In an effort to reduce life-cycle costs, new major concrete structures are being designed for service lives of 100 years or more; longer than the 75 years typically assumed in bridge design codes.
The New NY Tappan Zee Bridge
The New NY (Tappan Zee) Bridge is a major North American project under construction that requires the design to achieve a 100-year service life for non-replaceable components. COWI Bridge provided the structural design of the main span cable-stayed bridge and the service life design for both the approach and main span bridges.
Optimzing Life Cycle Costs
Advancing bridge design to extend service life as well as increasing awareness and understanding of deterioration mechanisms are vital factors in optimizing lifecycle costs of structures.
North American structural design codes do not explicitly consider the durability and service life of reinforced concrete structures. Prescriptive “deemed-to-satisfy” rules in these codes are oversimplified and do not quantify the service life of a structure in a given environment for given materials. Reliable long-term performance in aggressive environments is not achieved, and serviceability failures occur earlier than the assumed design life.
Designers can take steps in the design and contractors can adopt best practices to enhance service life. However, these steps alone are not sufficient.
Methods for Addressing Service Life
Multiple methods have been recently developed to address service life of concrete structures; however, there is no widely accepted standard in North America.
Some methodologies require the use of proprietary software in which analyses are based on “closed” algorithms and “in-house” databases that are difficult or impractical to independently verify.
Others require a number of lengthy and onerous durability tests that are incompatible with fast-paced construction schedules. Typically these methods have not been used for long enough time periods to validate the input parameters and the resulting service life performance. Better defined design standards and methodologies for service life are needed so that owners can rely on them.
Don Bergman, P.Eng, is Vice President, Major Projects & Senior Project Director at COWI Bridge North America, Anne-Marie Langlois, P.Eng., is a bridge engineer at COWI Bridge North America.