arena fonte nova - low prestress for a lightweight roof

8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof

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10 TheStructuralEngineer

November 2014 Arena Fonte Nova

Project focus

Arena Fonte Nova,Brazil: low prestress for a

lightweight roofJorge Chenevey Project Manager,RFR Stuttgart

Yu HuiProject Director,RFR Shanghai

Mathias Kutterer Offi ce Director, RFR Stuttgart

SELLARPROPERTYG

ROUP


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www.thestructuralengineer.org

IntroductionDesign criteria for stadia have expanded significantly

in the past 50 years: to the basic requirements for

stands and circulation has been added the need

for numerous extended features and facilities for

spectators around the sports venue. The stadium

roof, which must provide effective protection for al l

stands against sun and rain, has become one of the

primary aspects of stadia design, in terms of both

architectural quality and cost.

An obvious criterion for roof structures, which

cover ever greater spans, is roof self-weight, which is

related to costs. In the past two decades, s poke-

wheel systems have allowed designers to achieve

record-low self-weights for roofs spanning ever

greater distances. Work to develop and optimise

this type of structure continues, giving rise to a large

number of new and sometimes unique solutions

each year.

One such solution is the Arena Fonte Nova (Figure

1)in Salvador de Bahia, Brazil, which was inaugurated

in April 2013. The design of the arena, which hosted

several matches during the 2013 Confederations

Cup and 2014 FIFA World Cup, includes a lightweight

roof based on a spoke-wheel system. This relies

on the combination of a steel structure and cable

system with comparatively low prestress. As the

authors show, its hybrid structural system employs a

series of features that make the Arena Fonte Nova a

unique stadium, not least in terms of cost.

Background

On 25 November 2007, only a month after being

named one of the venues for the 2014 World Cup,

a tragic accident occurred at the existing Estadio

Fonte Nova, with the failure of part of the concrete

stands. The accident killed seven people and injured

40. Faced with the need to find a solution quickly,

the authorities decided to demolish the old stadium

and build a new one, with a capacity of 56 500. An

international design competition was launched.

The winning design was provided by German

architects Schulitz in partnership with structural

engineering firm RFR. The design included a new

lightweight roof, covering all the new stands, which

kept the original, horse shoe-shaped bowl with

an opening towards Toror Lake at the southern

end. The roof is supported by short columns on the

highest level of the concrete stands; the opening on

the south side required 13m high columns (Figure2).

By the end of 2010, the old stadium had been

demolished, meaning the foundations of the new

Arena Fonte Nova could be built. The stadium is

situated near Pelourinho, the oldest part of the city of

Salvador and a UNESCO World Heritage Site.

Geometry

The Arena Fonte Nova is built on a north-south

orientation. The lightweight roof, which covers the full

perimeter of the stadium as well as part of the inner

area, is entirely supported on the concrete stands.

The roof structure, which is oval in design, is

approximately 260m long by 216m wide (Figure3). It

is divided into 36 bays, while the concrete stands are

divided into 72 bays. Instead of skipping one axis and

only putting roof columns on every other concrete

column, the roof axes were shifted by half a bay and the

radial cables were split at the outermost node in order to

bring an equal load down to the two adjacent concrete

columns. This offset required the inclusion of split nodes

at each radial roof end. These transfer the loads from

the upper and lower cables to the concrete axes.

The inner part of the roof, covering the stands, is

clad with a polytetrafluoroethylene (PTFE)-coated

glass fibre membrane, while the outer part is covered

by a lightweight metal deck. Radially, the roof slopes

outwards by approximately 3.5 and 1.5 in the inner and

outer parts respectively. The south side of the stands

has a large opening, which the roof structure flows over

on long, slender columns.

Synopsis

The new Arena Fonte Nova in Salvador de Bahia, Brazil offers an innovative

lightweight solution for a large-span roof. The spoke-wheel system is

enhanced by drawing on stiffness and load-bearing capacity from bracings

in the vertical plane of the tension rings and in the horizontal plane of the

compression ring. This allows prestressing to be reduced to 50% of the

usual level.

