UPDATE: Turns out this was a legitimate question to ask — the feds did examine this issue in detail, but ultimately concluded it was not a factor (see their final report here, page 126).
I got a little flak when I made a similar suggestion back in August, prompted by my Minneapolis-based brother. So I will try to report as neutrally as possible on an article in the Minneapolis Star Tribune just sent to me by said brother, headlined:
Did heat, rusted plates doom bridge?
Federal investigators are trying to determine whether 91-degree heat caused expansion that put too much pressure on the corroded gusset plates that held the I-35W span together.
What does the article say?
… authorities are analyzing what role the 91-degree heat on Aug. 1 might have played in increasing stress on the already-weakened L-11 gusset plate, which connected four steel beams located near the bridge’s south end.
Like the New Orleans levees that failed during Katrina, this bridge was not well-designed. In particular, it apparently could not handle the consequences of the cold and heat that Minneapolis is subjected to:
One of the structural engineers who has viewed the wreckage and knows the design of the bridge said runoff of salt and de-icing chemicals from the bridge deck could have contributed to the corrosion in the L-11 gusset-plate connection. That’s because a diagonal, H-shaped beam running into the joint could have acted to channel the liquid toward the gusset plate, the engineer said.
If so, the response to cold Minneapolis winters set the stage for a hot summer failure:
Both structural engineers interviewed by the Star Tribune … say federal authorities are examining whether intense heat on Aug. 1 triggered a chain reaction of force that overpowered gusset plates in crucial locations — such as the one at L-11.
The I-35W bridge was designed to flex, to handle expansion in extreme heat and contraction in bitter cold. But that design assumed that roller bearings would move accordingly.
Roller bearings are mounted on top of bridge piers. They support the weight of the bridge and contain steel cylinders that allow the bridge to roll smoothly back and forth as it expands and contracts with temperature changes.
MnDOT officials had long known from their inspections that the bearings were not working correctly because of corrosion and buildup of debris. And in July 2006, a consultant’s report highlighted the problem. “The bearings are not allowing the structure to move linearly with changes in the … temperature,” the report said.
Roller bearings recovered from the river are stacked neatly in the NTSB’s secure area for inspection. One of the structural engineers who asked not to be named said the bearings appear to be lacking marks of wear, indicating they may have been locked up or their movement restricted by debris and corrosion.
Does this mean climate change contributed to the Minneapolis bridge collapse? We don’t know the final answer to that yet, but it certainly seems now like it was a not unreasonable question to ask after all.
One thing is clear — global warming means more extreme weather in the future, and our infrastructure designers need to plan accordingly. Reassuringly, Minnesota public radio ran a segment titled, “Can the new bridge stand the heat?” (available here) where they interviewed Linda Figg, the lead designer of the I-35W bridge replacement project, “about how the new I-35W bridge will be designed to withstand Minnesota’s extreme winters and summers.”
I am not an “I told you so” type of person — well, maybe a little (isn’t that one of the central points of blogging?), but my brother is a “your blog on the matter was well ahead of the curve” type of person. Okay, ’nuff said.