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« Thursday 13 #4---13 Quotes from H.L. Mencken. | Main | Today: Radio Interview re Outsourcing America's Intelligence (and National Security) »

August 02, 2007


mike brown

1. This failure was not caused by static or dynamic overload (of the original design), as the bridge would have had to have been designed to carry worst case loads, such eight lanes filled with loaded dump trucks. 2. Fatigue loading is the main contributor to road and bridge aging. The number of heavy trucks going over the bridge is the main contributor to fatigue loading. The number of cars is inconsequential--one tire of a dual wheel of a semi-tractor trailer can carry a load equivalent to the weight of a typical large SUV. Fatigue causes cracking and crack growth in bridge structures just as in the Hawaiian island hopping plane that lost its roof a few years ago. The more cycles, the more fatigue damage. Age is determined for the most part in cycles, not years. Corrosion only accelerates crack initiation. 3. From the live video, the southern end of the bridge dropped down first indicating the failure occurred near the southern piers supported by the fact that the bridge moved laterally about 50 feet from a freefall position on that end, so I would guess a structural element failed causing buckling and hinging of the other elements on this end. 4. Fatigue cracks are easily distinguished from overload tears, so unless the main contributor of the failure was a shifting pier, it should take a whole lot less than the one year that has been discussed to figure out the origin of the failure. Material testing and comparison to the original material certifications should take only weeks. 5. The failure cannot be blamed on the bridge redecking (unless there was jackhammering directly to the bridge truss). If the collapse occurred after the removal of some decking, then the decking was carrying part of the truss load and truss would have already started to fail. 6. All of this talk of thoroughly inspecting all other structurally deficient bridges in this state and around the country, sounds like a whole lot of passivication to me. Just to inspect all of these bridges is going to take years. To thoroughly inspect would involve, as a start, any or all of the following: examining every critical joint or element with x-ray equipment, disassembling components to inspect, and removal of paint which may be hiding cracks or heavy corrosion. Visual only inspections don't cut it. 7. Structurally deficient or structurally obsolete designations are harsh sounding, but I see that as a good thing. I think a structurally deficient bridge should have large yellow and black signs indicating to travellers that it is considered "STRUCTURALLY DEFICIENT". I bet ya those deficiencies would be fixed in a real hurry instead of after decades.


Thanks for this explanation. I particularly appreciate the suggestion about the sign. I bet you're right.

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