KẾT CẤU MỚI LARGE ENVIRONMENTAL ENCLOSURES, THE ROOF OF THE MILLENNIUM DOME

There is a long history of dreams of creating large enclosures to ameliorate the climate in inhospitable parts of the world. This trend started in the 18t h century when the owners of fashionable country houses built heated glazed enclosures to grow Pineapples and grapes. These stoves or conservatories were generally built of stone with the glass in conventional wooden frames. As the glass making technology improved and iron working techniques developed the greenhouses became larger. All iron glass houses appeared at the start of the 19th century. There is a classic example remaining at Bicton in Devon. This structure is more like a shell with small panes of glass set in fine wrought iron ribs. The classic examples of this type of construction were the Palm house at Kew built in 1848 and of course the Crystal Palace for the 1851 exhibition. The Crystal Palace was a relatively simple modular construction based on an 8 foot module for the floors, walls and roof and a 24 foot grid for the columns and introduced the idea of factory construction. Iron and glass structures were used for the great 19th century railway stations and increasingly for winter gardens or pleasure palaces. The largest span was 60m at St Pancras station. In the 1950s Buckminster Fuller was working on developing larger and larger geodesic domes. Climatron at St Louis was based on his ideas. One of his futuristic ideas was to build a big geodesic dome over Manhattan. To quote his reasoning at that time.

LARGE ENVIRONMENTAL ENCLOSURES, THE ROOF OF THE MILLENNIUM DOME

Ian Liddell CBE, FREng, MA, DIC, FIStructE, MICE

Partner of Buro Happold and Visiting Professor of Engineering Design, University of Cambridge

This paper describes how ideas for enclosing very large

areas have been around for some time though without

being brought to completion The Millennium dome is

the first structure to be in this category The paper goes

on to describe the engineering and construction of the

Dome

Fig 1 A Victorian Winter Garden

There is a long history of dreams of creating large enclosures to ameliorate the climate in inhospitable parts

of the world This trend started in the 18t h century when the owners of fashionable country houses built heated glazed enclosures to grow Pineapples and grapes These stoves or conservatories were generally built of stone with the glass in conventional wooden frames As the glass making technology improved and iron working techniques developed the greenhouses became larger All iron glass houses appeared at the start of the 19t h

century There is a classic example remaining at Bicton

in Devon This structure is more like a shell with small panes of glass set in fine wrought iron ribs The classic examples of this type of construction were the Palm house at Kew built in 1848 and of course the Crystal Palace for the 1851 exhibition The Crystal Palace was a relatively simple modular construction based on an 8 foot module for the floors, walls and roof and a 24 foot grid for the columns and introduced the idea of factory construction

Iron and glass structures were used for the great 19t h

century railway stations and increasingly for winter gardens or pleasure palaces The largest span was 60m at

St Pancras station In the 1950s Buckminster Fuller was working on developing larger and larger geodesic domes Climatron at St Louis was based on his ideas One of his futuristic ideas was to build a big geodesic dome over Manhattan To quote his reasoning at that time

"The way the consumption curves are going in many of our big cities it is clear that we are running out of energy Therefor it is important for our government to know if there are better ways of enclosing space in terms of material, time, and energy

If there are better ways society needs to know them Domed cities can be illuminated by daylight without direct sunlight That part of the dome through which the sun does not shine directly would be

transparent In summer the dome would be protected

by polarised glass; during the sunny hours it would not hold heat but in winter the sun would penetrate all the dome The atmosphere will be dust free

Controlling the environment through domes offers the enormous advantages of the extroversion of privacy and the introversion of the community"

(Reference 1)

