Bridge deck design optimised by new research

Collaboration between the Technical University of Denmark (DTU) and Cowi could result in lighter and longer span bridges than was previously possible.

“We applied different methods for examining how to better use materials, which primarily consist of steel and concrete,” said Cowi bridge engineer Mads Jacob Baandrup, who undertook the study as part of his PhD project. “Initially, we sought to optimise their use in traditional structures by using transverse diaphragms in the bridge deck, thereby achieving a theoretical weight reduction of up to 14%.”

With a view to achieving additional savings, the researchers looked at the possibility of altering the structural design. That was done by using topology optimisation, a method known in car and aircraft industries, that had not previously been used for large-scale building structures.

DTU mechanical engineering associate professor Niels Aage added: “In popular terms, it’s about ‘emptying’ a bridge girder of its existing elements, providing complete freedom for choosing a new design. The inner volume of the bridge girder is then divided into a structure of very small voxels (3D pixels), like small dice. The topology optimisation method is then used for determining whether each individual voxel should consist of air or steel material. The result is a bridge girder design that uses the least possible steel without impairing the strength of the structure.”

The team analysed a bridge element measuring 30m by 5m by 75m, which was divided into two billion voxels, each no bigger than a 20mm. The team said that this resulted in an extensive calculation performed by a supercomputer that would have taken an ordinary computer 155 years to do.

According to Cowi, it is the largest structural optimisation ever carried out.

The computer calculation presented input for how to best structure the design space of the bridge deck. Among other things, that meant curving part of the currently straight transverse diaphragms, making it possible to shave off 28% of the material that is used for bridge decks.

“We adjusted calculations to ensure that the bridge girder structure has the optimum design, and that it can be carried out without too costly production methods. The economic aspect is important in order for the design to be a realistic option for future bridge projects,” said Baandrup.

Cowi has said that further analysis of the design is needed before being put into use but it believes that the findings are key to the future design of suspension bridges.

Cowi technical director Henrick Polk said: “The new bridge girder design can be converted into a weight and CO₂ reduction of up to 20% for the entire bridge, which of course benefits the climate.”

DTU mechanical engineering professor Ole Sigmund added: “We believe there are huge perspectives to using topology optimisation for ensuring sustainable design of other large building structures, such as high-rises, stadiums or highway bridges. We want to explore that field, and since the construction industry accounts for 39% of global CO₂ emissions, almost any reduction can be of interest.”

Source: https://bit.ly/2YRoSj5