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Original & Concise Bullet Point Briefs

How Sensors Keep Bridges From Collapsing (and other structures too)

Exploring Sensors and Strain Gauges to Improve Infrastructure Performance

  • Sensors can be used to measure the performance of infrastructure
  • Sensors allow us to compare the predicted and actual performance of structures
  • Structures must work perfectly on the first try, but with sensors we can measure how they perform over time
  • Polybridge is a popular video game which allows players to build bridges, but unlike in reality, if the bridge collapses in the game players can try again
  • Strain gauges are an alternative to dial indicators as they don’t need a reference point.

Engineers Rely on Strain Gauges to Monitor Stress in Structures

  • Strain gauges can measure the internal strain of a beam when subjected to load
  • Concrete beams or slabs can also be measured with a strain gauge cast into the concrete
  • A strain gauge measures a tiny change in distance between two parts of the steel, showing an imperceptible 0.002% change in length to the human eye
  • Engineers can use these gauges to validate design calculations and check if a structure is overloaded
  • Strain gauges are also accurate in measuring changes over large periods of time, such as cracks in concrete structures.

Advances in Civil Engineering: Monitoring Structures with Sensors

  • Sensors are used in civil engineering to monitor performance of structures over long periods of time
  • Crack meters, soil moisture sensors, and piezometers are used for measuring distance, moisture content, and water pressure respectively
  • Load cells and inclinometers measure force in anchors and shifts in embankments
  • Vibration, temperature, flow rate and other parameters are also monitored.