Iron and glass remained the preferred materials for large

environmental enclosures and are still frequently used

today Today the glass technology has developed with

large panels of toughened and laminated glass supported

on ever more daring steel structures but the spans and

the scale of the enclosures has not increased

significantly

In thel950s new translucent polymers and plastics

became available One development in particular was

coated fabrics which offered new freedoms in form and

span for large enclosures Starting in the mid 1940s

Walter Bird developed air inflated structures initially for

radomes but later for tennis halls and large sports halls

The main translucent fabric materials developed at this

time were PVC coated polyester and later in the 1970s

PTFE coated glass fibre cloth The big breakthrough for

very large covered areas was Walter Bird's low profile*

cable dome (Reference 2)

Fig 3 Walter Bird's Cable Dome for spans greater than 300m

This was adapted by David Geiger for the US pavilion

for the Osaka expo in 1970 The principle was then used

in a reduced cost form for several large football stadia

Unfortunately these structures had large valleys along the

cable lines which collected snow and caused local

ponding occasionally leading to loss of pressure and

deflation to a stable down-hanging position These

deflations caused unacceptable co-lateral damage to the

fabric leading to the abandonment of the form At the

time the air supported structure seemed to point the way

towards the city scale environmental enclosure

envisioned by Buckminster Fuller and concepts were put

forward by Walter Bird

In 1970 Frei Otto and his colleagues at the IL put forward

a scheme for a covered city in the arctic The covering was to be an air-supported fabric structure 2km in diameter with an area of 3,000,000 m2 (Ref 3) The primary structure was to be a net of "Trevira" Polyester ropes

Fig 5 Walter Bird's Cable Dome for spans greater than 300m

I helped Peter Rice with some calculations of the forces under wind load The effects of snow were happily glossed over with the assumption that because of the smooth shape of the roof the wind flow conditions would be close to potential flow where the wind would sweep the roof clear of snow The provision of building services and the management of an enclosure of this scale were not considered in great detail at that time

Ten years later in 1980 we in Buro Happold had the opportunity to undertake a feasibility study for covering

a town in Northern Alberta The leader of this design team was a Canadian architect called Arne Fullerton and again we worked with Frei Otto The study was exceptionally interesting in that it included the human response of living in such a space as well as the servicing requirements and other considerations such as the impact of fire One of the designs for this enclosure was

a 150,000m2 air-supported roof that was to have steel strand cables at 0.5m spacing and would use ETFE foil cladding This design was taken to concept stage and was supported by calculations and reports This time the snow loading was not glossed over but we did not know how the building would respond to the extreme snow falls for the area

Fig 6 58°N 18 Ha Air Supported Structures

Experience with large air-supported roofs in North

America had demonstrated how snow on such roofs

could initiate ponding and cause severe maintenance

problems (Ref 4) Our roof would have had a much

lower rise to the foil cladding and hence not such deep

valleys to initiate snow drifting The cushions would

also have had higher pressures to support the snow Even

so there would have been a considerable risk of problems

with snow

Fig 7 Chelsea and Westminster Hospital Atrium, part clear and part

tinted ETFE for cushions

There was no experience with the use of ETFE foil as a

cladding material at that time 10 years later in 1990 we

engineered a roof for the atrium of the Chelsea and

Westminster hospital with this material Subsequently

we designed a roof for a tennis hall which consisted of

foil cushions on a tensioned cable structure

Now 20 years on we have built the Dome on the

Greenwich peninsular for the Millennium Experience

which is to be held in the year 2000 This roof is 80,000

m2 and is the nearest structure yet to these dreams of

covered urban environments It will enable us to

evaluate the performance of such a space

P R I N C I P L E S O F T H E D O M E R O O F

S T R U C T U R E

The structural concept for the roof is based on the innovatory principle of using straight tensioned cables and flat fabric for the structure rather than adopt doubly curved surfaces which had become the accepted form for such structures In 1994 Buro Happold pointed out that there was considerable advantage in using straight tensioned cables which could carry both the uplift and down loads with resultant forces in the same direction (Ref 5) The pretension stiffens the cables against deflection allowing high strength tensile materials to be used to create very large spans Flexible fabric or foil can be used as cladding between the cables provided it is pre-stressed in the same way as the cables The advantages of the arrangement are:

