Mehaffy on Skyscrapers

Over at New Urban Network, Michael Mehaffy has posted an in-depth account of the drawbacks of tall buildings, echoing some of the points made here previously but with much greater depth, detail and scholarly support.  Most interesting to me is the suggestion of a density optimum, a point at which density and livability are maximized while minimizing construction costs and other negative externalities of tall buildings:

"[R]esearch shows that the benefits of density are not linear, but taper off as density increases. In other words, there is an optimum density, above which the negative effects of density start to increase over the positive ones. That "sweet spot" seems to be in the neighborhood of about 50 people per acre. And many cities around the world achieve this density without tall buildings, and while creating a very appealing, livable environment (e.g., Paris and London, as well as the aforementioned parts of New York, Vancouver et al.)."

Now, since people per acre is an unreliable predictor of form, and because the negative characteristics of tall buildings Mehaffy describes are mostly attributable to their form, rather than their population density (if any, in the case of office buildings), it would be interesting to see numbers couched in terms of floor area per acre, ground coverage and building heights rather than population density.   One of the studies cited in the article mentions a six-story "sweet spot," and the House of Commons report includes illustrations of a few hypothetical configurations, but this is an area that deserves further exploration.

Thursday Old Urbanism

Tübingen, Baden-Württemberg, Germany, from the river Neckar

Skyscrapers, Height and Density

Going back the issue of skyscrapers, density and residential affordability once more, it seems to me that part of the problem with any discussion about density is a lack of a clear, consistent method by which to measure and understand it.  Population density does not tell us about urban form; units per acre does not disclose the average size of the units, the designed capacity of the units or how much of an area is actually devoted to residential use; and Floor Area Ratio (FAR) is typically only used to measure individual parcels, not whole neighborhoods, and again doesn't tell us much about form (as Andrés Duany has observed). 

The following "density quiz," part of a longer presentation by Redwood City, CA planner Dan Zack, highlights the inadequacy of the units per acre ratio as a measure of density given that the physical form in which similar levels of density are contained can vary widely:

One point which comes across for me in this segment is how building height, although it may be the most immediately noticeable building trait, and the one we instinctively associate with "high density," is far from the only, or even most important, contributor to overall density.  The compounded effect of high lot coverage, modestly-sized units and limited on-site parking can produce exceedingly high densities in the absence of great height, while the Corbusian towers-in-a-park design approach produces much lower density even where buildings are very tall.

The pro-density urban advocate who focuses on the relaxation of height limits to the exclusion of other factors such as parking requirements, mandatory setbacks, minimum street widths and minimum unit sizes addresses only one component of overall density, and one which, from a city-wide perspective, may not have an especially large impact on it. 

Finally, a mindset which holds up the increase of density, by whatever means, as an absolute good, ignores important non-economic costs and benefits of each means of increasing densification.  The negative impacts of very tall buildings on urban life, beyond the obstruction of views and natural light immediately noticed by city residents, have been covered at length by Nikos Salingaros and Léon Krier, among others.  Greater lot coverage and narrower streets, meanwhile, confer numerous benefits, both economic and not, in addition to their positive impact on density.   

Thursday Old Urbanism



By pivatana


























Side street in Bangkok, Thailand.

Can New York Build Its Way Up to Affordability?

What proportion of New York's housing would you guess is contained in skyscrapers?  My best answer to that question is below (it may not be what you think!), but by way of answering it I'd like to return to the following excerpt from Ed Glaeser's recent article, How Skyscrapers Can Save the City:

"Again, the basic economics of housing prices are pretty simple—supply and demand. ... The actual marginal cost of adding an extra square foot of living space at the top of a skyscraper in New York is typically less than $400. Prices do rise substantially in ultra-tall buildings—say, over 50 stories—but for ordinary skyscrapers, it doesn’t cost more than $500,000 to put up a nice 1,200-square-foot apartment. The land costs something, but in a 40-story building with one 1,200-square-foot unit per floor, each unit is using only 30 square feet of Manhattan—less than a thousandth of an acre. At those heights, the land costs become pretty small. If there were no restrictions on new construction, then prices would eventually come down to somewhere near construction costs, about $500,000 for a new apartment. ... In a sample of condominium buildings, I found that more than 80 percent of Manhattan’s residential buildings built in the 1970s had more than 20 stories. But less than 40 percent of the buildings put up in the 1990s were that tall."

