Page 98 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 2 DIAGNOSTIC DESIGN AND SELECTIVE RECONSTRUCTION 73

2.6.8 Revival of Covered Bridges Is Possible
Covered bridges are structural icons representative of the New England countyside. A cov-
ered bridge is a timber, steel, or reinforced structure supporting a deck surface that carries loads
over an obstruction (e.g., a river). Its structural components are protected from the elements by
various coverings: walls, roofs, and decks.
Covered bridges have been built in many different situations and in widely varied settings.
There used to be many covered railroad bridges, but only a few still remain in the U.S. According to
the World Guide, there are only eight North American railroad covered bridges still standing.
Timber structures were covered simply to help protect the timber from the ravages associ-
ated with periodic wetting. Timber truss structures without coverings would often fail after 10
to 20 years of service. Coverings quickly proved their worth by greatly extending the life of the
structure, so much so that the use of timber structures without coverings was only accepted for
a brief period of time. The author has composed a short poem on the need for maintenance:

A bridge is not so inanimate,
It owes its fi tness to man;
It obeys all the acts of God,
Will sway much like a fan;
And with diligent looking after,
Will serve you all it can.

As reported in literature, Table 2.3 from World Guide gives details of historic covered bridges
built in timber or masonry with as many as 880 surviving in 30 states.
Apart from a sense of aesthetics, the advantages are protection from deicing salts of the
deck slab and deck joints and reduced thermal effects.

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