Compared with a two-way cable net one set of cables

is eliminated along with the cross clamps and terminations

Whether the load is upward or downward the cable tensions are in the same direction which can be a great advantage if the tensions are resisted by a funicular arch or ring beam

Connections to the foil or fabric cladding can be greatly simplified

It was claimed that taken together these benefits would result in very economical large span roof structures However the stressed fabric resists local loads by relatively large deflections rather than by simply increases in stresses Concentrated snow drifts could possibly create a deflection which would be so great that water did not drain out from

it This would be potentially disastrous The key to adopting this concept is to develop a form where there is adequate drainage to avoid ponding problems and details which will allow for the deflections Wind stimulated dynamic oscillations are not a problem provided the fabric

is fully tensioned as there is a high degree of damping from the fabric and the attached air

•

Fig 9 RSSB Tent, the fabric and cables can be seen responding to

wind from the right

In 1994 opportunities arose to utilise this concept on two

structures, the Eastleigh tennis centre and a very large

demountable tent of 20,000 m2 for RSSB The latter

proved to be extremely economical and met the owner's

requirement of ease of installation

T H E D O M E R O O F D E S I G N

The structural concept for the Roof of the Millennium

Dome is apparently very simple 72 tensioned steel

stringer cables in pairs of 032mm steel spiral strand are

arranged radially on the surface The stringers are

supported at a radial spacing of between 25 and 30m by

an arrangement of upper hanger and lower tie-down

cables that are arranged around the 12 100m tall primary

steelwork masts Circumferential cables keep the stingers

on their radial lines

Fig 10 Dome Primary Structure

The forces in the radial stringer cables are taken by a

central 30mm diameter cable ring supported by forestay

cables, which run through to the centre point, where they

support a flying mast, which in turn supports additional

radial cables which carry the cladding The central area is

formed by a cable truss connected to a 30m diameter cable

ring The level of stress within the cable ring leads to a stiff

structural form

Between the cables, tensioned coated fabric is used as cladding Both the tensioned cables and cladding carry the loads by deflection accompanied by increase in tension This concept is simple but there are dangers associated with the deflections particularly ponding caused by snow or heavy rain When loaded by wind or snow, the upper hanger, the lower tie-down and the stringer cables carry the loads from the fabric down to the ground The stringer cables are restrained at the perimeter by the perimeter masts and large boundary cables attached to 24 anchor points The vertical components on the forces at these points are resisted by ground anchors grouted into the London clay and the horizontal forces are resisted by a compression ring beam under the external wall

Tension structures rely on the shape of the stressed surface for their performance under load Forces are resisted by the tension and the curvature, the greater the curvature the less the tension required to resist a given load Both the radial stringer cables and the fabric are prestressed with sufficient tension to stiffen them against imposed load deflections The prestress levels and cable geometry were selected to provide adequate deflection control, with the materials and structural sizes selected to provide high stiffness

Fig 11 Ring Beam

In the event of a loss of a panel of fabric, it is important that some of the in plane prestress is maintained in the panels adjacent to prevent the deflections rising to unacceptable levels During the normal operation of the structure, all fabric forces pass through the plane of the surface and are resisted by equal and opposite forces that arise in adjacent panels In the event of a panel failure (or removal) this balance is upset and the radial cable connection nodes would be forced out of line Circumferential cables through the nodes were required

to maintain their spacing and resist these forces

The dome roof shape with tapering segments has an advantage in resisting ponding in that the span of the fabric panels increases as their slope increases so the fabric surface gets progressively softer However, If the

