KẾT CẤU MỚI DEVELOPMENT OF THE NEW WEMBLEY STADIUM ROOF

The configuration of the new Stadium was primarily developed through the brief complied for the competition, and expanded to suit the requirements for expected use. The new Stadium is to be capable of hosting events similar to that of the existing. Together with the primary sporting events of football and rugby, major athletics events were to be capable of being held. Recent national publicity on the topic of athletics at Wembley has shown this to be a very emotive subject, one that thankfully is not for this paper. Other events that complete the portfolio are concerts, pageants and exhibitions. The brief called for a world class Stadium which would continue to uphold the traditions of the existing Stadium. A full bowl arrangement for the terracing is utilised as the whole of the stadium was being reconstructed. This provides the opportunity to ensure all the are spectators are wrapped around the action creating an intimate environment, full of atmosphere. With this arrangement, the Wembley Roar would continue A spectator capacity of 90,000 was eventually chosen. During the development of the business case for the operation of the stadium a 200 bed hotel, interactive museum, 100,000 ft2 of offices a 5,000 seat (the largest in London) banqueting suite and full spectator hospitality and corporate facilities are to be provided.

DEVELOPMENT OF THE NEW WEMBLEY STADIUM ROOF

Michael J Barker Director Mott MacDonald Ltd

A B S T R A C T

This paper describes the current development of the new

Wembley Stadium Roof The final design has not yet

been completed at the time of writing, however the

principles of the scheme are established Once

constructed, this roof will be one of the largest in the

world The paper describes the background to the project

and the reasons behind the evolution of the current

scheme for this large area, long span roof

B A C K G R O U N D

The New Wembley Stadium was conceived from the

sense that whilst the current stadium could boast a

glorious past it had little, if any future As the

centrepiece of the 1924 British Empire Exhibition, it has

staged many celebrated football matches, since the

famous 1923 "White Horse" FA Cup Final The most

notable football match ever staged however, was the

1966 World Cup Final The stadium was also the venue

for the track and field events during the 1948 Olympics

It has also become the traditional home of the Rugby

League Challenge Cup Final By default therefore it has

become the National Stadium

All agree that the current stadium has clearly passed the

point at which it can usefully continue to serve the as the

National Stadium Faced with this position and the

prospect of a number of applications for major new

lottery funded stadia, Sport England established with the

Football Association, (FA) the Football League, The FA

Premier league, the Rugby Football Union and the

British Athletic Federation, a competition for the

development of a New National Stadium In July 1995

bids were received with Wembley emerging as the

preferred location

It was intended that the owners of the Stadium, Wembley

pic would take this forward with a body especially

established for the purpose, The English National

Stadium Trust The Trust would lease the site from

Wembley pic and be responsible for the design,

construction and finance for the new stadium This

arrangement ultimately could not be made to work, and

in the face of bids from private competitors such as

Arsenal FC, the lottery grant assigned to the project

would be used to purchase the existing stadium and it's business The finance would be raised by a development company, Wembley National Stadium Ltd (WNSL) a wholly owned subsidiary of the FA This finance would

be secured on the basis that the FA would continue to hold it's flagship events at the new stadium After negotiations with Wembley pic the existing Stadium and business was purchased on 15 March 1999

T H E N E W S T A D I U M

The configuration of the new Stadium was primarily

^developed through the brief complied for the competition, and expanded to suit the requirements for expected use The new Stadium is to be capable of hosting events similar to that of the existing Together with the primary sporting events of football and rugby, major athletics events were to be capable of being held Recent national publicity on the topic of athletics at Wembley has shown this to be a very emotive subject, one that thankfully is not for this paper Other events that complete the portfolio are concerts, pageants and exhibitions

The brief called for a "world class" Stadium which would continue to uphold the traditions of the existing Stadium A full "bowl" arrangement for the terracing is utilised as the whole of the stadium was being reconstructed This provides the opportunity to ensure all the are spectators are wrapped around the action creating an intimate environment, full of atmosphere With this arrangement, the Wembley "Roar" would continue! A spectator capacity of 90,000 was eventually chosen During the development of the business case for the operation of the stadium a 200 bed hotel, interactive museum, 100,000 ft2 of offices a 5,000 seat (the largest

in London) banqueting suite and full spectator hospitality and corporate facilities are to be provided