Other important features that simplify the design and reduce costs are

concave radial cables with compression elements, using fewer arches with

non-uniform membrane prestress, and a flat overall roof slope. The low

prestress brings cost savings both by reducing the weight of all primary

structural elements and also by incurring smaller forces during installation.


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Project focus

Designing a lightweight roofSeparation of skin and structureThe architectural preference for a low, flat roof

without predominant structural elements, which

would integrate well into its hilly and densely

populated setting, led to a strictly horizontal three-

ring system with a strong outer compression ring and

two inner tension rings.

The further geometric layout of the roof was

governed by several somewhat contradictory

criteria. Spectator comfort required the position of

the roof skin to be as low a s possible, while structural

effi ciency requires height. Other criteria dictating

the overall form and slope of the roof were the

aerodynamics and rainwater evacuation. To all ow

the maximum amount of room for simultaneous

adjustment and optimisation of all these criteria, the

design team decided to separate the roof skin from

its supporting structure by positioning it midway

along the upper and lower cabl e chords (Fig.1).

This meant that the skin surface would b e at a low

level, providing the maximum amount of protection

from sun and rain, while the structural height between

the upper and lower tension ring could b e increased

and optimised without any compromise (Figure4).

Parametric approach

To accommodate frequently changing architectural

features, and to maximise structural optimisation,

successive calculation models were based on

several parameters, each entered as an independent

input, e.g. slope, convex/concave radial shape, height,

and bay division.

This allowed the team to easil y produce, study

and compare a wid e range of options. From the very

early stages, the slope was carefully assessed using

computational fluid dynamics simulations in order

to provide minimum wind resistance while offering

an adequate drainage angle. The parametrisation

of the model also a llowed control to be maintained

over each node and el ement numbering. This proved

to be of great benefit when exchanging 3D model

information with the contractors.

The curvature of the radial cables turned out

to be one of the most interesting parameters in

terms of stiffness and effi ciency. It was describe d

as the f/L ratio varying between concave and

convex in the range of 0.6 to 0.6 (Figure5). The

output parameter was the deflection under a

characteristic uniform load, measured at the tip of

the cantilever (black line) and at mid-span (blue

line). The mechanical interrelationship is clear in

the diagram. For the deflection of the tip of the

cantilever, the best configuration is the one which

is almost straight the curvature parameter is

close to zero. For the deflection at mid-span, it is

obvious that the stiffness increases in an almost

linear fashion as the curvature becomes more

and more convex. This is due to the direct load

path: a downward load d irectly activates the lower

radial cable whereas in a convex configuration

the load is first transferred to the upper cable and

then, via a flying column, to the lower cabl e. The

dotted line, with a characteristic system change at

the neutral point (f/L = 0), represents a structural

system with tie hangers at the concave (negative)

range, whereas the continuous lines correspond

to a system with strut hangers in both concave and

convex configurations.

The best compromise for mid-span and end

point rigidity can be found somewhere between

f/L= 0.02 and f/L = 0.03, which corresponds to a

convex stitch of about 1m in both the upper and lower

radial cables.

Hybrid system

The roof is based on a closed-ring cable system

with a compression ring concentrating all the heavy

parts of the structure along the outer perimeter. This

principle the spoke-wheel system allows large

spans while keeping the self-weight down . Although

the principle is not new, the Fonte Nova stadium

roof is innovative due to the fact that it requires

considerably lower prestressing forces than previous

examples.

The compression ring is laid out as a two-chord

horizontal truss, providing high in-plane rigidity for

the entire roof system (Figure6). This counteracts

the non-uniformity of the radial forces, which are due

to the oval shape in plan view and the non-uniform

distribution of wind loads.