Fig 12 Wishbones to raise circumference cables

circumferential cables were in the surface of the fabric

they would cause a dam at each circumferential line so an

arrangement was required which would take these cables

out of the surface

This was achieved by raising the circumferential cables

above the surface with rigid members (wishbones) and

connecting them to the nodes with criss-cross cables

Lower circumferential cables were also required to

control the tiedown cables; these were also spaced off the

surface but with out the criss-cross cables

It was also necessary to control the deflection of the

radial cables Their length is very long, 150m from the

perimeter to the centre Because of this if one 25m span

were loaded the remainder of the cable in the line would

act as springs so the loaded span would not be as stiff as

if it was fixed at each end The only way to gain the

necessary stiffness is to use a high pretension In fact the

planned pretension in each radial line is 400kN, about

2/3 of the peak tension The last element in preventing

ponding is the patterning and prestress in the fabric

panels

During the tender period some development of the design

continued We decided to change the central node for a

30m diameter cable ring This was constructed with

12-48mm diameter cables Because of the redundancy

implicit in the 12 cables, failure of one of these cables

would not compromise the overall safety of the roof

These changes were brought in to the contract package

before the contract was finally placed

S T R U C T U R A L D E T A I L I N G

With cable structures it is essential that the details respect the system lines and system points of the cables and their intersections, as well as the likely movements of the cables at the connections If the radial cables were continuous through the node points the flexing at those 'points would cause the cables to fail prematurely in fatigue At every hanger location, the radial stringer cables are connected together at a node detail This detail allows the high radial forces to pass directly thorough into the adjacent cables and allows the hangers (both upper an lower) to be connected into position

The vertically oriented connection plates allow the radial cables to rotate on their end fittings as they deflect under load The flat top plate stiffens the node against shear forces and provides a surface that the fabric can be clamped onto to form a weather seal

D E S I G N V E R I F I C A T I O N

As is usual for major building structures the safety of the design was verified by calculations These relied on using our "Tensyl" program for calculating the forces in the fabric and cable structure A Vj2 model of the cable system was modelled using Tensyl, to investigate the effects of cable prestress and the environmental imposed loadings upon the structural system The tensyl analysis was then expanded to a V2 model to allow us to predict the maximum cable and fabric forces, the compression loads in the supporting structures and the deflected shape

of the 'total' system

Analysis of the cable system has shown that the behaviour of the structure is very sensitive to cable stiffness The radial stringer cables rely upon a high level

of prestress (about 400 kN) to ensure that deflections are

controlled during each load case Under load it is critical

that the stretch in the mast hanger cables is minimised

Each cable has been sized not only for strength and

ultimate load capacity, but also for axial stiffness in order

to ensure that the cable system does not 'go soft' when

under load This is the reason why we have used spiral

strand cable, a much stiffer product than standard IWRC

wire rope

Wind loads were derived initially from published data They

were then confirmed by wind tunnel testing at the BMT

wind tunnel at Teddington The safety of the components

was investigated following normal design rules

Resistance of the whole structure to accidental damage is

provided by redundancy, i.e the structure can tolerate the

loss of an individual component without collapse This

principle also applies to the support pyramids which are

designed to withstand the removal of a leg

The 90m long masts were constructed with 8 323mm

diameter tubes braced with rings at 2.5m spacing Their

overall diameter was limited by transportation

requirements and a great deal of computer calculation

went into verifying their load capacity

The limiting load was calculated using LUSAS in a non

linear mode Since the masts are leaning deflections

under self weight and icing have to be taken into account

as well as initial out of straightness Wind loading is also

significant but this does not occur with the peak down

loads from snow and icing

Fig 14 Masts

S T E E L W O R K C O N S T R U C T I O N S T A G E

The selected steel contractor, Watson Steel, were obliged

to develop the engineers design drawings into shop

drawings for the production of the components This

process involves an element of detail design of the

components and connections The shop drawings show

the cutting and holing dimensions of all the plates as well

as the welding and connection details These drawings

are reviewed by the engineer and architect for approval

prior to the start of fabrication of each particular part

The cable work was subcontracted by Watson to Bridon Ropes of Doncaster The cables have to be wound from wires that have been previously drawn and galvanised For the dome project, class A galvanising, the lightest, was specified for cables which were beneath the roof and Galfan, a mixture of aluminium and zinc galvanising which is much more durable, for the external cables The cable has to be pre-stretched to eliminate the construction stretch and then marked to the correct lengths under the specified pre-stress load Most of the cables are dead length without any provision for adjustment, consequently great care has to be taken to ensure that the cables are made up to exactly the right lengths

