Why suspend?

Large span structures are often expensive:

• 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 pounds.

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.

Bridge comparison

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

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 significance.

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.

Tensile structures

All buildings have a balance between forces of compression and tension.

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

Two dimensions raises the possibility of air suspension. A number of large air-supported structures have been constructed.

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.