The inner roof consists of two tension rings which

are tied in the radial direction to the compression ring

by a set of upper and lower radial cables. Downward

loads (e.g. dead load, rain and wind pressure) are

transferred from the l ower tension ring (consisting

of three, parallel, fully locked 95mm diameter

Figure 1Arena Fonte Nova

roof viewed from inside

Figure 2Longitudinalsection of stadium

Figure 3Plan of roof

ERIK

SALESVANER

CASAES

AG.BAPRESS

RFR


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cables) to the compression ring via the lower radial

cables. Upward loads (e.g. wind suction) activate

the upper tension ring (three, parallel, fully locked

70mm diameter cables) and the upper radial cables

(Figure7). The tension rings are separated by 22mtall inclined flying columns. To increase vertical

rigidity against non-uniform loads and to reduce

the required cable prestress level, the upper and

lower tension rings are braced by C55 cables which,

together with the flying columns, form a truss a ble

to take unbalanced loads. These bracings introduce

relatively high friction forces at the tension rings,

which are absorbed by strong clamp plates.

Similar stadia usually rely on very high tension to

withstand unbalanced loading. The hybrid system

of the Arena Fonte Nova roof, with vertical bracing at

the inner ring and horizontal bracing along the outer

ring, allows lower prestressing (half that of other

similar stadia), a significant reduction in the total cable

tonnage, and the use of smaller and lighter tension

ring nodes. Machined S355 steel plates could be

used for these nodes.

Wind loads

Stadium roofs which include form-found membrane

cladding rarely conform to the standard geometries

found in either the codes or the l iterature for

assessing wind loads. Factors such as roof shape,

concrete bowl shape, openings and scale of the

structure may result in incorrect assumptions being

made about wind load. In particular, in the case of the

Arena Fonte Nova, the la rge opening at the sout h end

of the stands meant that strong winds were expected

to be funnelled into the stadium, creating high uplifts

on the north side; these would not be covered by the

codes.A wind tunnel test(Figure 8)was therefore carried

out by specialist engineers (Wacker Ingenieure,

Germany). Based on the architectural and structural

drawings, Wacker built a 3D p hysical model of the

roof and concrete bowl at its laboratory at a scale of

1:300.

The most important aspects of the stadiums

immediate topography were also modelled. Rough

elements on the tunnel floor were used to simulate

the wider surroundings in order to provide an

accurate assessment of the wind speed profile and

turbulence of the approaching wind. 450 pressure

taps were installed on the top and bottom surfaces of

the cladding.

The wind pressu re coeffi cients obta ined from

the test were later combined with the expected

reference pressure at the project l ocation over thestructural life of the stadium, i.e. a 50-year period. This

resulted in a design gust wind pressure of 0.70KN/m

at a height of 40m. Wacker then produced a series of

plots showing wind pressure and wind suction from

eight different directions, distributed on roof loadi ng

regions. The vertical loads were all multiplied by a

resonance factor of 1.05 a figure calculated from

the eigenfrequencies of the roof.

In addition, a study was carried out on the different

erection phases of the membrane roof. An installation

sequence was agreed upon with the membrane

contractor, and a set of wind loads was provided for

the chosen sequence. This made it possible to detect

critical stages and determine the sizing of temporarybracing elements.

Membrane roof

Reducing self-weight in a large span requires a

cladding material which is l ight and flexible, yet

strong and durable. Membrane textiles such as

PTFE-coated glass fibre comply with all these

requirements: weighing only 1.3kg/m, PTFE-

coated glass fibre is non-combustible and has a

lifespan of 25 years. Some claddings are still in use

even after 30 years or more of service. However,

special care needs to be taken during handling and

transportation. Only limited folding of glass fibre

Figure 4Typical cross-section of roof

Figure 5Convexconcavestudy results

Figure 6Two-chord horizontal compression ring and two braced tension rings

Figure 7

3D rendering ofstructural system of roof

SCHULITZ

RFR

RFR

RFR

RFR


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Project focus

columns, the split cables at the end, and the truss

build-up of the compression ring together bring very

high fail safe redundancy to the structure.

