[Re]Thinking Airhalls; evolution of a tennis
dome product.
Paul Romain1
1
Ingenu Limited, Bellotts House, Bellotts Road, Bath, BA2 3RT, England
Abstract
Air-supported structures come in many forms and sizes. Their economy of materials
and large span capability is as relevant now as it was when Lanchester filed his first
patent in 1917. The long clear spans are particularly appropriate to sports enclosures
from stadia to swimming pool and tennis court covers. However, their very nature
lays them vulnerable to damage and collapse and even though good design can
mitigate this, previous failures and current perceptions have hindered their
widespread adoption.
An opportunity arose in 2004 to rethink the airhall for a tennis club in Surrey, UK
where the Client wished to replace a damaged two-court demountable cover using
existing components. This project led to a subsequent commission the following
year, to replace an existing airhall at another tennis club in London. Here, a
complete design package was required addressing the key components of
foundations, envelope, inflation controls and access and the formulation of a
‘product’ began to evolve. Despite its success, attempts to market the product - an
engineered mid-budget tennis dome – were thwarted by non-technical issues.
However, other related projects arose, including the replacement of a larger multisport structure at a school in Wales.
The paper discusses the three air supported projects and identifies key engineering
and design features which were re-thought. Obstacles to the advancement of the
tennis airhall product are discussed. It is suggested that consideration of these may
be relevant to a wider range of air-supported structures and along with thoughtful,
robust design, instrumental in their next cycle of popularity.
Keywords: airhall, air-supported structure, tennis dome, product design
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1
Introduction
Of the many types of air-supported structures, it is the inflated single skin, wide span
type of enclosure to which this paper relates and which echoes the designs shown in
the 1917 patent of FW Lanchester [1].
Structural engineers often spend many hours developing a bespoke structure for a
specific set of design criteria. Whilst they can be enthusiastic in wishing to
commercialise the final design, or an aspect thereof, this is often not possible. Yet in
2004 an unexpected opportunity arose to develop a product for an ‘airhall’.
The evolution of a tennis dome product (known as ‘Airplay’) is presented through
the process of three linked projects which took place over a period of seven years.
Their success is considered in the concluding paragraphs.
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Historical Background
The purity of form offered by air-supported structures was exemplified by the US
Pavilion at Expo’ 70 in Osaka., combining a column-free clear span with minimal
material weight. These benefits had been equally appropriate to the development of
Radomes in the 1950’s. The design principles were then commercially exploited in
small and medium sized enclosures for swimming pools, tennis courts and other
smaller sports venues.
Since their conception nearly a hundred years ago, the popularity of these smaller
‘bubbles’ has been particularly cyclic, having been influenced by materials
development, fashion trends and failures, the latter usually as a result of poor
maintenance or extreme weather events.
It is intriguing that the Wikipedia entry for Lanchester [2] makes no mention of his
patents for air-supported structures, yet his description of the components and
observations on design are just as relevant today. He describes the key components
of an airhall as the enclosure membrane, the inflation blowers, the foundations and
the access. In a subsequent patent, Lanchester even indicates the design for an
enclosure for two tennis courts – foresight indeed.
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Tennis Dome Design
The first two projects described in this paper are demountable, single skin PVC
polyester membranes restrained and reinforced by a net of steel cables over the top
surface. The third project deviates from this approach, has no cables, incorporates a
second inner liner membrane and is a permanent installation.
The stability of the outer membrane is gained by an internal pressure of around
250Pa generated by a blower unit. To resist higher winds or snow loading, this
pressure can be increased. Foundations resist the uplift forces around the perimeter
of the airhall, with access via air-locks using revolving doors or an airtight lobby.
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For an enclosure of cylindrical section, a relatively straightforward calculation can
be made using the equation T=PR. The perimeter tension (T) will be at an angle and
not purely vertical; the pressure (P) will be a function of inflation and wind uplift
pressures; and the radius (R) will be a function of the geometry (span and mid-span
rise).
The Lawn Tennis Association
(LTA) stipulates minimum
clear zones around a standard
tennis court within an airhall
enclosure. As a result, typical
geometry and thus structural
requirements can be derived
easily. These zones are
greater than for a traditional
building form and it is usually
difficult to satisfy all the
Figure 1. Tennis court
dimensional parameters when
dimensional requirements (LTA)
covering existing courts.
For a double court enclosure, the typical plan dimensions are approximately 36m x
36m, with a maximum height around 10m and with cables running parallel to the
length of the courts.
Key design guidance was taken from a number of references but particularly the
proceedings of the IStructE conference of 1984 [3] and the state-of-the-art report by
the ASCE published in 1979 [4]. Despite the age of these documents their content is
still relevant.
