Saturday, May 26, 2007

New Covered Bridge Opens in Union County

This new covered bridge may appear all wood but the truss it utilizes is a combination of wood and steel. This is not a new innovation as many old covered bridges used a combination of iron or steel and wood in their truss. Most notably - and probably the best CB truss - the Howe truss used metal as an adjustable vertical tension member. As metal got cheaper wood was done away with altogether in the truss, which meant the wood roof was no longer necessary to protect it from rotting and covered bridges became history!

John Smollen, former County Engineer of Ashtabula county is leading a minor rebirth by building several new wood CBs in Ohio, which he argues can outlive some modern materials. These are not replicas of historic CBs but utilize a modern variant of a Pratt Truss of his design. A Pratt truss was rarely used in all wood bridges because wood is less suitable for use under tension as required in the long (diagonal) members of this truss. The Pratt truss did come to dominate in metal bridges because metal is better suited for tension loading.

Now if we just had a similar champion for iron truss bridges.