The LaPlatte River Bridge, designed by PERCH and opened last fall, still stands after a severe Halloween rainstorm brought the highest flood stage in at least 25 years. The pedestrian suspension bridge in Shelburne’s LaPlatte Nature Park was designed for a high-water level of 7.63 feet, achieved in 1996 and 2017. Early on November 1, USGS recorded a water level of 8.07 feet, high enough to put the entire bridge deck underwater. The lower west tower withstood water at least 4 feet deep.
The storm washed out roads and flooded homes in northern and central Vermont; over 100,000 customers lost power according to the Burlington Free Press. Vermont Emergency Management is seeking FEMA assistance to pay for repairs. Among the storm’s casualties was the nearby Lewis Creek Bridge, which served as a model for the LaPlatte Bridge. The Lewis Creek Bridge towers had a rubble foundation that apparently succumbed to the boulder-moving power of high water. The LaPlatte Bridge’s ground anchors, giant screws buried 7 feet below grade, remain intact.
PERCH feels great respect for the laws of physics, as well as all the building codes, design guides, and outside engineers consulted in the design of this bridge. The LaPlatte’s amazing construction crew often wondered if the bridge was overbuilt with a design live load of 90 psf (equivalent to about 70 people standing on the bridge at once) and a 3.0 safety factor applied to the bearing strength of the clay soil. But extreme events like this flood demonstrate why structural engineers demand high standards.