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I'm a bit shocked that in 2021, students rely on physically building the bridge and simulating earthquakes with moving platforms. This feels like dummy engineer

I'm a bit shocked that in 2021, students rely on physically building the bridge and simulating earthquakes with moving platforms. This feels like dummy engineering. I would have expected a mechanical engineer to actually be able to prove or disprove the feasibility through math, and create a CAD model on which to apply forces simulating earthquakes, heatwaves, etc.

rini17 5y ago
On contrary, I am actually delighted to see the hands-on approach. Pure math is vulnerable to missed preconditions/unknown unknowns, with potentially catastrophic ramifications when it's carelessly applied to real building. You must know in advance what you're going to prove.
- unearth3d 5y ago
  Like London's Millennium Bridge https://arstechnica.com/science/2018/10/new-study-sheds-more-light-on-what-caused-millennium-bridge-to-wobble/ https://arstechnica.com/science/2018/10/new-study-sheds-more... I agree, engineering is art + science, impossible to iron everything out theoretically.
- Galanwe 5y ago
  > Pure math is vulnerable to missed preconditions/unknown unknowns, with potentially catastrophic ramifications Interestingly I see the opposite. I expect the replica to be full of dangerous approximations: scaled weight of the stones not realistic, friction of the material not taken into consideration, earthquake ground vibrations not properly emulated by just shaking plates, etc
  - klyrs 5y ago
    I take issue with the term "pure math" here. Engineering simulation is strictly the realm of applied math, a dark art of floating point calculations, finite element analysis, idealized material properties, etc. Pure math demands much more rigor, and gets exact answers, but the methods available cannot handle the complexity of a bridge in this lifetime.
chrisseaton 5y ago
"Beware of bugs in the above [bridge]; I have only proved it correct, not tried it." is some wisdom to go by.
R0b0t1 5y ago
Why spend a lot of money making an error prone program when you can have reality simulate it for you? This is a problem I had at my last job, actually. We'd spend a lot of time on simulation that was kind of accurate but still need to do a fair bit of rapid prototyping to make up for simulation deficiencies afterwards. For simple stuff like a prototype jig to hold equipment mockups can be faster to make out of rough cut materials instead of spending 2 weeks making everything fit in a CAD program.
fuzzfactor 5y ago
>create a CAD model I would think that could commonly be done without any university. Making the actual physical model could be considered a better use of MIT resources while they are there.
wiredfool 5y ago
They probably did both. You end up remembering physical lab stuff differently than problem sets. I still remember some of the shake table stuff I did 30 years back.
- icegreentea2 5y ago
  They did do both. From the linked abstract: > Both of these factors are tested through analytical means and a 3D physical model supported by moveable abutments http://congress.cimne.com/formandforce2019/admin/files/fileabstract/a870.pdf http://congress.cimne.com/formandforce2019/admin/files/filea...
ColinWright 5y ago
It's the "Unknown Unknowns" that bite you. Via simulations and calculations you can be fairly sure that everything you've thought of will be fine, but there may be things you haven't thought of, and they may prove fatal to the design. In a later comment you say: > "I expect the replica to be full of dangerous approximations: scaled weight of the stones not realistic, friction of the material not taken into consideration, earthquake ground vibrations not properly emulated by just shaking plates, etc" How do you know that your model accurately represents the earthquake ground vibrations? How do you know that your model accurately represents the friction? Compressibility and friability of the materials? The scaled weight of the stones? How many times have you written a large and complex program, only to find that there are real world cases you haven't considered? I'm sure they did the computer modelling, but personally, I'm happy to see people building real-world versions to verify their (software and mathematical) models.
- Galanwe 5y ago
  > Via simulations and calculations you can be fairly sure that everything you've thought of will be fine, but there may be things you haven't thought of, and they may prove fatal to the design. I don't deny that, the theorical approach will surely not be 100% accurate. Still, I expect that I would give me thousands of time more confidence than a 3D printed replica. > How do you know that your model accurately represents the earthquake ground vibrations? Because I expect the earthquake simulations in architecture CAD software to be made by physicists that spent time carefully modeling the realistic and complex set of forces that a structure would feel during an earthquake. > How do you know that your model accurately represents the friction? Again, because architecture CAD software are specifically crafted for that purpose, and can theorically simulate the friction and adherence of various materials much more realistically than a mockup 3D printed replica.
  - spenczar5 5y ago
    You say that architecture CAD programs simulate things better than physical models with a lot of confidence, but I am not sure why. One way to think of this is that the physical world is a vastly superior computer. We cannot make a fluid simulation that runs as precisely as an actual river, for example. Tribological models have to make tons of approximations since friction is insanely complicated - but the physical universe manages to calculate the real friction effect basically instantly!
jaclaz 5y ago
Only as anecdata, many years ago, circa 2000, I was marginally involved in a project for the re-building of Stari Most (the bridge at Mostar): https://en.wikipedia.org/wiki/Stari_Most https://en.wikipedia.org/wiki/Stari_Most the initial team of "modern" structural engineers were convinced - after mathematical analysis and computers simulations - that the bridge could not possibly be re-built exactly as it was. Then we contacted to verify the calculations an "old" structural engineer that believed that if something has been built and resisted (before the bombing) some 400 years, it can be built and is resistant enough. It came out that due to some limitations of the software used at the time, and to some approximations in the design of the computer model, the resistance (and influence) of the lead fixed steel pins (that were - for the time of building - a technical marvel) was greatly underestimated, as well as the effect of the steel strips. See here for some details on how the bridge was built: https://www.litosonline.com/en/article/old-bridge-mostar-reconstruction-symbol https://www.litosonline.com/en/article/old-bridge-mostar-rec...
IfOnlyYouKnew 5y ago
In a biology course, less than ten years ago, we cut out the graphs we sketched and weighed them, to calculate area-under-curve.
cseleborg 5y ago
We're all speculating here, of course, but I would expect it to be rather difficult to simulate both the static properties of the finished bridged and the dynamics of putting it together, which is obviously an important part of the process (see the keystone anecdote).
deleted 5y ago
[deleted]
sollewitt 5y ago
Arches are tricky. The undergraduate way to model arch bridges is as three hinged structures - free to rotate at both ends and with a virtual "pin" in the middle, in order to make the math tractable. That's the kind of approximation that goes into mathematical models.

