Any week that starts in a 15th century dining hall in Cambridge at a table full of Construction History all-stars and ends watching the Blackhawks win a playoff game in the company of Chicago preservation engineers and contractors is, by definition, a good week. Last Saturday I was honored to give a keynote address to the Third Annual Conference on Construction History at Queens College, a now-regular UK event that draws a consistently good crowd from throughout Europe. The topics were, as always, varied and fascinating, ranging from stone architecture in Petra to inflatable formwork for concrete domes in the 1970s, and from medieval nails to the organization of the contracting industry countrywide throughout Europe. I am no longer surprised, but always consistently happy, that these conferences manage the difficult tasks of depth and breadth in their papers.
One or two highlights. I always find myself drawn to the sessions on vaulting at CH events, in part because there is always a good debate about the fine-grained performance of medieval and renaissance structures. The two sides, as I understand it, generally disagree about whether masonry structures are best understood as solid shells, which lend themselves to analysis fairly easily, or as more ductile structures that move and adjust constantly by cracking and settling–a more difficult process to understand, but an idea that is attractive in that it allows us to look at a cracked structure and to relax a bit (depending on where the cracks are and whether they’re getting larger or not); in the words of Jacques Heyman, “it’s actually quite difficult to design a masonry arch that won’t stand,” since an arch that thrusts or settles will usually find, at some point, a configuration that permits equilibrium. (Don’t try this at home). In this context, a paper by Danilo Capecchi and Cesare Tocci, from Università di Roma and the Politecnico di Torino, was our version of a lighting rod, as it took the famously confident analysis of St. Peter’s dome by Giovanni Poleni and contrasted it with the lesser-known but somewhat more accurate report by R.G. Boscovich. Poleni argued that the shape of St. Peter’s was near enough to the curve of a catenary, and that the dome had sufficient double curvature, that the cracks that had developed in it by the early 1700s were not as worrying as they appeared. Boscovich, however, pointed out that the cracks indicated more serious problems in the lack of thrust resistance offered by the relatively tall drum, and that the double curvature assumed by Poleni only worked if the dome and drum were monolithic. In fact, because of the division of the drum into sixteen sections by windows, this couldn’t quite be assumed. Poleni, according to a slightly mythological history, capitulated to the desire to do something, and suggested additional iron hoops to contain the dome’s thrust above the drum. Capecchi and Tocci’s conclusions are that these hoops were, in fact, critical and that the addition of a hoop placed at the base of the dome that restrains the top of the drum, may have been the intervention that has kept the dome intact.
Back to Iowa earlier this week (after a visit to the Foster mothership in London to see former colleagues and students…) and then to Chicago and Northbrook yesterday for the annual meeting of the Western Great Lakes chapter of the Association for Preservation Technology. Wiss, Janney, Elstner hosted the session in their Northbrook offices, which are attached to their testing lab. WJE does a ton of preservation and forensic work all over the country, from the Washington Monument to Wrigley Field, and they do much of their own materials testing, which they showed off during the afternoon break. That’s a piece of steel rebar in the rig in back, wired up to show a picture-perfect stress-strain curve on the screens in front. Did it get tested to failure? Oh, yes, it did. WJE engineer, Iowa State alum, and loyal architecturefarm subscriber Rachel Will suggested I take advantage of the knowledgeable crowd to test out some new ideas about the postwar Chicago project, and I was grateful for an audience who was willing to sit out the first really great spring day in the city to talk about air conditioning, double glazing, and heat-rejecting glass during the late 1940s and early 1950s.
So, about 10,000 miles logged this week, 2000 years of history, a couple of good meals in good company, and just over 80 kips of tension on that rebar before it gave way.