KẾT CẤU MỚI THE MILLENNIUM STADIUM, CARDIFF

The architect HOK Lobb have balanced a series of factors to achieve the optimum configuration that will ensure that the spectators are close to the pitch and have excellent sight lines, seating comfort and safety. High quality facilities for all the family have been provided, including restaurants, shops, bars and fast food outlets. Behind the scenes, below the entrance concourse level there are changing rooms with state of the art physiotherapy and medical facilities, offices, kitchens, storage and parking. Unique to the project is a fully palletised system of interlocking turf modules which can easily be lifted out and replaced when worn or damaged (fig. 2). The whole system can also be completely removed to create one of the largest covered arenas in Europe, capable of hosting almost any indoor event.

THE MILLENNIUM STADIUM, CARDIFF

Mike Otlet Director of Engineering Design

WS Atkins - Oxford

I N T R O D U C T I O N

The Millennium Stadium is located on the site of the

original Cardiff Arms Park stadium in the heart of Cardiff

the capital City of Wales Conceived as a prominent and

attractive landmark, it received £46 million of lottery

money from the Millennium Commission and became

one of the major projects to mark the new Millennium

(fig 1)

Fig 1

It is the first opening roof stadium in the United

Kingdom and took four and a half years from conception

to completion

In order to hone the design and refine the details to suit

the Arms Park site, budget and programme, many

structural forms were considered

The Rugby World Cup was to be hosted by Wales in

October 1999 and this event, provided both a catalyst and

a completion date for the project

This paper reviews some of the key stages in the work of

the design office and fabrication workshops, which led to

the final spectacular solution Nowadays, the design

process relies heavily on the use of computers and, in

this, the Millennium Stadium was no exception They

were used extensively throughout the design process for

analysis purposes and to express the design proposals

B A C K G R O U N D

The new stadium, which seats 72,500, was built by John Laing Construction, over a three year period on the restricted inner city site of the original Cardiff Arms Park rugby ground It has close neighbours on all sides, including the River Taff In order that the stadium can host significant events besides rugby or football, two sections of the roof can be moved across to completely cover the spectator and pitch areas and form a weather-tight arena This closing roof is the first of its kind in the United Kingdom and the largest in Europe The quality

of the acoustics ensures that noise breakout is reduced to

a minimum, neighbours are disrupted as little as possible and there is, within the stadium, an atmosphere that will attract top performers and large audiences to the venue

The architect HOK Lobb have balanced a series of factors to achieve the optimum configuration that will ensure that the spectators are close to the pitch and have excellent sight lines, seating comfort and safety High quality facilities for all the family have been provided, including restaurants, shops, bars and fast food outlets Behind the scenes, below the entrance concourse level there are changing rooms with state of the art physiotherapy and medical facilities, offices, kitchens, storage and parking

Unique to the project is a fully palletised system of interlocking turf modules which can easily be lifted out and replaced when worn or damaged (fig 2) The whole system can also be completely removed to create one of the largest covered arenas in Europe, capable of hosting almost any indoor event

Fig 2

In these respects as an advanced technological building

and as a focus of urban activity and renewal, the new

Millennium Stadium can be considered to be one of the

first of the "Fourth Generation" stadia - a stadium for the

new Millennium

S T A N D S

In order to hold the required seating capacity and comply

with the space restrictions around the site, the stands rake

outwards as they rise The interesting structural solution

needed to achieve this, led in turn, to a dramatic

architectural form

The structure above the entrance, which is at concourse

level, is constructed from 6,500 tonnes of steelwork in

CHS, RHS, open sections and plate girders It comprises

a series of frames at typically 7.3 m centres The frames

are stabilised radially by concrete shear walls and,

although there are only two basic frame types with shear

walls, either close to the pitch or remote from the pitch,

the shape of the stadium means that virtually every one

of the 76 frames is different

The steel frames are supported by a reinforced concrete

substructure and piled foundation system (fig 3)