These facilities could not all be fitted into the concourse areas under the spectator bowl structure and are housed mainly on the North side of the building, facing Olympic Way The roof of the Stadium is designed to cover in one sweep the both the spectators and the other facilities housed in the building This has resulted in a very large roof, one of the largest in the world, containing long clear internal spans

The roof of a stadium is a very highly visible structure

and which dominates all views of the building, both

internally and externally It therefore demands special

consideration from the design team The existing

stadium's roof boasted the "Twin Towers" This very

emotive and powerful, national icon was the subject of

much debate To keep the Towers or not? It was not a

viable option to retain the Towers in their original

positions The new Stadium needed to be moved to the

North to create a piazza around the whole of the building

It is possible to relocate the Towers, however it was felt

that a new image should be provided for the Stadium

The roof for the new Stadium was the subject of a very

intense and detailed option study It was necessary to

provide a column free space within the spectator bowl

and to cover the additional facilities housed in the North

of the building The brief required all the spectators to be

covered There was not however a requirement to have a

completely retractable roof, as the primary sporting

events for the Stadium had to be held in the open air

However, could the capability for a closing roof be

excluded and the roof not "future proofed"? It would be

very difficult and prohibitively expensive to try and retro

fit a fully closing roof once the Stadium was constructed

Future proofing is discussed later in this paper

The configuration of the spectator bowl adopted with it's

much shorter sight distances and a roof covering all the

spectators also generates problems of shadow lines on

the pitch and lack of air movement over the playing

surface Both of the above cause problems with grass

growth Healthy grass needs both direct sunlight and

fresh air

The current Stadium configuration with it's low set back

roof, running track between the spectators and the pitch,

and the shallow seating tiers allows easy air movement

over the pitch Even at 4.45pm in mid May (FA Cup

final) there is only a small portion of the pitch covering

the South Western corner flag in shadow Apart from

pitch health, this is very important to the television

companies as their cameras cannot cope very easily with moving in and out of shadow Given this existing condition it is imperative that the new configuration gives a similar performance for both pitch health and shadow lines The solution for the roof needs to be able

to address these issues satisfactorily

S C H E M E D E V E L O P M E N T

The starting position for the scheme was that there should be no columns in the spectator bowl The perimeter of the roof therefore needed to be supported at the high back edge of the bowl The saddle shape of the bowl edge reflects the capacity requirements together with maintaining the required standards for the pitch and scoreboard sightlines The edge of the roof will be supported on the perimeter truss This element of structure is formed from the extension of the bowl primary raking beams triangulated with V props to form

a continuous perimeter truss on which the roof edge elegantly rests

It was immediately apparent that the North side of the building would be the dominant area and would need to contain the major supporting elements of the roof The roof structure could then align primarily North South If

a line of support could be gained at the internal leading edge of the North Roof, main trusses could be utilised to span from this edge to the Southern edge of the bowl Further advantages to the overall solution could be gained by following this concept As discussed, the problems of shadow lines and air movement over the pitch needed to be addressed These problems could be largely eliminated if the roof along the Southern side of the Stadium could be retracted back to allow sunlight onto the pitch They could be closed before or during a match if the weather deteriorated The main North South trusses could support retractable roof panels running along the truss top chord The requirement to cover all spectators would still be achievable, albeit only during inclement weather

