Machine Design

Berke on safety: The I-35 Bridge: The truth is out there

There are many theories, but most lead to the question: “Who was watching the store during repairs?”

In past columns, I often refer to hazard analysis, a technique for proactively avoiding product failures and unsafe conditions. It should be used for all designs, including bridges.

Prior to building this bridge, the design should have been analyzed for all reasonably foreseeable situations. This would include the expected life of the bridge. Was it? I do not know. This information is not readily available, but I hope it is included in the accident report when it finally comes out.

Without design information other than an occasional statement in the press, I believe that the initial design and maintenance had nothing to do with the failure. It was more likely the flexing and fatigue of the bridge deck during repairs that caused the accident. During repairs, the west side of the bridge was stripped of concrete and closed to traffic; the east side carried all the traffic, and during rush hour, it was always stop and go, bumper to bumper. So although I have no evidence, I believe flexing caused the bridge to collapse. Here is how I came to this conclusion.

A security camera showed that the bridge initially broke from the supports at the south end. Subsequently, the bridge fell northward into the Mississippi River. It was flexing faster and with greater amplitude than if the bridge roadway had been uniformly covered with concrete. The covering and weight of the concrete would have slowed the flexing and limited its movement. The bridge span was cantilevered on main supports (one at each end). Flex or bend a cantilevered beam heavily and it will fatigue and fail. (You can see this in action whenever you repeatedly bend a paper clip back and forth. Before you know it, you’re holding two pieces of wire instead of one.) In addition to the bridge flexing along its length, it was also twisting due to the uneven load across its width, i.e., concrete on just one side of the roadway. This accelerated fatigue in the bridge and made the failure more severe. Eventually we should know:

  • Details of the analysis of the bridge’s original design.
  • Whether or not the contractor who built the bridge took any shortcuts.
  • What bridge inspections over the years showed and what recommendations inspectors made.
  • If the repair contractor evaluated the approach being used to repair the bridge under all reasonably foreseeable conditions, and if he was carefully watching as work on the bridge progressed.
  • And hopefully what caused the bridge to collapse.
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