Fig 3

Pre-cast concrete stepping units sit on raking steel plate

girders around the bowl to form the seating areas At the

back of the stands, these girders carry not only the seats

but also some of the roof weight and, by means of tie rod

hangers, the extensive level 6 upper concourse Tubular

steel props assist in limiting bending moments and

deflections in these girders

Level 5 (Box and Restaurant level) and level 4 below

(Club level) are of pre-cast concrete slabs and are

supported by deep plate girders on steel columns Holes

are provided in all the horizontal plate girders for

services penetrations

A horizontally propped raking plate girder supports the

seats for the dramatic middle tier This cantilevers 14

metres out from the floors at levels 4 and 5

R O O F D E S I G N D E V E L O P M E N T

The stadium needed to be about 50 metres larger than the pitch in all directions to accommodate the 72,500 seats and the opening had to be at least the size of the pitch This gave roof dimensions in the order of 220 metres long and 180 metres wide with an opening of approximately 120 metres x 80 metres

At the outset, following discussions with the various members of the team, a number of design criteria were decided upon;

1 To keep the roof as low as possible to reduce the stadium's impact on adjoining buildings e.g Westgate Street flats

2 To keep the edge of the opening as low as possible to reduce the extent of shading on the pitch bearing in mind the requirement for roof falls for rainwater drainage

3 To make any structure around the edge of the opening

as small as possible, also to reduce the effects of shadows on the pitch

4 To make the track for the retractable roof to move along, as near to flat as possible, again bearing in mind the roof falls for water run-off and drainage, and also to assist with making the retractable roof mechanism simple and therefore less problematic

5 It must be a quality design

T H E R E T R A C T A B L E R O O F

The direction and form of the moving roof was an initial concern The drive systems however were not considered

to be a significant factor in this decision and have not unduly affected the structural form since

Due to the plan shape of the stadium seating bowl, and the aim to create a roof as flat as possible, dome forms were dropped in favour of linear "sliding door" style systems running on straight rails

Most schemes have involved two sets of 5 similar sections combined in some manner to form a total unit at each end of the opening

Initial ideas centred around methods for concertinaing sections so that they could be stored in a shorter length, than the area to be covered, clear of the pitch

One of the original sketches produced at the time of the studies is shown (fig 4) The third scheme (fig 5) was pursued in the greatest detail and certainly could have been made to operate successfully but the cost was

Fig 4

prohibitively expensive Instead, the efforts were

concentrated on creating two 55 metre x 76 metre

"doors" to cover the 110m long opening

Fig 5

F I X E D R O O F A N D S U P P O R T I N G

S T R U C T U R E S

Design Evolution

There was insufficient space on the site both at the ends

and each side to allow any arch forms starting at ground

level and it was decided not to follow the tied arch and

deep truss route used on the Ajax stadium in Amsterdam,

due, again, to the shadows created by such a high

structure Instead the schemes investigated all made use

of masts and tension systems in an effort to improve

structural efficiency

Scheme 1

Over the first weekend of the project we sketched some

ideas and started putting rough numbers to the member

sizes and depths, for a two mast solution, picking up 2

large lattice trusses for the retractable roof track to sit-on

(fig 6) From this we started to get a "feel" for the scale

of the problem and the magnitude of the various elements

involved

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Fig 6

An initial idea produced in the first two weeks of the design process, in April 1995, was eventually to bear a surprising resemblance to the final form

The first scheme was developed over the following weeks, ready for the first submission for Millennium funding, which was made in May 1995 This, unfortunately, was not successful

Scheme 2

Following lengthy discussions and the consideration of alternative sites for the stadium through the summer of

1995, a new location, partly on the existing Arms Park site and partly on the site of an existing BT building and