To enable this option to work, a major supporting

element needed to be introduced which could provide

support to the internal leading edge of the North roof and

could be supported outside the footprint of the building,

ensuring still a column free spectator bowl The initial

public scheme utilised 4 masts at the front (North) of the

stadium with twinned forestay cables attached to internal

edge of the roof Twin backspans cables were anchored

back to foundation blocks to the North of the masts

This scheme worked well as a structure, however it was

felt that the intrusion of the masts directly in front of the

building, together with their cables, and the same mast

type solution with other similar structures would not be

special or unique enough for the new National Stadium

Subsequently, a solution that found favour with all was

produced This utilised a massive arch, which was

positioned over the Northern roof and spanned the whole

building East to West This solution provided an efficient

and elegant solution to both problems of roof support

whilst giving the necessary icon to the Stadium

A series of forestay cables is attached to the arch

supporting the internal leading edge of the Northern roof,

and backstay cables anchor to the edge of the bowl,

neatly eliminating all the externally anchored cables of

the mast solution Refer to Figure 1

The arch quickly became the accepted image of the new

Stadium, replacing the existing Twin Towers as the icon

for the new National Stadium

Once basic agreement on the arch solution was reached

working models of the Stadium were constructed, Figure

2 A full wind tunnel test model was prepared tested to

confirm existing design data, determine accurate wind

loads and highlight and quantify any special effects on

the roof and arch for this configuration

T H E A R C H

The arch takes the form of a 7m dia 138m high, 315m span open "basket weave" unclad lattice structure It is formed of 457 dia CHS longitudinal chords with diaphragms at approximately 20m centres Alternate diaphragms are primary and support the stays Steel grades are S355 JO or J2 to BS EN 10025 Rolled Hollow Sections S355 J2H to BS EN 10210 Protection

is 400 dft micron epxoy primer / buildcoat and a 75dft micron finish coat, over a blast clean surface to Sa 2.5 of

BS 7079, giving the period to first maintenance of 30 years

Access to the arch will need to be undertaken for the following reasons:

• Structural Inspection

• Lighting maintenance / replacement

• Repainting (30 year interval)

• Festivity / celebration (eg pyrotechnics)

• Dressing the arch with flags or banners

It is anticipated that maintenance will be through the centre of the arch For the more thorough maintenance tasks a pre fabricated platform that is launched from the North Roof would be winched up under the section requiring attention, providing a safe working platform for the maintenance crew

The position of the arch and it's inclination have been the subject of an intensive iterative analysis process This to try and ensure that under all load combinations the arch acts as far as possible in it's most efficient state, direct compression

The treatment of the springing points of the arch will be important as it is expected that these points will be used

by the visiting spectators as favourite spots for

photographs In the design of the knuckle springing

points, there is also the security and safety issues to be

considered from people potentially being able to scale

the arch Figure 3 below

Fig 3 Arch Support Detail

The stays are spiral strand galvanised wires grade 1570

Cables Cable interior and exterior corrosion protected

First inspection within 5 years of initial coating and a

major inspection after 15 years from initial coating

Touch up will be required at the time of the initial

inspection, with complete re coating a likely option at the

time of the major inspection Under the approved'

maintenance regime the cables have a guaranteed life of

60 years

Generally the forestays cables range between 110m and

135 mm dia The backstays between 55 and 95 mm dia

There are 8 support points provided along the North roof

leading edge The first 2 supports at the East and West

ends are primary picking up the main North South T3

and T4 trusses These trusses span to the South edge of

the bowl and are the main elements which support the

whole of the Southern roof area

T H E R O O F P L A T E

The roof plate main structure runs North South Refer to

figure 4 for the member references The surface is

profiled to fall from the North South centreline away to

the East and West The roofing material is to be a

mixture of standing seam aluminium (eg Kalzip) and

30% translucent polycarbonate sheeting (eg Lexan) The

polycarbonate sheeting is introduced to allow diffused

light through the roof towards the leading edges This

provides a light gradient of open to fully solid roof which

improves hard shadow lines

Soffit treatment (lining) will be provided in certain areas

to hide walkways and services This lining will also have acoustic benefits during concerts Further to this there will also be areas treated with specific acoustic lining panels to both absorb sound and modify the general acoustic properties of the spectator bowl

350 mm cold rolled nested purlins at 3m centres running East West, support the roofing spanning between the main roof trusses The centre 10 bays are at 13.5m reducing to 10.5m for the end 6 bays

