier, thereby maintaining identicalcolumns on the entire structure. Thisconfiguration did not create any prob-lems for the general contractor on site,but was a major simplification in theproduction, transportation and erectionof the columns.

The frame consists of prefabricatedcolumns, ledger beams and double tees.The cross section of the interior columnsin the five-story main structure is 500 x500 mm (1 ft 7% in. x 1 ft 7% in.), whilethe facade columns and the columns inthe two-story wing is 400 x 400 mm (1 ff3% in. x 1 ft 3 3/4 in.).

The columns in the main structurewere prefabricated in one piece withoutfield splicing for a total height of 16.15m (53 ft). The cross sections of theledger beams and double tees are shownin Fig. 2. The ledger beams span ap-proximately 6.4 m (21 ft), and the doubletees have a span of about 15.5 in (51 $).

In order to keep the torsion in theledger beams within acceptable limits,two methods were used;

1. For the outer beams, with supportledge on one side only, the ribs of thedouble tees were welded to the ledge,and a wedge was put between the top

flange of the double tee and the side ofthe beam. This is a steel wedge which iswelded to the beam only. The supportdetail is shown in Fig. 3. This entiresupport welding was carried out whilethe double tee was still hanging fromthe crane cable, just barely touching thebeam. The welds on the support for thedouble tees, and the anchorages of thesteel plates are designed to transmit thehorizontal force necessary to counteractthe torsion induced in the beam by thetotal dead load of the slabs, plus that ofthe live load they carry. All cast-in steelunits are corrosion protected, and allwelds were treated with corrosion re-sistant paint.

2. For the interior beams, with sup-port ledges on both sides, it was possi-ble to avoid steel wedges and thewelding. This was done by making sure

Fig. 3. Support detail on spandrel beam.

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Resultant load

ELastomericbearing pads

Resultant support

-Hreuc Lion

Fig. 4. Support detail on interior beam.

Fig. 5. Plan, location of shear walls.


105 100

Imo] c3

^J _J^^ r 3

- Threa ded bars anchoringc; r the see plate

4 ^+ o_, i ;___ ;^ Steel plate

_ _ _ _ ^ _ _ __ Threaded bar anchoreda 1- , Irs F i In the insert

c I ,',r' ' Threaded insert

I i I I ^ F

I' ^' 1in=25.4 mmto Ld

Fig. 6. Adjustable connections in horizontal joints.

the torsional moments could be carriedby the supports for the beam with ac-ceptable deformations of the elas-tomeric hearing pads. Consequently, itbecame principally a matter of thegeometry of the ledges relative to thegeometry of the supports. The torsionalmoments were, of course, only due tolack of symmetry of the live loads,for — during erection — dead load ofslabs on one side only. The arrangementis shown in Fig. 4.

The double tees were cast with anadditional flange thickness. Normally,the flange is 50 mm (2 in.), but on thisproject the thickness was increased to 80mm (3 1/4 in.) This was done for structuralreasons, and eliminated the need forconcrete topping. Consequently, a lessexpensive wearing surface could beused. This will be elaborated upon later.

The stability of the structure isachieved by means of relatively fewshear walls, strategically located. The

chosen location of the shear walls elim-inated the need for expansion joints,which are both expensive and might bepotential problem areas. The locationsof the shear walls are shown in Fig. 5.

For the shear walls crossing the di-rection of the beams, the columns werecast as an integral part of the shear wall.The shear walls in the direction of thebeams were placed between the col-umns, but with relatively heavy weldedconnections to the columns, the col-umns were activated as part of thestabilizing structure. The inherent ad-vantage with this solution is that thedead loads of the building will heutilized in stabilizing the structure,thereby reducing the demand on allother connections in the shear walls.

In the horizontal joints of the shearwalls, a conventional bolted connectionwas used. The connection facilitates theadjustment of the elements during erec-tion, and will transfer both compressive

56

and tensile forces. This connection isshown in Fig. 6.

In summary, the main part of theparking facility is a rather traditionaltype of structure. However, the in-teresting elements are located outsidethe building, namely, the prefabricatedspiral access ramps, one for traffic en-tering and one for traffic leaving theparking facility. Although spiral rampshave been used before, to our knowl-edge they have never been used in pre-cast concrete construction.

