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Comment by jlgreco | Hacker News Reader

How tall does a building have to get before all the other "utility" stuff becomes a limiting factor for any reasonable floor-space/building footprint? I imagine the space you need to dedicate on each floor to water/sewage, electrical, ventilation, etc goes up approximately linearly with the addition of floors. You also need to add more elevators as you add more floors, just to service the additional people each floor implies. Add too many floors and eventually some of the floors become little more than shafts running to other floors.

I don't really know anything about architecture so maybe my intuition is off. Maybe these concerns are never realized with the number of floors that are practical for other reasons?

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Check out this plan for the (pre-9/11) World Trade Center:

http://en.wikipedia.org/wiki/File:World_Trade_Center_Buildin...

Roughly 1/4 of the area is taken up with elevators, stairwells, and so on. So here's a back of the envelope calculation:

Elevator to office ratio: 1 : 3.

Proportion of floor area that is usable: 3 / (1 + 3) = 3/4.

To join the mile-high club, you need a building that's about 4 times higher. I will make the very crude assumption that, for a building that is 4 times higher, you will need 4 times as many elevators.

Elevator to office ratio: 4 : 3

Proportion of floor area that is usable: 3 / (4 + 3) = 3/7

Usable floor area compared to floor area of the old World Trade Center, occupying the same site:

4 * (3 / 7) ÷ (3 / 4) = 16 / 7; or about 2.3

So it's doable, but it's ridiculously inefficient. Where you draw the line is an interesting question. 100 stories is already pushing the limits, but you can alleviate this problem by tapering the building, like the Burj Khalifa. This is remarkably similar to how the pyramid builders solved their problems.

But what the pyramid builders really needed was the arch. And what modern engineers really need is an efficient vertical transport system. The obvious thing to do is have more elevators per shaft. Maybe something like a vertical cable car would work. Or even ditch the cables and make each elevator independently mobile within a network of interconnected shafts, like a vertical train system.

One high-throughput system is the http://en.wikipedia.org/wiki/Paternoster. Unfortunately, it looks extremely dangerous, and in fact has caused a large enough number of accidents that it's banned in most modern countries.

That's pretty much what I had in mind as a "vertical cable car". Looks like somebody actually built one in ... um ... 1884!

On the same note, in the last century brick buildings were limited in height by the thickness of the walls. If you went much beyond 20 stories, the ground floor walls filled the entire floor, making a sort of pedestal. So further height got zero further floor space. Steel buildings got around this. Hanging each floor from a steel skeleton means the walls don't get thicker at all.

As you calculate, elevators are the new brick. The number needed to serve higher floors increase to the point the ground floor is nothing but elevators. So further height is pointless.

Its exacerbated by the need to fill/empty the building in bursts - people want to work regular hours, and everybody works the same hours. So you need enough elevators to essentially evacuate the building in under an hour.

Ways to work around this? Well, a pyramid structure would help I imagine. Elevators to server higher levels could be distributed evenly about the ground level, more in the center and fewer around the lower edges. That also increases windows-per-volume.

And of course if people lived and worked in the building and rarely left it, then the need to 'evacuate' from the ground floor goes away.

I imagine an arcology where people live, work and entertain in alternating levels. Fire and services can be located internally in strategic volumes. Air can be drafted up through the building using thermal gradients instead of fans.

Need to write a storey involving this kind of structure sometime I guess.

The Burj received some pretty unflattering coverage a year or two ago for their "poop trucks", which were necessary for moving however many tons of feces 30,000+ people produce per day. However in many cases the building "cheats", i.e. the floor area shrinks rapidly the higher you go up and so the strain is better distributed throughout the system. I say cheat only because it's much more challenging to manage a uniform (or close to it) floor area from base to top such as the old WTC. The GE Building at 30 Rock is a good example of a building that used the building code (NYC setbacks, a good topic in itself if you're interested) to its advantage in planning the elevators and ventilation. It was mentioned in another comment above I think but in reality the economics are a much more controlling factor than hard engineering, and most of the engineering problems are about doing X for $YYY rather than XYZ for glory and innovation.

Edit: This link might be of interest... http://www.bloomberg.com/news/2013-01-02/double-deck-elevato...

The problem has more to do with Dubai than with the building. Many buildings in Dubai employ similar schemes because the sewerage system wasn't scaled as the city grew.

This is like a lot of things in Dubai: roads, public transport and the airport which is currently being more than doubled.

You're quite right, but it's still worth noting that most supertalls are emphatically not residential towers. Office towers are much easier to manage when there's a workday pattern. I wonder if the rampant debt and consequent vacancies around Dubai has lightened the loads so to speak on their sewage costs...

The Burj is a joke. It also seems to be mostly empty, because it priced itself out of the real estate market after the receession. It is so 'prestigious', no one can afford to use it. There are miles of empty lots starting just a few hundred yards away. If the Burj toppled over to the east, the top of it would land on empty ground. It is a temple to the vanity of the rulers of Dubai and their property developer lackeys. It is also an embarrassment, because Sheikh Khalifa of Abu Dhabi (the one with all the oil) had to bail them out to get it completed after the property bust of 2008 - that's why it's called Burj Khalifa, not Burj Dubai, as originally planned. And not to cap it all, the observation deck is only half way up!

I know this happens, with places pricing themselves out of the market, but it surprises me. Wouldn't they want to lower the cost of owning/renting a unit until the can actually get some buyers? It would still be a loss, but wouldn't it be much less loss, and isn't that better?

One idea I keep returning to is a "laned" system for elevators/lifts. Two shafts for going up, and two for going down. One of the two in each direction is the "passing lane"-- and you fit dozens of elevator "cars" in the whole system. For very high altitudes it may even make sense to have a third, high-speed lane.

Obviously this would require a completely different mechanism as you can no longer have cables attaching each car to the roof. In fact I have no clue how it could work! This is what I keep pondering...

You might be able to do it with linear motors. The problem there would be the reliance on the safety brakes in the event of power outages (not really a problem with counterweight elevators).

Mount what amounts to an elevator system on the linear motor, but it never goes up and down. So you suspend a standard otis elevator from the linear motor.

Power fails, linear starts to fall, lands on top of the traditional otis with its emergency brakes holding the whole works up. There's no reason the cable between the otis and the linear has to be 200 feet for the linear to build up speed... make it, perhaps, 3 inches and a nice spring arrangement.

I bet it would still be pretty scary to be in one of those during a power failure. So put hybrid batteries on the linear motor so it can just keep on running during a power fail, perhaps programmed to do nothing but a normal controlled descent to the bottom doors. if ($powerfail == 'Y') { ignore_all_floor_selection_buttons_higher_than_this_floor; }

I've had exactly the same thought.

I remember hearing that elevators took up a quarter of the floor space of the old World Trade Center - only because they used express and local elevators, it would have otherwise been half the floor space. So it would seem a "vertical train" system could be quite valuable for saving real estate.

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