On the North roof, the primary fink type trusses are 6m deep spanning up to 75m These are supported from the arch stays and the Northern edge of the bowl The top chord is a box section generally 750 x 500 mm deep with

a cable bottom chords and CHS V struts at third points

A longitudinal truss T i l is provided to support the alternate secondary North South trusses and to provide in plane rigidity to the roof plate when considering assymetric and dynamic loading

The main T3 and T4 trusses span to the Southern edge of the bowl T3 spans 155m and T4 129m These trusses, together with the edge T5 trusses support the main Southern roof and carry the rails for the moving roof panels Lateral stability of these main trusses is provided

by a series of horizontal cable ties

At the leading edge of the Southern roof truss T13 is located This spans 135 m between the T3 trusses to support the central section of the Southern roof As the truss is curved in plan, diagonal cable ties are introduced back into the roof plate to counteract the bottom chord kick out

The main North South 6 m deep fink trusses are utilised

as runway beams for the sliding roof panels Fabricated box sections top chords are provided, as in the North roof Refer to Figure 5 for a complete isometric view of the roof structure

S L I D I N G R O O F

In order to meet the requirement to provide maximum covered seating whilst still allowing daylight for pitch health

a moving roof over the whole of the Southern side of the stadium is required The roof panels are nested over the static section of the roof and at each end double stacked The permanent roof structure running North South provides the runway beams supporting the track for the panels

The area of roof that moves is split into 5 bays, one middle section extending the length of the pitch (135m between trusses T3), and two bays at each end, covering the end stands These panels are supported of the T3,4 and 5 trusses The end bay panels are subdivided to allow them to double stack on top of the fixed roof without projecting over the Southern edge of the building

ARCH OVER ARCH OVER

Fig 4 Roof Plan

During operation, in order to maintain a positive wheel

friction to the rails, it is proposed that a wind speed limit

of 20 m/s (approximately 50 mph) is imposed This is in

line with common practice at other operable roof stadia

This limitation to operation pertains to operational

reasons only The panels themselves are designed to

withstand the design wind loading at any location, fully

closed, part open or fully open

The panels are generally framed by fabricated box

sections (up to 3m deep for the central large cantilever

panel) which are connected to the running bogies

Secondary framing UB sections are utilised with full

diagonal rod bracing for each panel to ensure racking of

the panel does not occur A full cycle for the roof to open

or close will take 20 minutes

F U T U R E P R O O F I N G

Whilst there is no current requirement for a fully closing

roof for the Stadium, there may be, in the future cause to

want this facility The existing design is able to be

adapted to cater for this by the strengthening of certain

key elements

the T3 trusses To cope with this additional load the T3 trusses, the supporting arch stays, the arch and the arch foundations would all need to be strengthened

It is considered that if this strengthening was not carried out at the time of construction, any retro fit would be virtually impossible and expensive to carry out The technical problems could be overcome however any future fully independent structure over the top of the roof would be visually intrusive and would be unlikely to gain planning permission This would effectively deny the retro fit route for this option

S U M M A R Y

The roof, especially the arch of the New National Stadium provides the icon that will take over from the Twin Towers and continue the tradition of Wembley for the next 50 years The stadium will be a truly magnificent stage for the major events that will be held there It will elevate England into a country capable of hosting almost any international sporting event in style

As the kingpin for bids for the World Cup, and the Olympic Games it is considered that any such bid would

be very strong built around the New Wembley Stadium The fully closing roof would take the form of two

additional central overlapping panels clear spanning the

length of the pitch (135m) These would be supported on

N O T E

The structural engineering for the New National Stadium

is being undertaken by the Mott Stadium Consortium.*

This consortium is lead by Mott MacDonald and contains

Connell Wagner Pty, Modus Consulting Engineers and

Weidlinger Associates Inc

R E F E R E N C E S

1 Wembley National Stadium, House of Commons

-Culture, Media and Sport - Fourth Report

2 March 2000

2 The New English National Stadium, World Stadium Team February 2000

3 Key Development Criteria, Technical Development

Criteria, English National Stadium Development

Company Limited March 1999

KEEPING THE DOORS OPEN:

