Tag Archives: skyscrapers

Building Height and Density in Center City Philadelphia

With a hat tip to this tweet from John Ricco, linking to this compendium of tall buildings in Center City Philadelphia from the Philadelphia City Planning Commission. The document provides a brief profile of each building, showing building height, site size, gross floor area, floor area ratio, year of completion, and floor count.

Example of information from the Philadelphia FAR catalog. Screenshot from the document.

Example of information from the Philadelphia FAR catalog. Screenshot from the document.

Pulling the data into a spreadsheet allows for some quick charts to show the relationship between building height and density.

Height v density

It’s generally true that taller buildings are more dense, but not universally so. Buildings with the same density come in different shapes. Both the Liberty Place complex and the 230 South Broad St have an FAR of ~19.5; but Liberty Place includes a 960′ and 783′ tall towers. 230 South Broad St is just 250′ tall, but the building’s floorplates occupy 100% of the site.

By comparison, the densest zoning in DC is for 12 FAR (the C-5 zone), located in one of the few exception areas for DC’s height limit (allowing 160′ tall buildings along some blocks of Pennsylvania Ave NW). Quite a few blocks are zoned for up to 10 FAR, but nothing in DC can be built to an FAR of 15, 20 or 25, as in Philadelphia.

Considering DC’s effective downtown height limit of 110′ to 130′ combined with a maximum FAR of 10, it’s not hard to understand why DC has so many boxy buildings forced to occupy entire parcels. Likewise, DC’s height limit is indeed a hard limit on office density. Beyond 10 FAR, any additional density requires more height than the law currently allows.

In New York, the Empire State Building has a FAR of about 28. At less than half the height, the Equitable Building (inspiration for New York’s 1916 zoning code) has a FAR of 30.

Note: almost all of these very dense buildings are offices.

Back in Philadelphia, a more obvious example: the obvious relationship between building height and floor count (taller buildings have more floors).

Height v floors

Looking at building height by decade, you can see the clear trend of taller buildings emerging following the end of Philadelphia’s ‘gentleman’s agreement’ on building height – that no building should be taller than the Statue of William Penn atop the City Hall clocktower. This agreement left plenty of room for tall buildings; at 548 feet tall, City Hall was the tallest building in the world between 1901-1908. The agreement was breached by the construction of 1 Liberty Place in 1987.

Height by decade

This particular data set doesn’t include any buildings shorter than the City Hall tower; it’s not a complete record of all construction in Center City, just high rise buildings (the document was published in 2010). You can clearly see the approximate 500′ limit prior to 1987.

If you put all of these characteristics into one chart, you get something like this:

height GFA FAR year

The size of the circles indicate the gross floor area of the project.

The power of skyscrapers

Chicago SkylineChicago skyline CC image from 1’UP on flickr

Several friends have pointed me to this Atlantic piece by Ed Glaeser on the power of skyscrapers and density in shaping the city, and the role cities play in our economy.  Some snippets:

On the micromanagement of zoning codes:

New York slowed its construction of skyscrapers after 1933, and its regulations became ever more complex. Between 1916 and 1960, the city’s original zoning code was amended more than 2,500 times. In 1961, the City Planning Commission passed a new zoning resolution that significantly increased the limits on building. The resulting 420-page code replaced a simple classification of space—business, residential, unrestricted—with a dizzying number of different districts, each of which permitted only a narrow range of activities. There were 13 types of residential district, 12 types of manufacturing district, and no fewer than 41 types of commercial district.

Each type of district narrowly classified the range of permissible activities. Commercial art galleries were forbidden in residential districts but allowed in manufacturing districts, while noncommercial art galleries were forbidden in manufacturing districts but allowed in residential districts. Art-supply stores were forbidden in residential districts and some commercial districts. Parking-space requirements also differed by district. In an R5 district, a hospital was required to have one off-street parking spot for every five beds, but in an R6 district, a hospital had to have one space for every eight beds.

On gentrification, growth, and the hidden costs of height limits and other restrictions:

The relationship between housing supply and affordability isn’t just a matter of economic theory. A great deal of evidence links the supply of space with the cost of real estate. Simply put, the places that are expensive don’t build a lot, and the places that build a lot aren’t expensive. Perhaps a new 40-story building won’t itself house any quirky, less profitable firms, but by providing new space, the building will ease pressure on the rest of the city. Price increases in gentrifying older areas will be muted because of new construction. Growth, not height restrictions and a fixed building stock, keeps space affordable and ensures that poorer people and less profitable firms can stay and help a thriving city remain successful and diverse. Height restrictions do increase light, and preservation does protect history, but we shouldn’t pretend that these benefits come without a cost.

On the inherent dynamism of cities and the shapes of growth:

Great cities are not static—they constantly change, and they take the world along with them. When New York and Chicago and Paris experienced great spurts of creativity and growth, they reshaped themselves to provide new structures that could house new talent and new ideas. Cities can’t force change with new buildings—as the Rust Belt’s experience clearly shows. But if change is already happening, new building can speed the process along.

Yet many of the world’s old and new cities have increasingly arrayed rules that prevent construction that would accommodate higher densities. Sometimes these rules have a good justification, such as preserving truly important works of architecture. Sometimes, they are mindless NIMBYism or a misguided attempt at stopping urban growth. In all cases, restricting construction ties cities to their past and limits the possibilities for their future. If cities can’t build up, then they will build out. If building in a city is frozen, then growth will happen somewhere else.

It’s a fantastic read.  I imagine this piece is a prelude for Glaeser’s recently released book, “The Triumph of the City.”  I will have to pick up a copy.