Allan McRobie enjoys a life of the audacious engineer who pioneered the windproofing of bridges and skyscrapers.
Wind Wizard: Alan G. Davenport and the Art of Wind Engineering
- Siobhan Roberts
Siobhan Roberts' Wind Wizard is an unlikely gem, a biography of both a man and a field. It tells the story of Alan Davenport and the 50 years he spent creating the discipline of wind engineering to span the gaps between fluid dynamics, meteorology, structural engineering and architecture.
The book reveals the backstory to many of the world's more iconic structures. Here, for instance, are the World Trade Center; the Sears, CN and John Hancock Towers; the Citicorp Center; and a comparable compendium of epic bridges, all from the perspective of their ability to withstand windstorms. Early wind-tunnel tests of the World Trade Center revealed the need for more realistic wind modelling, spurring Davenport to establish the Boundary Layer Wind Tunnel at the University of Western Ontario in London, Canada. This facility was designed to replicate the turbulent conditions of the lower atmosphere.
Such projects are massive investments, often of billions of dollars, with thousands of lives at risk when extreme winds hit. The physics is complex and uncertain, the mathematics intractable and the definitive experiment — building the full-scale structure and seeing what happens — cannot be done. It is a difficult problem, and the book describes how Davenport pieced together pragmatic theory and painstaking model testing to give rational, reliable predictions of performance.
Roberts charts how each challenge led to improvements in procedure and theory. For example, in her descriptions of the young Davenport's meetings with Leslie Robertson, the World Trade Center's structural engineer, as early as 1964, you detect both the creation of a prudent yet record-breaking design and the emergence of a field. Davenport replaced rudimentary rules of thumb for static pressures with a discipline. This tackled the complexities and uncertainties of the atmospheric boundary layer and the dynamic complications of wind flows such as galloping, vortex shedding, buffeting and wake buffeting. It was from that design process — which inevitably makes for poignant reading given the events of 11 September 2001 — that the Boundary Layer Wind Tunnel emerged. It went on to become central to all such studies.
Two of the projects studied show the potential for disaster posed by skyscrapers.
Two of the projects studied in the tunnel show the potential for disaster posed by skyscrapers. In 1978, a phone call from an inquisitive student to the structural engineering firm behind the 59-storey Citicorp Center (now the Citigroup Center) — already built, and balancing on four huge columns high above mid-town Manhattan — prompted the shocking realization that the supporting calculations had omitted to take into account 'quartering' winds, which hit the building at 45 degrees.
This story, well-known in structural-engineering circles, represents one of the nightmare scenarios. Roberts captures the heart-thumping horror of the moment, and the parts played by Davenport and Robertson in the testing and emergency remedial action to strengthen the bracing that followed. Perhaps my only criticism of the book is that the student who telephoned is not named. I believe her to be Diane Hartley, who was then studying under David Billington at Princeton University — a surprising omission for that institution's own press.
The other difficult case is the John Hancock Tower in Boston. Its problems were more subtle, although equally alarming. Again, the issue involved wind forces from directions that had not been considered, and required the retrofitting of stiffening and dampers, at great expense, to make the structure safe. From now on, I shall refer students and professors alike to Roberts' clear account.
I did begin to wonder whether the ultimate outcome of Davenport's life-long effort was allowing financiers to inhabit lofty eyries without overly endangering the people below. But the last chapter focuses on his determined efforts at disaster mitigation for the vulnerable. For example, in the Caribbean, he has worked on hurricane-resistant houses and was involved in numerous international initiatives that worked on disaster mitigation at a human scale.
Roberts has written a largely equation-free book in which technical subtleties such as aeroelasticity and Davenport's statistical description of turbulent buffeting are set out clearly, engagingly and accurately. Her precise, vivid phrases, such as vortices “pushing and shoving the structure this way and that like a gang of bullies”, will enliven my future lectures.
Before opening the book, I had decided to look out for two potential pitfalls. First, would the book acknowledge the alternative to Davenport's statistical theory of buffeting — the rapid distortion theory developed by Julian Hunt? It does. Second, would the story of the famous 1940 Tacoma Narrows Bridge collapse in Washington state fall back on the lazy and inaccurate 'resonance' description that most physics textbooks adopt? It does not. Instead, Roberts gives faultless coverage of work by engineers Robert Scanlan and, more recently, Allan Larsen to explain the physics of what actually happened.
This is my field, but I learned much from Roberts' admirable book, and emerged with great respect for both Davenport and his chronicler.
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McRobie, A. Engineering: Turbulent genius. Nature 491, 668–669 (2012). https://doi.org/10.1038/491668a