TA centre to the south, looked to be feasible This had the advantage of improved access from Park Street Again we opted for two masts to support the main structure and retractable roof track, but this time to the south of the stadium (fig 7) Effectively, it was the same

as the first scheme but turned through 180° To avoid the road, the masts were moved towards the centre line of the Stadium and transfer structures were incorporated This second scheme was submitted for Millennium funding and following close scrutiny by the Millennium Commission and its representatives, received £46 million

of lottery money on 23 February 1996

Fig 7

Scheme 3

Through early 1996 we had been having increasing difficulties with the foundations and buried services that would have been too costly to move elsewhere When these problems were combined with uncertainty

regarding the availability of the Empire Pool site to the

south, we started to investigate alternative mast

arrangements that did not involve such a large site By

going back to the beginning again and considering the

options available it became clear that four masts could be

successfully employed, one in each corner at 45°, to lift

the corners of the opening Being symmetrically loaded,

the ability to offer a more efficient design also became

possible (fig 8) After lengthy discussions with the

client, the architect et al, the four mast scheme was

eventually adopted by all in the summer of 1996 and

developed in conjunction with the contractor John Laing

Construction, through to the signing of a Guaranteed

Maximum Price, in March 1997

Fig 8

Scheme 4

One or two adjustments in early 1997 lead to the final

arrangement we have today These were:

i The seating bowl that originally varied in its row

numbers to the sides of the pitch, and was deeper and

therefore higher to the long sides than at the ends of

the pitch, was rationalised to a constant level This

deepened the radiused corners and pushed the masts

further outboard at this point requiring large diameter

columns externally to transfer the loads to ground

ii A section of the original North Stand was retained,

cutting into the roof zone adjacent to the Cardiff

Rugby Club This required structure to spread the

loads onto the existing concrete stand and an

adaptable solution to allow the roof to be extended at

some time in the future if required

iii.The masts to the north were rotated by approximately

22° to ensure they did not encroach on adjoining

properties' land Unfortunately the masts to the south

could not be similarly rotated and so a less efficient

asymmetric structure was the only solution

T H E F I N A L S O L U T I O N

The Roof Covering

Both the fixed and retractable roofs are clad in a standing seam aluminium sheet with about 120 mm of insulation which is supported by a 128 mm deep profiled aluminium sheet (fig 9) This was all manufactured by Hoogevans and installed by Kelsey Roofing Industries Ltd on-site

This type of make-up and weight is unusual for a stadium, but was necessary to comply with the acoustic criteria noted earlier and allow more concerts to be held annually

Fig 9

The top sheet continues from the roof opening out to a perimeter gutter, which runs practically all the way around the perimeter of the bowl A syphonic drainage system, made by Fullflow, then takes the water away from the gutters to the ground

Roof Services

Because the roof is closed completely for special events which require protection from inclement weather, there are a greater number of services suspended from the roof than would otherwise be necessary

There are two rings of walkways running around the stadium to access these The first is located back from the edge of the opening and the second in the middle of the fixed roof, 24 metres back from the edge of the opening Both walkways support heavy pitch lights and speakers weighing up to 165kg each, together with cabling (fig 10)

Fig 10

M A I N S T R U C T U R E S

Purlins

The roof cladding is supported by 14 lines of purlins that

run circumferencially around the roof at 4.0 metres

centres The surface created is very much like that of an

egg with varying radii in both directions As a

consequence, the purlins twist from one bay to the next

as they pass over the structure below The roof deck

provides lateral restraint at top boom level and small

CHS tubes provide lateral restraint at bottom boom level

These tubes also provide support to the metal ductwork

suspended from the roof

Tertiary Trusses

The Tertiary trusses support the roof deck purlins and

walkways for the fixed area of roof (fig 11) There are 44

in total generally at 14.6m centres around the stadium

With a span up to 50 metres, they are supported at one

end by the back of the stands and at the other by the

Primary/Secondary trusses which surround the opening

To achieve good sight lines the trusses reduce from 4.3

metres deep at the junction with the Primary/Secondary

Trusses next to the opening, to only 400 mm deep at the

back of the stands Here, the trusses sit, via individual

sliding bearings, on a perimeter truss (fig 11) The

perimeter truss spreads the end weight of the Tertiary

Truss uniformly onto two adjacent stand frames

Fig 11

The bearings ensure that differential horizontal

movements between the roof and the stands will not have

an adverse effect on either element, e.g under wind loads

and thermal expansion/contraction

Primary Trusses

Two major pieces of structure, known as the Primary trusses, are located on each side of the pitch in a north/south orientation

