-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
1/7
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
-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
2/7
11
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.
-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
3/7
12 TheStructuralEngineer
November 2014 Arena Fonte Nova
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
-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
4/7
13
www.thestructuralengineer.org
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
-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
5/7
14 TheStructuralEngineer
November 2014 Arena Fonte Nova
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
-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
6/7
15
www.thestructuralengineer.org
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
RFR
-
8/10/2019 Arena Fonte Nova - Low Prestress for a Lightweight Roof
7/7
16 TheStructuralEngineer
November 2014 Arena Fonte Nova
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