17 February 2009

Bridge criticism 10: Of Mice and Menn

In recent posts, I've discussed the views of Santiago Calatrava and Fritz Leonhardt on what makes a good bridge design. This time, it's the turn of the great Swiss designer Christian Menn, who was responsible for the excellent Sunniberg Bridge, amongst others.

Some engineers are highly prescriptive in their views on bridge aesthetics, and Menn is undoubtedly one. In his paper "Functional Shaping of Piers and Pylons" (in Structural Engineering International, 1998), Menn wrote:

"A truly well designed bridge balances economy and aesthetics while responding to the functional requirements and technical and environmental boundary conditions."
Menn considers the "functional requirements" to comprise the traffic, alignment and state-of-the art construction technologies. The "technical and environmental boundary conditions" include topography, geology, clearances, available programme, emission limits, impacts on adjacent buildings etc.

Menn continues:

"On the basis of the above considerations, the real art of bridge design is to elaborate a suitable technically appropriate structural system that aims at achieving an optimal balance of economy and appearance …This pragmatic, simple and purely functional approach not only leads to technically proper structures but also to aesthetically convincing ones."
According to Menn, any significant increase in cost above the "least expensive functional solution" (a 5% premium for larger bridges, or 20% for medium bridges) is unacceptable and "should be abandoned". (Image, right, of Menn's Boston bridge courtest of Ken Douglas at flickr, showing non-functional pylon caps).

Menn’s opinion is common amongst bridge engineers, with one typical example being the Billington & Woodruff paper discussed in a previous post. This moralistic position is also shared by Leonhardt.

Menn’s is the language of moral puritanism – bridges must be "proper"; ornamentation is improper; cost must be minimised; the most appropriate structural system will inevitably produce the best bridge. This back-to-basics approach continues to offer much of value in an age where architect-led bridge design has produced schemes which are unaffordable or unmaintainable, but it should not be the only game in town.

It's doubtful that in the modern era the assumption that a "simple and purely functional approach" automatically leads to "aesthetically convincing" structures. Writers such as Billington offer the greatest praise for the structures of designers such as Maillart, Candela, Nervi or Isler, but the conditions of production for such structures have changed irrevocably. These structures, for which the identification of an optimal form allowed forces and materials to be minimised, come from an age where least cost arose from least materials, and hence a technically efficient design would often coincide with one which is slender and elegant. Even in this pre-modern period, however, there were structures which are highly inefficient structurally yet which have become much loved icons, such as the Forth Railway Bridge (pictured above right, courtesy of Simon Bradshaw at flickr).

Modern technology has changed the conditions for least cost. In particular, mass-production, pre-fabrication, and automation mean that in most cases the least-cost solution is one that minimises site labour and maximises the use of off-site fabrication and assembly. In this situation, a parallel-flanged beam may cost less than a beam shaped to fit its bending moments, even though more material is used. Many structures which were efficient to construct have also been found to be expensive to maintain, and a structure with a lower whole life cost may well have higher initial cost e.g. the use of concrete and hence heavier foundations to avoid the cost of repainting structural steelwork.

The further difficulty with Menn’s prescription is the issue of public opinion. While there may be some common ground amongst structural engineers as to what constitutes a good design, it is far from clear whether our idea of good design is shared by the public who benefit from a bridge and who fund its construction. For example, the public may be thought to delight in Calatrava’s white skeletal frameworks, even though the costs of fabricating these complex geometries must frequently result in a project that greatly exceeds Menn’s "least expensive functional solution".

Ironically, of course, Calatrava was Menn's student, and Menn was rich in praise for him initially.

Who are engineers to judge the success of such a bridge if their opinion departs from the end-user? While clearly there are areas where engineers have expertise the public do not (on both capital cost and particularly on the likely maintenance liabilities), we should not deceive ourselves into thinking that a concentration on functionality automatically produces the "best" bridge.

Next: Michel Virlogeux

1 comment:

Anonymous said...

Menn appears to have thrown his "least expensive functional solution" theory out the window on touchdown in the middle east. His concept design for the Al Sowah Island bridge 4 in Abu Dhabi sports a 110m high arch over a 170m span! That's just the height above the deck, its another 30m down to water level.