Infrastructure seems so permanent and mundane that most of us never give it a second thought. Maintenance doesn’t make for a flashy news story, but you will frequently find a nagging story on the inside pages of the news cycle discussing the slowly degrading, crumbling infrastructure in the United States.
If not given proper attention, it’s easy for these structures to fall into a state of disrepair until one suddenly, and often catastrophically, fails. We’ve already looked at a precarious dam situation currently playing out in California, and although engineers have that situation under control for now, other times we haven’t been so lucky. Today we’ll delve into a couple of notable catastrophic failures and how they might be avoided in future designs.
Gaining Weight While Delaying Repairs
Most of us take infrastructure for granted every day. Power lines, roads, pipelines, and everything else have a sense of permanence and banality that can’t be easily shaken. Sadly, this reality shattered for most people in Minneapolis, Minnesota in August 2007.
A massive truss arch bridge carrying interstate highway traffic over the Mississippi River suddenly collapsed, tragically killing 13 people and injuring another 145. Rush hour traffic and an ongoing construction project had overloaded the corroded, failing bridge to the point that the center span of the bridge abruptly fell into the river, followed by the adjacent spans. The casualties could have been much higher, too, but four of the eight lanes were closed for resurfacing.
While engineers had noted major structural issues in the early 1990s, no substantial measures were ever taken to improve the bridge. In fact, changes to the bridge may have exacerbated its issues. A de-icing system was installed in the late 90s which may have increased the rate of corrosion of the steel components of the bridge, including the bearings. Additional concrete was laid on the bridge for resurfacing throughout the years, and sturdier guard rails were added as well. Additionally, at the time of collapse, a construction crew was on the bridge resurfacing the travel lanes.
However, the structural deficiencies of the bridge were no secret. The US Department of Transportation (DOT) rates bridges on a 100-point scale, and for two years before its collapse this bridge scored a 50, putting it at the bottom of a list of bridges in the US. Additionally, a plan to reinforce the bridge was cancelled in the early 2000s because it was found that drilling into the existing structure to install the reinforcing steel would actually weaken the bridge further.
After the investigation was completed, it was found that the gusset plates — used to connect beams and girders together — were the main cause of the failure. They were undersized for the original weight of the bridge, which had increased around 20% over the course of its lifetime. Gusset plates don’t just connect the structural members of bridges and buildings (including traditional wood-frame buildings), but they make the connections stronger than they would otherwise be. For this reason it is crucial that gusset plates be sized correctly and repaired or replaced if needed. Additionally, the bridge’s bearings were partially frozen that day which may have contributed to the overloading and failure of the gusset plates, but the main cause of the failure was their overload.
Since the collapse, a new bridge has opened to carry the interstate traffic over the river. The pre-cast concrete box girder bridge was completed ahead of schedule in 2008 and includes a number of modern technologies, such as an array of sensors to measure movement of the bridge. Additionally, the concrete was poured with a number of additives that will increase the lifespan of the bridge to an estimated 100 years.
Structure Flaws Plus Human Error
While the collapse of the I-35 bridge in Minnesota was tragic and arguably preventable, it’s far from the only major infrastructure failure that has happened in recent memory. Just a day’s drive south from Minneapolis is the Taum Sauk Hydroelectric Power Station, a dam that is used as a pumped storage facility. In 2005, the dam was overtopped which resulted in its complete failure. Approximately one billion gallons of water was released when the dam wall collapsed, and the 20-foot wall of water that poured from the dam destroyed everything in its path. Luckily no one was killed as a result, although one family was swept away by the flood and suffered major injuries.
Part of what makes this dam unique is that it wasn’t built into any of the surrounding topography like traditional dams are, aside from being built on the top of a hill. The dam makes a complete loop to impound the water that is stored there and acts as a huge battery for the power grid. During the night when power is cheap water is pumped into the reservoir, and then is allowed to flow out of the dam during times of peak demand on the grid during the day. While the cause of the dam failure in Missouri was mostly operator error (known flaws with gauging how full the reservoir was, purposefully filling the dam beyond its capacity), these bad practices combined with construction flaws in a catastrophic way.
The dam has since been repaired, using more robust roller-compacted concrete instead of earth fill. Additionally, and perhaps more importantly, a more accurate gauge system was installed and it seems that the dam’s operators are less likely to operate the plant in the same way that they were before the failure. This is in part due to a $15 million fine levied against them by the Federal Energy Regulatory Commission (the second-highest fine ever levied by FERC).
While the failures of the I-35 bridge and the Taum Sauk dam aren’t the only modern infrastructure failures we’ve seen in recent history, they are among the worst and are a reminder that flawed design coupled will poor maintenance practices and operating procedures are a recipe for literal disaster.
If there is another piece of infrastructure near you that isn’t receiving the attention it deserves, tell us about it in the comments or email the author directly at firstname.lastname@example.org.