(This diary is a part of the Infrastructure Kos Series. You can read last week’s diary on the Flint Water Crisis here. The hope is for this to become a weekly series, because it is always be Infrastructure Week here at Daily Kos. Always.)
This past Monday, news came of another bridge collapse in the United States, this time in Chattanooga. The fact that a bridge collapse has to be qualified with the determiner ‘another’ in the richest Country the World has ever know is distressing, even more so considering said bridge was also part of the largest infrastructure project the World has ever know.
Late Monday morning, the side of an overpass on I-75 collapsed, tumbling onto the ramp headed to Chattanooga. This bridge had been built in the 1950’s, and was recently inspected in July 2018. The condition of the bridge was found to be ‘Fair’(1), which sounds more like a weather report than something very large that can collapse and kill you.
Fortunately, no one was killed (this time), although one person was seriously injured. And fortunately, this particular failure didn’t appear to be directly related to neglect or structural deficiency.
Engineers were quickly able to determine that the bridge had been previously weakened by an impact from an oversized load. Engineers were able to observe that all connecting steel rebar between the concrete that ultimately failed and the concrete that remained in place had all been laterally deflected (meaning they had been bent or sheared to the side) to the same extent prior to the collapse, and in the same lateral direction(3). The only way this could have happened was is from a lateral impact to that part of the bridge that did not result in its immediate failure. But once the rebar was bent, it was weakened, and it was only a matter of time before it would fail.
This must have been some impact.
How a conflict between a bridge and a semi-trailer always ends.
There’s no record of what type of load struck the overpass(2). And the topic of how our trucking industry is insufficiently patrolled and largely relegated to self-regulation is the topic for another diary. And it’s possible that the time between impact and failure was too brief to be caught through inspection. Then again, Tennessee only inspects its bridges annually. That’s a long time for a problem to fester.
Structurally Deficient Bridges, and Where to Find Them
But the Chattanooga Overpass failure, while not directly a result of structural deficiency, has been just one of all too many. A report came out just two days later by the American Road and Transportation Builders Association (ARBTA). The timing was coincidence, but the message all the more salient: it will now take 80 years to repair all the bridges in the United States at our current (and abysmal) rate of funding. From the latest report by ARBTA:
This year's report, released Monday and based on 2018 data, found:
- There are 616,087 bridges in America
- Of those, 47,052 (nearly 8%) are "structurally deficient" and need urgent repairs
- 235,020 bridges (38%) need some sort of repair
- Americans cross structurally deficient bridges 178 million times a day, including such landmarks as the Brooklyn Bridge and the San Mateo-Hayward Bridge over the San Francisco Bay
- The average age of a structurally deficient bridge is 62 years
What is a structurally deficient bridge?
On the same 0-9(4) scale, a structurally deficient bridge has a rating of ‘4’ or less. The Virginia DOT(5), gives a pretty approachable definition of what this term means:
Bridges are considered structurally deficient if they have been restricted to light vehicles, closed to traffic or require rehabilitation. Structurally deficient means there are elements of the bridge that need to be monitored and/or repaired. The fact that a bridge is "structurally deficient" does not imply that it is likely to collapse or that it is unsafe. It means the bridge must be monitored, inspected and maintained
While that doesn’t mean that bridge is in danger of imminent collapse, again, for 47,052 bridges nationwide to be in this state of disrepair is pretty alarming. You can even find the list of structurally deficient bridges near you. The same ARTBA report has a breakdown by Congressional District.
In the above map, a congressional district with 0 — 4.9% structurally deficient bridges is in blue ,5.0% to 8.9% is in yellow, and 9.0% and higher is in red. Image from the ARTBA.
The list of the bridges in the worst shape are here(6), and you’ll quickly notice that these bridges are exclusively urban interstate bridges. Why are highway bridges in urban areas the most dangerous of all in America?
- Decades of neglect of urban infrastructure by state and federal authorities who feel taxpayer dollars are better spent on rural and suburban constituents.
- Lack of public transportation, or overcrowding of public transportation, forcing these structures to carry far more vehicles than they were ever designed for.
- Replacing these structures in an urban environment, where service cannot be interrupted and there is no available real estate to build a new bridge alongside, is extremely costly.
What Does it Matter if A Bridge is Structurally Deficient?
