
Suspension Bridges of Disbelief - szczys
http://hackaday.com/2015/11/18/suspension-bridges-of-disbelief/
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encoderer
This is a really well done piece.

Totally unrelated, but I've always been fascinated by suspension bridges. When
I was 12 I built one out of Lego blocks with a road deck 5 feet long, using
fishing line for the cabling. I couldn't get past a few feet at first, my
towers would break in half. I glued the blocks. It helped, for a bit. Finally
I realized that the cables shouldn't be affixed to the towers, that they
should be draped over them and anchored at the ends.

Then I got my first knex set and learned how cantilevers work.

~~~
GotAnyMegadeth
The idea of gluing Lego makes me feel really uneasy for some reason.

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api_or_ipa
Great article, but the following line irked me a bit.

> played by Vancouver’s Lions Gate Bridge, the only non-iconic bridge on this
> list.

Vancouvers still the 3rd largest city in Canada and I think most Canadians
would consider Lions Gate to be ionic in its own right. It's certainly a
stunning location spanning across the Burrard Inlet into Stanley Park.

Anyways, it's not important enough to worry too much about.

~~~
mikeash
The other bridges discussed are iconic throughout the world. Merely being
iconic to most Canadians is not really in the same league.

~~~
Symbiote
> The other bridges discussed are iconic throughout the world.

I don't think they are. The only iconic North American bridge is the Golden
Gate Bridge — this is widely recognised.

Worldwide, I'd add Tower Bridge [1], but I'm struggling to think of another
bridge that is so widely known that it's instantly recognizable by a
significant number of people.

[1] If you don't the name and don't recognise it, then I'm probably wrong:
[http://cdn.rsvlts.com/wp-content/uploads/2012/12/Tower-
Bridg...](http://cdn.rsvlts.com/wp-content/uploads/2012/12/Tower-Bridge-at-
night-2.jpg)

~~~
locopati
Perhaps the Brooklyn Bridge?
[https://en.wikipedia.org/wiki/Brooklyn_Bridge](https://en.wikipedia.org/wiki/Brooklyn_Bridge)

~~~
Symbiote
If shown a picture yesterday, I wouldn't have been able to name it. I think I
would have known it was in New York, but when I visited it wasn't one of the
sights — unlike Tower Bridge and the Golden Gate Bridge. (Perhaps I don't
watch enough American TV shows; it's the kind of thing that would be in the
intro-credits showing the setting is NYC.)

I think it's like the Forth Bridge near Edinburgh or Charles Bridge in Prague.
They're extremely well known nationally, and may well be tourist attractions,
but would most people know about them before visiting the area?

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nsxwolf
Why is the shape of the cable a parabola and not a catenary? I've always been
taught that the shape a cable makes hanging between two supports is a
catenary. Isn't a suspension bridge the same thing?

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TeMPOraL
Wikipedia says [0] it's "due to the weight of the roadway being much greater
than that of the cable".

[0] -
[https://en.wikipedia.org/wiki/Catenary#Suspension_bridge_cur...](https://en.wikipedia.org/wiki/Catenary#Suspension_bridge_curve)

~~~
msandford
Yeah, catenary is the shape a cable makes when under its own weight, and with
only its own tension holding it up.

If you hang a single 1lb weight in the middle of a clothes line, it forms very
nearly a perfect V. That's because the 1lb totally dominates the 1/2 oz that
the rest of the line weighs.

~~~
function_seven
Right, but isn't the roadway's weight uniformly distributed along the length
of the main cable?

Maybe a linear weight distribution favors the parabola, while a curved weight
distribution (i.e. the cable's own weight when hanging free) causes the
catenary? That makes sense to me, there's more weight per linear meter at the
ends than in the center when considering just the cable's weight.

~~~
abduhl
The reason for this is how the force distributions are applied.

A catenary forms when self weight is the driving force. In other words, when
the weight per unit length of the cable is the same. For a bridge, the weight
of the deck is much alrger than the cable. The weight of the deck is the same
per unit horizontal length which is not the same as the unit length of the
cable due to the curvature.