The lifting of the masts was planned by Watson with great care This involved selecting a suitable crane and devising lifting positions which would not overstress the masts Each mast was lifted and guyed with the two permanent backstays and two temporary forestays There is also an intermediate position while the crane was released when only one forestay could be used and a short term guy was added from the centre of the mast to the adjacent base While the mast is guyed with the temporary forestays the central ring is lifted by the permanent forestays During derigging of the crane and the operations of changing the guy positions the tensions

in the guys had to be carefully controlled to maintain the stability of the mast

Following the lifting of the ring, the guy system was moved so that the rest of the cable net could be assembled and lifted to its place This was done using hydraulic cable jacks with the hoisting cables running over sheaves on the top of the masts

When the net was completely assembled and all the cable lengths checked, each of the 72 pairs of radial cables had

to be tensioned This was achieved in several steps using

a 550 kN capacity 'Enerpac' pull jack in the pre-designed jacking points at the front of the perimeter masts Because of the flexibility of the central ring and the boundary cables the tensioning of the radial cables had to

be done to specified dimensions rather than to specified loads with final adjustments made at the end

S E L E C T I O N O F C L A D D I N G A N D T H E

I N T E R N A L E N V I R O N M E N T

The roof is to provide a controlled environment for the exhibition and for what other uses it may be put The human response preference is for a bright translucent roof with a light spectrum as close as possible to daylight This requirement conflicts with the needs for the central show and some of the exhibits for which lower light levels would allow greater impact from exhibition lighting Coated fabrics tend to change the spectrum to a brownish hue rather like tungsten lighting This of course affects the perception of colours within

Fig 15 Perimeter masts with jacking points between

rigging screws

the dome and according to our researches for the 58° N

project can affect the physical performance of people

within the dome

It is difficult to have a translucent fabric roof with

insulation but with out any insulation condensation will

occur on the underside which, in certain conditions, will

fall as rain This situation would be totally unacceptable

in a building that will have a lot of electrical displays To

reduce this risk a lining can he installed under the main

fabric There has been a considerable amount of

experience with fabric roofs with linings where

condensation has not been a problem Checks were run

on the risk of condensation as part of the environmental

modelling and they demonstrated that with two

membranes the risk of condensation on the underside

was very low

The available materials for cladding the dome were

PTFE coated glass fibre cloth, PVC coated polyester

cloth or ETFE foil cushions Our preferred material for

the roof as an environmental enclosure would have been

ETFE foil This would have provided a high translucency

roof with three layers of foil which would have a

considerable amount of insulation effectively eliminating

the risk of condensation Unfortunately we did not

consider that there was sufficient experience with

detailing this material in this situation and we considered

that it would be too risky to try to develop a suitable

system within the very tight time scale

The necessary properties of durability and flame

resistance are provided by PTFE/glass without the need

for any additives The glass fibres are not affected by UV

light but they are damaged by water

The function of the PTFE coating is to protect the fibres

from water and abrasion, the PTFE itself is nearly inert

and is not affected by the weather The fabric is seamed

by heat sealing using a FEP interlayer which melts at a

Fig 16 PTFE/Glass fabric

temperature of around 350°C A benefit of this is that the material can be repaired on site with a permanent seam that is the same as those done in the factory

