Clarity of design – giving things a name
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otably represented by the East Bay signature n span of the San Francisco-Oakland Bay Bridge. Bill Baker observed that, for buildings, there are many structural systems – for example, “belt truss”, “bundled tube”, “buttressed core” – the names of which fall into the format of a noun modified by an adjective (noun + adjective), and this may be a preferred way to name any structural system. It could be seen as not unlike the binomial genus-species convention of biologists. However, rather than the species following the genus in the name, in structural engineering the species precedes it, at least in English, as in the examples given. This could be seen as appropriate, since it is in the qualifier of the noun that the innovation is captured. Yet not every structural system necessarily elicits the same noun + adjective pair from every structural engineer. As if there were a principle of relativity at work, the same tall building may be seen in different ways by different observers. Fazlur Khan saw the Sears Tower as a bundle of nine tubes acting together as a single “bundled tube”, but another engineer might see the building’s “noughts-and-crosses” ground floor plan and its curtailed versions in the building’s upper storeys as the basis for a framed tube or a modular tube (Fig. 2.3). Baker sees the layout as the cross section of a giant beam with four webs. However the structure is viewed and no matter what it is called, an engineer’s understanding of its behaviour as a tall cantilever emerging from the streets of Chicago will be shaped by how it is imagined. Given SOM’s emphasis on simplicity and structural clarity, the structural system of the Exchange House in London (see “Exchange House in detail”, p. 76 – 81) virtually names itself, as the hybrid building-bridge structure exposes the dominant tied arch (note the noun + adjective) that is central not only visually but also structurally. The appropriateness of this unique building-bridge structure over the wide expanse of railway tracks north of Liverpool Street Station might also be seen as a playful interpretation of the concept of a railway bridge. Similarly, the
long-span roof of McCormick Place Convention Center is a bold expression of a cable-stayed roof (taking its name from the bridge system of the same form; Fig. 2.6, p. 44). Both Exchange House and McCormick Place are long-span structures that share many of their traits with bridges, so it is not surprising that their structural systems evoke familiar bridge typologies. But what of buildings that do not so directly suggest a bridge or any other familiar structure? Naming the structure The frontispiece to Bill Baker’s Beyond Tall essay is an updated and augmented version of a graphic used by Khan in the 1960s to show how structural systems (and their names) evolved as the number of building storeys increased. As with bridges, a certain type of structural system is appropriate for a range of building heights. As the limit (whether structural, functional, economic or aesthetic) of an existing system was reached, a new system had to be created to build beyond the maximum height allowed by the old. As these systems evolved, they needed names to serve as a shorthand means of capturing the essential feature that distinguished them from one another. For concrete office buildings, the names of the structural system as the buildings grew from 20 to 75 storeys are shown in the graph to have been, successively: frame, shear wall, frame-shear wall, framed tube, tube-in-tube and modular tube (Fig. 2.5, p. 44). The systems for steel structures, which are shown to have grown from 30 to a possible 140 storeys, have been given the names, successively: rigid frame, frame-shear truss, belt truss, framed truss, truss-tube with interior columns, bundled tube and truss-tube without interior columns. (These names had evolved somewhat from those Khan had used, but that was to be expected as structural types were reinterpreted.) Certainly, all of these designations are clearly descriptive, but some are more mellifluous than others. How preferable it would be, from an aesthetic point of view, to stick to simple noun + adjective names.
2.3 Bundled tube structure of Willis (formerly Sears) Tower, Chicago, Illinois (USA) 1974 2.4 Shaped truss, International Terminal, San Francisco International Airport, California (USA) 2000
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