The concept is to build the bridge much stronger than necessary, ie "overbuild" it in the sense of reinforcing its structure. See below:

Montgomery Countym Arkansas low water bridges
... Such bridges come functionally obsolete because they are not wide or tall enough,
they tend to be sturdy because most were "overbuilt" to handle heavier loads ...
www.rootsweb.com/~armontgo/bridges.htm


and:


]01.05.01: The East River Bridges of New York: An Expression of ...
... The Queensboro Bridge seems absurdly overbuilt for our time when the far greater ... Perhaps
as interesting as a waterborne visit to the bridges is a walking tour ...
www.yale.edu/ynhti/curriculum/ units/2001/5/01.05.01.x.html


and:

The Miami Herald | 07/22/2002 | State drawbridge inspections in ...
... As an added safety precaution, bridges are ''overbuilt'' -- such
as by using more beams than are needed. Thus, if one part fails ...
www.miami.com/mld/miamiherald/3708738.htm