The need for an economical bridge system has never been greater than today. The combination of sharply increased costs and concurrent decrease in highway revenues has had a serious impact on the highway construction program. Most seriously affected are our old bridges since, unlike the rest of the highway, their life cannot be indefinitely extended by maintenance. As the bridges get older and older they finally become unsafe or they fall down. An example of this is shown in Fig. 1. Approximately 150 bridges in the United States collapse every year. Many of these are out on the back roads where we hear little or nothing about them. Unless the bridge is large or involves fatalities, the collapse is considered just another "fact-of-life" occurrence.