The following footage depicts the collapse of a section of the I-35 Mississippi River Bridge in Minneapolis, MN. The bride was 8 lane, steel truss arch bridge that carried I-35W across the Saint Anthony Falls portion of the Mississippi River. The bridge failed catastrophically during the evening rush hour on August 1, 2007. 13 people were killed in the collapse, and another 145 were injured. A security camera for a parking lot located at the north end of the span captured the collapse, although the bridge is not centered in the video, and the resolution is poor.
If you go examine the first frame of this security camera footage, you will see that the central span of the bridge is fatally deflected downward prior to collapse. The following still is from KARE 11.
Horrifying still image from security camera, via KARE 11. Note deflection of beams in circled region and downward movement of the roadway. This is immediately prior to collapse beginning.
Collapse would be a forgone conclusion once the bridge had failed to the point in the still image, above. Motorists wouldn’t have been aware of the danger until minutes or seconds before collapse., But motorists don’t inspect our bridges. The State does. And there were warning signs over the years.
- In 1990, the bridge was designated structurally deficient for the first time.
- In June 2003, the following image is from under the roadway. You can see the bowing plates(7).
Photo from NTSB. Note bending of gusset plates.
- In 2005, the bridge was again rated as structurally deficient, and in possible need of replacement.
- In 2006, a steel reinforcement project was planned for the bridge.
- In January 2007, the steel reinforcement project was canceled in favor of periodic safety inspections. Engineers had determined that the reinforcement project would, in fact, weaken the bridge further. Engineers also worried that if they began the work necessary to reinforce the bridge, they might uncover deficiencies that could force it's closure.
- In 2007, The bridge ranked near the bottom of a separate federal inspection rating system, and still classified as structurally deficient.
- By 2007, the bridge was scheduled to be replaced ca. 2020, an eternity in politics and long beyond the time frame needed to design and build a replacement. Meanwhile, construction work began on the bridge to make it until 2020, including joint work and the replacement of lighting, concrete and guard rails.
Conservatism Strikes Again
The I-35 Bridge Collapse shows many the problems with infrastructure spending in the United States, touched on in the last diary of the series regarding the Flint Water Crisis. In particular, we again see devolution of infrastructure funding to the lowest levels of Government, which cannot run deficits or print money like the Federal Government. There was a known problem, and nobody was willing to spend the money to fix it. Tim Pawlenty (R) was governor in the runup to the crisis, who campaigned on attempting to balance the budget without raising taxes, thereby starving municipal agencies of the funds needed for infrastructure inspection and repair.
The bridge had design issues dating back prior to its actual construction, and a contributing factor in the collapse was that the state had placed new pavement on top of old, instead of removing the old pavement first, thereby increasing the static load by 20%. The bridge was under construction at the time of collapse, and an additional 262 tons of material were being stored on the bridge. Combined with the evening rush hour, where vehicle loading was at its highest, the bridge finally gave way.
But in drawing a comparison to the crisis in Flint, one can’t help but wonder that with proper funding of inspections, both the tragedy this week and the tragedy in Minnesota could’ve been averted.
___________________________
(1) Federal bridges are rated on a 0 to 9 scale. There are only three descriptors used for bridge condition: Good, Fair, and Poor. A fair rating has a score of 5 or 6.
(2) And that is a separate infrastructure problem in and of itself. Highways are very clearly marked, yet our municipalities and federal government must repair dozens of bridge strikes each year. Tennessee DOT noted that they see about 50 bridge strikes a year.
(3) The collapse under vertical load would show vertical deflection and shear without any lateral movement, so the presence of lateral deflection without shear is proof positive that the bridge was deflected laterally prior to collapse.
(4) You can tell an Engineer came up with this because it is not 1 through 10 like a normal human being would conceive.
(5) For the purposes of this diary, it is safe to assume this definition applies Nationally.
(6) Ranking of the worst of the worst is not be condition, but by loading in terms of average daily travel. The number 1 bridge on the list is simply a structurally deficient bridge with the highest loading, not necessarily in the worst condition.
(7) These are termed gusset plates.
Monday, Apr 8, 2019 · 3:16:19 AM +00:00
·
NoFortunateSon
Thank you all for the wonderful comments!
This diary was inspired by another user’s comment from last week, about a bridge in Joliet, IL. The bridge in Chattanooga would collapse the next day, so I though the atrocious state of our bridges would be a good topic.
Also, please let me know if you would like me to put “Infrastructure Kos: ...” at the beginning of the title of the diary (I did last week / didn’t this week) to differentiate these from other topics.
