long span steel cage match

I’m filling in for Rob Whitehead this year as designated structures professor for our SCI-TECH sequence (he’s got a well-deserved internal fellowship to teach the rest of the University how to do project-based learning–or, as we call it, learning), and like any good substitute I’m calling a few audibles. For a while now, the final structures module in our five-course sequence has covered long spans, and for a cumulative assignments students have done a case study on a classic long span project. While that’s paid some dividends, in particular integrating history into our tech coursework, both of us thought that some hands-on summative work might be a good experiment.

Enter the Long Span Steel Cage Match. For five weeks, students have been proposing model structures to meet a very specific set of criteria: carry a 10-pound weight over a 48″ span at a height that allows a regulation soccer ball to pass under its 1/3 points. They were given height limits and a base condition–two 2×4 timbers forming ‘rails’ that were the only things their structures could touch. We also limited the materials they could use, to cardboard (not corrugated), basswood, string, and hot glue. Final models would be tested and ranked, and grades for the final lab (1/4 of the module’s grade) would be based on how much their structures weighed, relative to the rest of the class.

This last bit was critical. In the past, we’d done competitive model testing to see how much a structure could carry, but this invariably led to models that were way, way over designed. Requiring the class to design to the same applied load and judging based on weight allows us to emphasize long span principles–in particular reducing unnecessary or under-stressed material, and using funicular shapes. It also encourages them to work iteratively to gradually remove material or elements until the structure just barely carries the load.

Sam Rushenberg, Javier Rodriguez, and Grant Olson

The five week module had four lab periods scheduled. For each of these, we gave students an intermediate assignment that roughly paralleled how a design experiment would actually proceed: Hypothesis, Modeling, Prototype Testing, and Final Testing. In Week 1, students had to propose, on paper, three possible solutions to the problem. For each one, they had to sketch their scheme and predict possible modes of failure. They presented these to the class, and we gave them feedback–without making any overly specific suggestions.

Danica DeWit, Nick Stenslie, Tike Akintan

In Week 2, we tested 1/4 scale models of their designs to destruction, asking them to explain in static terms how each of their options failed. Many teams noted that their models performed better than expected–and most teams understood that this wasn’t actually good news. If a structure carried much more than its required load, it meant that the structure was probably over-designed.

The most productive thing we tried was an optional prototyping session two weeks prior to the final test. We provided exactly the conditions under which we’d test the final models, and teams could subject as many options as they wanted during the lab period. Not every team showed up, but those that did were able to see not only how their models performed, but also how every other teams’ worked. Sure enough, after a dozen or so structures that carried the load and weighed between 600 and 1000 grams, one team came in with a simple four-legged structure that carried the load–and weighed in at just 234 grams. Those teams that were there realized that all of the arches, trusses, and cable-structures they’d brought had been the results of over-thinking; the ten-pound weight was a point load, and the funicular shape for a point load at midspan is a simple triangle. Sure enough, the lightweight option, by following the funicular shape in both the longitudinal and the cross-sectional direction, cut more than 2/3 the weight of the next-lightest scheme. We could not have asked for a better demonstration of long span principles.

On testing day, we saw a pretty solid variety of schemes. Many of the teams who had been present for the prototyping session arrived with some version of the funicular pyramid–which we endorsed. There’s no “stealing” in evolutionary design, just “research,” I suggested. But there were plenty of other solutions that worked–the one above, for instance, which used a compression arch formed of truss elements. But, very quickly, a lead pack emerged that was some version of the pyramid:

Once all the schemes had been tested (only two failures!) we had a near tie for first place at 230 grams, and we allowed teams to keep working and testing throughout the two-hour lab. What finally won (above) was the result of a quick bit of structural surgery, with a team cutting one leg off of their pyramid to create a true tripod. At 163 grams, nothing else came close, and they were clever enough to wait until time was nearly up to test it. Noting that the iterative process had cut about 70% of the weight from the initial round of prototyping, we suggested that there were lessons here not only about long spans, but also about adopting a truly experimental process for design; by introducing real competition, we were able to provide an incentive for innovation. By holding the tests in public, the knowledge gained from testing those innovations was diffused throughout the teams, enabling truly iterative testing until the clearly ‘correct’ idea won out. As pedagogical proof, every team in the top five had attended the prototyping session, which meant that they arrived with a clear advantage over those teams testing for the first time.

