In August of 2007, the 40 year old I-35 West bridge in Minneapolis collapsed into the Mississippi River, killing 13 people and injuring more than 140. The engineers were unaware the bridge was about to buckle, but if they had been alerted of the problem, this crisis could have been averted. The United States has 614,387 bridges making up our roads and bridges infrastructure, four out of ten of which are 50 years or older. In 2016, 9.1% of these bridges were classified as structurally deficient despite the fact that 188 million trips are made across these bridges every day. In Nashville, the majority of our bridges are in good shape. But as the population increases with around an additional 100 people a day, more cars will be on the roads and crossing bridges, putting larger loads and more stress upon them. This is threatening disaster similar to the 1-35 West bridge for many of the older bridges in our city. But, with engineering solutions and new technologies such as sensors that can measure changes in the structure and a special kind of paint that detects vibrations, we can greatly improve bridges - an essential component of the transportation infrastructure.
There are 1,119 bridges on the public road system in Nashville. Since there is a major river flowing through the city, these bridges are an important part of our infrastructure and are crossed by many people each day. The population is growing dramatically and with it the number of vehicles on roads and bridges. As more and more cars pile onto bridges, the loads the bridges are under are becoming greater. This puts more stress on the structure. As the stress becomes too great for the bridge to handle, it experiences a large amount of tension and compression which causes it to bend and then collapse. If the problem is internal (with no visible signs of bending or cracking), like the I-35 West bridge, engineers may have no way of knowing which bridges are being affected which could cause unanticipated failures. This scenario could possibly happen to many of our older bridges.
In order for engineers to be alerted of any changes in the structure, they need to know what and how much any parts are being altered. One device that can help solve this problem is a fibrotic sensor such as a strain gauge or a tilt meter. These sensors are integrated into the structure and from there are able to monitor changes in the bridge by sending beams of light throughout it. This measures acceleration, rotations, tilts, and internal stress. Using cellular technology it allows engineers to see how well the materials and the structure are doing over time as well as alerting if the bridge is under to much pressure or strain. Although the sensors themselves are low in cost, the installation and maintenance of them is much more expensive. Because of this issue, most sensors are only installed on landmark bridges that would be more expensive to replace if they were to collapse.
Smart paint is another solution that may help indicate a problem before it happens. Many engineers will make bridges bulky when designing them to avoid collapse. “Especially where failure could mean lives are lost, engineers over-design to ensure they don’t get failures,” says Jack Hale, the developer of the paint. Although this does not cause any particular problems, the issue with this is that it does not make anything better either. As a solution, Dr. Hale came up with the idea of smart paint. Smart paint is made up of lead zirconate titanate (PZT) which is a piezoelectric material. Whenever the structure is stretched or squeezed, the PZT crystals send out an electric signal proportional to the force being exerted on the bridge. If the bridge is being put under tension, the crystals are squeezed together causing a thinner layer of paint. The opposite happens with compression. Engineers would be able to look at the changes in the paint to determine what is happening to the bridge. Smart paint was tested on the Gateshead Millennium Bridge in 2005. Although the calculations made using smart paint are extremely accurate, it is unable to determine the direction of the force, so scientists and engineers may be unable to identify the issue. Fortunately many strain patterns are unidirectional so it may not matter in some cases.
To solve the increasing problem of Nashville’s bridge infrastructure, I propose we install strain gauges and tilt meters on bridges as well as use smart paint. Both of these solutions have accurate calculations of forces being exerted on the structures, but each would be better suited for different bridges. Since the sensors are more expensive to use, we should use them for our more heavily traveled and important bridges because it would be a larger and more costly disaster if they were to fail or collapse. Mayor Barry has proposed to raise the sales tax in July of 2018 that would reach 10% in 2023. This money has been earmarked for funding of a light rail. My suggestion is that we use 25% of this money to use for the installation and maintenance of sensors for some of our bridges. As for the remainder of our bridges, smart paint may be a better solution. Since it is cheaper, it will be better suited for use on a larger number of structures. Of course we will not use these solutions on every single bridge in Nashville, but we will use it on bridges that are greatly relied on. Because we will need specialists to read and analyze the results these technologies produce, we can set up more jobs and training programs with expertise in this area.
The infrastructure of Nashville’s bridges will need to be improved if we are going to have more vehicles on the roads and putting greater loads on our bridges. Although it may cost a lot of money, if we are able to support the funding for this proposal, it will be well worth the cost. With these new technologies, we will be able to avoid failures of bridges before they occur thus preventing disasters, closings, collapses, and even greater expenses with our bridge infrastructure.
References:
Bridges. ASCE, 2018. 2017 Infrastructure Report Card, www.infrastructurereportcard.org/cat- item/bridges/. Accessed 11 Jan. 2018.
Dalton, Aaron. "Smart Bridges: New Structures Detect Problems before They Occur by Monitoring Themselves." Popular Mechanics, vol. 182, no. 10, Oct. 2005, p. 24. Science in Context, http://link.galegroup.com/apps/doc/A136565861/SCIC?u=tel_k_harpeth&xid=8d83a678. Accessed 9 Jan. 2018.
"Living on Earth: Smart Bridges." Living on Earth, 17 Feb. 2012. Science in Context, http://link.galegroup.com/apps/doc/A280946007/SCIC?u=tel_k_harpeth&xid=34af974f. Accessed 9 Jan. 2018.
"Mayor Barry unveils sweeping $5.2 billion transit proposal for Nashville with light rail, massive tunnel." Tennessean, 17 Oct. 2017. Tennessean, www.tennessean.com/story/news/2017/10/17/nashville-mayor-megan-barry-unveils-ambitious-mass-transit-plan-for-music-city/759973001/. Accessed 18 Jan. 2018.
PUBLIC INFRASTRUCTURE SECURING THE FUTURE OF NASHVILLE’S INFRASTRUCTURE. Nashville.gov. Nashville.gov, www.nashville.gov/Portals/0/SiteContent/Planning/docs/NashvilleNext/next-report-Infrastructure.pdf. Accessed 10 Jan. 2018.
Sample, Ian. "Smart Paint Will Make Bridges Cheaper, Lighter and Thinner." New Scientist, vol. 170, no. 2290, 12 May 2001, p. 22. Science in Context, http://link.galegroup.com/apps/doc/A75286375/SCIC?u=tel_k_harpeth&xid=e7c32d71. Accessed 9 Jan. 2018.
Tennessee 2016 Report. ASCE, 2018. 2017 Infrastructure Report Card, www.infrastructurereportcard.org/state-item/tennessee/. Accessed 10 Jan. 2018.