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pcl
My favorite lines:

 _The soldiers continue to battle Godzilla and the drivers motor across the
bridge to safety, all oblivious to the physics errors that spared their
lives._

and the parenthetical:

 _(I must note that the bridge featured in the movie’s climax is a cantilever
bridge and therefore outside the scope of this article.)_

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tempestn
This is really great. A couple of my favourite quotes so far:

"Most of the structural elements have no purpose, and bridges are instead
supported by a mix of perplexing whimsy, directorial ignorance, and nothing."

"(Not that it really matters because apparently nothing matters, but the
Golden Gate Bridge isn’t long enough to span the distance depicted even if the
structural forces could be resolved.)"

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ghaff
Tangentially related. What caused cable-stayed bridges to so increase in
prominence over the past few decades? I did some reading and they've become a
popular choice for bridges and have advantages (e.g. not requiring really
solid anchoring for the ends of the cable) but I haven't run across a good
explanation about what made the design popular where it hadn't been before.

~~~
oasisbob
Does the increased use of pre-stressed concrete have anything to do with it?

T Y Lin discusses some neat aspects of bridge design that touch on this in his
oral memoirs. [1]

IIRC, cable-stayed bridges have the advantage of being able to be cantilevered
from the tower out during construction. Their disadvantage of compressive load
on the deck goes nicely with prestressed concrete's strength.

The whole memoir is really good.

> Lin: This was all concrete. Except the cables. See, cable bridges are really
> prestressed concrete. To tighten the cables against the concrete, but put
> the cables outside; for ordinary beam, the cables are inside. When the
> bridge gets too long it is cheaper to put cables outside, so it s really a
> type of prestressed concrete bridge. They call it cable-stayed bridge. It's
> post-tensioned concrete. So, okay.

[1]
[https://archive.org/details/fatherprestressed00tylirich](https://archive.org/details/fatherprestressed00tylirich)

~~~
vlehto
Then the underlying reason would be price. Prestressed concrete can't compete
with steel in strenght/weight ratio. But the strenght/price ratio makes it
good choise for some budget designs.

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Animats
There was a terror plot against the Brooklyn Bridge in 2002.[1] It didn't get
that far, but the plan was feasible - to get inside the anchorage where the
cable strands fan out to be individually anchored into concrete, and cut them
one at a time, using gasoline-powered saws, until the cable failed.

Security had been tightened up, though. From 1985 to 2001, the anchorage was
used for art exhibits, but that stopped after 9/11\. The plotters gave up on
that plan, since they would have needed hours or days of uninterrupted access.

[1]
[http://www.nyc.gov/html/nypd/html/pr/plots_targeting_nyc.sht...](http://www.nyc.gov/html/nypd/html/pr/plots_targeting_nyc.shtml)

~~~
cousin_it
Sounds like a suicide plot. Standing next to a steel cable that's holding up a
bridge, and cutting it until it breaks? They'd probably have more success
using thermite on the cables, though they'd need to smuggle lots and lots of
it.

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rootbear
In the "I, Robot" movie there is a ruined suspension bridge that ignores
physics. That was not the most serious problem with that film.

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sanoli
About a decade ago I was thinking about spider-man swinging down a NY Avenue,
his web going from building on one side to a building on the other (you know
the scenes I'm talking about), and I realized it just couldn't be. Pretty
quickly he'd touch the ground. Gave a good explanation to my friends and half
of them didn't get it, the other half thought I was wrong/stupid because of
course you can swing down an avenue like spiderman. Now I got the itch to call
these same friends and tell them about these bridges. Thanks!

~~~
Steko
Sounded like all of your friends didn't get it and I don't get it either,
enlighten me!

~~~
lsd5you
My take is this - to maintain height he would have to switch sides on the
upswing. If he's attached to the side of a building he would hit that building
first before achieving any upswing.

(Unless of course the web is pulling him in. Possibly it contracts after it
hits the buildings, or it reels in somedow)

~~~
vlehto
You can turn forward momentum directly into upswing. So if he does switch at
the low and high point of a swing, it might work. At low point he would shoot
bit backwards, at high point bit forwards.