In addition to the basic sizing of all elements,

with suffi cient margin under the ulti mate limit state

(ULS), several failure scenarios were considered:

failure of a lower radial cable; failure of one flying

strut; failure of one flying column; and failure of one

of the three lower tension ring cables. For each

individual scenario:

a non-linear analysis was run under acorresponding worst load case

if the analysis converged (no geometricalcollapse), deflections were verified to evaluate

whether the deformed structure clashed with

the surrounding elements (spectators, concrete

stands etc.)

a member check was then carried out on cables(breaking load) and steel elements (plastification

or buckling)

in case of elements plastfifying or cablesbreaking, a second analysis was run with non-

linear geometrical and material parameters

Figure 11shows the effect of one lower radial

cable failing and demonstrates that the roof offers

alternative load paths while avoiding a clash with

the concrete stands.

Anticipated design life and maintenance

The primary structure was designed with a

minimum lifespan of 50 years. This was achieved

is possible or it may break. For the Arena Fonte

Nova, a type III PTFE membrane, with characteristic

strengths of 7000N/50mm in the warp and

6000N/50mm in the weft directions, was applied to

the whole roof. A series of tests was carried out on

the membrane to ensure that it met all performance

requirements.

Geometrically, the membrane roof consists of 36

bays each divided into five panels. The low number

of arches per bay was compensated for by having

different membrane prestresses in the warp and

weft directions. This allowed an optimum double

curvature to be achieved for the effective wind

pressure and suction.

Each panel (except the first and last) consists of

two arches in the ring direction and two radial trus ses

on each side (Figure9). Downward loads activate the

warp (radial) direction of the membrane, which in turn

introduces compression in the arches. For upward

loads (wind suction), the weft (tangential) direction

of the membrane is a ctivated. This creates bending

in the radial isostatic trusses w hich transfer upward

loads to the flying struts in the direction of the upper

radial cables (Figure 10).

Robustness

The roof structure, with its two inner tension rings

connected to the outer compression ring, offers a

clear primary load path. Failure somewhere along

this path represents one of the critical scenarios.

However, should an element fail, the roof als o offers

several alternative load paths: multiple tension

cables, the flying cross-bracing between the flying

partly by requiring a high level of corrosion protection

for all steel and cable elements. All steel elements

are protected with a paint system reaching corrosion

class C4H according to ISO12944. Cables are

GALFAN-coated with a minimum weight of

300g/m; this offers better corrosion protection

than zinc coating. The cladding was designed with a

minimum lifespan of 25 years.

A maintenance and inspection programme was

also developed. This provides core instructions for

ensuring public safety during use of the building. It

includes a description of the considered loads in

order to assess the structures alteration capacity,

e.g. to what extent audio and lighting equipment can

be modified from the initial design.

The maintenance manual also sets out a

detailed inspection schedule. This includes an

initial inspection, an annual general inspection, a

full inspection every six years, and an exceptional

inspection to be carried out following accidental

loading conditions, e.g. heavy storms close to or

above the 50-year reference wind speed. It also

describes critical details which require special

attention during an inspection (connections

under heavy loadings, cable clamps or water

accumulation on the membrane).

The maintenance manual is an essential tool

for guaranteeing good serviceability of the roof

structure.

Installation

Erection sequence

In order to provide access to the stands and pitch

Figure 83D modelat 1:300 scale ofstadium withinwind tunnel of12m length, 2.5m

width and 1.85mheight

Figure 9Membranegeometry isoheightlines and lines of greatestslope

Figure 10Load paths withintypical panel unit (blue:downward acting loads;red: upward acting loads)

WACKER

RFR

RFR


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as quickly as possible, the design team decided

to install the main structure using a lift operation

the first time this had been attempted in Brazil.