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Project Background
The apparent simplicity of smaller airhall structures attracts opportunistic companies
offering low cost ‘budget’ demountable structures – attractive to those Clients with
minimal finance. Whilst some budget products operate acceptably, others have not
lasted and lack the formal engineering basis of more expensive permanent structures
available from established suppliers.
After the failure of one such poorly executed dome at a small tennis club in Ash in
Surrey, UK, we were approached to pattern a replacement membrane. A good
relationship with the sub-contract fabricator existed and we shared an enthusiasm to
further involvement in this form of sports enclosure. We had also established contact
with the LTA who were instrumental in our involvement with the Coolhurst Tennis
Club in North London when they sought to replace their single court cover.
Coolhurst was a complete airhall package, including foundations, and spawned the
idea of a product which could be developed and marketed. Although the product
idea faced challenges, our fresh experience of air-supported structures led to another
project at a school in Haverfordwest, South Wales, whose multi-sports dome had
collapsed.
These three projects are described in more detail below.
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5
Ash – Analysis and Membrane Detailing
The Ash Manor club is a small private club which also provides facilities to the
adjacent school. Two pairs of tennis courts are each covered with airhalls for the
winter season. The lower pair were covered successfully by a budget dome and
served as a useful benchmark for the re-covering of the two upper courts. Whilst the
Ash project started as a simple patterning request, there were no as-built records. It
was clear that a greater level of engineering input was required and so it was agreed
to replace the fabric and cables, but to re-use the existing two revolving door
assemblies, the ground anchors and the inflation fans.
5.1
Analysis
In order to prove the design of the enclosure components, some rudimentary
analysis was required. An assessment of the snow and wind loading was made to
British Standards and used to set the target inflation pressures of 250Pa at steady
state and 600Pa to resist peak snow loading and storm winds. These levels agreed
closely with those proposed in air-supported structure codes such as BS6661,
although this had been withdrawn.
5.2
Inflation
The original enclosure interfaced with the tennis clubhouse via a poorly assembled
fabric link. There would have been significant leakage at this interface which would
have contributed to low inflation pressures and consequential loss of airhall stability.
The low inflation could also be attributed to twin blower units which were in a poor
state, and ran off a single phase electricity supply. Whilst similar to those used on
the lower courts, we insisted that a new fan be used for the new enclosure.
Figure 2. Typical fan
curve.
Fan selection is probably the most
complex component to specify.
The key parameters are volumetric
output and pressure, with each fan
design having its own unique fan
curve. A system curve can be
derived considering the ducting
losses and leakage through the
membrane and its connections
based on the target inflation
pressures noted above. Overlaying
the two curves gives the first step
towards appropriate fan selection
although other factors including
power consumption, noise and fan
efficiency need to be considered.
On this project, with a single phase electrical supply, even the largest available fans
would have struggled to meet the stipulated criteria. Consequently, some
compromise on the target pressures was made but the eventual unit was still superior
to the old fans. These were incorporated within the design to augment the pressure
for the snow and storm conditions.
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5.3
Fabric and Natural Lighting
Although a heavy grade PVC fabric can satisfy design requirements without the
need for cable reinforcement, these enclosures must be lit from the interior which
carries a risk of damage to the envelope if the dome deflates, or safety issues if the
lighting units are suspended from the enclosure itself. Lighting from outside the
enclosure using existing floodlighting is possible with a clear outer membrane and
simplifies installation, as well as making full use of natural light during the day.
Figure 3. Ash - Interior
5.4
Introducing a network of 10mm
diameter cables over the top
surface reduces the span of the
fabric and compensates for its
lower
strength.
A
lightly
reinforced clear PVC material with
an 85% light transmission was
selected for the majority of the
enclosure, combined with a more
typical Type I solid PVC/polyester
around the perimeter and a Type II
for a durable skirt in contact with
the ground.
Perimeter Detailing
The cables linked directly to the top of the existing ground anchors. These had been
installed at nominal centres of 1.2m (sides) and 1.5m (ends) but varied wildly.
Where anchor pairs were significantly out of alignment across the dome, steel
transfer angles were introduced. To avoid the usual slack areas in the rectangular
corners, the corners were truncated at 45 degrees in plan using a similar steel
member. Although only 1.5m long, this chamfer created a much improved form,
more uniformly stressed fabric and better access internally.
Traditionally, the bottom edge of the fabric would be clamped down to a fixed
perimeter beam but this was outside the budget of this project. Hence, to minimise
movement of the fabric relative to the cables, reduce air leakage at ground level and
transfer the fabric tensions direct to the anchors, a webbing belt was placed around
the perimeter, attached to the skirt and retained with d-rings sharing the cable/anchor
connection. The bottom of the skirt tucked under the carpeted playing surface.
