Jon Kraqczynski reports for the Associated Press,
“Authorities lowered the death toll from an interstate bridge collapse to four Thursday, but warned the final number could change as divers comb the twisted steel and chunks of concrete that crashed into the Mississippi River.”
A bridge collapses in Minneapolis, with no apparent fire, earthquake or other shock. In one of the BBC pictures, a twisted steel beam with a rusted and/or burned end is seen. The news story notes that fires occurred as a result of the collapse (probably from vehicle fuel tanks). All the green metalwork is a truss (now twisted and sundered).
One picture shows one of the two concrete footing-plus-columns, which support the bridge at each bank, to be tilted in toward the river. Either this concrete foundation was torqued inward by the downward motion of the span over the river (before it broke off), or there was some softening (by water penetration, as in dikes and damns?) of the soil around the footing of the concrete foundation allowed it to tilt, and then the gravity load of the truss and roadway were no longer aligned with the concrete columns; this would increase the tendency to tilt, and such a “positive feedback” could easily accelerate the rate of tilt leading to “sudden” collapse. Whatever the cause of the collapse, it will ultimately have to involve some kind of failure of the weight-bearing supports on each bank.
Of course, it could be another masterstroke of “black ops” by a nefarious secret government trying to distract the public from its evil doings. Not a rational possibility, but one that makes for a better movie.
MANUEL GARCIA, Jr. is a recently retired physicist from a US Department of Energy laboratory. He is presently on holiday, and his technical interests involve fluids, electricity, heat flow and energy. His non-technical interests are varied, one being the social responsibility of scientists, another being the social dimensions of choices for the energy technologies that power a community. He can be reached at firstname.lastname@example.org.