Redundancy and Resurgence

by Don Friedman on August 11, 2017


Apparently, traffic is up on the Erie Canal. The canal has had a strange history: as the first practical transportation link directly from the midwest to the east coast, it helped make New York City what it is. Traffic, mostly agricultural produce in the early 1800s, came here instead of floating down the Mississippi to New Orleans. The route was so important that it was naturally copied by the New York Central Railroad just a few years after the canal opened. The railroad took the passengers and high-value freight, leaving the canal with the big heavy objects. As the article describes, a lot of the current freight consists of things too big to fit on a railroad flatcar.

Anyone in my line of work is an easy target for an accusation of antiquarianism. I believe that I don’t like old things simply because they are old but rather when they are old and useful. The fact that the canal serves to transport objects too big for road and rail is enough reason to say it’s useful, but there’s another: it provides redundancy. Engineers like redundancy because we deal with real-world failures. Nothing every goes as planned, nothing is ever built perfectly, our ability to predict loads is imperfect, and our models of structural action are a little fuzzy. Redundancy increases our safety, not by changing the calculated load capacity of a structure but by providing alternate load paths if our design was wrong or if the construction was wrong.

The canal, the railroad, and the western leg of the New York State Thruway (I-90) run parallel for hundreds of miles. The vast bulk of road traffic was never considered for the canal – pretty much no one driving to Buffalo says “maybe I should take a boat up the canal” – but that doesn’t mean that the canal has no effect. If there were a problem with driving, anything ranging from an accident cutting off road access to extreme environmental legislation that made the use of diesel trucks impossible, the canal and railroad are there, capable of taking more traffic. The redundancy, as in structural redundancy, is back-up.

One of the worst criticisms an engineer can make about a system is “single point of failure.” The classic example of a system with a single point of failure is a chain, which has as many of those single points as there are links. The failure of any link destroys the system. The opposite example is a net, which can have holes where nodes are missing (picture a fishing net with rips in it) and still function, because it is redundant. Any system we rely on, such as transportation, is safer as a net than a chain.

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