Thomaz Paschoal
Writ 340
Prof. Townsend
05/01/2012
Illumin Article Portfolio

Abstract: This paper is aimed at highlighting the benefits of installing structural health monitoring systems on structures. Through a case study, I examined the various issues with todays outdated visual inspection techniques and showed how the installation of SHM systems on new bridges solves many of these issues.

Bio: Thomaz Paschoal is a junior majoring in Civil Engineering at the University of Southern California. He was born in São Paulo, Brazil. Outside of class he is involved with Engineers Without Borders and the American Society of Civil Engineers.

Structural Health Monitoring Systems

Imagine for just a second that you live in Minneapolis, Minnesota; you work in one of the many shiny downtown skyscrapers. It’s Wednesday, 5:30 p.m., and you decide to go back home to the northern suburbs of the city. You are about to get on the I-35W Bridge over the Mississippi river, a bridge that you take at least twice a day. But this crossing is not like the hundreds of other previous crossings. On August 1, 2007, this bridge, a major commuter route, simply collapses into the river. 13 people die and 145 people are injured and, even more frightening, it could have been worse.
The infrastructure in the United States is in need of a major upgrade. There are roads, bridges and tunnels all over the country that are badly in need of repairs. According to the Federal Highway Administration there are currently 69,223 bridges that are classified as “structurally deficient.” In other words, they require significant maintenance or repairs in order to be deemed safe (Transportation for America). One of the big problems with bridges is that it is usually difficult to tell which bridges are in worse condition since engineers rely on unreliable visual inspections. This means that there are bridges in desperate need of repair that are being overlooked, while bridges that could last another decade are being replaced. This becomes an even more serious issue when there are so many bridges that are being used way past their expected lifespan of 50 years (Transportation for America).
The I-35W bridge carried 144,000 cars per day (Stachura). The only reason why there weren’t more cars stuck in the bumper-to-bumper traffic that covered the bridge deck at the time of the accident was because of the construction that the bridge was undergoing, which had closed 2 lanes. This is the same construction to which many experts attribute the collapse, as it was adding a tremendous amount of weight to the bridge in both equipment and materials. This was a very old bridge, built in 1967, and one that had been undergoing yearly inspections for quite some time. These inspections, however, did not reveal the severity of the bridges’ condition and did not prompt engineers to place restrictions on the bridge. Had the bridge been equipped with a Structural Health Monitoring System, the unreliable visual inspections would not have been as vital and the engineers would actually have known the bridges’ condition instead of making a guess based solely on visual evidence.
The reason why visual inspections are so unreliable is that you can never be sure about the severity of what you are looking at. In the last three inspections (2005, 2006 and 2007) of the I-35W bridge, engineers had noted with some concern the condition of the truss members. They made the judgment call that it was not a cause for alarm and allowed it to continue normal operation. This proved to be a mistake.
Figure 1 was a picture taken by the inspectors of a plate on the I-35W Bridge before its collapse. You can see that the gusset plate, which holds everything together, is bent a little bit out of shape. Even this slight bending is enough for the plate to not behave ideally. The engineers didn’t think it was a big deal, however, because it was not a commonly occurring issue on the rest of the bridge. The engineers did not know that with the extra weight of bumper-to-bumper traffic, the stress of construction, and the two-inch thick slab they had added to the deck a couple of years before, the gusset plate would become what you see in Figure 2.

Figure 1: Gusset plate before the collapse Figure 2: Gusset plate after collapse
Source: www.civilengineering.com Source: www.civilengineering.com

The collapse of the bridge is not to be blamed entirely on the engineers performing the inspection, however. They noted the issue, as you can see in Figure 1, and made an educated decision, based on engineering calculations, and kept the bridge running. Without a SHMS, there is no way for engineers to get actual load and stress data. They instead must use estimated loads and predict how the material will behave based on what they see in the field. Installing a SHMS is the best way to make this information available to engineers so that they can make more educated decisions about the conditions of structures in the future.
The collapse of the I-35 Bridge served as a wake-up call; people started to realize that they needed to pay more attention to the bridges and roads that they drove on every day. This is something that most people may take for granted. Engineers kept this in mind when they were designing the new bridge to take the I35-W’s place. The new I-35W Saint Anthony Falls Bridge opened on September 18, 2008, over a year later, in the same location. But it was nothing like its predecessor. BusinessWeek called the bridge America’s Smartest Bridge because of the Structural Health Monitoring System (SHMS) that was installed in the bridge, making it the first of its kind in the U.S. (Hamm). These SHM systems do exactly what their name suggests: They monitor the structural integrity of the structure that they are attached to. SHM systems work through an array of sensors, which measure different forces and displacements on a structure. These are the two main parameters that the engineers looking at the data have to look at to determine if the bridge is behaving like the designers intended it to.
Every time a car or a truck gets on a bridge, it makes the bridge move ever so slightly downward and makes it vibrate a little. The bridge is designed to do both of those things. It deflects downwards because a certain degree of elasticity increases the bridges ability to take more load cycles. The vibrations help to distribute the loads more evenly throughout the structure and it makes the bridge more earthquake-resistant. This ability that the bridge has to move under certain conditions is one of the reasons the bridge is so strong. The SHM system takes measurements such as the vertical deflection and the vibrations experienced by the beams and girders, which you can see running laterally and longitudinally under a bridge in Figure 3.
The SHMS also measures the tension on the truss members, which you can see in Figure 4. The truss members are where the bulk of the weight is carried. A structure is nothing more than a path for a load to take from its point of support on the structure to the ground, where the structure itself is supported. More specifically, for a bridge, this represents the load of a truck on the deck of a bridge. The load will travel through the trusses towards either of the ends of the bridge where it will then go into the foundation and be transferred into the ground. This is why the truss members are so important for the structural health of a bridge.

