Suspension Bridges of Disbelief
hackaday.com
hackaday.com
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.
> 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.
https://en.wikipedia.org/wiki/List_of_filming_locations_in_t...
With the current exchange rate we'll have another wave of movies pretending Canadian cities are American cities.
Shame, because aside from real estate prices, Vancouver is a jewel.
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...
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?
[0] - https://en.wikipedia.org/wiki/Catenary#Suspension_bridge_cur...
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.
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.
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.
As for why, it's been a few years since I had to derive this, but it goes something like this:
* The horizontal component of the force in the cable doesn't change along its length
* This leads to the fact that the extra vertical load picked up in the cable, per unit horizontal length as you approach the support, is proportional to the change in slope of the cable per unit length
* The change in slope is the second derivative of the cable height, and since it is a constant the cable shape is parabolic
Fun extra fact - the parabola is the equilibrium shape because the cable can resist tension but not bending. If you reverse the sign on all the equations then they still work, but your parabola is pointing away from the direction of gravity and you have only compression, no bending, in the material. That's why arch bridges are shaped like a parabola too.
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.)
"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.)"
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.
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...
(Unless of course the web is pulling him in. Possibly it contracts after it hits the buildings, or it reels in somedow)
He doesn't need to maintain height, but energy.
> 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.
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
http://www.popularmechanics.com/technology/infrastructure/ne...
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.