5.5
Patterning
Empirical patterning methods for airhalls tend to constrain the shape to a regular
form. To compensate for smaller plan dimensions additional height in the corner
zones is required if LTA guidelines are to be met. Formfinding techniques usually
employed on anticlastic tensile structures were adopted, manipulating membrane
stresses and cable dimensions to gain an extra one to two metres height. This
approach also enabled the generation of patterns for the two different ends to match
the asymmetry of the revolving door placement and the variable anchor spacing.
Fabric stretch compensations were introduced not only to allow for the elastic
stretch of the fabric, but also to allow for the rise of the fabric between cables. The
dome was constructed as a separate middle and two ends for ease of handling in the
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factory, with the union of the three pieces as the final welding operation before
delivery to site as a single piece. Links to the doorways were tailored on site.
Figure 4. Ash - External view
Coolhurst – The Complete Package
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The Coolhurst Tennis and Squash Club was founded in 1903 and comprises 11
courts. Only one court at the West end of the site had a seasonal cover, albeit the
first tennis dome in the UK, originally built in 1964 for the princely sum of £4000.
The fabric had been replaced in the 1980’s but with the inflation equipment also
nearing its end of life, the decision to replace the complete structure was taken. A
club committee member, Mike Russum, acted as project manager. As an Architect,
he could see the advantage of employing professionals to tailor a bespoke enclosure.
Much of the enclosure design echoed that of the Ash airhall above ground using the
same materials and a new rationalised cable net arrangement. The key challenges
were the foundations and the inflation unit.
6.1
Foundations
The use of tension anchors came into question when the Coolhurst site investigation
uncovered a particularly complex set of conditions for such a simple enclosure:
1.
Courts had been newly tarmaced so minimal disturbance was required.
2.
A new concrete tree root barrier had been installed along the West
boundary which would inhibit placement of any foundations.
3.
To the South, a 0.9m diameter surface water main at 5m depth ran the full
length of the site, on the line of an old stream, restricting anchor placement.
4.
The site was old meadow lands, with the top 8m of soil being made ground
or alluvial deposits, requiring ground anchors to be at least 9m in depth, as
opposed to the anticipated 3m to 4m.
A study comparing the cost of a dead weight concrete ring beam to deep penetration
ground anchors concluded the latter to be cost competitive (around 50% at 2005
prices) although a hybrid concrete beam and anchor solution was still necessary
adjacent to the water main. The study also prompted the club to choose a two-court
enclosure which was better value.
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6.2
Inflation and Controls
Building on the experience of fan selection from the Ash project, a bespoke inflation
unit was designed. It features manual controls to respond to high snow fall, and an
automatic stepped control linked to a wind anemometer to increase inflation
pressure during sustained high winds. Using electronically controlled invertors, fan
speed is adjusted to produce inflation pressures between around 230Pa and 600Pa
from a single fan. Two 3-phase powered fans are cycled to ensure equal wear, and
the whole unit is mounted on wheels to allow it to be stored away from the courts in
summer. The Club opted not to invest in a back-up power supply or heating.
Figure 5. Inflation unit.
6.3
Figure 6. Control box.
Access
The airhall features a single access point located midway down the East edge.
Combining a revolving door and airtight lobby (for emergency stretcher use) into a
single unit, the bespoke design addressed the difficult site constraints. It provided a
high level of detailing, reinforced glass viewing panels, and a smooth transition
between the differing floor levels. The link between the door and the dome was
specifically patterned (unlike Ash) to improve the appearance and minimise leakage.
Figure 7. Coolhurst – Light and airy interior
To meet minimum travel distances in the event of fire or collapse, two gas-zipped
emergency exits were also provided, integrated within the chamfered corner forms
of the main enclosure at either end of the West side.
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Figure 8. Coolhurst - Exterior.
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Figure 9. Exit
Tasker Milward School – Patterning and Detailing
The Haverfordwest dome is larger than the two-court enclosures of Coolhurst and
Ash, measuring approximately 38m x 55m covering two tennis courts and a multisurface area. Originally built in 2000, it incorporated a lining fabric, a raised
perimeter concrete wall and fully controlled inflation unit with back-up. The original
dome had been irreparably damaged when the back-up generator had failed and the
dome deflated onto an independent framed structure inside. The scope of the project
was to replace the double skin enclosure including the links to the perimeter
structure. The inflation unit and door frames were to be re-used. The new challenges
were the successful patterning of the links to the two storey entrance building, the
incorporation of suitable stretch compensations, and detailing the liner.
7.1
Patterning
The concrete perimeter wall and other adjacent structures meant that access to the
courts was limited. Consequently, the outer membrane was welded in five separate
assemblies, each measuring approximately 44m x 12m, which could be manhandled
through the entrance doors. These were joined on site with a clamped connection.
The location of the joints was dictated by the fabrication constraints and the position
of the doorways, which could not be split across a joint.