All this makes me curious as to just how much of New York's residential space is actually located in tall buildings.  After all, if skyscrapers are to be the remedy for New York's high housing costs, they must be capable of providing a substantial fraction of the city's housing supply.

Fortunately, the Council on Tall Buildings and Urban Habitat has published a list of all New York buildings 20 stories and taller (the arbitrary cut-off used by the Council, but mentioned by Glaeser as well).  In New York City, including all the five boroughs, there are 552 such buildings completed as of 2010.  Of these, only 169 are devoted exclusively to residential use, with an additional 26 being partly residential.   The average height for these buildings is approximately 41 stories.

In all of New York, there are a total of 932,706 residential parcels, according to the New York Property Tax Report for 2010.  That is, buildings 20 stories and more with at least some residential component comprise two-hundredths of one percent of all taxable residential parcels in New York City.  Granted, very tall buildings are likely to have far more units than the average residential parcel — examples include the 40-story Chelsea Stratus with 235 units, the 41-story One Sutton Place North with 334, the 43-story Platinum with 185, the 43-story America Apartments with 211 and the 42-story 325 5th Avenue with 250.

The same tax report puts the number of individual residential units in New York at 2.93 million (this of course omits a very large amount of New York-accessible housing in New Jersey, Connecticut, and Westchester and Nassau Counties,  very little of it in tall buildings).   Assuming the average residential tower to have 250 units, and factoring in the 26 part-residential buildings, we get (169*250) + (26*125) = 45,500/2.93 million = 1.6%.  That is, units located in "tall" buildings comprise 1.6% of the residential units in the City.  But wait: if the claim is that tall buildings are needed to supply additional housing, not the absolute number of units should be counted, but only those provided by the "tall" portion of the building  — that portion 20 stories and over.  Reducing the figures accordingly, only .80% of all units in the city are located above a 19th floor level, or, 1 out of every 125 units.  And even this is probably a substantial overestimate, as it is likely that proportionately more units are located on lower stories than higher ones (due to a narrowing of towers as they rise, in combination with larger floorplans being located on higher floors — see below). 

A few observations on all this: first, it's clear that skyscrapers' visual impact is all out of proportion to their real estate impact.  Even an unprecedented residential skyscraper building boom, such as the one during the past decade, constitutes a very small portion of the overall increase in New York's housing supply.  The great bulk of the increase evidently derives from low and mid-rise construction.  Second, the seemingly low number of units in several of the high rises appears to be a result of their ultra-luxurious nature: the expense of building very high necessitates very high prices, which in turn requires ample floorplans on desirable higher floors to attract high-income buyers.  I suspect this produces a declining return on the total number of units per floor as one builds higher (other reasons for internal space constraints on high-rise construction can be found here).  Finally, in the cases of many of these towers, but particularly those from the 1960s and 70s, the buildings do not make efficient use of their lot (for various and debatable reasons).  In some cases, lot coverage appears to be as low as 30 or 40 percent.  Were the entire lot to be used, the same number of units could be accommodated at one-half the height or less.

As to Glaeser's observation about the percentages of towers built in the 70 and 90s: the actual numbers for those are 20 towers built in the 1970s and 21 in the 1990s.  Unmentioned are the 2000s, in which 56 such towers were built in New York.  Why the 90s were used by Glaeser for comparison instead of the 2000s, I can only guess.

