This is the third part of a multi-part book review, see parts 1 and 2.
The second chapter of "Kurilpa Bridge" discusses "Hybridity and chaos". The authors suggest that a key design heuristic for engineers is the avoidance of hybridity, and a preference instead for structures which express a single form as purely as possible. This is to some extent a straw man - few engineers fail to understand that there are many simple but visually unappealing bridges.
The book suggests that the engineering "fear" of hybridity partly arose as a response to structural indeterminancy - until recently, hybrid structures (e.g. those which combine cable-stay and suspension systems, rather than being one or the other) were difficult if not impossible to analyse accurately, and may therefore have been seen as lacking in robustness. If the designer does not know what proportion of the load each structure will bear, it is difficult to assess how safe each system is.
There are counter-examples. One that the authors acknowledge is the use by Roebling and others of hybrid cable-stay and suspension bridge systems (most famously in the Brooklyn Bridge), where the cable-stays provide essential stiffening but also carry a share of the load.
Indeed, counter-examples are more common than the book accepts. On one hand, there are a number of cases where hybridity has been embraced through the trial-and-error process applied by untutored, experimental pragmatists, such as John Justice Jr or James Dredge. On the other, hybridity has resulted from hard-headed rationalism, where it offers an elegant way to separate out different functions - Robert Maillart's deck-stiffened arches seem to me a prime example of the rational development of a hybrid design, where the deck is a relatively deep beam used to stiffen a slender arch, analogous to the use of truss-stiffening in suspension bridges.
The authors also suggest that a taboo against hybridity is part of a wider cultural taboo. Given how puritan many engineers appear to be in their aesthetic outlook, I can understand this, but I think that if it is true at all, it is driven by simple economics. Even if a distaste for hybridity is part of the wider culture, you would have to ask why it persists so strongly in engineers and not, for example, in architects. Hybrid forms are less straightforward to build, a point well-proven by Kurilpa's complex erection engineering. They may be less safe, economic or reliable, and these are the engineer's primary concerns.
The third chapter is one with more questions than answers, asking whether a bridge can be art. The book asks whether Kurilpa Bridge can be seen as the distillation of a cultural zeitgeist, and point that it may be an exemplar of the digital age, a period where our culture has become dependent on tools that few of us understand or can make. That sounds about right for the Kurilpa bridge, where even the specialist bridge engineer has to undertake considerable head-scratching to understand quite how it actually stands up. The bridge, however, is not like the mobile phone, which must provide good value at low cost or fail in the marketplace - with major infrastructure like this, a clearer understanding of whether the cost is reasonable is harder to obtain.
I suggest there's also a political zeitgeist at work here, the way the complexities of democratic government can severe any clear connection between value and cost. The less an organisation directly experiences the financial consequences of its activities (for example, where it relies on external development funding), the less interest it shows in measuring the outcome of its expenditure. The result is the flashy monument predicated on an assumption that it brings value, without ever understanding whether the same value could have been purchased at a lower cost. This disconnect bedevils consideration of all iconic footbridges.
Other zeitgeists which might be thought relevant to the Kurilpa Bridge are the concepts of celebrity and bling - conspicuous consumption as a way to deal with status envy.
land art, particularly Walter de Maria's Lightning Field, or the work of Kenneth Snelson (pictured, right). Clearly, there is some relationship there, which might support the idea that Kurilpa Bridge's aesthetic is on a higher level than merely a bridge. What is left largely unsaid is that art is defined by being non-functional - the less utilitarian an object is, the more it aspires to the status of art. I'm not convinced that this way of thinking does the bridge any favours, but I'm glad to see the authors aren't seeking a definitive answer either.
To be continued ...
(Images of Kurilpa Bridge courtesy of Jan Smith; image of Kenneth Snelson sculpture courtesy of Barbara Eckstein)