Reducing the environmental impact of footbridges through smart under-deck cable-stayed systems

The construction sector is responsible for more than a third of energy-related CO2 emissions. This fact has motivated the search for structural solutions with minimal environmental impact. In this research, smart cable-stayed systems are presented as a solution to reduce the carbon footprint of footbridges. The use of prestressing through external tendons has been extensively adopted in bridges to optimize the internal stress distribution, which leads to important savings in terms of materials. However, these configurations only allow an optimal structural behaviour for a specific load distribution. This paper proposes to extrapolate the advantages of external tendons for any acting live load. This is achieved by using a smart system, which is gradually tensioned as the live load increases; hence it is possible to modify bending moments and deflections to obtain an overall reduction of material. A new methodology is presented to design smart (also called responsive) cable-stayed structures. To evaluate the efficiency of the responsive system, a parameter defined as degree of responsiveness is used. This parameter represents the portion of the load carried by the smart system. Results prove that a relevant material reduction can be achieved using partially responsive systems. The slenderness (span-to-depth ratio) of the deck can be increased from 27 (passive structure) to 53 (partially responsive structure). In terms of carbon footprint, this translates as a 28.5% reduction, a result that shows the potential of using smart structures for the reduction of their environmental impact. Finally, the applicability of the system is evaluated through the construction of a 6-meterspan prototype.