Beginning this week, I switch from the fifth class in the sequence to the third, which means less exotic topics–instead of truss arches and cable-stayed roofs, we’re going to spend most of our five weeks on basic beam and column theory. But in among the labs on shear-moment diagrams and slenderness ratios there is one open lab period that seems tailor-made for some beam-busting. As Rob’s new book, Structures by Design puts it, “Think, Make, Break, Iterate.” It’s become our version of medical education’s “watch one, do one, teach one,” and I’m convinced it’s the best possible way to build up intuitive knowledge of structural principles…

Removing dead weight, by any means necessary…
This one lasted exactly one second past the required testing time–which the team rightly interpreted as “just barely” design.

secret sky

This past weekend I joined an impromptu reunion of my 2013-2014 American Academy Fellows in Pinnebog, Michigan for the opening of Secret Sky, the latest piece by our colleague Catie Newell. Her work deals forthrightly with materials, architectural form, and how these can be manipulated to create experiences that are at once richly engaging and productively unsettling. Secret Sky is one of three barns around Port Austin, in the tip of Michigan’s ‘thumb’, that have been re-conceived by Detroit artists, and it provided a backdrop for a dinner, conversation, and party that provoked some deeply enjoyable questions…

Over two years, Catie and her team sliced through their barn, turning it into a pair of structures with a wedge-shaped gap between them. It’s a subtle move–from the road the barn seems normal at first, and it’s only on approach that the deeply strange geometries of the slice become apparent. The long, wedge-shaped voids seem physically impossible, and from the front the view of the sky through the barn takes a minute to understand–it occurs right where the post-and-beam structure of a typical barn would be most vital, and the combined stoutness of the gambrel-shaped roof and the apparent fragility of the two pieces underneath it make a sort of invitation to figure out what’s going on.

And close up, things get interesting, because it’s clear that the slice isn’t casual, but it’s been immaculately worked over–‘tailored’ was the best way I heard to describe the detailing of the slice’s walls. The void is the result of a careful re-construction, the original siding re-purposed and re-cut to match the faceted geometry needed to make the slice appear like a clean opening through the barn’s volume. Its scale and shape make walking between the sloping and vertical walls an uncanny experience and a structural riddle, which is answered by the last stop on a mowed path, at the entrance to the barn on the opposite side.

Here the ‘tailoring’ is apparent, with new timber and steel rods that do the work of supporting the slanting, re-constructed wall of the slice. Showing off the stitching that makes the clean lines possible is a bold move, but it’s a generous one, emphasizing the fragile construction that the barn shares with most agricultural outbuildings. The inseams are thoughtfully laid out but not overworked, and the ‘reveal’ of the steel rods contrasts with the weathered timbers supporting the roof.

It’s a rare combination of formal, structural, and material virtuosity–a moving meditation on how delicate and temporal building can be, and how much a simple defiance of architectural expectations can affect us. We’re used to buildings that shelter, that are sturdy, and that can be readily understood or appreciated, and coming across such an articulate enigma is a rare thing.

There are comparisons here to the sliced or cut buildings of Gordon Matta-Clark, but Catie’s work goes deeper than the shock value of his controlled demolitions; the attention she’s paid to the reconstruction of the barn into an intentional set of forms adds a sense of stewardship and, maybe, of hopefulness. Plans to preserve the barn by installing a new roof are underway (you can contribute through the Port Austin Artist-in-Residency website here…include in the memo “for Secret Sky roof”), which would mean that this exercise in sublime fragility would be around for a few more generations…