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Almost immediately after I started making videos about engineering, people  started asking me to play video games on the channel. Apparently there’s roughly a billion  people who watch online gaming these days, and some of them watch silly engineering videos too!  And there’s one game that I get recommended even more than minecraft: Polybridge. So I finally  broke down one evening after the kids went to bed and gave it a try. I’m really not much of a gamer,  but I have to admit that I got a little addicted to this game (hashtag not-an-ad). I admit too that  there really is a lot of engineering involved. You have different materials that give your structure  different properties. The physics are RELATIVELY accurate. You get a budget to spend on each  project. And your score is based on the efficiency of your design. But there’s one way this game is  not like real structural engineering at all: if your bridge collapses, you get to try again!In the real world, we can’t design a dam, a building, a transmission line pylon,  or a bridge, spend all that money to build it, watch how it performs, tear it down,  and build it back better if we’re not happy with the first iteration. Structures have to  work perfectly on the first try. Of course we have structural design software that  can simulate different scenarios, but it’s only as powerful as your inputs,  which are often just educated guesses. We don’t know all the loads, all the soil conditions,  or all the ways materials and connections will change over time from corrosion, weathering,  damage, or loading conditions. There are always going to be differences between what we expect  a structure to do and what actually happens when it gets built. Hopefully engineers use  factors of safety to account for all that uncertainty, but you don’t have to dig too  deep into the history books to find examples where an engineer neglected something that  turned out to matter a lot, sometimes to the detriment of public safety. So what do you do?We can’t build a project then watch the cars and trucks drive over with the pretty green  and red colors on each structural member to see how they’re performing in real time… except you  kind of can, with sensors. It turns out that plenty of types of infrastructure,  especially those that have serious implications for public safety,  are equipped with instruments to track their performance over  time and even save lives by providing an early warning if something is going wrong.I love sensors. To me, it’s like a superpower to be able to measure something about the world that  you can’t detect with just your human senses. Plus I’m always looking for an opportunity to exercise  my inalienable right to take measurements of stuff and make cool graphs of the data.  So I have a bunch of demonstrations set up to show you how engineers employ these sensors to  compare the predicted and actual performance of structures, not just for the sake of delightful  data visualization, but sometimes even to save lives. I’m Grady, and this is  Practical Engineering. In today’s episode, we’re talking about infrastructure instrumentation.And what better place to start than with a big steel beam? In fact, this is the biggest steel  beam that my local metals distributor would willingly load on top of my tiny car. One of  the biggest questions in polybridge and real world engineering is this: How much stress is  each structural member experiencing? Of course, this is something we can estimate relatively  quickly. So let’s do the engineer thing and predict it first. Beam deflection calculations  are structural engineering 101, so we can do some quick recreational math to predict how  much this thing flexes under different amounts of weight. And we can use my weight as an example:  about 180 pounds or 82 kilograms. The calculation is relatively simple. You can choose your  preferred unit system and pause here if you want to go through them. Standing at the beam’s center,  I should deflect it by about 2 thousandths of an inch or about 60 microns, around the diameter of  the average human hair. In other words, I am a fly on the wall of this beam (or really a fly  on the flange). I’m barely perceptible. In fact, it would take more than 100 of me to deflect this  beam beyond what would normally be allowed in the structural code. And it would take a lot more than  that to permanently bend it. But 2 thousandths of an inch isn’t nothing, so, let’s check our math.I put my dial indicator underneath the beam, and added some weight. I started with 45 pound or 20  kilogram plates. Each time I add one, you see the beam deflect downward just a tiny bit. After three  plates, I added myself, bringing the total up to around 315 pounds or 143 kilos of weight. And  actually, the deflection measured by the dial indicator came pretty close to the theoretical  predictions made with the simple formula. Here they are on a graph, and there’s the point at my  weight, with a deflection of around 2 thousandths of an inch or 60 microns, just like we said. But,  we can’t always use dial indicators in the real world because they need a reference point,  in this case, the floor. Up on the superstructure of a bridge, there’s no immovable reference point  like that. So an alternative is to use the beam itself as a reference. That’s how a strain gauge  works, and that’s the cylindrical device that I’m epoxying to the bottom flange of my beam.A strain gauge works by measuring the tiny change in distance between two parts of the steel. You  might know that when you apply a downward load to a beam, it creates internal stress. At the top,  the beam feels compression, and at the bottom it feels tension. But it doesn’t just feel the  stress, it also reacts to it by changing in shape. Let me show you what I mean. When I put one of the  plates on top of the beam, we can see a change in the readout for the strain gauge. (Of course,  I had the gauge set to the wrong unit, so let me overlay the proper one with the magic of  video compositing.) For each plate I add to the beam, we see that the flange actually lengthens,  in this case by about 3 microstrain. That’s probably not a unit of measure you’re familiar  with, but it really just means the bottom of the beam increased in length by 0.0003%.  When I add another weight, we make it 0.0003% longer again. Same with the third weight. And  then when I stand on top of the whole stack, we get a total strain of about 0.002%, a completely  imperceptible change in shape to the human eye, but the strain gauged picked it up no problem.Imagine how valuable it would be to an engineer to have many of these  gauges attached to the myriad of structural members in a complicated bridge or building  and be able to see how each one responds to changes in loading conditions in real  time. You could quickly and easily check your design calculations to make sure the structure  is behaving the way you expected. In my simple example in the studio, the gauge is measuring  pretty much exactly what the predictions would show, but consider a structure far  more complicated than a steel beam across two blocks, in other words, any other structure.  What factors get neglected in that simple equation I showed earlier?We didn’t consider the weight of the beam itself; I’m not actually a one-dimensional  single point load, like the equation assumes, but rather my weight is spread out unevenly  across the area of my sneakers; Is the length exactly what we entered into the equation? And,  what about three-dimensional effects? For example, I put another strain gauge on the  top flange of the beam. If you just follow the calculations, you would assume this flange would  undergo compression, getting a tiny bit shorter with increased load. But really what happens in  this flange depends entirely on how I shift my weight. I can make the strain go up or  down simply by adjusting the way I stand on top, creating a twisting effect in the beam,  something that would be much more challenging for an engineer to predict with simple calculations.  Putting instruments on a structure not only helps validate the original design,  but provides an easy way to identify if a member is overloaded. So it’s not unusual  for critical structures to be equipped with instruments just like this one,  with engineers regularly reviewing the data to make sure everything is working correctly.Of course, we don’t only use steel in infrastructure projects, but lots of  concrete too. And just like steel, concrete structures undergo strain when loaded. So I  took a gauge and cast it into some concrete to measure the internal strain of the material. This  is just a typical concrete beam mold and some ready-mix concrete from the hardware store. And  even before we applied any load, the gauge could measure internal strain of the concrete from the  temperature changes and chemical reactions of the curing process. Shrinkage during curing is one  of the reasons that concrete cracks, after all. Luckily my beam stayed in one piece. Once the beam  had cured and hardened for a few weeks, I broke it free from the mold. Compared to steel, concrete is  a really stiff material, meaning it takes a lot of stress to cause any kind of measurable strain.  