rini17 5y ago

On contrary, I am actually delighted to see the hands-on approach. Pure math is vulnerable to missed preconditions/unknown unknowns, with potentially catastrophic ramifications when it's carelessly applied to real building. You must know in advance what you're going to prove.

unearth3d 5y ago
Like London's Millennium Bridge https://arstechnica.com/science/2018/10/new-study-sheds-more-light-on-what-caused-millennium-bridge-to-wobble/ https://arstechnica.com/science/2018/10/new-study-sheds-more... I agree, engineering is art + science, impossible to iron everything out theoretically.
Galanwe 5y ago
> Pure math is vulnerable to missed preconditions/unknown unknowns, with potentially catastrophic ramifications Interestingly I see the opposite. I expect the replica to be full of dangerous approximations: scaled weight of the stones not realistic, friction of the material not taken into consideration, earthquake ground vibrations not properly emulated by just shaking plates, etc
- klyrs 5y ago
  I take issue with the term "pure math" here. Engineering simulation is strictly the realm of applied math, a dark art of floating point calculations, finite element analysis, idealized material properties, etc. Pure math demands much more rigor, and gets exact answers, but the methods available cannot handle the complexity of a bridge in this lifetime.

chrisseaton 5y ago

"Beware of bugs in the above [bridge]; I have only proved it correct, not tried it." is some wisdom to go by.

