What makes a bridge a bridge?

[GenAnnoyance]

Not sure if this is the correct forum but...

So last night I was watching True Lies and when it reaches the bit on the Overseas Highway my Father (who recently visited Key West) said that it was the longest bridge in the world. Intrigued, I looked into the veracity of this claim. Wikipedia's list gives the accolade to a bridge in China which is substantially shorter than this 'bridge'. I asked him if it was one continuous bridge all the way and he said yes. 5 minutes of googling did not deliver an answer on why it did not count as a bridge.

So, great and wise engineers of TWC, what makes a bridge a bridge?

[Sphere]

If it was designed using the AASHTO manual its a bridge.

[Tacitursa]

The oversea's highway is not a bridge it's a Highway (Large parts of it stretch over miles of land). It does contain several bridge sections though and the longest of them is the Seven Mile bridge.

7 miles is quite average in terms of the longest bridge in the world some bridges are 20+ miles long.

[removeduser_4536284751384]

I edited that Wikipedia article so the Overseas Highway is now #1. Some of those chinese bridges aren't really bridges either, some bits cross land and valleys, they're really to go from A to B at the same elevation.

[Tacitursa]

Some of those chinese bridges aren't really bridges either, some bits cross land and valleys

Which ones? The largest ones listed are 90% on water

[Gaidin]

I edited that Wikipedia article so the Overseas Highway is now #1. Some of those chinese bridges aren't really bridges either, some bits cross land and valleys, they're really to go from A to B at the same elevation.

It doesn't matter that some of them aren't bridges. It just matters that one of them is, and is 26 miles long, recently finished, and knocked out Louisiana's bridge from the standings to #2.

[GenAnnoyance]

Thanks for the replies guys, really been informative.

So a bridge definitely has to cross water? In other words, a bridge across, say, a valley doesn't count?

[Gaidin]

Thanks for the replies guys, really been informative.

So a bridge definitely has to cross water? In other words, a bridge across, say, a valley doesn't count?

It can be across a valley. It just has to be a bridge the whole length for it to count towards the record. That's why the most the Key West highway can manage is 7 miles.

[Helm]

So a bridge definitely has to cross water? In other words, a bridge across, say, a valley doesn't count?

If it's solid and it doesn't have solid ground underneath it it's a bridge.

[Turtle Hammer]

I don't think the ground underneath needs to be non-solid. There are bridges that have land under them, but traverse valleys so as to allow passage at one elevation without having to go up steep hills or even cliffs. Also, does a bridge stop being a bridge if the river it crosses dries up?

[Sphere]

What makes bridge design unique are the long spans and dynamic loading. Whatever is underneath the bridge is meaningless.

[Helm]

Well you have to be able to walk or float underneath it, if it's in contact with the surface it becomes some kind of boardwalk or track.

[Turtle Hammer]

Well you have to be able to walk or float underneath it, if it's in contact with the surface it becomes some kind of boardwalk or track.

What about bridges over roads and railways?

[Helm]

What about bridges over roads and railways?

You can walk underneath those as well it's just that you could be squished by a car or train if you did.

[Plant]

Surely the definition of a bridge are the parts of a construction with the aim of transport, which aren't in direct contact with the ground?

[Sphere]

There is no Packer game today, so I am going to present some information on bridge design.

First I would like to talk about the problem caused by long spans in general, for which I have created a simple little illustration...


The Problem

Spoiler Alert, click show to read: 

In the final equation that needs to be true to keep a bridge from collapsing, R(L/2) = C(D), has on the right hand side two big terms, that is "R" which is essentially the load you place on the span, and the span itself "L". Loads like trucks, cars, pedestrians etc. which are big loads because they are dynamic loads, and bridge spans are long spans.

But on other side of the equation you have what would be small terms, that is "C" the internal forces developed within your bridge, and "D" the distance between these internal forces, also know as the internal force couple. "C" is limited by material properties; concrete and steel can only be made so strong, while "D" is limited because of size; it is harder to build a deeper structure.

There are several ways of finding a solution to this inherent problem of having large values on the left hand side (spans and loads), up against what are inherently small values on the right hand side of the equation (material strengths and structure geometry). And I'll be presenting them in future posts, including some real world solutions.

[Plant]

It helps when explaining something, to define the letters.

For instance;
P = Load
R = Reaction Force
C = Shear Force?
D = Thickness?

I'm not familiar with the equation, but according to the free body diagram, surely the top and bottom C forces will be unequal in order for the section to be not rotating?

Edit: Actually I've been staring at the diagram for a while now, and I don't understand it. What type of diagram is this supposed to be? It appears to be an equilibrium diagram with a side dashing of internal shear and moment equation, which makes no sense in context of the diagram. On the other hand it has been a while, but I don't believe my abilities have been depreciated this much due to underuse.

[ash874]

dont have anything to contribute to this discussion
just wanted to say i have built a bridge myself a couple months ago
college assignment
held decently