Bridge criticism 7: Moralists on the march

In the previous posting in this thread, I discussed the public criticism (and it's lack of assistance to the unfortunate client) surrounding the Glasgowbridge competition. For this post, I'm going to cover an instance of criticism from a technical paper, very different in tone, of no assistance to the designer or client, but possibly of wider interest to the design community.

The Sundial Bridge is a cable-stayed footbridge designed by Santiago Calatrava and opened in 2004 in the Turtle Bay Exploration Park near Redding in California (shown right; all images of the bridge are courtesy of informedmindstravel on flickr). The bridge’s 66m pylon is inclined away from the deck, which it supports without the use of back-stays. It has some similarities to Calatrava's Alamillo Bridge, although at Turtle Bay the pylon is offset to one side of the deck rather than aligned with the centreline.

Sundial Bridge cost $23 million to build, mostly funded by a private foundation. According to Turtle Bay, “in addition to being a functional work of art, the bridge is a technical marvel as well”.

Much of the public commentary on the bridge has been very positive. To take just one example:

"It’s a wonderfully calibrated delight, an artistic feat of engineering that resembles nothing around it except perhaps the angled neck of an egret rising from the riverbank towards the sky." (San Francisco Chronicle)

There has been some public criticism of the bridge’s cost, which increased considerably from the earliest estimates. The cost is substantially in excess of that for many other comparable architect-led footbridges. However, the bridge is more unusual in that it has been the subject of a detailed published critique by fellow professional engineers.

In a paper titled "Aesthetics and Ethics in Pedestrian Bridge Design" (Footbridge 2005 conference, sadly not available online), David Billington and Shawn Woodruff argue that "the drive for landmark bridges has led some engineers to disregard the engineering ethic of economy with some recent footbridges". In their extensive paper, they note that its cost per square metre is more than double that of the Solferino Bridge (while still less than half that of the London Millennium Bridge). Their table of costs (in US$) is shown below:

In criticising the bridge, they make very clear their assumptions about the philosophy of structural engineering:

"A couple of engineers have made the assumption that due to their small size and form, pedestrian bridges are not necessarily constrained by economics. This is simply incorrect. It is the engineer’s ethic to design with economy in mind, whether one is designing for a public entity or for a massive private corporation … the most successful footbridges are those that satisfy the ideals of structural art. Structural art is a disciplined art form independent to architecture. It has three dimensions: scientific, social, and symbolic, with each having a specific measure: scientific = efficiency, social = economy, symbolic = elegance. To be a work of structural art, a structure must satisfy each of these three ideals."

I'm not entirely sure from which holy scripture comes the dictate that economy is ethical, nor that the best bridges are those which match Billington's prescriptions for "structural art". These considerations deny the simple fact that in ordinary life value is not merely monetary, or that the quality of a bridge should be judged by the experience of its users rather than the interests of its designers.

To explain the structure’s high cost, Billington and Woodruff note several issues with its engineering design:

"Enormous bending moments are created in the deck near the tower and also in the tower due to the lack of back stays for both pylons. The omission of these stays creates a need for large pylons with much material to take the large bending moments. Ironically the structure which appears light when viewed from afar is actually much heavier than it could be with the inclusion of just a single backstay."

Their criticism of the structural behaviour is left implicit; it's the lack of efficiency and hence high cost to which Billington and Woodruff clearly object.

Their paper is unusual in that it is a formal critique on engineering grounds, but it focuses mainly on cost and is not thorough in its criticism. The authors imply they have carried out a finite element analysis but offer no details. They mention the "irrationality of the entire form", and on the subject of construction they say:

"The understanding and visualization of the tower was aided through actual physical models, but the intense three dimensional geometry and the multiple planes of plates, stiffeners, and cables still made the process unnecessarily difficult."

That "unnecessarily" is odd: if the geometry is unusual, then clearly fabrication will be less straightforward. But where does the necessity come from? To a structural engineer, the offset pylon is "unnecessary", because a central pylon would have reduced torsion and simplified design detailing (considerably). But is the pedestrian experience on such a bridge better? (For more on the difficulties of construction, including more extracts from the design drawings like the one shown left, see Scott Melnick's Sun Sculpture [PDF]).

In many ways, the criticisms read like the complaint of the stolid engineer whose inherent conservatism has been offended. They take as read the assumption that it would be better to simplify the geometry than to invest additional money to achieve it, however this assumes the bridge’s function to be entirely that of carrying people across an obstacle.

Billington and Woodruff don't really offer any way to evaluate the bridge's aesthetic success, and the economics are taken solely as the cost of construction, with no attempt to quantify the benefits to the local economy in terms of tourism or other revenue.

The possibility that a piece of sculpture could hold intrinsic value separate from its role in the economy or position in the art market is not entertained – for the bridge engineer, it seems everything must come down to the relationship between cost and functionality.

Billington & Woodruff offer a direct contrast to the Sundial Bridge in Robert Maillart’s reinforced concrete arch Töss footbridge (shown right), which they rightly describe as elegant, efficient, and economical. In this, they rely to a great extent on their own structural analysis to demonstrate its material efficiency. However, no attempt is made to ask whether a bridge of this style would have provided the same value to Turtle Bay as did Calatrava.

It may seem that the criticism of the Sundial Bridge shows engineers still to occupy the modernist paradigm of the mid-twentieth century, while the broader culture has moved on and accepted that form and function have a more complex relationship. As suggested by the title of their paper, Billington & Woodruff are moral puritans who regard "correct" structural engineering as a matter of basic ethics.

Should structural engineering criticism limit itself to technical matters or can it attempt to address wider concerns? Even Billington & Woodruff draw aesthetics into their set of base criteria by citing elegance as a mark of good engineering, while their concept of economy can readily be expanded to address matters beyond cost and functionality, particularly if recast as value rather than cost.

I have considerable sympathy for the criticisms of this bridge: from the structural point of view, it's a monumental folly, with the 14m deep foundations alone grossly out of scale to the needs of a simple river crossing. There's little doubt that an attractive, striking bridge could have been built for a fifth of the cost.

But I don't think that extravagance is inherently unethical - nor, no doubt, does anyone admiring the pyramids, Mount Rushmore, the Roden crater, or any of thousands of other artistic interventions in the landscape. If engineering criticism is to take its place alongside architectural debate, more than just the standard modernist dogma will be needed.