Trimming the structural ‘fat’: The carbon cost of overdesign in bridges

Two hypothetical bridge designs are presented, one 'lean' and one with significant 'fat'. They are both highway bridges with the same overall geometry. Both are formed from pre-cast beams and in-situ fill using the same concrete mixes and beam types. For illustration, the 'fat' solution incorporates piles and lightweight fill, whereas the ‘lean’ design uses spread footings and a regular granular fill. Embodied carbon (CO2e) for the two bridges is calculated using How to calculate embodied carbon published by the IStructE. The 'lean' solution is shown to contain 38.9% less carbon than the 'fat' solution (885 tCO2e vs 1449 tCO2e). Commentary on the solutions is provided. Reasons for overdesign and options for discouraging overdesign are discussed.