Trends Winter 2019 | Page 23

the inspector noted that bridge piers were not included in the varying degrees, compromise integrity of the entire structure. inspection due to high water levels at that time. The disaster that NBIS requires U.S. bridges to be inspected above water at least followed a year later profoundly changed the way U.S. bridges are once every two years and a maximum of five years below water. inspected and maintained. Those showing evidence of scour are typically inspected on a more frequent basis and after major storm events, both above As a direct result of the New York bridge failure, the Federal water and below water, until repaired, replaced, or re-evaluated. Highway Administration (FHWA) revised National Bridge Inspection Standards (NBIS). Since 1988, all state, county, Ayres’ structural inspection group is well versed in evaluating municipal, and other owners of public bridges of specified length bridges and other structures for the presence of scour, are required to conduct underwater evaluations for evidence developing plans of action, and designing effective control of “scour,” riverbed conditions similar to those that caused the measures. Through thousands of bridge inspections conducted Schoharie Creek incident. throughout the country, Ayres’ engineers and certified divers are familiar with this issue as well as the logistical Michigan Department of Transportation (MDOT), for example, challenges of examining bridge foundational components currently inspects nearly 6,000 bridges in accordance during flood conditions. with FHWA standards, and more than 400 of these are considered at increased risk of failure due to the persistent threat of bridge scour. The depth of the problem Bridge scour is a phenomenon by which high velocity water removes streambed material from around a bridge’s foundations, creating holes in the river bottom that erode support and, in Bottom left photo: Traditional underwater inspection when river conditions permit safe diver access. Right: “Snooper bucket” inspection that allows inspectors a close view of the underbridge structure. 23