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.
Pulling the data into a spreadsheet allows for some quick charts to show the relationship between building height and 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.
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).
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.
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:
The size of the circles indicate the gross floor area of the project.