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.
