Building the Skyline

The Birth & Growth of Manhattan's Skyscrapers

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Mining the Wind? The Economics of Wind Turbines on Skyscrapers

Jason M. Barr             December 19, 2019

Swaying in the Wind

In the early-1990s, my wife worked for WH Freeman and Co., a publisher of scientific and academic textbooks. Her office was on the 35th floor at 41 Madison Avenue in Manhattan. The building, opened in 1974, is a 41-story modernist glass box designed by Emory Roth & Sons. On extremely windy days, she and her co-workers could feel the building sway—so much so that junior employees would claim motion sickness and request permission to go home. Doors and drawers would sometimes close on their own. Regarding the elevators, my wife recalls, “They told us they were safe, but when windy, they made a weird scraping noise that freaked people out.”

For a skyscraper, wind is a big concern. When a building’s height rises above ten stories, the so-called lateral forces—those usually generated by wind—drive the structural design, as the wind speeds rise dramatically at higher altitudes. A 1,500-foot (457-meter) skyscraper must be fifty times stronger against the wind than a 200-foot (61-meter) one. In very tall buildings, up to ten percent of its structural weight and costs go into wind bracing. Occasionally, the developer doesn’t get it 100% right, and the people inside feel it.

The “Windustry”

But as the demand for skyscrapers has increased, a sophisticated industry has formed to help conquer the wind. In the old days, giants, like the Empire State Building, simply used more and thicker steel to stiffen the structure. But today, there’s a quest among developers to employ methods that use less material to get the same result. For example, before construction begins, a skyscraper model is placed in a wind tunnel to simulate its performance under local conditions. If one style is unsuitable, its shape or structural elements is altered.

Another technique is to hang a massive pendulum on the top floor, which dampens the sway. In general, the aim is not to perfectly stiffen the building, but to slow it down, so its movement is imperceptible (and the elevators don’t screech). Taken together, these innovations reduce the amount of steel or concrete needed, thus lowering the total construction costs. The art and science of wind bracing have progressed rapidly in the last decade or so. So much so that today 1,000 feet or taller buildings are quite common around the world (see graph below).

Skyscrapers with Wind Turbines. From Left to Right: The Strata SE1 (London), Bahrain World Trade Center, Pearl River Tower (Guangzhou), Hess (Discovery) Tower (Houston).

Energy Efficiency and Tall Buildings

One of the big complaints about skyscrapers is that they use a lot of energy—carbon generating energy at that. The question remains, however, about whether tall buildings are, in fact, good or bad for the environment. On the one hand, by virtue of their size, they consume more energy than low-rises. And because many of them are covered in glass, they can require more heating and cooling than smaller masonry clad buildings. On the other hand, by concentrating economic activity in one place, and promoting mass transit use or walking, they can reduce the harmful environmental impacts of sprawl.

Users versus the Structure

But when discussing the energy use of tall buildings, it’s essential to keep in mind the difference between the building itself and the occupants. Buildings with different heights, shapes, materials, plant and equipment, and so on, will use varying amounts of energy, holding constant what the people inside are doing. If we have two identical buildings side-by-side and one is used by finance firms, who engage in high-frequency trading, while the other has low-tech folks, the energy usage will be very different.

The Bank America Tower in New York City offers a noteworthy example. Completed in 2010, it was designed with the latest energy efficiency technology. For instance, it included a 4.6-megawatt combined heat and power plant, which runs on natural gas. The wasted heat created for electricity is recycled to heat and cool the building in winter and summer, respectively, thus reducing overall natural gas usage. However, after a few years in operation, the skyscraper was found to be one of the highest energy users in the city. A large part of this was from the occupants. The tower is filled with stock and bond traders who require intensive computing.

Skyscrapers and Sustainability

But given their size and visibility, city officials, architects, engineers, developers, and building owners are concerned about reducing the carbon footprints from skyscrapers. To do so in an economical way is both good for the bottom line and society. Many cities have policies that incentivize greenhouse gas reductions. And, the non-profit organization, the U.S. Green Building Council, certifies buildings as meeting one of several energy-efficiency categories. Their Leadership in Energy and Environmental Design (LEED) certification program encourages developers to include innovations that make their buildings more sustainable. Many new tall buildings in the U. S.  gain some form of certification because tenants are willing to pay more to be in them.  But LEED certification, in and of itself, may not necessarily mean a reduction in carbon emissions. If the building’s fuel comes from oil or gas, its occupants will produce carbon. Since building owners often have limited control over whether their energy provider is generating power from sustainable or renewable methods, they need to either reduce their power consumption or find a way to create green energy themselves.