He doesn't need to maintain height, but energy.

~~~
sanoli
The thing is, he doesnt swing in a straight line. he goes forward but also in
a zig zag path, so the web would end up getting longer and longer each time.

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Shivetya
Other than the silly gore that clip from Final Destination V was really an
interesting portray of a suspension bridge coming apart. I would think it
would come apart faster after watching other bridge accidents actually caught
on video

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igonvalue
What would actually happen in the pictured Dark Knight scenario where a
relatively small part of the middle of the the bridge disappears? It seems
like if the road deck is sufficiently rigid the vertical suspenders near the
middle would realign and provide at least some of the tension force previously
supplied by the removed suspension cable. Is there any reason why the bridge
_must_ fall?

~~~
eridius
It's not clear from the picture, but the article says

> _Explosions sever the suspension cables and the road decks all at once_

If the cables weren't severed, then it sounds reasonable to me that the rest
of the bridge would stay up, although the outer parts would probably droop
slightly as the cable in the middle lost some weight.

But if the explosions did actually sever the cables, then the article makes it
pretty clear that the whole thing should have collapsed.

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zobzu
"Americans don’t build real bridges anymore, or maintain the ones that we
have"

I can't speak for all bridges in general, but since the author takes the
Golden Gate bridge as an example, I can tell you it's maintained. Many pieces
have been replaced over the (past) 4 years I've crossed it, including
structural pieces. Heck they even document some of it / how its done on the
bridge itself...

They also re-built half the Bay bridge from scratch (other side of San
Franciso). Granted, for this one the work-force was mainly Chinese (but a
large part of it was still American).

So yeah, don't start off with false facts, specially if you have valid points
afterwards, since I'd generally just stop reading at this point and figure
you're just full of yourself throw stuff you think sound cool and can't be
verified.

Also here's some REAL suspension bridge collapses... it's not so far off in
some (obviously not all) movies...

[https://www.youtube.com/watch?v=3mclp9QmCGs](https://www.youtube.com/watch?v=3mclp9QmCGs)

[https://www.youtube.com/watch?v=zWtYfHS-
UL4](https://www.youtube.com/watch?v=zWtYfHS-UL4)

[https://www.youtube.com/watch?v=DOvLzExOsmA](https://www.youtube.com/watch?v=DOvLzExOsmA)

~~~
icegreentea
The author partook in hyperbole (I thought it was obvious that it was
hyperbole.. but heh), but was referencing stuff like this:

[http://www.popularmechanics.com/technology/infrastructure/ne...](http://www.popularmechanics.com/technology/infrastructure/news/a16532/america-
crumbling-infrastructure-interstate-10-bridge-collapse/)

[http://www.infrastructurereportcard.org/bridges/](http://www.infrastructurereportcard.org/bridges/)

~~~
Sanddancer
The first article really is a fairly sensationalized piece on bridge
infrastructure failures. The headline bridge, the one on I-10, was
"functionally obsolete" because the guardrails were of a design that wasn't up
to what modern bridges would have. The scouring that occurred because flash
flooding isn't completely predictable, and washes can and do occasionally try
to change course.

The second bridge in the example was even worse. Someone died /demolishing/ a
bridge. Unless I'm missing something, rebuilding a deficient bridge is kinda
the opposite of neglecting infrastructure. Yes, there's an infrastructure
problem, but the article in question just muddies the issue with near classic
yellow journalism.

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dcre
The scene he mentions from The Dark Knight Rises (the bridge is at the very
end):
[https://www.youtube.com/watch?v=UsoYwysCdfY](https://www.youtube.com/watch?v=UsoYwysCdfY)

~~~
cousin_it
Yeah. Hollywood proves that storytellers don't need to do research to be
popular. Just watch any movie about hackers.

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sohkamyung
I think the same bridge problems also occur for the 1998 movie version of
Godzilla. There, it gets stuck in the Brooklyn Bridge which refuses to
collapse despite the cables being pulled out.

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lloydde
Suspending belief?

