Climate Makes Bridges Shrink and Grow.

• Position: Brief Article

In the summertime, as temperatures become warmer, people usually notice that their fingers are subject to swelling, making it difficult to remove a ring. When the air cools, the hand returns to its normal size. These same phenomena can affect building materials.

For example, bridges are subject to fluctuations in length, notes J. Michael Duncan, University Distinguished Professor, Virginia Polytechnic Institute and State University, Blacksburg. There are many types of bridges, and one in particular--the integral bridge exacerbates the expansion-contraction problem. The structure may become inches longer in the middle of the day. Nonetheless, it is considered by many engineers to be a better way to build bridges, mainly because it offers a possible solution to another problem facing the highway industry.

An integral bridge has no joints in its deck, and the structure is connected rigidly to the abutments, so they act as a unit. The abutments usually are supported on piles. The variations with integral bridges occur with the horizontal movement of the piles or the abutment against the adjacent soil. "As the bridge pushes on the approach fill and then recedes, the fill settles, and then a driver of a car can feel the bump at the end of the bridge," Duncan explains.

A conventional bridge has expansion/contraction joints to accommodate the change in length as temperatures rise and fall. Salt and dirt will enter the...