With PVC/polyester the fibres are damaged by UV light and they burn so the function of the coating is to protect the fibres from UV light as well as providing the flame proofing The PVC itself is light stable and does not burn well but it requires a number of other compounds such as pigments, UV stabilisers, plasticisers, fungicides and flame retardants to meet the functional requirements Since 1987 several of these compounds especially fungicides and heavy metal stabilisers have been banned and this has led to an increase in problems of fungal growth in the yarns which severely discolours the cloth This situation has recently been improved by the use of anti-wicking treatments to the yarns The other big problem with PVC coatings has been dirt retention The PVC coating is porous and the plasticisers absorb dirt This has recently been improved by the use of fluoropolymer surface lacquers which give it a durable sealed surface After investigating the products of the three best coaters in Europe an outer fabric was selected which gave 15% translucency and an inner lining fabric which gave 75% The combination gave the highest translucency, about 12%, and a good colour rendering The fabric selection was changed to PTFE/glass after a political decision to build the dome with a long life

F A B R I C W O R K

The contractor who had made the best offer for the PTFE/glass material was Birdair from Buffalo, New York State They have been producing structures in PTFE for over 20 years including some 12 covered stadiums of approximately half the area of the dome

The fabric patterning and attachment details had to be modified to accommodate this alternative material and since time had been lost in the programme, this had to be done in a very tight time scale Because of the

arrangement of the panels within the cable net, and the

fact that the cloths were to be fitted in to dead lengths, the

patterns had to be extremely accurate Since the warp

direction of the panels of the outer fabric ran radially on

the roof with 25m long cloths, it was necessary to model

the fabric as an equal mesh net to represent the warp and

fill lines of the cloth This was a much more time

consuming method than the standard method of

representation using triangular elements These basic

geometry patterns were converted by Birdair into cutting

patterns They also built in the stretch compensations,

which were agreed after biaxial tests on the actual

production cloth and added in all the edge details.which

were agreed after biaxial tests on the actual

productioncloth and added in all the edge details

Fig 17

The fabric attachment detail proposed by Buro Happold ,

was a double luff groove extrusion fitted onto the radial

cable pairs to accept a roped edge on the fabric Birdair

proposed a 12mm edge cable in the fabric which would

hook into special clamps fixed to the cables The clamps

were developed into a two part extrusion cut into 50mm

lengths and retained by two 12mm bolts Fabric sealing

flaps were closed over the top of the site joints and sealed

together using a hot iron at 380°C and an fep inter-layer

Fig 18

T U N N E L V E N T A R E A

A 50m diameter hole was required in the roof around the Blackwall tunnel vents, to accommodate the 'air rights'

of the ventilation structure After considering a number

of ways of leaving a hole in the fabric, Buro Happold adopted a net of 8mm cables at l m spacing which would replicate the stress-carrying capacity of the fabric but would allow the vent air to pass through The cable net was attached to the fabric with clamp bars at the edges The net patterns were developed directly from the typical fabric patterns with the boundary line being defined to align with the top of the enclosure The net arrived on site

in rolls and was erected in the same way as the fabric using the same extruded hooks modified with a steel plate to which the cable terminations were attached

L E A R N I N G F R O M T H E D O M E

There are two questions; is a large environmental enclosure of 10 or 15 hectares feasible If so would it be energy efficient and provide a improvement it living conditions in extreme climate zones The zones would be either semi-arctic or desert requiring either raising or lowering the internal temperature

F o r m

The selection of the form of the Dome as a shallow spherical cap is beneficial for wind and snow loading The smooth profile generates a smooth airflow over the surface with of local turbulence It also reduces the uplift pressures so reducing the tensions in the cables

Snow will always be a problem on transparent or translucent roofs If not from the load effects then because

it excludes the light In windy conditions the snow will mostly blow off Dividing the roof surface up into individual panels with valleys between has been shown

to give trouble because of the concentration of snow which can build up there The smooth Dome form avoids this, it also has the useful property of having increasing slope with increasing fabric span that reduces the risk of ponding This form could be increased to say double the area without compromising the structural behaviour

Unfortunately we may have to wait several years to get a few snow storms to prove or disprove the behaviour of the Dome under snow loading