THE OLYMPIC STADIUM, SYDNEY

S Morley Principal Modus Sinclair Knight Merz

Fig 1 Aerial view of Stadium Australia

Sports facilities, where shelter is provided, invariably fall

into the category of widespan enclosures Stadia and

arenas tie up vast amounts of a client's capital and the

planet's resources and therefore should be designed to be

suitable for many uses as far as practicable This may

mean providing a range of environments through altering

their widespan enclosures

Where field sports played on natural turf form some of

those uses the enclosure needs to be open centred to

promote grass growth and comply with current

regulations for 'outdoor' sports Having the ability to close this central opening can greatly increase the possible uses and also provides event surety at least in the face of a temperamental climate

It is, to a degree, possible to quantify the benefits of this flexibility of extent of enclosure by assessing the revenue from additional usage and avoidance of lost revenue from cancelled events (although the latter is difficult to assess as past events at other facilities statistically represent such a small sample of total available event

days that little guidance can be drawn from "I've never

known an event cancelled in thirty years") However,

this judgement will necessarily be made based on current

knowledge of the forthcoming developments in the event

markets in which the new facility is intended to compete

and the only certain thing about such markets is that they

change, often unpredictably Hence it can be of benefit

to provide for flexibility of enclosure or at least for

adaptability to be able to introduce such flexibility at a

later date

Two of Australia's most recent multipurpose sports and

entertainment facilities - Stadium Australia at the

Olympic site in Homebush Sydney, and Colonial

Stadium, Docklands, Melbourne, are good examples of

the application of this strategy Stadium Australia, the

host venue for the 2000 Olympics, is designed to operate

in two distinct modes In Olympic mode the North and

South sections of the bowl are unroofed to allow space

for large temporary grandstands, each accommodating

15,000 people

After the Olympics these end tiers are due to be removed

an the perimeter enclosure completed with the addition

of North and South Roofs At the same time the lower

tier of seating will be moved inwards by nearly 16m on

the sides and 20m on the ends to greatly improve their

proximity to field sports This strategy was part of an

innovative financial package which helped secure the

project for the Multiplex led team in a B.O.O.T (build,

own, operate and transfer) competition Under the

auspices of such a procurement method the development team was acutely aware of the need for long term financial viability Whilst, based on 'current' understanding of how the stadium might be used, there as insufficient justification for providing the flexibility of full as well as partial enclosure, it was considered important to 'keep this door open' by allowing adaptability for this flexibility in the future

The roof of Stadium Australia follows , and in fact was borne out of the geometry of the seating bowl In the longer term post Olympic mode, from the lowest point

on the end stands to the highest point on the side stands there was a height differential of perhaps 40 metres and the natural shape to fit this saddle perimeter was the hyperbolic paraboloid With this geometry the roof over the side stands curves gently downward maximising weather protection for every dollar spent on the roof whilst hugging the higher sightlines There was also potentially a construction advantage in that this iconic doubly curved form is generated from two sets of straight lines parallel to but progressively rotated from two principal generators at 45 degrees to the main axes of the stadium Structurally such a surface can be very materially efficient as loads can be transferred by in plane or membrane forces

However this geometric and structural purity is rudely interrupted by the roof plan form in Olympic mode when just crescents of roof over the side stands are required In plane action across the stadium is not possible in this

in bending to some form of edge stiffening along the

front edge of each crescent Nonetheless the other

advantages of the hyperbolic parabaloid remained and it

was decided to pursue this geometry True to this

surface, the front edge of the crescents were parabolic

which suggested a form of arch as edge stiffening Once

provided the arch stiffening would attract a large

proportion of the load in both Olympic and post Olympic

modes and to increase the efficiency of this system still

further the main arch was lifted above the HP surface by

up to 12 meters at its apex

The concentrated lateral load components generated by

arch forms are best taken directly to foundations on a

continuation of the line of thrust (curve of the arch)

