From the Union Bridge—the oldest suspension bridge still in use—built on the border between England and Scotland in 1820, to San Francisco’s iconic Golden Gate Bridge and the Great Belt Link that opened in 1997 in Denmark, many suspension bridges have become globally recognized landmarks. They also play a key role in connecting people and civil infrastructure.

Now, ever longer bridges are being envisaged. The Strait of Messina Bridge would connect the Italian mainland to Sicily. In Norway, a project to replace the ferries that are part of the north-south European route E39 will involve some of the longest proposed bridge spans worldwide.

However, the main spans (the length of the suspended roadway between the two lofty bridge pylons) of these planned bridges are fast approaching the limit of what is possible using the conventional design of suspension bridges originating in the 1950s.

In addition, the construction of bridges and infrastructure consumes a lot of energy and produces considerable CO2 emissions. According to the 2019 Global Status Report of the UN Environment Programme, the construction industry is responsible for nearly 40 percent of total global CO2 emissions.

A large portion of these emissions arises from the production and transport of building materials, primarily steel and concrete. Consequently, a way to reduce environmental impact is to find methods that use less of those materials.

Supercomputing reveals possibilities for super-long bridges 

These pressing problems are why Mads Baandrup and his colleagues in the group of professor Ole Sigmund and associate professor Niels Aage at the Technological University of Denmark (DTU) have reinvented the design of the bridge deck, the traffic-bearing element of suspension bridges.

To ensure industrial applicability, the research was done in close collaboration with Technical Director Henrik Polk from COWI. The goal was to maximize the load carrying capacity of the bridge deck to enable a longer main span, while at the same time minimizing material consumption. 

To achieve this, the scientists used topology optimization, a computational method already used extensively in the car and aircraft industries to optimize combustion engines or wing shapes. 

“With the recently increased power of supercomputers we could adjust the method to apply it to large-scale structures,” says Baandrup.

Using the PRACE Joliot-Curie supercomputer at GENCI in France, Baandrup and his colleagues analyzed a bridge element measuring 30 x 5 x 75 meters—a repetitive section that represents the whole bridge deck.

This element was divided into 2 billion voxels (the 3D pendant of pixels), each no bigger than a few centimeters. Existing components were stripped out to remove any trace of conventional design. The topology optimization then determined whether each individual voxel should consist of air or steel.

“In this way, the optimized structure is calculated from scratch, without any assumptions about what it should look like,” Baandrup explains.

To make the calculation work, the scientists modified an algorithm previously used to find the optimized shape of an aircraft wing, to instead impose the symmetry inherent to all bridge decks.

“In a process working towards optimization in iterations that were parallelized on thousands of nodes, this was not trivial,” says Baandrup. The symmetry constraint provided the advantage of reducing computational time. The complete calculation would have taken 155 years on an ordinary computer but took only 85 hours using 16,000 nodes on Joliot-Curie.

Less material means more sustainable construction

The topology optimization resulted in what looks like an organically grown bridge. Instead of the traditional girder of straight steel diaphragms placed inside the bridge deck to reinforce and provide stability, the algorithm came up with a net of curved steel elements.

“The software identifies the optimal structure but does not take into account if the structure is actually buildable,” Baandrup explains.

Out of that ideal design, however, he and his colleagues extracted a concept which is constructible—and at a reasonable cost. This interpreted design consists of a girder made of bundles of curved steel plates that are thinner than the plates constituting the conventional design.

The curved plates transfer the loads on the bridge deck much more directly into the hangers (vertical cables that absorb the loads of a suspension bridge deck) than traditional steel girders. That’s why bridges designed in this way can be constructed to span a longer distance than conventional bridges while requiring less material. In fact, the new design reduces steel consumption by 28 percent, resulting in a reduction of CO2 emissions of a similar magnitude.

In principle, a similar topology optimization could be applied to other large building structures, such as high-rises or stadiums, in order to reduce the consumption of steel and concrete and thereby work towards a more sustainable construction.

“Our results reveal a huge potential in rendering construction more ecological,” says Baandrup. “In the future, the construction industry should not only think about how to reduce cost but also how to reduce energy consumption and CO2 emissions. With our results, we believe we can initiate this discussion.”