Why Prefabricated

Spiral Ramps?In the current building industry in

Norway, there is an acute labor shortage.This situation may not last, but at thetime the parking facility was being bid,the general contractor was not surewhether such a facility could be built asa cast-in-place structure. The only re-course left was to use prefabricationwhich was probably correct as far as themain structure is concerned. However,it took some time before the concept ofprefabricating the ramps was fully ac-cepted. In hindsight, we can sayemphatically that prefabrication notonly solved the labor shortage problem,but also provided significant savingsboth in construction time and directcosts.

A comparison of the expendituresshows that the total cost of the rampswith the prefabricated solution wasabout 70 percent of the cast-in-placescheme. There were three major con-cerns that were important in the choiceof this solution, namely:

1. Cost2. Time3. ResourcesAs far as time is concerned, it is esti-

mated that a total savings of between 3and 4 months per access ramp was at-tained. This means that the parkingstructure can he put into service be-tween 6 to 9 months earlier. The

Fig. 7. Erection of spiral ramp.

economic implications of this earlierusage is evident.

The Spiral Ramps

Fig. 7 is a picture taken when theerection of one spiral ramp was nearingcompletion. The central core is 6 m (20ff) in diameter, with an outside spiralledge. The total circumference of thecentral core is divided into four precastsegments, each covering one-quarter ofthe circle. The thickness of the coresegments is 300 mum (12 in.) and theledge section is 200 x 250 mm (7 7/s x 97Ain.). The main dimensions of a centralcore segment are shown in Fig. 8.

The spiral deck part of the ramp ismade of warped, wedge shaped panels,each unit covering 15 degrees of the cir-cle. The ledge on the central core andthe deck panels are, of course, horizon-tal at the entry to each level of the park-ing structure. These solid deck panels

PCI JOURNAL/March -April 1988 57

A

300 200'^ 1 }Oq

t J Ln

9o°

Section

A 4--^ 1 in 25.4 m mPlan

s

'

i

' o^ , o

Section A-A

Fig. 8. Segment for central core.

span approximately 5.8 m (19 ft) andhave a thickness of 240 mm (9 1/2 in.). Astack of panels is shown in Fig. 9.

The outer supports for the spiral deckslabs are at this project a steel structure.The slabs are supported on steel beamsfollowing the spiral curvature of the ac-cess ramps.

Originally, the wedge shaped deckpanels in the spiral ramps were plannedas flat slabs, with the difference of ele-vation in the joints to be evened outwith a concrete topping. However, asthe geometry of the spiral was moreclosely examined we found that thegeometry of a spiral deck is actuallyrather simple, as all radial lines are hori- Fig. 9. Stack of panels.

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Fig. 10. Framework of form for warped deck panels. Beams of variable depth were laidout in a fan-like pattern on a horizontal surface.

Fig. 11. Geometry of warped deck panels. Although the underside surface of the panelswas developed by straight fines, the actual surface was curved.

PCI JOURNAUMarch-April 1988 59

Fig. 12. Casting of prefabricated panels.

zontal. Similarly, when unfolding thecylindrical central core, the spiral ledgebecomes a straight line, which can eas-ily be transferred to the curved mold.

Considerations forMold Design

Because the geometry of the spiralramps was relatively simple, the moldfor the warped panels could easily beproduced by constructing the frame-work for the bottom of the form withbeams of different depths being laid outon a horizontal surface in a fanned pat-tern, as shown in Fig. 10. These beamswere then covered with a 4 mm (^'is in.)thick steel plate, flexible enough to fol-low the curvature created by the beams.Thus, the underside of the deck panelsassumed a smooth surface of an apparentwarped shape, but actually consisted ofstraight lines in the pattern shown inFig, 11.

The warped deck panels had to have a

"right" and a "left" twist, due to thedifferences in direction of the two spiralramps. However, it was not possiblewith one form to find a practical way toreverse the twist. Consequently, a moldwas made for the "right" twist, and onefor the "left" twist. The choice of twoforms also made it possible to meet thedelivery schedule with only one castingin each form every day. Fig. 12 showsthe two forms in use. They were built ona tilt table that was slightly raised duringproduction in order to get the averageslope of the concrete surface as flat aspossible.

For the central core it was just as sim-ple. As mentioned before, when thecylindrical central core was unfolded,the spiral ledge became a straight Iine,as illustrated in Fig. 13. The bottom ofthe form was then constructed as one-quarter of a cylinder, laying on its"back." The measurements to the spiralledge, determined as the distances tothe straight line on the unfolded central

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rig, ii. untoiaea central core.

core, could then be transferred to thecylindrical form.