The compression ring, which could be installed in

self-supporting modules, served as the supporting

system for the cable lift (Figure12).

Before commencing the operation, the

compression ring was closed and all cables were

laid out on temporary platforms (Figure13a).

The first phase of the lift began by pulling on

the upper cables (radial and tension) and pinning

all radial split cables (Figure13b). The next step

involved hanging flying columns and struts to the

upper cables (Figure14). The jacks were then

connected to the lower radial and tension ring. Prior

to this, all pulling had been against the self-weight of

the raised structure.

The second phase of the lift began by pulling on

the lower cables which, via the flying columns and

struts, pushed the structure in the air (Figure 13c).

A transfer of forces occurred at this stage between

the upper and lower tension ring (Figure 15). The

flying struts were progressively connected as the

roof was lifted. Towards the last stages of the lift, the

double ring cable stiffness was activated and forces

increased exponentially (Figure13d). The progress

of forces in the tension rings and jacks can be seen

in Fig.15.

Once the lower cables were pinned, the structure

became self-supporting and secondary elements

(e.g. arches, gutters, the membrane and, lastly,

equipment such as audio systems, lights and video

screens) could be installed.

Figure 11Check forstructural robustness

Figure 12Installation of free-standing compression ring

Figure 13Installation phases

a) Start ofphase 1

b) End of phase1 pinning ofupper system

c) Start of phase 2 flying columns in place,lower system lifts off

d) End of big lift prestress applied,lower systemspinned

RFR

RFR

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Project focus

Jacking strategy

The Arena Fonte Nova has only 36 inner roof axes and benefits from a

comparably low prestress level. This made it possible to use 28 small

120t jacks and 8 220t jacks for the lifting operation. The jacking strategy

(Figure16)involved:

pulling simultaneously on all axes until the structure could be pinneddefining the optimum pinning sequence by comparing different pullingscenarios

pinning the corner axes M2 and M3 with the taller 220t jacks

ConclusionThe new Arena Fonte Nova in Salvador de Bahia offers an innovative

lightweight solution for a la rge span roof. The spoke-wheel system is

enhanced by mobilising resources in stiffness and load-bearing capacity

from bracings in the vertical plane of the tension rings and in the horizontal

plane of the compression ring. This allows prestressing to be reduced

to 50% of the usual level. Other important features which simplify the

structure and reduce costs are concave radial cables with compression

elements, the use of fewer arches with non-uniform membrane prestress,

and a flat overall roof slope. The low prestress level brings cost savings

both by reducing the tonnage of all the primary structural elements,

and due to the smaller forces involved in the installation procedure. The

project involved the use of 1300t of steel, 200t of cables and 28 000m2of

membrane.

To contact the authors, email: jo [email protected],

[email protected] or [email protected]

Architect:Schulitz Architects

Structural engineers:RFR Stuttgart

Client: Arena Fonte Nova consortium (joint venture between OAS and

Odebrecht)

Main contractors:

Steel: Martifer

Cables: Redaelli

Membrane: Taiyo Birdair

Lifting operations: VSL

Supervisors:

Structure: Nelson Szilard Galgoul

Lifting: Schlaich, Bergermann und Partner

Membrane: Tensys

RFR team:

Project director: Mathias Kutterer

Project managers: Yu Hui, Jorge Chenevey Planella

Engineering: Yu Hui, Jorge Chenevey Planella, Michael Bauer,

Pranjal Saraswat

Draftsmen: Illya Osherov, Volker Hass, Hartmut Haker,

Ccile Gosselin-Neubert

Acknowledgments

Figure 14Intermediatestage ofinstallation flying columnsbeing installed;lower cable restingon platform,waiting to be

connected to flyingcolumns

Figure 15Forcediagram of biglift (red: uppertension ring; blue:lower tension ring

Figure 16Jackingforces for twosequences (greenultimately chosen)R

FR

RFR

RFR

arena fonte nova - low prestress for a lightweight roof

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