Viaducts

* See also Construction
* See also Tunnels (Viaducts are kinda Tunnels too...)
* See also Streets
* See also Trolley System
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Viaduct ... "Why a Duck ??" - Marx Brothers Joke (Chico Marx - "The Cocoanuts", 1929) :
An important element of Rapture to systematically connect different buildings as if they were Street sidewalks ( though not the most effective way to move for anything other than walking -- Like for Goods Deliveries ). Better ARE the 'Streets' - much larger/more-open linear structures with a central path ( and usually a Trolley line ), and rows of building facades facing it. Those 'Streets' would have been built criss-crossing Rapture, allowing convenient Public Transit to connect the many spaced out building clusters. In them would also be located a substantial number of Rapture's population.
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Connecting Between Buildings :
See the nice heavy metal girders within some of the game Viaduct structures ? Such would be needed to support the great force pressing in on all sides due to the water's pressure at Rapture's depth. The curving together forms an arch, so that the inward force can be opposed by the force on the opposite wall ( and between floor and ceiling ). There should be more of that thick metal as those girders extend outward beyond the window panes ( opposing the pressure force in tension versus compression ) on the inside - to properly brace/support the window. 
The window panes themselves need to be quite thick, and preferably something far stronger than glass. The "Rapture" Novel talks about "Ryanium", as the author realized the limitations of glass ( and says this 'Ryanium' material is fairly expensive ). You notice the buildings aren't fully covered in windows ( as later buildings on The Surface were frequently built ). That material could be some plastic like PolyCarbonate ( which still has only 1/5th to 1/10th the strength of ordinary Steel ). Those HUGE window panes, we saw in a number of places (Neptunes Bounty and even larger in BaSx), would still be too incredible.
That rotating 'lock' thingee on the Securis Door is largely for show -- Even then that would only tighten seals, as such a Door doesn't NEED to be locked into place by bolts like a Bank Vault. Explained elsewhere, what we see move as those doors Open or Close is just one of a pair of Outer Blocking Doors ( acting as a 'door' for the passage through its portal, fire and airtight at normal pressures ), which serves to keep the doorway clear. This is in-case the REAL Pressure Door needs to engage when there is a major leak - The required strong/thick emergency door simply weighs too much (as in tons) to have them constantly open and close quickly.
Viaducts made for pretty Sea Scenes, linking buildings ( better hope that glass is REALLY strong )
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Game Viaducts Versus Realistic Ones - Stairs :
You want them to be relatively FLAT, no Stairs, and few sharp turns if possible. Going up and down stairs becomes bothersome when people are forced to used them routinely/constantly.
These are important primary connectors between buildings in a city cluster, and besides being used by walking humans, they are commonly used for simple short distance transport of goods. If you have to adjust levels, then you use a ramp incline with as mild a grade as possible ( and designing the connected buildings 'from the start' with entranceways at around the same depth ). 
This actually goes for the interior of many buildings as well, where utility is more important than aesthetics.
Game level designers use Stairs and unneeded corners to break up the ordinariness of straight lines, and don't have to be concerned with how their passage would be used in real life.
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Real World :
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Those LONG Viaducts Seen in Distance
 
Why these are not at the common base level (lower stories) to be in common with other building connections is not clear ( Except -'artists' filling in a scene they were told to illustrate ).
You WANT connectivity maximized to have effective passage for those that use them.
Tall pillars to support them are inefficient. Those would be minimized in the MMORPG, and being closer to the sea floor would be the most typical.
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No Problemo
It would be No Problemo for the MMORPG system to stitch together new Viaducts connecting Expansions of Rapture where needed in the 'On The Fly' parts of the MMORPG Rapture Map. They are built of fairly standard parts, with a few 'Adjustable Sections to allow them to be fitted where needed.
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