So I got out my trusty hydraulic press for this one. I slowly started adding force from the jack,  then letting the beam sit so the data logger could take a few readings from the strain gauge  inside. After the fourth step, at just over 50 microstrain, the beam completely broke.  Hopefully you can see how useful it might be to have an embedded sensor inside a concrete  slab or beam, tracking strain over time, and especially when you know about the amount of  strain that corresponds to the strength of the material. This is information that would  be impossible to know without that sensor cast into the concrete, and there’s something almost  magical about that. It’s like the civil engineering equivalent of x-ray vision.One of the most amazing things about these sensors is their ability to measure tiny distances. 1  microstrain means one millionth of the original length, which on the scale of most structures,  is a practically impossible distance for a human to perceive. But in addition to tiny distances,  they also are excellent in measuring changes that happen over a large period of time. A perfect  example is a crack in a concrete structure. You can look at grass, but you probably can’t perceive  it growing, and you can watch paint, but you won’t perceive it drying. And, you can watch a crack in  a concrete slab, like this one in my garage, but you’ll probably never see it grow or shrink over  time. So how do you know if it’s changing? You could use a crack meter like this one,  and take readings manually over the course of a month or year or decade. But in many cases,  that’s not a good use of any person’s time, especially when the crack is  somewhere difficult or dangerous to access. So, just like strain gauges measure distance,  you can also get crack meters that measure distance electronically. I put this one  across the crack in my garage slab and recorded the changes over the course of a few months.And, I know why this crack exists. It’s because the soil under the slab  is expansive clay that shrinks and swells according to its moisture content. I thought  it would be fun to use some soil moisture sensors to see if I could correlate the two,  but my sensors weren’t quite sensitive enough. However, just looking at the rainfall in my city,  you can get a decent idea about what might be driving changes in the width of this crack,  which grew by about half a millimeter over the course of this demonstration. Cracking  concrete isn’t always something to be concerned about, but if cracks increase in size over time,  it can be a real issue. So, using sensors to track the movement of cracks over long  durations can help engineers assess whether to take remedial measures.And, there are a lot of parameters in engineering that change slowly over time. Dams are among the  most dangerous civil structures because of what can happen when one fails. Because of that,  they’re often equipped with all kinds of instruments as a way to monitor performance and  make sure they are stable over the long term. One parameter I’ve talked about before is subsurface  water pressure. When water seeps into the soil and rock below a dam, it can cause erosion that leads  to sinkholes and voids, and it also causes uplift pressure that adds a destabilizing force to a dam.  Instruments used to measure groundwater pressure are called piezometers. They often resemble a  water well with a long casing and a screen at the bottom, but instead of taking water out, we just  measure the depth to the water level. That’s made a lot easier with electronic sensors, like this  one, but I don’t have a piezometer in my backyard. So, to show you how this works, I’m just hooking  my pressure transducer to the tap so we can see how the city’s water pressure changes over time.  I hooked this up to a laptop and let it run for about a day and a half, and here are the results.The graph is a little messy because of the water use in my house throwing off the readings every  so often, but you can see a clear trend. The pressure is lowest when water demands are high,  especially during the evenings when people are watering lawns, cooking, and showering. In  the middle of the night, the pumps fill up the water towers, increasing the local pressure in  the pipes. This information isn’t that useful, except that it gives you a new perspective of  thinking about real-world measurements. Recently I had a plumber at my house who took a pressure  reading at the tap, which seemed like a totally normal thing at the time. But now, seeing that the  pressure changes by around half a bar (or nearly 10 psi) over the course of a day, it seems kind  of silly to just take a single measurement. And that’s the value of sensors, giving engineers more  information to make important decisions and keep people safe after a structure is built.By the way, the engineering of these instruments is pretty interesting on its own. Most of the  sensors I’ve used in the demos were sent to us by our friends at Geokon, not as a sponsorship  but just because they enjoy the channel and wanted to help out. These devices rely on a wire inside  the case whose tension is related to the force or strain on the sensor. The readout device sends an  electrical pulse that plucks the wire and then listens to the frequency that comes back. You  can see the pluck and the return signal on my oscilloscope here. Just like plucking a guitar  string, the wire inside the instrument will vibrate at a different frequency depending on  the tension, and you can even hear the sound of the vibration if you get close enough.  Of course civil engineers use lots of different kinds of sensors,  but vibrating wire instruments are particularly useful in long-term applications because they are  incredibly reliable and they don’t drift much over time. They’re also less vulnerable to interference  and issues with long cables, since they work in the frequency domain. In fact, there are vibrating  wire instruments that have been installed and functioning for decades with no issues or drift.And the demos I’ve shown in this video just scratch the surface. We’ve come up with  creative ways to measure all kinds of things in civil engineering that don’t necessarily  lend themselves to garage experiments, but are still critical in performance monitoring  of structures. Borehole extensometers are used to measure settlement and heave at excavations,  dams, and tunnels. Load cells measure the force in anchors to make sure they don’t  lose tension over time. Inclinometers detect subtle shifts in embankments or slopes by  measuring the angle of tilt in a borehole along its length. Engineers keep an eye on vibrations,  temperature, pressure, tilt, flow rate, and more to make sure that structures  are behaving like they were designed and to keep people safe from disaster.Here’s another engineering measurement you may not have ever considered: the exposure  length of a blade in a handheld razor. Luckily, today’s sponsor Henson has thought about it,  and in fact come up with a design that holds the blade within an extremely tight tolerance - that  tiny green area is the spec. Henson reached out to me last year about sponsoring an episode,  and I thought, “What does a personal care product have to do with engineering?” So I said send me a  razor and let me try it out. And I don’t even know where my old razor went because I haven’t  used it once since I tried the one Henson sent. Is a new razor going to change your  life? Probably not. But, shaving’s a chore (at least to me), and using a precision tool makes  it feel less like a chore, and instead a part of my day that I actually enjoy.I had never used a safety razor and figured they were old technology. Totally not true - these are  made in an aerospace machine shop. I also figured there would be a learning curve,  but that also wasn’t true. This razor is so easy to use, I don’t think I could ever go back to a  cartridge razor with their flexible blades and difficulty in rinsing out. If you’ve ever been  on the market for a tool and splurged on the nicest brand, this is that, except,  it's not really a splurge. The blades for the Henson razor are so cheap you could probably  put a new one on for every shave and still save money. And in fact, if you use my code  PRACTICALENGINEERING at checkout, you can get a 100-pack of blades on me. Just make sure both  the razor and the blades are in your cart, enter the code, and the discount will be applied right  away. There’s no subscription service or a monthly fee, it’s just a cool razor that I  really like and I think you will too. Thank you for watching and let me know what you think.