As Miami-Dade just tragically learned, the collapse of any bridge can lead to injuries, loss of life and property damage on a scale equal to plane crashes, terrorist attacks and natural disasters. That’s why bridge designers, engineers, construction workers, managers and inspectors take their jobs so seriously.
The best way for them to prevent catastrophic accidents is to understand the factors that cause bridges to fail. As the National Transportation Safety Board team investigates the collapse of the FIU pedestrian bridge under construction, here are the top 6 reasons bridges collapse.
1. The top reason bridges fail is a mix of factors that, if they happened individually, would not cause a bridge to collapse. However, when they take place all at once, they result in devastating consequences. For example, severe winds may not be enough to cause a structure to come down. However, when they hit a bridge that’s structurally too rigid to withstand them, it leads to failure.
The I-35W Mississippi River Bridge in Minneapolis collapsed suddenly in August 2007. The official cause was attributed to gusset plates that were too thin and tore along a line of rivets. The plates had supported heavy traffic volume for 40 years. It took a secondary factor, the additional weight of construction equipment parked on the bridge at the time, to trigger the failure. Another contributor: Wear and tear on the gussets had not been identified prior to construction starting because bridge inspections had been missed.
So, while the gussets were identified as the root cause of this devastating collapse, it was really three separate factors coming together that led to disaster.
2. Infrastructure issues: According to the American Society of Civil Engineers, bridges in the United States earn a mediocre C+ rating for maintenance and safety. The group reports that one out of every nine bridges in the country is considered structurally deficient, and the average age of bridges in the U.S. is more than 42 years old. The age and condition of bridges is a contributing factor to many recent collapses.
3. Unexpected events: When it comes to bridge construction, engineers simply don’t know what they don’t know. Countless bridge incidents happen because of unanticipated structural or design-related issues.
The good news is that computer modeling and testing make it easier for engineers to see how different bridge designs hold up against a broad range and combination of unexpected conditions. Still, it’s important for them to build redundancies into structures to cover unknown possibilities in today’s fast-changing world.
4. Accidents: Whether it’s a truck hitting a support post, a train falling off the tracks or a boat colliding into a foundation, accidents are one of the leading reasons bridges are damaged or come down. Bridge engineers must plan for all types of incidents, including those caused by vehicles that exist today and ones like driverless cars, larger ocean tankers, and cruise ships, along with pilotless drones that could impact bridges in the near and distant future.
5. Construction incidents: Some bridges never make it to completion. They fail during construction. A lesson learned from these incidents is that it’s just as important for designers and engineers to plan all aspects of bridge construction step by step, analyzing the impact new phases will have on previous ones.
Back in 1907, an epic bridge failure happened during construction in Quebec City. Designers were made aware that the bridge weighed eight million pounds more than estimated at a certain point in the construction process.
However, they didn’t feel this was a significant enough issue to make adjustments. Soon after, an onsite engineer noticed the frame of the bridge was starting to bend. However, others didn’t take the observation seriously, claiming the beams were bent when they were delivered.
Not long after, the structure came down, killing 75 workers. In the end, it was determined that the beams were not adequate to handle the additional eight-million-pound load.
6. Design flaws and manufacturing errors: While it’s becoming less common than in the past, some bridges fail almost immediately after completion due to significant design errors or issues associated with materials used in the construction process. Frequent inspections throughout the construction process can provide the “fresh eyes” needed to identify problems and flaws.
Oregon-based Bridge Masters has installed hundred of utility conduits on bridges across the country. This article from their website is reprinted with their permission.