However, the arch line had to remain above the HP

surface to the edge of the roofed area which left it more

than 20 metres above precinct level There was simply

not enough site area to continue on this line and therefore

some form of cantilever thrust block was required

Interestingly a degree of mitigation was provided by

allowing the arch line to change in response to areas of

concentrated gravity loads Because of the geometry of

the opening such a concentration occurs where the roof

ends meet the arch (here there was also a requirement to

support 50 tonne video screens) and the arch line was

therefore deliberately diverted inward and downward at

the ends like the profile of a crab This lowered the thrust

blocks slightly such that the arches could spring from a

point 17 metres above the precinct

The resulting roof geometry therefore was the hyberbolic

paraboloid surface curtailed to a vertical cylinder

defining the back of the seating and with two crabbed

arches stiffening the edges of the central rectangular

opening in post Olympic mode and providing full edge

support in Olympic mode

Early on in the development of this design the client

instructed the designers (Architect Bligh Lobb Sports

Architect and Engineers Modus with Sinclair Knight

Merz) to consider how a fully closing roof could be

provided and what steps might sensibly need to be taken

now to ensure this remained a possibility for the future

It was found that geometrically the hyberbolic parabaloid could readily accommodate a fully closing roof The arched edge reinforcement running East West provided natural trackways for a simple sliding system albeit on a constant radius curved track Furthermore, the fact that the top surface of the end roofs dishes down following the HP surface between these trackways provided additional space for the depth needed for the moving roof panels to clear span the 100 metres plus between the rails This property was particularly useful as it proved necessary to stack a pair of moving leaves above arch and fixed roof within the curtilage of the stadium Inclusion of a closing roof would therefore increase loads

on the arches and their supporting thrust blocks and foundations requiring commensurate strengthening Also as this additional loading is to one side of the arch centreline, the roof diagrid supported off the arch which restrains the arch horizontally also would require strengthening locally The design work instructed allowed informed decisions to be taken on which aspect

of their strengthening could and should be carried out during initial construction to 'keep the door open' whilst minimising the amount of capital tied up in steel and concrete for a potentially lengthy period before any revenue is gained from this benefit The strategy for

Fig 5 End segment of arch truss

strengthening the arches was to add a new central

compression element within the arch trusses This could

conceivably be carried out at a later stage by unloading

the arches using the same temporary towers employed

for their construction and feeding in the new

compression member in pieces small enough for

assembly by site welding (used extensively for initial

construction of the arches)

However, strengthening of the arch supports would be

altogether more involved to the point of being

impractical Fortunately it was found that the cost of

strengthening the pins at the ends of the arches, the pin

plates, thrust blocks and foundations was reasonable and

therefore these measures were included in the initial

development Also it was found that the diagrid

strengthening local to the arches could be achieved

simply and with minimal cost by deepening the diagrid

in this region

By this means the option of installing the future closing

roof has been kept open in a logical way and with the

minimum of additional expenditure and construction

beforehand

With Colonial Stadium, in Melbourne's Docklands, the

decision was taken that the roof should be closeable,

subject to the additional costs involved being supported

by the business plan Thereby a stadium designed as an open stadium complete with an AFL sized natural turf pitch could become fully enclosed to operate as a multipurpose venue to attract a greater number or events Although having a moderate seating capacity of 52,000, the infield is exceptionally large by European or US standards, to accommodate the AFL pitch and closing the 1.7 hectare retractable roof creates an enclosed arena of vast proportions

Several forms of retractions were explored and a simple sliding mechanism with two 50 x 165 m span doors selected for cost efficiency and design efficiency Such

an arrangement naturally concentrates loads on two distinct lines at the ends of the doors and recognising this the Architectural team of Daryl Jackson Architects Pty Ltd and Bligh Lobb Sports Architects, proposed that the upper tier be divided into quadrants allowing four corner supports to be 'pulled in' under these loads

This arrangement could readily be accommodated as the moderate capacity compared with the long infield perimeter meant seats would be redistributed without detriment to viewing quality or efficiency

The Engineering Team of Connell Wagner with Modus used this opportunity to the full to produce a very efficient structure which, even with the 165m span doors