Wind Power to the Rescue?

Given that tall buildings are surrounded by wind, in theory, it could be cheaply harvested for energy. This idea has motivated a handful of developers to embed wind turbines directly into their skyscrapers. Around the world, there are currently a handful of cases.

Completed in 2010, the Strata SE1, a/k/a the Razor, is a 43-story residential skyscraper in London that has three turbines on the roof, making the building look like an electric razor. The turbines were expected to generate 8% of the building’s total energy consumption.

The Bahrain World Trade Center is a twin-towered projected, which opened in 2008. In between the two fifty-story buildings are three turbines rotating around three horizontal beams connected to each tower. The turbines were designed to add a relatively modest cost of 3% and were expected to generate 11% to 15% of the towers’ energy consumption. The buildings were also shaped to funnel the wind directly to the turbines.

The Pearl River Tower is a 71-story skyscraper in Guangzhou, China. Completed in 2011, it has vertical turbines placed at about one-third and two-thirds the way up inside the belly of the structure. They are placed on mechanical floors, which have no windows and are exposed directly to the wind. Initial studies predicted that the façade openings would accelerate the wind speed by 2.5 times, resulting in more than eight times the power generation when compared to a turbine located in an open field.

The Hess (Discovery) Tower, a 29-story high-rise in Houston, completed in 2010, included ten vertical-axis turbines on the roof.

Number of Worldwide Completions and Cumulative Total of 1000-foot or Taller Skyscrapers, 1929-2019.

The Results

What have been the results of these cases? It seems that wind turbines in skyscrapers are not quite ready for “prime time.” The learning-by-doing process has suggested that the economics don’t quite work as  hoped or expected. They currently don’t appear to be economically efficient, as the amount of energy they typically produce is low, and the savings do not pay for their additional costs.

What was Learned?

To get more information on what was learned, I talked with the wind engineers, Brad Cochran and Roy Denoon, of CPP Wind Engineering & Air Quality Consultants. Over the years, they have been directly involved with wind turbine projects in tall buildings. According to Cochran and Denoon, there are several reasons why turbines have not met expectations.

Turbulence

First is that along the edges of buildings, the wind tends to “split,” generating a circulating wind cavity. Inside the cavity, the wind slows down; while just above it, the flow can be turbulent. The ideal conditions are high-speed but steady winds. These cavities create the opposite effect. To make rooftop turbines work, they need to be extended much higher up, but this dramatically increases their costs, assuming that Federal Aviation Administration or local zoning requirements don’t prohibit them at such heights.

Blade Size

The second problem relates to the size of the blades. For wind to generate sufficiently large quantities of energy to produce a satisfactory return on investment, the blades need to be quite long. Brad Cochran says,

For wind to produce significant energy, you need a large surface area. Even a 50kW wind turbine, which would likely produce on average closer to 10kW (20% capacity factor), requires a blade diameter on the order of 20 meters (65 feet). The roofs of most tall buildings don’t have a lot of extra space where these size devices would likely fit. Nor are they designed to handle the additional structural loads that the turbines may impose on the roof. Furthermore, to make even a minor impact on achieving a net-zero building would require several of these wind turbines being mounted on a single roof. So, the metrics just don’t work out.

Maintenance

The third issue is the maintenance problem. Wind turbines, by their very nature, are placed in extreme environments. This means they require extra maintenance by specialized personnel, significantly increasing the costs and reducing the return on investment.

Accidents

Finally is the possible harm to residents. Wind blades can become dislodged. If they are atop a structure, they can fall onto the street, hitting pedestrians or damaging property. Additionally, the rotating blades  produce noise, which may be uncomfortable to tenants.

Learning by Doing

The handful of cases discussed above seemed to have cooled the enthusiasm from about a decade ago. Evidently, the turbines have been turned off on the Strata SE1. They never even went into operation on the Hess (Discovery) Tower, after one of the blades fell off the roof and landed in a pickup truck. And there is little reporting on the energy production from the Bahrain towers or the Pearl River Tower, again suggesting little to boast about.

