Why the I-35W Bridge Fell: The Plates Were Half as Thick

The I-35W bridge in Minneapolis was built just as it was drawn, from steel that tested fine, and on August 1, 2007 it fell into the Mississippi River. The mistake was on paper: the steel plates that held one joint together, called gusset plates, were about half as thick as they needed to be. This case file follows the weight of the road down through the bridge to that joint, shows why nobody caught it for forty years, and what every bridge like it had to check afterwards. How a frame of steel triangles carries a road, the extra concrete that made the bridge heavier, the 2003 photo of a bending plate, the afternoon the plates tore, what the safety board found, and the checks that changed. Chapters 0:00 The moment 1:12 How it carried the road 2:42 Half an inch 4:11 Warning signs 5:49 The day 7:24 What the investigation found 8:58 What changed Report: National Transportation Safety Board, Collapse of I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007, Highway Accident Report NTSB/HAR-08/03 (investigation HWY07MH024), adopted November 14, 2008: https://www.ntsb.gov/investigations/P... Also: FHWA Technical Advisory T 5140.29, Load-carrying Capacity Considerations of Gusset Plates in Non-load-path-redundant Steel Truss Bridges (January 15, 2008): https://www.fhwa.dot.gov/bridge/t5140... ; FHWA Technical Advisory T 5140.31 (January 29, 2010): https://www.fhwa.dot.gov/bridge/t5140... ; MnDOT, the replacement bridge: https://www.dot.state.mn.us/i35wbridge/ Image credits Aerial photo of the collapsed center span, NTSB (U.S. government work, public domain), from the NTSB investigation page Photo of the bridge in May 2006: Todd Murray, CC BY-SA 3.0 (https://creativecommons.org/licenses/..., https://commons.wikimedia.org/wiki/Fi... Bowed gusset plates, June 12, 2003: NTSB photo, public domain, https://commons.wikimedia.org/wiki/Fi... Fractured gusset plate: NTSB photo, public domain, https://commons.wikimedia.org/wiki/Fi... Security-camera images of the collapse: U.S. Army Corps of Engineers, public domain, https://commons.wikimedia.org/wiki/Fi... Figures 17, 21 and 23 and the quoted lines: NTSB report HAR-08/03 (U.S. government work, public domain) Report and advisory pages: NTSB and FHWA (U.S. government works) The truss, joint, load and timeline drawings are our own illustrations. Narration uses a synthetic voice. #engineering #bridgecollapse #structuralengineering