The gusset plates were undersized:

The collapse of the Interstate 35W bridge may have originated with the failure of gusset plates that were sized a half-inch too thin in the original 1960s design, the National Transportation Safety Board (NTSB) said Tuesday.
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The NTSB also warned bridge engineers nationwide... to review the designs and recalculate maximum loads on the estimated 12,600 similar steel deck truss bridges.
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The sizing error occurred in the original design process at Sverdrup & Parcel, a St. Louis firm that was later acquired by Jacobs Engineering. Because original work papers are missing, investigators don't know whether it was a math mistake or other miscue, [NTSB chairman Mark] Rosenker said.

He said no deficiencies have been found in the bridge's steel or concrete.

- Star-Tribune, January 15, 2008

The findings confirmed forecasts by investigators from three months after the collapse—plus engineering experts in the immediate aftermath— and underscored the dire state of America’s crumbling infrastructure.

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