Figure 10. Tasker Milward - 3D showing form and link bubbles
Fabric stresses in cable reinforced airhalls are relatively low meaning that stretch
compensations are also minimal and typically accounted for by the bulge of the
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fabric between the cables. However, fabric stresses were much higher for the Tasker
Milward dome as the Type III fabric was carrying all the tension. Consequently,
more care was necessary to incorporate compensations as high as 3% within the
patterns and to allow for the necessary de-compensation down the site joints.
In addition to the four emergency exits located at the midpoint of each side, two
further links were patterned; one to the airtight store room, and one to the two-storey
entrance lobby and changing facilities. The latter measured approximately 8m across
and 4.5m high, attaching some way up the arch of the dome. At this height, the
potential sway of the dome is quite substantial and it was necessary to balance this
sway with the movement of the link fabric without compromising its own integrity.
Figure 11. TW Exterior.
7.2
Figure 12. TW Interior.
Detailing
As on the previous airhalls, a chamfer to the corner was introduced to improve the
form and fabric assembly. The assembly was re-thought to improve the air-tightness
and integrate better with the re-used perimeter angle.
The attachment of the link forms was also an evolution of those used in Coolhurst,
and which were required to accommodate cables up to 22mm diameter in tightly
curved pockets. The Type I liner was simply suspended from the outer using cable
ties attached to welded plastic eyes, each of which had to be located and scheduled.
At the perimeter, the liner was attached to an eyelet strip incorporated within the
clamped edge detail of the outer membrane.
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Conclusions
The programme and cost constraints on a typical building project often preclude the
furthering of designs originating with that project. In contrast, a series of related
projects, with essentially similar goals, does provide the opportunity to evolve those
designs and a ‘product’ can result. This was the case for the airhall design of the
Airplay tennis dome where the knowledge and experience from successive projects
was built upon. It is also clear however, that as with all products, the evolution is
organic and that improvements can always be made.
Applying engineering rigour to develop a mid-range product fitted well within the
marketplace and the aspirations of the LTA to fund affordable, demountable
facilities. Key aspects of all the airhall components were re-appraised but the
success of a product relies on other factors beyond its functionality and design.
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On the supply-side, the team needs to have a long term commitment. Without this,
valuable effort can be wasted finding alternative partners or ultimately, production
can stall as it did with the Airplay dome. Costs need to be compatible with the
intended purpose and competing products; budget domes offer a very minimal
specification to keep costs low but do not per se attract grant funding for Clubs from
the LTA, whilst the fully specified permanent domes with heating, lighting and
back-up generators require larger Clubs who have the ability to match-fund LTA
grants. This directly relates to the customer-side consideration of potential market
size; a bigger marketplace will encourage the supply team commitment and loyalty
and provide greater product exposure. A higher volume of sales will also enable
further investment in the development of the product.
As individual projects, each of the above airhalls satisfied its own brief and was a
success in its own right. As a product, the Airplay responded well to its target brief
of an engineered, mid-range, demountable tennis dome. However, this focus limited
its potential market size and is considered another reason its progress stalled.
Each building project will remain influenced by its environment and the aspirations
of the Client. In this context, it may be more appropriate to refer to the Airplay
tennis dome as a ‘bespoke product’ – ready to be customised to suit. Certainly if the
rigour of good design and engineering helps to improve the lower end of the market,
confidence will grow in this form of air-supported structure and help it compete with
other solutions as a means of providing long span enclosures.
References
[1] LANCHESTER, F.W., An Improved Construction of Tent for Field
Hospitals, Depots and like purposes. GB Patent 119339A, 1917
[2] WIKIPEDIA, Frederick W. Lanchester
http://en.wikipedia.org/wiki/Frederick_W._Lanchester , Jan 2013
[3] HAPPOLD, E. et al, The Design of Air-Supported Structures,
Conference proceeds., Bristol, Institution of Structural Engineers, 1984
[4] BOWER, E.J. et al, Air-Supported Structures, ASCE Report, 1979.
Image credits (all images copyright): FW Lanchester, fig 1. LTA (Manipulated), fig
2. Paul Romain figs 3-11. J&J Carter fig 12. Pembs CC fig 13.
Project credits
Ash Manor (2004): Client: Tennis Together; Project Management: Dragon Covers
& Domes; Structural Design/Patterns, Ingenu; Fabrication/Installation: AJ Tensile.
Coolhurst (2005): Client: Coolhurst Tennis & Squash Club; Project Management:
M. Russum; Design & Patterning, Ingenu Ltd; Fabrication/Installation: AJ Tensile.
Tasker Milward (2010): Client: Tasker Milward School; Project Management,
Fabrication & Installation: J & J Carter; Structural Design & Patterning, Ingenu Ltd.
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