R0b0t1 5y ago

Why spend a lot of money making an error prone program when you can have reality simulate it for you? This is a problem I had at my last job, actually. We'd spend a lot of time on simulation that was kind of accurate but still need to do a fair bit of rapid prototyping to make up for simulation deficiencies afterwards. For simple stuff like a prototype jig to hold equipment mockups can be faster to make out of rough cut materials instead of spending 2 weeks making everything fit in a CAD program.

fuzzfactor 5y ago

>create a CAD model I would think that could commonly be done without any university. Making the actual physical model could be considered a better use of MIT resources while they are there.

wiredfool 5y ago

They probably did both. You end up remembering physical lab stuff differently than problem sets. I still remember some of the shake table stuff I did 30 years back.

icegreentea2 5y ago
They did do both. From the linked abstract: > Both of these factors are tested through analytical means and a 3D physical model supported by moveable abutments http://congress.cimne.com/formandforce2019/admin/files/fileabstract/a870.pdf http://congress.cimne.com/formandforce2019/admin/files/filea...

ColinWright 5y ago

It's the "Unknown Unknowns" that bite you. Via simulations and calculations you can be fairly sure that everything you've thought of will be fine, but there may be things you haven't thought of, and they may prove fatal to the design. In a later comment you say: > "I expect the replica to be full of dangerous approximations: scaled weight of the stones not realistic, friction of the material not taken into consideration, earthquake ground vibrations not properly emulated by just shaking plates, etc" How do you know that your model accurately represents the earthquake ground vibrations? How do you know that your model accurately represents the friction? Compressibility and friability of the materials? The scaled weight of the stones? How many times have you written a large and complex program, only to find that there are real world cases you haven't considered? I'm sure they did the computer modelling, but personally, I'm happy to see people building real-world versions to verify their (software and mathematical) models.

Galanwe 5y ago
> Via simulations and calculations you can be fairly sure that everything you've thought of will be fine, but there may be things you haven't thought of, and they may prove fatal to the design. I don't deny that, the theorical approach will surely not be 100% accurate. Still, I expect that I would give me thousands of time more confidence than a 3D printed replica. > How do you know that your model accurately represents the earthquake ground vibrations? Because I expect the earthquake simulations in architecture CAD software to be made by physicists that spent time carefully modeling the realistic and complex set of forces that a structure would feel during an earthquake. > How do you know that your model accurately represents the friction? Again, because architecture CAD software are specifically crafted for that purpose, and can theorically simulate the friction and adherence of various materials much more realistically than a mockup 3D printed replica.
- spenczar5 5y ago
  You say that architecture CAD programs simulate things better than physical models with a lot of confidence, but I am not sure why. One way to think of this is that the physical world is a vastly superior computer. We cannot make a fluid simulation that runs as precisely as an actual river, for example. Tribological models have to make tons of approximations since friction is insanely complicated - but the physical universe manages to calculate the real friction effect basically instantly!

jaclaz 5y ago

Only as anecdata, many years ago, circa 2000, I was marginally involved in a project for the re-building of Stari Most (the bridge at Mostar): https://en.wikipedia.org/wiki/Stari_Most https://en.wikipedia.org/wiki/Stari_Most the initial team of "modern" structural engineers were convinced - after mathematical analysis and computers simulations - that the bridge could not possibly be re-built exactly as it was. Then we contacted to verify the calculations an "old" structural engineer that believed that if something has been built and resisted (before the bombing) some 400 years, it can be built and is resistant enough. It came out that due to some limitations of the software used at the time, and to some approximations in the design of the computer model, the resistance (and influence) of the lead fixed steel pins (that were - for the time of building - a technical marvel) was greatly underestimated, as well as the effect of the steel strips. See here for some details on how the bridge was built: https://www.litosonline.com/en/article/old-bridge-mostar-reconstruction-symbol https://www.litosonline.com/en/article/old-bridge-mostar-rec...

IfOnlyYouKnew 5y ago

In a biology course, less than ten years ago, we cut out the graphs we sketched and weighed them, to calculate area-under-curve.

cseleborg 5y ago

We're all speculating here, of course, but I would expect it to be rather difficult to simulate both the static properties of the finished bridged and the dynamics of putting it together, which is obviously an important part of the process (see the keystone anecdote).

deleted 5y ago

[deleted]

sollewitt 5y ago

Arches are tricky. The undergraduate way to model arch bridges is as three hinged structures - free to rotate at both ends and with a virtual "pin" in the middle, in order to make the math tractable. That's the kind of approximation that goes into mathematical models.