Both spiral ramps should be curvingleft, one going up and one going down.Since the recess in the mold forming theledge was a fixed part of the mold, thiswas achieved by extending the form onboth sides of the recess, and then duringproduction switching the top and bot-tom limitations of the mold from oneend to the other. The ledge thenchanged direction from up to down.

In order to facilitate all the variationsneeded, the horizontal joints werestaggered. The ledge could then he atthe bottom of the element, curving up,or at the top and curving down. Theelements that should have the ledgehorizontal were produced with the re-cess for the ledge being built outside theend of the form. The stagger of the jointsis shown in Fig. 13.

Thus, only one form was needed toproduce all the elements for the centralcore of the spiral ramps. In actuality, all48 elements for the two central coreswere produced in this one mold during51 working days, allowing a few days forthe necessary changes in the form.

The fabrication of the mold and the cast-ing operations were executed verysmoothly.

The JointsThe horizontal forces acting on the

spiral ramps were relatively small, sincethe ramps were not considered contrib-uting to the stability of the main parkingstructure. Consequently, the centralcore only had to take care of the stabilityof the spiral ramp itself. For the sake ofsimplicity, though, the same connectiondevice was used in the horizontal jointsas in the shear walls in the main parkingstructure (see Fig. 6).

The transfer of shear forces in thevertical joints is accomplished bywelded plates combined with a frictioncontribution From the grouting of thejoints.

The connection between the warpeddeck panels and the central core, includ-ing also the connection to the steelframe on the outside, was not requiredto transfer forces of any significant mag-nitude. Bolted connections were con-sidered, but the necessary inserts andbolts complicated the production pro-cess, and also placed considerable de-mands on the accuracy of the productionand erection operations. Consequently,welded connections were chosen, withsteel plates cast in both the central coresegments and the warped deck panels.

PCI JOURNALMarch-April 1988 61

Fig. 14. Proud foreman showing perfect fitof components of spiral ramp.

Unfortunately, the welded connectionscreated some additional finishing work.Therefore, it is suggested that on futureprojects bolted connections be usedsince they are structurally superior andsafer than welded connections.

Erection Time.__ The erection of the ramps proceededvery smoothly. The curved elements ofthe central core were erected at a rate ofabout six per day. The erection of thesteel structure (which was done byothers) took some time, but the neterection time was about 2 weeks for oneramp. In the beginning, we erected thepanels at a rate of about fifteen per day.The total erection time for one spiralramp, five stories high, then became ap-proximately 5 weeks, This time periodcan probably be reduced as experience

is gained. In Fig. 14 a proud erectionforeman is showing how perfectly all theelements fit.

Cost of Spiral RampsAs mentioned earlier, the method of

prefabricating the spiral ramps not onlysolved the problem of labor shortage,but also provided significant savings intime (3 to 4 months per five-story ramp).From an economical viewpoint, a com-parison of the bids with the actual con-struction cost shows that the cost of theprefabricated spiral ramp is about 70percent of the cost of a cast-in-placesolution.

The basic dimensions of one spiralramp is as follows:Diameter of central core: 6 m (20 ft)Diameter of outer steel frame: 18 m (60

ft)Width of traffic lane: 6 m (20 ft)Story height: 3.1 m (10 ft 2 in.)Total height: 16.4 m (53 ft 9 in.)

The total cost of one such access ramp,including the central core, the deckpanels, the steel structure at the outerperimeter, all produced, transported anderected, but excluding the foundations,the wearing surface in the traffic lanes,the railing and the roof structure, wasapproximately NOK 2,100,000($325,000). This amounts to NOK 1.850per sq m of access road, or $26.50 per sqft.

The use of a concrete frame instead ofa steel frame at the outer perimeterwould not change this figure signifi-cantly. Fig. 15 is a graphical presenta-tion of a cost analysis comparing prefab-ricated and cast-in-place spiral accessramps.

Wearing SurfaceAs mentioned earlier, the double tees

were cast with an additional flangethickness. This was done for structuralpurposes to avoid a concrete topping. Asa result, it opened up the possibility touse mastic asphalt as a wearing surface.

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100 Finishing90

80 Materials

=• Finishing60 Erection

n 50 : Transportation

.5 40 = Formwork

30 Materials LI and

2a ; Manufacture

10 Rigging

Prefabricated Cast in place

Fig. 15. Cost comparison for prefabricated andcast-in-place solution of spiral ramps.