Rising 35 metres above the pitch, these are continuous over the full 220 metre length of the stadium (fig 12) Support is provided at two intermediate positions (at the corners of the opening) via cables up to the corner masts which are then tied down to anchors outside the stadium With a 1067m diameter top and bottom boom, the trusses range in depth from 4 metres at each end to 13 metres in the centre

Fig 12

A 778 dia middle boom, four metres above the bottom boom, provides a connection point for the tertiary truss top booms and resists high compression loads from the mast structures, (ref analysis)

On one side the trusses provide the support and rigidity for the continuous runway beam which support the moving roof On the other side they provide support for the fixed roofs on the east and west of the opening

Secondary Trusses

The secondary trusses run in an East-West orientation

and trim the North and South edges of the opening They

traverse the full 180 metres width of the stadium and are

formed from a 915 diameter top boom and 550 diameter

bottom boom (fig 13) Support is provided at each end

by the stand structures, and at the intersection points with

the Primary Trusses, by the corner mast and cable

assemblies They principally provide support to the pitch

end of the North and South Tertiary Trusses and also by

a lesser extent, to an area of roofing to the corners

Fig 13

Bracing and Lateral Restraints

The fixed roof is connected together to perform

structurally as one homogeneous unit The straight,

rectangular, roof areas are braced in both directions on

plan at top and bottom boom level for stability and lateral

restraint purposes (fig 14)

FIXED P O S I T K J N S <RJ

The purlins perform the role of lateral restraint, at top

boom level, with CHS tubes at bottom boom level

In addition the bracing holds the track for the moving

roof in position laterally This requires both levels of

bracing to resist the torsion effects of the moving roof

loads being applied eccentrically to the Primary Truss

The corner tertiaries are restrained back to the adjacent parallel roof section (either east-west or north-south) The total roof is trimmed by a 4060 CHS which supports

an eccentrically applied cladding load and holds the shallow Tertiary trusses vertical at the bearing positions

on the perimeter trusses

T H E C O R N E R M A S T S

Four corner mast structures are key to both the vertical support and horizontal stability of the roofs Each mast structure is made up of a pair of lower columns (concrete filled steel tubes 12190) which sit upon a 16000 fabricated steel tensioning chamber which, in turn, rest

on reinforced concrete foundations (fig 15) The tensioning chamber is connected to an 8m deep reinforced concrete shear wall via 10 no 750 Mac Alloy bars cast into the wall On top of the pair of lower columns is a complex series of connections commonly known as the elbow and knuckle The elbows form the link between the roof and the stand structures providing total stability horizontally to the roof via the eight elbows, in 4 pairs, and the cross-bracing between them

Fig 15

The A-frame mast rests on the knuckle and is held down

by the high tensile forces in the cables which on one side lift the main roof and, on the other, are tied down to the tensioning chamber at the base of the pair of concrete filled steel columns

The high tensile forces in the cables generate compression in the two horizontal structures On the pitch side these are known as the Mast Tertiaries These are fabricated units 2.6m deep and are made of 60mm thick plate to form a Tee-shape section which were then welded to a 6600 CHS tube at the bottom Pairs of Mast