But the more significant issue, I would argue, is that while experimentation is necessary and vital, it’s hard to generate a sample of many cases. Wind turbines are expensive to embed and maintain. They create a risk that only a few developers are willing to bear. In those cases, the failed economics generates a kind “once bitten, twice shy” type of situation, especially when the press is particularly unkind. Ideally, one would like to create a large sample of buildings with different designs and technologies and study how well the turbines performed across may cases. But given the high costs, such a large sample is not being generated. Unlike an automobile or a smartphone, one can’t first build and test a prototype skyscraper and then redesign it after its actual performance is understood. Once constructed, it’s, more or less, stuck with the infrastructure it has, at least for several years.

A Breath of Fresh Air?

Nonetheless, we are headed for a climate catastrophe as world temperatures continue to rise. Skyscrapers are here to stay, and they play an important role in the growth of cities. So, government policies should encourage more experimentation in green designs and technologies. From them, we can learn from both the successes and the failures. There’s no direct path to a carbon-free economy. It must come through both trial and error and making carbon much more costly to produce. I say it’s not time yet to give up on befriending the wind.

 

Continue reading related blog posts: NYC skyscrapers and greenhouse gas emissions here, or urban sprawl and CO2 emissions here.

by Jason Barr 7 Comments

The Bedrock Myth and the Rise of Midtown Manhattan (Part II)

Jason M. Barr         August 6, 2019

Note: This two-part series is adapted from chapters seven and eight of my book, Building the Skyline: The Birth and Growth of Manhattan’s Skyscrapers (OUP, 2016), which is based on research (here and here) performed with Troy Tassier of Fordham University.

The Bedrock Myth

An aerial view of Manhattan reveals the shape of its curious skyline: a cluster of skyscrapers Downtown and another one in Midtown, but few in between. The conventional wisdom is that skyscraper developers gravitated to these areas because access to bedrock near the surface made it easier to create foundations.

As discussed in Part I of this series, this conventional wisdom is false—a confusion of correlation with causation. Lower Manhattan, in fact, had some of the worst geological conditions in the island. Because the bedrock there is below the waterline, it is covered with wet sand. This required expensive foundation technologies—most notably caissons—to anchor skyscrapers to the bedrock. Developers willingly payed these costs because the rewards were so great—high rents from the many corporations clustered Downtown.

In the region just north of City Hall, where the bedrock was deeper down, developers had no interest in constructing tall buildings because they contained dense tenement districts. Here, the residents had no ability to pay the high rents needed to justify the costs of high-rise construction.  If there was a demand for skyscrapers there, builders could have used caissons to create the foundations.

Looking back at the history of the skyline in the 1960s, geologists saw no skyscrapers where bedrock was deepest and made the false conclusion that it was their depths that scared off developers. But the truth is just the opposite: places with the deepest bedrock also had the lowest demand for skyscrapers.

The Midtown Myth

But this begs the question, if geological conditions where not influential, then what caused the rise of Midtown as the nation’s largest skyscraper district?  Another conventional wisdom permeates the discussion: the location of Grand Central Terminal.  Forced by city officials to keep its terminal north of lower Manhattan, in 1871, the New York Central Railroad opened a depot at its current location on East 42nd Street.

The story holds that this area developed as a natural focal point because it created a giant transportation hub. However, the problem with this argument is that the tracks were exposed until 1913, when they were electrified and covered. Before then, the neighborhood north of the station was polluted by coal-infused steam from the locomotives. Out of the seven crosstown streets passing through the railroad’s property, north of 42nd Street, only two were open for vehicles; the rest were for pedestrians.

The Earliest Skyscrapers of Midtown. Left: The Flatiron Building (1902). Middle: The Metropolitan Life Tower (1909). Right: The Croisic Building (1912). The first skyscrapers north of Downtown were mostly in the neighborhood around Madison Square, bound by East 23rd Street, East 26th Street, Fifth Avenue, and Madison Avenue.

