R. MOSES ANSWERS:
All infrastructure will deteriorate over time and will require maintenance activities to retain its level of performance and value to the traveling public. Bridges are a unique highway asset in that their life span is much longer than others -- design life is often 75 years and it is rare for a bridge to be fully replaced in less than 50 years. Whereas it is not uncommon for assets such as pavement to be replaced in 10 years.
When it comes to bridges there are two terms most commonly used when major rehabilitation or replacement is discussed: structurally deficient and functionally obsolete.
Structurally deficient is the more critical case, it means the bridge has deteriorated to the point where it is no longer able to withstand its design loading. Don't let that statement frighten you, as often the design loading is two to four times higher than the actual loading placed on the structure. Structurally deficient does not mean imminent collapse.
Functionally obsolete typically doesn't deal with loading capacity of the bridge, but with how well it performs its function. Does it cause a traffic bottle neck? Is it accessible to bicycles and pedestrians?
Many of the bridges we have were designed in the '50s-70s. Since that time, automotive usage has increased tremendously, standards lane and shoulder widths have changed, and guidelines for pedestrian access have improved. If a 50-year-old bridge does not have adequate deck area to move the cars or meet current lane/shoulder with requirements, (or doesn't meet other standards) it cannot perform its function and is thus labeled "functionally obsolete." There are federal standards for bridge inspections and many states will add additional criteria on top of these.
In my opinion, a bridge being replaced because of inability to move cars is a poor usage of public funds. This issue is symptomatic of automotive dependent development and serves to further expand single mode transportation. If automotive drivers were to utilize busses or bicycles instead the bridge would not need to be replaced and it would move more people with less congestion.
Over the life of a bridge or viaduct, the requirements can change. There can be more use, less use, higher loads or a change in the rules defining acceptable minimum standards. However, as with historic buildings, that is not a reason to demolish anything. Limited use or adaptive re-use is a much less costly alternative. Railroad right-of-ways are converted to pedestrian and bike trails. Bridges are converted from train or vehicle to pedestrian and parks such as New York City’s highly successful Highline Park. Bridges and viaducts can have exceptionally long lives as evidenced by the Roman viaducts that are over 2000 years old, and still standing and used.
To respond to the engineers who say something doesn’t meet current level of service, ask what level of service they do meet? Is there a lower standard that is still viable, perhaps more viable? If new service is required, would it be better somewhere else or by some other means?
Structural surveys are important as public safety should always be a prime concern. The important thing is how you ask the question. If you ask the engineer to survey the structure to assess whether it is adequate for all current standards and loading, you may not like the answer. If you ask what the condition is and what could it be used for, you have possibilities.
How would you respond to this question? Jump in with your answers in the comments!
Note: R. Moses is not meant to be professional engineering advice nor should be relied upon as such. Consult your own technical professional before proceeding with your own project.