Tertiaries are braced together for stiffness and buckling

resistance On the other side of the knuckle outside of

the stadium, is an A-frame outrigger made from 9150

CHS tube The tubes are restrained by a tensegrity

structure to stop the outrigger from bowing under its own

weight This also provides buckling resistance

The A-frame masts rise 40 metres above the edge of the

roof and 70 metres above the surrounding ground level

(74 metres above the pitch) Each leg is a fabricated oval

section 915 x 1415 overall, tapering down to 9150 at the

knuckle Again, as with the outriggers, the mast A-frame

is restrained by a tensegrity system of McCalls tie rods

and struts

The tension system, although loosely described as cables,

is in fact a group of 15 mm diameter high tensile steel

strands by PSC Freyssinet inside 6 No 2730 HDPE

sleeves

F T O M i r o or mmm

r r r r c s (70™. tuxi

Fig 17

M O V I N G R O O F

There are two moving roof sections that are generally

located one to the north and one to the south of the

opening over the fixed roofs Both sections are 76 metres

wide and 55 metres long and made up of 5 individual

units, each 11 metres wide (fig 16) The units are linked

together, principally at the ends with vertically orientated

sliding bearings Each unit is prismatic in cross section

and 8 metres deep at the centre The truss curved in

elevation has a single CHS top boom and two CHS

bottom booms The flat roof deck sits on purlins above

the bottom boom with all diagonals and the top boom*

exposed to the elements The units are allowed to move

differentially horizontally (fig 17) to accommodate

curvature of the track on plan Sliding bearings are

provided above the wheels to cater for variations in the

distance between the two retractable roof runway rails

The retractable roof units were assembled at ground level

and lifted onto the roof in 76 metre sections Since

positioned and connected, it has functioned well with

few problems

ttCTXX THROUOM WfSt S K *

T H E M E C H A N I S M

The moving roof sections have no power connection to them whatsoever The actuating mechanisms are mounted on the fixed roof between the track and the Primary Truss (fig 18)

Fig 18

Fig 16

These are on each side of the moving roof located at the

four comers of the opening to pull each half of the roof

open or closed via a cable loop (fig 19) This operation

takes 20 minutes in each direction, a speed of only 2.6

metres a minute

Fig 19

A system of hydraulic motors and gearboxes power the

drums back and forth Substantial brakes are provided to

each drum Hydraulic buffers are located at the centre of

the travel and where the units rest at the outer ends of the

track, as end stops (fig 20) The speed and actuation are

all computer controlled such that both sections move at

the same time and at the same rate

Fig 20

A N A L Y S I S

The roof was initially broken down into simplified 2D

frames before the assembly of a complete 4349 member

3D roof model (fig 21) The model was developed over

three months in order to reduce the highly loaded and

high displacement/deflection points to acceptable levels

Fig 21

Wind Tunnel testing was carried out prior to the commencement of the design to ascertain the most suitable wind loads to be applied to the roof analysis model This also provided an opportunity to check the effects of the new stadium on the surrounding buildings

The 3D model was loaded with self-weight, dead load, live load and wind load cases individually and then with combined load cases with the retractable roof units in open, closed and partly closed positions The output was then sifted to obtain the worst combination of axial and biaxial bending stresses for each roof member

The model was refined to improve structural efficiency and uniformity in truss boom sizes The Primary Trusses were pre-tensioned by varying amounts until the optimum location and pretension were determined This showed that maintaining the theoretical Primary/Secondary truss intersection node "level" following installation of all the dead load and retractable roof trusses was the best solution This meant that the cable system had to be shortened by approximately 500 mm at ground level to achieve the required position

During the construction period, sections of the model were deleted to reflect the partially constructed state Further analysis was undertaken to ensure that the partial stability and strength of the roof structure and its various components during the erection period were satisfactory These figures were then used during the tensioning sequence for comparison purposes with the actual figures measured on site

C O N S T R U C T I O N A N D C O N N E C T I O N S

All the steelwork for the stadium was manufactured in the

UK by British Steel (now Corus) and shipped to Italy for fabrication by Costrusioni Cimolai Armando SpA