Internal A i r

The biggest problem with the internal environment has proved to be the dust Firstly that generated by the construction operations which began immediately the roof was completed and continued for a year We think that the ongoing running operation of the dome will create a dusty environment generated by the visitors

moving around and by the cleaning operations with dry

brushing There is also the dust and dirt coming in with

the external air in the polluted environment The solution

would be to hose the surfaces down This is the method

adopted in large covered stadia but of course it causes

condensation on a single skin roof

The special qualities of fresh air are not well understood

but dust and pollutants are known to make it unpleasant

In a covered environment a plentiful supply of outside air

is normally required to keep it smelling sweet If the

temperature outside is sub zero any cold air introduced

will flow across the ground floor tending to defeat the

object of the covering It would be interesting to

investigate to what extent fountains and "rain" would

clean the air so that air changes can be reduced

Heating

In winter the warm air immediately migrates to the top of

the space until it looses heat so heating the air is not very

energy effective A better way is to heat the ground to

improve the local comfort and let the air look after its

self This is more or less the approach adopted in

Victorian stoves where the heat was introduced via pipes

in ground trenches Ground heating can be done with

low grade heat supplimented on sunny days by radiant

heat from the sun and heat from internal buildings

Because the covered ground area is so large little heat will be lost to the outside although it might be absorbed into the ground at a low temperature Heat exchange to the ground will change its temperature very slowly and

the deep ground temperature of 8 or 10 deg C will

provide limits

Cooling

In desert conditions the enclosure would need to provide shading This can be provided with a single skin of Teflon/glass fabric The problems of condensation would be very much less than in cold climates Cooling

at ground level is easier to achieve since the cold air tends to stay on the ground To gain a benefit from the enclosure it would be necessary for the ground to act as

a coolth sink Would this happen by natural means or will mechanical cooling systems be required?

Cladding

In the case of the Dome the dust has made the lining fabric unacceptably dirty This is largely due to the particular material supplied by Chemfab Called Fabrasorb It is marketed as a sound absorbent material because of its porosity In reality it is glass fibre cloth barely coated with teflon and the coating is easily damaged by handling The porosity improves the

translucency at first but the result is that the dirt gets into

the fibres and cannot be removed There are now more

translucent PTFE/glass fabrics around, the problem is

that the inner layer may not bleach out because of the

lack of UV penetrating the outer skin The porous lining

will also allow water vapour to pass through and

condense on the outer skin

The solution for a habitable enclosure has to be to treat

the roof as for a swimming pool This means that there

should be sufficient insulation to prevent condensation

The best way to acheive this is to use three layer ETFE

foil as was originally proposed for 58 deg North The

benefit of this approach is that a vapour barrier would not

be required on the ground so it could be treated is it is

outside Plants could be grown anywhere, water features

could be used and the interior could be cleaned with a

hose with water soaking into the ground and evaporating

from it

The Dome and other large enclosures such as the Eden

project will enable us to obtain data to evaluate the

benefits of such structures and define the limits to the

size This may be determined by fresh air, roof drainage

and access requirements as much as by the structure

R E F E R E N C E S

1 R Buckminster Fuller and Robert Maries, The Dymaxion World

of Buckminster Fuller, 1960, Anchor Books Edition, 1973

2 Walter W Bird, The History of the Air Structures in the USA IL16 Zette, publication of the Institute for Lightweight Structures, University of Stuttgart

3 IL2, Publication of the Institut for Lightweight Structures University of Stutgart, 1971

4 Liddell W I, 'Minnesota Metrodome' A study on the behaviour

of air supported roofs under environmental loads Structural Engineering Review 1994, Vol 6, No 3-4, pp.215-235, Pergamon

5 C Gill, I Liddell, C Schwitter, Straight cables for tension structures Procedings IABSE symposium, Birmingham 1994

6 Liddell W I, 'Creating the D o m e ' The 1997 Royal Academy of Engineering, Hinton Lecture published by the RAEng

7 Liddell WI, Miller PW, 'The design and construction of the Millennium D o m e ' , The Structural Engineer, Vol77, No7, 6 April

1999

8 Elizabeth Wilhide, The Millennium Dome Ted Smart 1999