https://www.captis.com/wp-content/uploads/2021/05/210517141525-hernando-de-soto-bridge-0514-restricted-super-169.jpg 619 1100 CAPTIS https://www.captis.com/wp-content/uploads/2021/01/captis_full_large.png CAPTIS2021-05-17 17:07:072021-05-17 22:13:17Memphis bridge's unusual design puts it at risk for future problems
The reason behind this optimistic first thought: I knew that fatigue cracks by nature take a long time to form and develop, and therefore I never thought a routine inspection for a major bridge would uncover a fatigue crack in its mature stage — at least not without being previously noticed during the periodic inspections. When the Tennessee Department of Transportation (TDOT) released the photos of the crack (more a complete fracture of three plates and a partial fracture of the fourth, bottom, plate of the bridge), I was totally shocked.
Shutting down the bridge is a major supply chain disruptor, as work on the bridge impacts hundreds of boats and barges running through the heartland on the Mississippi River. I live in the east end of Memphis (far away from the bridge) and have been working mainly from home due to the pandemic, so the closure of the bridge did not affect me directly in my day-to-day business.
As an occasional user of the I-40 bridge, which opened to automobile traffic in August 1973, I am confident that after all the local and national attention this incident has attracted, officials and experts will make the right decisions to keep the public safe. I have two main concerns going forward. First, this bridge — central to the local daily lives of residents in Tennessee and Arkansas and crucial to national shipping and trucking routes — has a unique design that puts it at risk for future problems. Second, this incident is a stark reminder of how vulnerable a state America’s bridges — and therefore our nation’s infrastructure — are really in.
This bridge’s structural system is defined as an arched truss bridge with a deck suspended from the truss. This system is not the ideal system for a bridge that long. For this type of bridge, it’s far preferable that the truss extend below the deck of the bridge at the pier (the upright structure that goes into the river to hold the bridge up) to provide better force distribution.
But because of this bridge’s unique “M” shape, the entire truss (the arch of the “M”) is above the deck and the pier is in direct contact with the bridge deck (which holds the highway). Because the arched “M” shape also requires the cables connecting the truss to the bridge to be shorter in places, this imposes high stress concentrations at the beams near the pier, specifically where this fracture was located.
Fatigue cracks occur when metals experience thousands or even millions of cycles of stress — namely, the instantaneous change in stress conditions when a vehicle passes over it. Take one truck as an example: As it crosses the bridge, it imposes several stress cycles due to its multiple axles on various points across the bridge. The more vehicles passing over the bridge and the heavier the vehicle weights are, the faster the crack development would occur.
The location of the fracture in the beam of the I-40 bridge would be considered by engineers as an ideal spot for a fatigue fracture based on these two conditions — high stress on a small space and large number of repetitive load cycles (due to heavy traffic on the bridge). If fatigue cracks are left unrepaired, the cracks will usually grow to reach a critical length and threaten the integrity of the structure. In this case, the Arkansas Department of Transit report shows that crack developed over time as a normal fatigue crack and was not noticed during the latest inspection in 2019.
To repair the crack, a bolted splice is an option, where steel plates are used to connect the fractured plates of the box beam. This could be completed within a few days and would guarantee that no crack will never develop at this specific location again.
However, it would not guarantee that other weak spots along the beam wouldn’t trigger additional cracks due to altering the existing load path of the structure. This means that when a specific section of the bridge is strengthened through repairs, the forces within the overall structure will redistribute, triggering failures in the weakest links. It is essential that other weak components within the system be identified before any repairs are made. This should be accomplished by engineers who must perform a robust analysis to understand the consequences of strengthening the fractured beam component on the overall behavior of the structure.
With the current technologies as well as the advances in computer simulations and sensors, failures like this could be prevented. In order to prevent similar incidents from occurring moving forward, reliable monitoring systems of the bridge’s safety must be implemented and maintained by state departments of transportation.
Structural health monitoring is a well-known technique to ensure the safety of in-service bridges and to assess the conditions and predict the fatigue life for both cracked and older bridges as well as collect true in-service data of bridge behavior. These data can then be used to calibrate detailed computer models to develop effective repairs to enhance bridge performance and prolong its life. Following the development of a repair strategy, bridges can be monitored again to confirm and validate the viability of the repair strategy.
When conducting a fatigue assessment, fracture assessment has to be considered to evaluate the critical crack size at which fracture is triggered. This will require additional state funding to purchase necessary monitoring equipment and to bring academic and professional experts together to develop this system.
Currently over 600,000 operational bridges, with an average age of 42 years, exist throughout the United States. Many of these bridges, designed for specific service life, have fallen into a state of degradation.
We learn from the I-40 bridge incident that additional resources need to be allocated to be able to fund additional efforts to continuously monitor our bridges in a robust manner for the safety of millions of passengers that use our infrastructure on daily basis.
This is not a CAPTIS article. Originally, it was published here.