Some of the most important foundation elements of a bridge are its least visible – the abutments, piers, piles, and footings submerged under the waterline. Regular inspection, both above and below the waterline, helps to keep aging structures solid and stable.
How Often Should Bridges Be Inspected?
The National Bridge Inspection Standards (NBIS) were established in the 1970s and set up criteria for which bridges need inspections, how often they should be inspected, and who is qualified to conduct an inspection. NBIS dictates that vehicular <href="https: www.ayresassociates.com="" does-my-bridge-need-inspection="" "="" target="_blank">bridges, greater than 20 feet long, open to public travel, must have regular inspections by qualified bridge inspectors.
That includes routine above water inspections every 24 months. Underwater inspections are required at a maximum interval of 60 months for bridges with elements that cannot be accessed without diving equipment.
Why Perform Underwater Inspections?
As mentioned at the outset, important structural components of the bridge exist beneath the waterline, and periodic inspections performed by certified underwater bridge inspectors help to protect bridges and the commuters who travel them.
But another incredibly important aspect of those inspections is looking for scour – the erosion of soil surrounding bridge foundation elements (footings, columns, piling, and abutments). During routine inspections, comparison of the current groundline elevation as compared to previous elevations is also carefully reviewed.
According to the Federal Highway Administration (FHWA), bridge scour is the leading cause of bridge failures in the United States (approximately 25 to 30 bridges per year). Thousands of bridges are listed as “scour critical” in the National Bridge Inventory (NBI). Without effective countermeasures in place, scour can cause catastrophic results.
What is Scour?
Scour, a naturally occurring process where flowing water removes riverbed sediment from around bridge piers and abutments, is recognized as the No. 1 cause of bridge failures in the United States. Without adequate design and countermeasures in place, scour can destroy infrastructure and undermine bridge foundations.
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