The use of mastic asphalt wearingsurface is actually rather common inNorway, and over the years the materialhas performed quite well in our ratherfrigid climate, where the use of steelstudded tires is common on all cars forabout 6 months of the year.

The standard procedure for installing

mastic asphalt wearing surface consistsof a bottom layer of 10 to 15 mm (about1/2 in.) thick asphalt concrete whichserves the purpose of a gas releasinglayer (nonadhering) and mastic asphaltlayer of 30 mm (1'/a in.). This method isused for all levels except the roof park-ing deck. There the bottom layer is as

ray. io. iviaau aspria,ir surraces ror interior ano exterior parking areas.


Fig. 17. Erection of spiral ramp. Fig. 18. Closeup of spiral ramp.

Fig. 19, Shot of spiral ramp and main parking structure on a winter night.

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Fig. 20. Overall view of spiral ramp and main parking structure nearing completion.

described above, then follows a masticasphalt membrane of 15 mm ('/a to s/4 in.),

then, as a drainage layer, a nonwovenpolypropylene geotextile, and finallythe mastic asphalt wearing surface. Theinstallation of mastic asphalt in the traf-fic lanes in the ramps is similar to that ofthe interior parking levels, except thatthe wearing layer is placed in two op-erations. Each layer is less than 25 mm(1 in.), but the two Iayers added are alittle more than 40 mm (I in.). Sectionsthrough the two types of mastic asphaltare shown in Fig. 16.

The mastic asphalt membrane has amaximum aggregate size of 10 to 6 mm(% to r in.), a filler content of about 40percent and a binder content of about 11to 12 percent. The bitumen is at straightrun 40/50 penetration. The mastic as-phalt wearing layer has a maximumaggregate size of 10 mm (% in.), about 30percent filler and a binder content of 7to 8 percent.

The mastic asphalt in the membraneas well as the wearing course has novoids and is waterproof The wearinglayer has joints for every 350 to 600 m2

(3763 to 6452 sq ff) in order to take careof temperature differentials. Betweenthe double tees and before the asphalt is

laid, the gap is sealed with a strip ofasphalt felt. The mastic asphalt is laidby a beam paver and by hand at a tem-perature of about 220°C (428°F).

The use of mastic asphalt eliminatescostly joint details in the concrete slab.Other advantages are wearing resis-tance, flexibility and skid resistance.Any large uneveness of the concretesurface must be smoothed out with con-crete or ordinary asphalt before apply-ing the mastic asphalt.

The cost of a mastic asphalt surface,adding the cost of the increased flangethickness of the double tees is approxi-mately NOK 140 per sq m ($2 per sq ft).This is 5 to 10 percent less expensivethan a concrete topping with joints. Themain advantage, however, is the elimi-nation of complicated joint details thatare potential problem areas. Normally,the only maintenance required is addingnew top layers as the old surface is worndown by traffic. In the access rampsdeterioration can be expected between5 to 10 years but in other parts of thegarage the life expectancy of the surfaceis considerably longer.

Figs. 17 through 20 show various con-struction phases of the spiral ramp andmain parking structure.


ConclusionWe believe that the new parking

structure at Fornebu will serve its pur-pose efficiently and economically byproviding a safe facility with a durableand long life. In the main structure nonew techniques have been tried. Theprefabricated spiral ramps, however,were new to us. The geometry and pro-duction concept presented a challenge,but that is now behind us, as the rampshave been erected. The solution of pre-fabricating the spiral ramps is notunique, and can he adapted to anyparking structure. We are confident ofthe success of this structure, both for theairport authorities and public and for usas the producer.

CreditsOwner: Civil Aviation Administration

(Luftfartsverket), Oslo, Norway.Architects: Nansen and Eskeland, Engh

and Seip, Oslo, Norway.Consulting Engineer: Taugbol and

Overland A/S, Oslo, Norway.Project Administration: Construction

Management A/S, Oslo, Norway.General Contractor: A/S Veidekke, Oslo,

Norway.Mastic Asphalt: A/S Spesialdekker,

Kjeller, Norway.All the prefabricated concrete ele-

ments were designed, manufactured,transported and erected by OstlandskeSpennbetong A.S., HGnefoss, Norway.

Note: Discussion of this article is invited. Please submityour comments to PCI Headquarters by December 1, 1988.

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spiral ramp

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