The structural designer’s new toolbox

It is said that “an engineer is a (wo)man who can do for a dime what any fool can do for a dollar.” While it is customary for an engineer to be responsible for achieving a specific technological need for the lowest economic cost, this saying is crippling to both the engineer’s creativity and the design’s potential. With the available new digital and numerical tools, the role of the bridge designer needs to be emphasized.

Traditionally, designers use tools such as back of- the-envelope hand calculations, physical experiments and tests, design charts, and 2D sketches to develop and advance the preliminary design process. The digital and numerical computational techniques are new tools in this existing toolbox; they allow for the rapid generation of a large set of design alternatives that fulfill specific requirements. These alternatives might be unusual and surprising to the traditional bridge designer, who is grounded in intuition and accumulated knowledge; yet they present unexplored feasible domains in the design space. I would like to demonstrate this statement with the design development of one of my favorite footbridges. The Knokke-Heist footbridge in Belgium (designed in 2008 by Ney and Partners), was designed unlike traditional structures, where the loads in the longitudinal and transverse directions were decoupled. The bridge features on the cover of our book “Laurent Ney: Shaping Forces” and was a key feature in the exhibition with the same name held at the Palais des Beaux Arts, Brussels.

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Cover of the book “Laurent Ney: Shaping Forces”
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Exhibition at Palais des Beaux Arts: Laurent Ney (third from the right), Sigrid Adriaenssens (right)

From a topological point of view, the bridge is based on a cutout, curved shape that efficiently carries external loads through membrane action and satisfies the site requirements. The shape of the steel bridge was numerically form-found, like a network of connected springs supported at the abutments and the mast heads. The grid is allowed to relax or “fall” under gravity loads applied at the spring connections. The idea behind the curved structural form is to carry all loads within the steel surface shell without needing additional structural elements.

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Knokke-Heist footbridge in Belgium

Once the overall shape has been found, the geometry is further refined to comply with the CNC manufacturing constraint of single-curvature steel sheet bending, and then it is numerically optimized to maximize the overall stiffness of the bridge. The latter task presents a typical topology optimization problem that consists of distributing a given amount of material in a design domain subject to load and support conditions, such that the stiffness of the structure is maximized. The figure below shows the optimal thickness distribution in the shell for different values of the thickness ρmean, which is a measure of the total material volume constraint. Topology optimization provided a powerful tool for the preliminary design of this thin-shelled bridge. By combining topology optimization with form finding and CNC manufacturing constraints, a 3D typology that might not have been conceivable in a purely analytical or intuitive fashion was generated. You can find out more about the topology optimization study we carried out on this bridge here.

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Optimal material distribution in the thickness of the shell

Author: Sigrid Adriaenssens

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