Figure 3: Beams and Girders Figure 4: Truss Members
Photo Credit: Karl Jensen Photo Credit: Rich Corbett
Some bridges are equipped with upwards of 100 sensors. The installed sensors connect to a central computer, which compile all of the data generated by the sensors. An engineer then analyzes the data from to make sure that each member is taking the loads that it was designed to. If it is taking more, it is at risk of failing. If it is taking less, then that is a sign that there is another member taking more loads than it was designed to.
Another instance when a SHMS is very useful is during and after an extreme event, such as an earthquake or storm. Having a system like this installed allows engineers to quickly assess the condition of the structure and to see if immediate repairs are needed. If engineers notice increased vibrations in a bridge in the aftermath of an earthquake, it is usually a sign that one or more members of that bridge have been damaged. If the bridge is equipped with an SHM system, this becomes a painless process since the bridge is already being constantly monitored. For a bridge that does not have any type of monitoring system, a visual inspection is needed to check for damage on the bridge, which as I mentioned before is not very reliable (Lockhart, AES).

Figure 5: Sensors installed on new I-35W Bridge
Source: BusinessWeek

In the specific case of the new I35-W Bridge, DATAQ installed 323 sensors. There were six different types of sensors as part of a SHM system that measures seven different parameters on the bridge. These sensors are pictured in Figure 5. Aside from the movement sensors that measure the deflection, the accelerometers that measure the vibrations, and the strain gauges that are used to measure the forces on members, there are also other less-crucial parameters measured on the bridge. This includes corrosion, ice and temperature measurements, as well as concrete strength gauges, which are used during construction.
The process of installing the sensors started even before any concrete was poured into the bridge, since they were used during construction as well. During the construction phase, strain gauges were attached to the forms of the bridge. This allowed the contractors to determine exactly when the concrete was ready to bear weight since they were measuring the compressive strength of the concrete. This made the construction faster, since work could start on the next parts of the bridge sooner. It also made the construction safer since the builders could be sure what they just poured was structurally sound before moving on to the next part of the bridge. Strain gauges were installed throughout the bridge to measure strain on the concrete as well as the steel members on the bridge. The engineers used accelerometers to keep track of vibrations on the bridge. This is what they look at after an earthquake to see whether the bridge behaved in the way that they were expecting it to under such conditions.
Aside from these sensors, whose main function is to ensure the structural integrity of the bridge, other sensors, such as thermometers, ice gauges and corrosion detection sensors, were also installed. These were mainly to help with the upkeep of the structure turning on deicing systems automatically and alerting maintenance crews when painting was needed (Hamm, Lockhart).
As you can tell, the benefits from SHM’s extend beyond ensuring the safety of the bridge. The ice gauge, for example, serves as a sensor to detect when there is ice buildup on the roadway and activates the anti-icing sprinkler heads over the roadway to keep the road safe. There are also movement sensors on the expansion joints that detect if one of the plates has or is coming loose so that they can send a team out to repair it. If these sensors were not in place, the maintenance costs for the bridge would be much higher. It would require a crew being sent out to deice the bridge and to perform a visual inspection of the expansion joints.
Research is being done perfect these systems and make them an even more reliable alternative to traditional structure inspection techniques. Another example of a bridge that had a similar SHM system installed is the Jindo Bridge in South Korea, shown in figure 6. In this case the system was put in place by researchers from University of Illinois at Urbana-Champaign in conjunction with researchers from the University of Tokyo. They installed a network of wireless sensors along the cable-stays of the bridge. In the case of a suspension bridge, such as the Jindo Bridge, manual inspections are very expensive, equally unreliable, and cannot be done frequently due to the difficult access to the cables. The sensors will measure sagging and tension in the cables to ensure that they are always doing what they should be doing.

Figure 6: Jindo Bridge in South Korea
Source: StructureMag

This particular system has two huge benefits over past SHM systems. The first of them is that the sensors are wireless. This is extremely important in a bridge such as this where they installed hundreds of sensors that all need to send data to one central location. If they were not wireless, a very expensive network of cables would need to be installed to connect each sensor to a central computer. The other huge benefit from this system is that researchers were able to create a program that takes all of the data from the sensors and displays it in an easy-to-understand format. This allows any civil engineer to use the system even without an extensive computer science or electrical engineering background (La Montagne).
Up to a few years ago structural health monitoring was something that was done through highly subjective manual inspection and testingNow that structural health monitoring systems are being installed on more and more structures, it will help engineers to be able to tell with much more accuracy the true condition of these structures, preventing tragedies like the one that occurred four years ago in Minnesota. We are at a crucial point in time right now with most major roads and bridges reaching the end of their lifespan. In fact, no major new infrastructure projects should be built without some form of SHMS. If not, 50 years from now, engineers are going to be performing visual inspections on the structure, just like today, in order to be sure that the structure is still safe for traffic. That doesn’t seem very futuristic, does it?

Works Cited
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