Due to the large size of all the trusses it was necessary to subdivide them into transportable sections no bigger that 5 metres by 17 metres It is interesting to note that seventy five percent of the roof structure is there purely to support its own selfweight

With this in mind it is obviously important to keep the

selfweight to a minimum Given that the steel was being

transported from Italy to Wales after fabrication it was

important that the connections between each piece were

both small and efficient During the fabrication drawing

period the Primary truss which was previously a

3-Dimensional prismatic form was redesigned as a

2-Dimensional element for ease of transportation and in

particular shipping Additional lateral restraints were

required as a consequence

From an early stage it was decided to follow the simple

principle for the connections of:

i The ends of complete trusses or members would be

emphasised architecturally within the practical

constraints of tolerance, fit-up and economy

ii All intermediate (splice) connections would be

hidden to give the impression of being a continuous

monolithic piece

In the majority of cases such as the tertiary truss ends and

lateral restraint/bracing member end connections, a

simple tapered tube detail was developed with single

plates protruding A plate each side with multiple bolts

in a circular arrangement then linked the pieces together

Tolerances for length and direction were achieved via

four interfaces each with 3mm oversize holes for M27

bolts (fig 22)

Fig 22

The connections for the mast assembles were considered

individually due to their varying requirements of

movement, tolerance and adjustments

The Primary node which connects the Primary,

Secondary and Mast tertiaries together as well as

numerous smaller bracing and lateral tubes is formed

from a single 100mm thick high grade steel plate cut to the external profile of the overall connection, (fig 23) The plate is orientated vertically in the direction of the cables to enable the tube and cable termination housings

to be welded directly to each side Short stubs for the incoming truss members were then welded at the appropriate angle onto the central plate to give sufficient space for bolted splice connections to be made Where forces were prohibitively high in-line butt welds were made on site, but these were rare

Fig 23

The mast top cable termination, outrigger end cable termination and base tensioning chamber all followed the same principle of the single central plate cut to the external profile of the connection All other plates and tubes were then welded to the sides of this (fig 24) Cover plates (bent plates) welded outside these have ensured that the external appearance is as smooth flowing as possible, within' the budget constraints The use of a central plate has ensured that the forces flow more evenly across the connection and high local bending moments and shear forces were kept to an absolute minimum

Fig 24

The knuckles which are located at eaves level on 1219 A

tubular columns form the focus and connection point for

the A - frame masts, Outriggers and Mast tertiaries (fig

25) This central knuckle (or hub) has to resist

approximately 40000 kN axial compression from each

incoming member During the cable tensioning process

and when the retractable roof sections close, or when it

snows, the forces in the cables (which join the outer ends

of those members) increase, causing them to elongate

significantly This in turn causes a rotation at the knuckle

necessitating a pivot at the same point

Fig 25

A 2.4m diameter cylinder 2.4 metres long orientated

horizontally, like the centre of a bicycle wheel has around

it removable steel plates with PTFE and stainless steel

contact surfaces to allow the small rotation to occur (fig

26) The drum was sized to accommodate the very large

circular end plates from each of the three incoming

members with the low working stresses in the PTFE

material

Figure 26

The splice connections between the Primary and Secondary trusses required the greatest development time Various options were considered (some of considerable weight and complexity) before the final detail was found Bolted connections were considered to

be essential for speed and ease of construction Due to the large diameter of the tubes involved (770 diameter and above) it was possible to climb inside to make a hidden bolted connection(fig 27)

Fig 27

The majority of tubes are highly stressed and an innovative detail was required to solve the problem The axial loads are transferred by 4 flat plates bolted to fabricated tees welded to the inside of the tubes (fig 29) The tees are of sufficient depth to allow the splice plates

to pass over an internal flange on the ends of each section The flanges are bolted together to transfer shear and torsion

Where tubes were too small to climb inside, port holes were formed to gain access to the bolts and flange plates

Figure 28