Elegant Simplicity in Engineering: I’m a structural engineer — can I play, too?

By: Dane Jorgensen
13 June 2017
“Elegant Simplicity in Engineering” is a recurring feature wherein engineering-
focused team members discuss the relevance of design in their work.

As a structural engineer, I view my contribution to the concept of “elegant simplicity” in a couple of ways. Can I not only design a structural system that it safe, constructible, and economical; but can I do it in a manner that requires no additional materials to cover the aesthetic sins my discipline is known to commit? Can I help design and build structures that are bare of extraneous coverings and blur the line between structure, architecture, and sculpture? I say yes, and this can be accomplished in a couple of ways.

First, there are those projects where the structural systems are obviously celebrated in the architecture. A quick Google search will turn up a myriad of results (Cube at Bricco Rocche and Pavilion of Portugal are a few favorites). These structures are sleek, clean, and impeccably detailed. I haven’t had the opportunity yet to work on structures like this, but I do sometimes daydream about the types of analyses, hours of calculations, and iterations of detail concepts necessary to achieve these simple-looking structures. Some day, perhaps.

Second, and I think more importantly, is that using my knowledge and skill sets in a creative manner can help a building’s architecture achieve its project design intent. As a structural engineer, it is imperative to have an understanding of the architecture and design intent. This allows one to know which design items need additional attention and which do not. Sometimes it is necessary to adjust whole building systems in order to accommodate a single important feature. Other times, we can quickly identify areas of less importance that can be framed with brute structural economy. This concept also extends into finish detailing and the connections of other building materials to the structural substrate.

On a recent project, I was excited by the opportunity to lend a hand in creating a structure which, to me, had the potential to meet my interpretation of elegant simplicity in structural engineering. One aspect of the project involved an exterior roof structure containing a large, sloped, and curved radius which bent the structure, in plan, at a 48-degree angle through the center. As the roof was exterior and isolated from the other conditioned structures, the question became, “How much of this roof system can we eliminate?”

We started with the typical framing mentality of adding sub-framing to reduce deck span and thin out the deck gage. This resulted in a framing pattern that felt heavy and was too busy. This idea was quickly dismissed.

We next considered use of a single-span deck condition. The deck would be cut in pie-shaped pieces so it could be oriented orthogonal to the primary framing members in each bay. At the edge of the radius, this resulted in a simple span that exceeded 12 feet in length. We were able to accomplish this using 18-gage, three-inch galvanized roof decking. After running a quick estimate, this solution turned out to be a few cents per square foot cheaper than the typical framing solution we had previously checked. In addition, this solution was more aesthetically pleasing as it gave the roof deck ribs the appearance of following the overall roof curve.

After figuring out the structural deck, we turned our attention to solving the roofing question. Since this roof was not a part of a conditioned space, what would be the bare minimum needed to construct it? The team decided on a standing seam roof system that did not require any other structure other than the cleats needed for attachment. A clean, simple solution.

This roof selection posed a slight problem to our deck orientation and selection. How would we fasten the standing seam system to the exposed deck, with enough fasteners for uplift, that doesn’t result in the roof system looking like the underside of an industrial building? This roof was a key architectural feature of the facility and was to be completely exposed to view on the underside. We didn’t want it to have the feel of a cave full of five-inch metal stalactites dripping morning condensation onto guests as they walked under the roof.

We considered several options for solving this design problem. An architectural acoustic deck, with hidden flutes, was too expensive and didn’t result in a pleasing profile when compared to the three-inch N-deck. Ultimately, it was decided to nest CFS sections inside the deck ribs to fasten the roofing to. These would act as a concealed purlin system for the roof cladding. This, however, created another interesting design problem to solve.

The component and cladding pressures for the roof purlins proved to be difficult to manage. The relatively long spans, uplift pressures, and unbraced conditions resulted in several miles worth of 3.625-inch, 97-mil studs. After several iterations and tweaks with the project architect and CTA’s Envelope group, we was able to optimize the purlin size, spacing, and layout. This resulted in a purlin plan proportional to a roof uplift diagram.

We incorporated a “bottom-up” fastening pattern that reduced the gage material of most of the purlins down to only 43 mil. The fasteners were specified to have a flat, wafer head to reduce their visible profile, and a fastener pattern was detailed to control the locations of the visible fastener heads.

This was my first earnest attempt at structural elegant simplicity. Through much discussion and coordination, and many iterations, we were able to provide a large, curved roof system, only 3.625 inches thick, with mostly concealed fastening that is built out of straight-cut members. Does it meet the design community’s definition of elegant simplicity? I hope so. All I know is it was fun to understand and be engaged in bringing a key project concept to life. It was a fair amount of work, but I can’t wait to try it again on another project.

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