The idea behind proposing that large span roofs be
constructed using similar principles to large-span bridges
is essentially one of efficiency.
Large span structures are often expensive:
Construction costs represent a substantial proportion of
- The Oita stadium cost around 210 million dollars.
- The new Wembley stadium looks likely to cost around 757
million UK pounds.
- The Cardinals Stadium cost 455 million dollars.
- Cardiff's Millennium Stadium came in at 190 million UK
Obviously it makes sense to pay attention to efficiency -
when costs are on such a scale - since even small efficiency
savings can amount to millions of dollars.
In the case of bridges, cable-stayed bridges and suspension
bridges are the cheapest large-span constructions - above a
If extremely large domes were to be constructed,
the designers would be forced to use the principle
of cable suspension - since that is the only technology
capable of producing such large spans.
However, fortunately the largest domes are relatively small
compared to the largest bridges. Today, no domes exceed
250m in diameter, while the largest bridge is nearly eight
times that length.
Today, cable-stayed bridges are generally regarded as an
economically favoured option for spans ranging from 150
metres to 450 metres - while suspension bridges are
economically favoured beyond that point.
An enclosed arena is not the same as a bridge - but the fact
that both involve a structure crossing a divide without
ground support makes them remarkably similar - similar
enough to raise the question of whether much the same
solution applies in both cases.
Mechanics of large spans
A few words on the superiority of suspension structures when
it comes to covering large spans from the point of view of
We can be confident empirically that suspension structure
are the most cost effective solutions to creating large
span one dimensional structures.
Cable-stayed bridges have the basic advantage of being over
other structures of being as close as is reasonably possible
to get to being an all-tensile structure.
Such bridges do have struts - but these rest with
one end on the ground - and do not have to be lifted against
gravity into the air by the rest of the bridge - making the
area of the bridge over the span pure-tensile.
Tensile elements are lighter than compression elements
capable of withstanding similar forces - and in large
structures considerations relating to weight have increased
Suspension bridges also score well on stability - due mainly
to having multiple anchors at widely separated points.
What of the idea that compression elements are best off
being short - and the longer you make them the fatter,
heavier and more expensive you have to make them to
prevent them from buckling?
This is true. Suspension bridges often have large piers.
However, those piers rest on solid ground. Their weight may
be large - but because the compression members rest on solid
ground, it is not very significant. The piers can
conveniently be made of materials such as reinforced
concrete, which is strong in compression and inexpensive.
If you have struts in the middle of your bridge, they can't
be made of concrete. They have to be light.
Suspension bridges are efficient because they are tensile
All buildings have a balance between forces of compression and
However, most tensile structures exploit the trick of using
the earth as one of the main compression members.
The earth can withstand vast compression forces, over
huge distances - and is available on site without additional cost.
By allowing the ground to play the role of handling compression
forces, structural elements that have to be lifted into the
air can be put into tension - making them lighter, cheaper,
and less likely to buckle in the process.
From bridge to dome
What about two dimensional structures?
Two dimensions raises the possibility of air suspension.
A number of large air-supported structures have been
However large air supported structures seem to have fallen
out of favour about twenty-five years ago - around the time
the first aspension cable network domes appeared.
Deflation and the need to seal the interior seem to have
been the pain problems.
Another difference with a two dimensional structure is that
it has to withstand the full force of rain wind and snow in
a manner that a one dimensional bridge does not.
Rain needs to find its way to the perimeter without getting
trapped, snow potentially needs to be supported - and the
area presented to the force of the wind is much larger with
a dome than with a bridge of a similar clear span.
While these considerations will mean that building large
domes is difficult, they don't really have much impact on
the issue of whether a suspension-based design will continue
to beat arch-based designs in the case of domes.
There is no significant difference between a dome and a
bridge in that regard.