Of the many reasons that the bridge failed, one major cause was controllable.  As explained in the analysis, if one major support structure in a bridge built with redundancy were to fail, the extra strain would be distributed among the other supports.  However, the I-35 bridge was built with only four pylons, supporting a single 458-ft steel arch.  The bridge could have failed due to a fault in any one structure.  “The textbook example of a non-redundant bridge is the Silver Bridge over the Ohio River. It failed shortly before Christmas in 1967 resulting in 46 deaths. A single piece of hardware failed due to a tiny manufacturing defect. But that piece was non-redundant, and the entire bridge collapsed into the icy river.”  The I-35 bridge had been opened just a month prior to the Silver Bridge collapse.  Although bridges designed after the incident included redundant structures, inspectors and engineers should have remained alert to the danger of existing non-redundant bridges collapsing.  To further illuminate this lack of foresight, the Mn/DOT watch list recently added three more bridges that used a non-redundant design, bridges which would have remained on standard inspection schedules if I-35 did not collapse.
On the day of the collapse, a construction crew had closed off the inner four lanes of traffic, and was doing deck repair to fill up potholes and fix the curbs.  Aerial photos of the bridge 3 hours before the incident show that piles of sand and gravel were dumped near the center of the bridge for these repairs.  Although the crew drove in several heavy trucks, the heaviest equipment they used for deck repair was limited to 45-lb. jackhammers.  There is some speculation that the presence of the construction crew put excessive weight on the bridge, but under normal circumstances, the bridge should have been able to support the extra weight with no problem.