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One bridge too far for engineering practice

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LETTERS

I would like to air my views on the London Millennium Bridge.

I see no particular virtue in 'stretching the boundaries' of design when applied to public artefacts. Originality should be the consequence of practical endeavour, not the intention.

Serviceable, elegant, well proportioned and economic, by all means, but not if it takes £5 million to put right after it has been declared ready for use.

A suspension bridge is implicitly unstable unless the catenary is combined with some additional stiffening member - usually the deck, either as a truss or a box girder and sometimes with triangulated hangers. This instability increases in proportion to the ratio of non-uniformly applied load to self-weight. This is why, with some notable exceptions, the form is best suited to longspan road-bridges, where the dead weight of the structure far outweighs the applied live loads.

Where they have been applied successfully to relatively short-span footbridges (eg the work of Jorg Schlaik) the consequences of secondary deformation have been carefully integrated into the design concept.

The greater the span-todepth ratio, the higher the primary bending stresses and the greater the deflection. The Millennium Bridge is analogous to a very shallow beam with high tension loads in the cables. These have to be reacted by equally high compression forces, either through the deck or by anchors at the abutments. How these forces are resolved is not apparent. I raise this point because the design appears to have been driven by an attempt to express the primary structural diagram, yet lacks a satisfactory visual resolution. 'No matter, ' cries the structurally illiterate architect, 'it looks right to me.'

All structures have to resist horizontal forces, usually with lateral bracing, a stiff homogeneous deck or a torsion-resistant box girder, none of which appear in the Millennium Bridge. The cables are curved in plan. This may contribute to the overall ability to resist lateral forces, but it means that if one cable is subjected to greater load than the opposite some lateral movement must result.

The swaying is ascribed by the designers to horizontal forces generated by large numbers of people walking in step. This form of loading triggers a harmonic response that sets in motion oscillations. But what came first, the stagger or the sway?

Thin, flexible structures are subject to harmonic instability similar to a vibrating string. This is usually recognised in the conceptual design, by including either structural or aerodynamic devices. The basic structural diagram of the bridge invites instability under conditions of asymmetrical load. It was never developed further.

Why did the world's greatest structural engineering practice not acknowledge the limitations of this concept at the outset? I can only conclude that the engineers, overawed by the reputation of the architect, omitted to attend to the essential quality of common sense. The problems, now acknowledged, will be overcome with diagonal bracing and sophisticated dampers. But far from being at the cutting edge of technology, it is repair work more analogous to adding stainless steel bracing to a mediaeval hammer beam.

I do not doubt that it will be a great asset to the capital.After all, it connects the two greatest free attractions in London. Also, it will resonate in history how the designers, like Icarus, failed through an excess of hubris but, unlike Icarus, who would not be advised on the limits of the technology he depended on, lived to see a flawed vision realised.

Finally, is the 'Blade of Light' beautiful? Well, it is grey, the colour of London and, from a distance, merges almost imperceptibly into the background. In another context, I can hear the architects explaining this as intention.

Bob Owston, Owston Associates

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