Midtown 1.0: Madison Square

A closer look at building patterns before 1913 reveals that the Grand Central Terminal neighborhood was not of interest to skyscraper (and office) developers during the earliest years. The first skyscraper north of Downtown was the Flatiron Building completed in 1902.  It was built on a triangular plot between Broadway, 5th Avenue, and 23rd Street.  The first world-record-breaking skyscraper north of City Hall was the Metropolitan Life Tower, completed in 1909, on the corner of Madison Avenue and 23rd Street, across from Madison Square Park.

Founded in 1868, the company originally opened an office there in 1893. MetLife introduced the strategy of having its agents go door-to-door, collecting monthly premiums.  Presumably, by having their offices at Madison Square, agents were much closer to their customers. Its success at that location prompted it to construct its tower; this strongly suggests that Madison Square was the center of business and commerce north of lower Manhattan. The Chrysler and Empire State Buildings, further north, were completed a generation later.

The “Jump”

The figure below shows the pattern of skyscraper construction during this generation. Each dot represents a building that is 80 meters (262 feet) or taller. The figure on the left shows that during the decade or so between 1890 and 1901, all the skyscrapers were constructed Downtown, the sole exception being the tower of Madison Square Garden. Then, suddenly, starting in 1902 there was a “jump” where skyscraper completions were evenly divided between Downtown and Midtown. The maps indicate that around the turn of the 20th century there was a tipping point, where developers saw opportunities north of 14th Street that did not exist in the decade prior, despite the fact that little had changed in the neighborhood around Grand Central Terminal, which would not be rebuilt and completed until 1913.

The graph also reveals that Midtown construction was split between two areas. The first was between Union and Madison Squares, from 14th Street to 26th Streets; the second area was along 42nd Street, running west from Grand Central to Times Square. But the greatest concentration was around Madison Square.

The corridor along 42nd Street was likely influenced by two changes occurring in the first two decades of the 20th century. First, as mentioned above, was Grand Central Terminal. Second was the completion of the first subway line, which began at City Hall, ran up to Grand Central Terminal, then cut across 42nd Street to Times Square, and from there up Broadway. The line may have influenced the early shape to some degree, since developers and businesses saw opportunities along the lines. But as I will argue below, these transportation developments, while playing a supporting role in the rise of Midtown, were not the cause of its emergence.

The Jump. Left: Skyscrapers (80 meters/242 feet or taller) built between 1890 and 1901. Right: Skyscrapers built between 1902 and 1912. The figure shows that the largest cluster of Midtown skyscrapers were around Madison Square (shaded oval). Grand Central Terminal (shaded box) was not a draw before 1913.

The Birth of Commuting

To understand the reason for Midtown’s birth, it’s necessary to go back in time—to the 1830s, actually. This was the period when the seeds of Midtown were being planted. Up until that time there was no such thing as mass transit. Most people got around on foot. Horse-pulled coaches were available, but they were relatively expensive and limited in capacity.

The pedestrian city, as a result, generated a particular urban landscape. The wealthy tended to live closer to the very tip of Manhattan, near the port and seat of government. Here they could easily walk between their homes and their employment. The poor, on the other hand, could not afford to live in the center; as a result, they tended to cluster along the outer fringes of the city, which was then the area just north of City Hall. Without mass transit, the city had the opposite layout as to what we normally see today. The rich lived in the center and the poor lived on the periphery.

But then in the 1830s, this began to change. First came the horse drawn omnibus in 1829—a large stagecoach outfitted for mass transit. Next, in 1832 came the horse drawn streetcar, which was driven on rails embedded in the road. The streetcar produced less friction and was thus able to move more people with fewer horses. In short, these transportation innovations created, for the first time in American history, the possibility for people to live further away from their homes and commute to work each day.

The First Inversion

New Yorkers embraced this opportunity. This resulted in what I call the First Inversion. In the 1830s, the wealthy began an exodus from their homes in lower Manhattan. But rather than simply shifting their residences a little to the north, outside the growing business district, they “jumped” over the working class neighborhoods, such as Five Points, and the Lower East Side into the less developed northern areas of the island, such as Washington Square and lower Fifth Avenue. The working classes remained in their historical enclaves because they could not, or would, not pay for mass transit. This transition created a new pattern. The business district was at the lower tip of the island. North of that were the crowded tenement neighborhoods. Above that were the middle- and upper-class residents, living in their townhouses.

