The economic case for service life extension of structures using structural health monitoring based on the delayed cost of borrowing

The economic case for extending the service life of structures using the combination of testing and structural health monitoring (SHM) is very strong. In the USA, it is estimated that there are 70,000 structurally deficient bridges. Surprisingly, 12,000 have no deficiency other than a low load rating, with only six reported to have a rating based on load testing. Bridges that are load tested are often, but not always, found to have higher load capacity than estimated by analysis. The underestimate of capacity may stem from the careful application of safety margins due to uncertainty about materials and construction. It may also stem from over simplified structural analysis, inaccurate modeling of boundary conditions and the load bearing capacity of “non-structural” elements, as observed in the Swiss bridge system. However, once results from testing are available, it is the updated estimate of the capacity that is the most appropriate to use. Therefore, one can conclude that the service life of many structures could be extended through testing to ensure capacity and the application of monitoring to ensure on-going capacity and safety. The economic return of extending service life in this manner is very high as the yearly borrowing minus the inflation rate cost for even modest structures significantly exceeds the cost of typical SHM system. Application to new structures or major rehabilitation is more challenging, as the value is discounted by the compounded borrowing rate minus the inflation rate over several decades. As a result for new structures, the capital cost and the decades of operating cost of the SHM system need to be <1 % of the structure cost.