Skyscraper height/cost tradeoff.

[sport200]

Is there a point where adding additional floors, while selling at the same price as the floors below, leads to a loss. In other words, what is the height and approximate floor number where the marginal cost of adding that additional floor is higher than the marginal revenue gained?

From what I have read, it appears at 50 floors, costs start to increase at a greater rate than the additional revenue gained, due to considerations such as elevators (cost) and wind stability (additional materials) among others.

Is this correct?

 

[marcus_a_j]

I would like to know as well, since I made that assertion in the 18 York thread. sport200, if you have any sources other than what you read in that thread then please post them, because my info is from people I've spoken to and general assumptions regarding building construction, nothing really substantial or documented.

 

[Peepers]

If we are talking about an office tower the optimal number of floors is around 50 stories. This is due to the increased requirement for elevators the higher you go which increases the size of the "service core". Beyond 50 floors designers must resort to double decker elevators and/or skylobbies which are less than optimal solutions since passengers must transfer to another elevator at a skylobby or stop at double the usual number of floors with the double deckers. In Toronto Scotia Plaza and "First Can" both employ double deckers. As you go beyond 50 floors and get into the category of the "supertall" (~1000 ft) wind loading becomes a major factor. On a 1000 ft tower the wind forces can be greater than the "dead load" forces. To deal with these forces designers have in the past resorted to placing most of the structural steel around the perimeter of the building to create a "Tube" in the case of the WTC or "bundled Tube" in the case of the Sears Tower.

If we are talking residential skyscrapers the optimal floor height is much higher because the requirement for elevator capacity is much less (and hence a smaller service core). Also because residential towers are constructed from reinforced concrete instead of steel they have much more mass to deal with the wind loadings.

 

[Johnzz]

Interesting Peepers, thank you.

 

[maestro]

IMO, Too many site specific variables for there to be some sort of generalized optimal height.

If we are talking residential skyscrapers the optimal floor height is much higher because the requirement for elevator capacity is much less (and hence a smaller service core). Also because residential towers are constructed from reinforced concrete instead of steel they have much more mass to deal with the wind loadings.

That doesn't take into account parking requirements or even elevators. I've seen the introduction of a fourth elevator change a condo developer's mind. I assume the commercial side deals more with banks of elevators.