Glaeser, Jacobs and Old Buildings

Economist Ed Glaeser has a lengthy new piece out in The Atlantic on the virtues of the skyscraper.  On the second page, perhaps unsurprisingly, Jane Jacobs' name appears as a presumptive opponent of vertical densification:

"In 1961, Jacobs published her masterpiece, The Death and Life of Great American Cities, which investigates and celebrates the pedestrian world of mid-20th-century New York. She argued that mixed-use zoning fostered street life, the essence of city living. But Jacobs liked protecting old buildings because of a confused piece of economic reasoning. She thought that preserving older, shorter structures would somehow keep prices affordable for budding entrepreneurs. That’s not how supply and demand works. Protecting an older one-story building instead of replacing it with a 40-story building does not preserve affordability. Indeed, opposing new building is the surest way to make a popular area unaffordable. An increase in the supply of houses, or anything else, almost always drives prices down, while restricting the supply of real estate keeps prices high."

It has been a while since I read Death and Life, but I was fairly certain Jacobs had not made such an argument.  A search of my copy of her book, specifically the chapter titled "The Need For Aged Buildings," yielded the following quote:

"A successful city becomes a kind of ever-normal granary so far as construction is concerned.  Some of the old buildings, year by year, are replaced by new ones -- or rehabilitated to a degree equivalent to replacement.  Over the years there is, therefore, constantly a mixture of buildings of many ages and types.  This is, of course, a dynamic process, with what was once new in the mixture eventually becoming old in the mixture.  ... Time makes the high building costs of one generation the bargains of the following generation."  The Death and Life of Great American Cities, pp. 246-47.

So Jacobs was not arguing against replacing old buildings per se, but simply against replacing them all at the same time, a position that served as a critique of the prevailing urban renewal methods of her day. 

Her point flowed from the common sense economic observation that "a depreciated building requires less income than one that has not paid off its capital costs;" (p. 248); and therefore is likely to charge less in rent than the newly-constructed building.  It is this variety in costs and types of buildings, Jacobs argued, that enables a diversity of uses within a city neighborhood.

Nor was Jacobs opposed to tall buildings and high densities, so long as density was not so great so as to result in standardization that would reduce or eliminate a diversity of uses (p. 277).  These points are made repeatedly and with great clarity in the book, so it is disappointing that Glaeser would represent them inaccurately.  Meanwhile, other legitimate arguments in favor of height limits, including non-economic ones, go unmentioned in the piece.  These deserve their own separate treatment in a new post.

Thursday Old Urbanism

San Francisco, 1905.

A "Verdict" On Landscape Urbanism?

The Landscape Urbanism movement has recently received a lot of attention in urban blogs (here, here and here, for example) as well as the mainstream press — much of it quite critical.   There soon may be at least one official verdict on Landscape Urbanist design, however, as a lawsuit is currently proceeding against the city of New York for negligent design of the High Line walkway, which allegedly caused the plaintiff, an elderly woman, to trip and fracture her ankle.

Source Here

At issue in the lawsuit are the peculiar concrete paving blocks, seen at right, which have a slightly upturned edge at path margins, and which taper off into raised, fork-like protrusions.  The purpose of this odd design feature?  In an essay by James Corner, the Landscape Urbanism proponent who was the lead designer for the project, a similar photo of the High Line is featured with the caption, "a view of hard and organic surfaces bleeding into one," which appears to tie into Landscape Urbanist ideas about breaking down the separateness of the agrarian and the urban.   Is it possible then that this design feature reflects purely aesthetic concerns?  The raised blocks, however, present a serious tripping hazard, according to Geoffrey Croft of NYC Park Advocates, one that should have been forseeable to designers of the $152 million project.

Having walked the High Line myself last September, my impression is that the blocks are a significant hazard, one which is magnified by the excessive narrowness of the pathways and the fact that pedestrians are understandably more interested in observing their surroundings than scrutinizing every step.  The staggered, uneven placement of the fork-like elements, as can be seen at upper right in the photo, almost seems designed to promote tripping and stumbling.  The city appears to have agreed, as the edges of the blocks are now awkwardly roped off, which adds a degree of safety but which destroys the "bleeding into one" aesthetic.

Thursday Old Urbanism

Source: denis martin

Shibam, Yemen.