Introduction

Looking at some of the existing large-span structures - e.g. on http://largedomes.com/largest/ it appears that quite a large number of different approaches have previously been employed to achieve large-spans.

The largest-span enclosed space in the world is in Oita, Japan - the dome there has a diameter of 245 metres.

Bridges

In the world of bridges, suspension bridges produce all the largest spans in the world.

In terms of their potential for spanning large distances, bridge designs stack up against each other something like this:

• Suspension bridge: the largest spans 1989 metres;

• Cable-stayed bridge: the largest spans 890 metres;

• Arch bridge: the largest spans 550 metres;

• Cantilever bridge: the largest spans 549 metres;

Suspension bridge

Cable-stayed bridge

Arch bridge

Cantilever bridge

Looking at the largest large-span arenas in the world, none of them use the catenary suspension technology used in suspension bridges.

A few use cable-stay technology - the largest of which are the Muna Bulk Reservoir Cover (210 metres), and The Millennium Dome (206 metres), and the Cardiff Millennium Stadium (181 metres).

What type of technology do the domes use? There is some variation - but here are some of the most common principles:

• Arches

  The principle of an arch is the principle on which the five largest span domes in the world are constructed on.

  From the perspective of large-span bridges, using an arch is not normally regarded as being very sensible. The largest arch in the world is the Lupu Bridge. This spans 550 metres - much smaller than cable-stayed bridges and much much smaller than catenary- suspension bridges.

  The reason arches are so poor at spanning large distances boils down to weight. Struts in compression have to be fat and heavy to avoid buckling - and the effect accumulates - if you make something in the middle of a bridge fat and heavy the stresses on the components on either side are increased - so they need to be fatter and heavier too.

  So: why are dome builders still using this technology for creating large spans? I have yet to hear a satisfying reason given in answer to this question.

• Aspension

  Another technology used by the dome builders that begins to take advantage of the tensile properties of materials is represented by structures such as the Tropicana field. This is distributes struts along the length of the span - rather than piling them up at each end as a suspension bridge does.

  I will refer to this type of design as an "aspension" structure - following the terminology of Buckminster Fuller, who - as far as I can see - came up with the idea originally.

  It is challenging to compare real aspension bridges to suspension bridges - because no aspension bridges have actually been constructed.

  However, probably the main reason no aspension bridges have been constructed is because the idea doesn't make much sense.

  Why would a designer distribute their compression members along the bridge - where their weight has to be supported by the cables - when they can pile them up at each end on solid land - taking advantage of cheap-but-heavy materials such as reinforced concrete - and in the process wind up with a bridge which is much stronger - due to being anchored to points far from the roadway?

• Air support

  The next most common type of dome technology is the air-supported dome.

  However, no large air-supported structures have been constructed for decades.

  The technology fell out of fashion - around the time when aspension structures came into vogue.

  Air-supported structures have some problems associated with the inconvenience of maintaing internal pressure - and can suffer from occasional deflations.

Suspension technology

For more details, go on to the "why suspend" page.