What to make of the news that the Aug. 1 collapse of the bridge carrying I-35 over the Mississippi River in Minneapolis was due to human error? And that all the inspection processes in the world wouldn't have prevented it?
Last month the National Transportation Safety Board (NTSB) attributed the failure to a design flaw: two little words that belie the gravity of the consequences. Some of the steel gusset plates that supported the bridge's two main trusses were roughly half as thick as required by 1960s design standards. The plates that were undersized on the bridge are undersized on the drawings, and the board can't find the original calculations. Therefore, it can't say whether the error was a miscalculation or a drafting mistake.
Nor could anyone have been expected to discover the mistake. The Federal Highway Administration's (FHWA) National Bridge Inspection Standards don't address design errors. Each time the bridge was modified, including when the concrete deck was increased from 6.5 inches to 8.5 inches, Minnesota DOT engineers recalculated stress levels for what they thought were the bridge's weakest members—which did not include the gussets.
Does this mean every structural element should be re-evaluated every time a major modification is made? If so, is that a wise investment of tax revenues?
England's Highways Agency thinks so. Since the 1970s, after four steel-box girder bridges collapsed, the agency has required that designs for both new construction and major upgrades be reviewed and approved by independent design engineers. The check is conducted during the design process, not once the design is completed. Sometimes the review includes an economic analysis of the proposed design, which sometimes leads to a more cost-effective—though equally viable—design.
Estimates for how much this step adds to overall project costs range from 30% of the design fee to less than 0.5% of construction.
The NTSB recommended that FHWA require owners of “non-load-path-redundant” steel truss bridges to conduct load capacity calculations to verify that the stress levels in all structural elements, including gusset plates, remain within applicable requirements whenever planned modifications or operational changes may significantly increase stresses.” In addition, Washington could make independent reviews mandatory for public works departments.
Much like a water manager justifying the expense of a new treatment process or plant to meet EPA standards, road and bridge managers would have an “excuse” for the extra cost involved.
And if FHWA declines to do so? How much is a human life worth to your community's residents? Your job is to make them aware of the options for maintaining their quality of life. It's up to them to decide.
To read The Highways Agency's language requiring design reviews, visit the Article Links page.
Stephanie Johnston, Editor in Chief
Editor in Chief