The First Inversion. Left: Blue dots are the residences of merchants in 1827. Green dots are residences of laborers in 1827. Right: Red dots are the residences of merchants in 1849. Purple dots are residences of laborers n 1849. Notice how between the two periods, a cluster of merchants formed to the north of the laborers’ neighborhood. During this period, many merchants “jumped” to the northern area of the city to live in the suburban upper-class parts of the city.

Geology and Manhattan

As discussed in Part I, depths to bedrock did not play a role in the formation of Midtown. But this is not to say that geology, more broadly, was inconsequential. In fact, the geological roots of the city are strong, but not in the way most people think. Rather, the geological story of importance is the long history of plate tectonics, and the coming and going of glaciers during the various ice ages. These events created Manhattan Island—two miles wide and 13 miles long. The “tubular” shape of Manhattan, combined with the First Inversion, is what created Midtown 1.0.

Through the 19th century, as Manhattan’s population swelled, it reinforced the pattern after the Inversion. The immigrants pouring into the city went to the tenement neighborhoods, where housing was cheap and lower-skilled jobs most accessible. The middle and upper classes tended to move north on the island, and commute to work.

Manhattan’s Shape

Manhattan’s shape is different than most cities. Chicago, for example, is like a semi-plain, bounded by Lake Michigan. And the London region is flat expanse centered around the Strand. These “open” landscapes tend to generate a city with one main central business district, and residential rings around the core. But in Manhattan, these rings were more concentrated because the rivers blocked their outward expansion. As Manhattan moved north, and population and business density increased, land values rose more than it would have in a more circular city because economic activity was concentrated in a narrower band. This was the cause of Midtown’s rise as a skyscraper district at the turn of the 20th Century.

The Economic Roots of Midtown

After the First Inversion, the Madison Square neighborhood in the 1850s was a quiet well-to-do suburb, with townhouses lining the park, and with shops, theaters, and restaurants nearby. As the population continued its northward expansion, Madison Square became a central location for shopping and entertainment. Its continued popularity triggered a change in the 1880s. Clothing factories moved in to be close to the stores that sold their products. Insurance companies and banks moved in to be close to their customers. Townhouses were converted to hotels for tourists. The Broadway theater district was born. And so on. Soon the area around Madison Square was a bustling, heterogeneous district catering to consumers. Around the turn of the 20th century, the popularity of the neighborhood put pressure on land values, which then made erecting skyscrapers rational investments. Hence, the first skyscraper of Midtown, was the Flatiron Building at 23rd Street and 5th Avenue, giving birth to Midtown 1.0.

The Evidence

How do we know this? The evidence comes from an analysis of a data set collected about where people were living and working, and where businesses were operating, during the second half of the 19th century. New York, like many cities, annually produced directories, which listed the locations of people and businesses. Fortunately for researchers in the 21st century, the directories often listed both people’s residential and work locations, along with the nature of their employment. Other directories listed just businesses and their office locations. By collecting samples from these directories, we can follow the commuting patterns of people and the location decisions of firms. While more details are provided in my book or in my paper with Troy Tassier, the following graphs offer some evidence.

Commuting in the 19th Century

The maps below show the residential and employment locations of people who worked in finance, insurance, or real estate (FIRE), or who were listed as corporate officers, such as president, treasurer, or secretary (we call them Corporate Service Workers). The maps demonstrate several things. First, the residential locations (triangles) of these workers were steadily moving northward over the course of the 19th century. In 1861, there was a large residential cluster around Washington Square Park and the bulk of the residences was between Washington and Madison Squares. But, the vast majority of job locations (dots) was in lower Manhattan. The 1879 map shows a steady northward movement of this population with the majority now living between 14th Street and Central Park South. Job locations, by and large, remained downtown.

The bottom two maps (1892 and 1905–1906) show a similar pattern: the FIRE and Corporate Service Workers were steadily moving northward. By 1905–1906, a large fraction was living on the Upper West Side and the Upper East Side, between Central Park South and Central Park North. Employment locations remained concentrated in lower Manhattan, though by 1892, we see a large presence of these office workers in midtown, providing evidence that white collar workers were steadily moving northward, while white-collar jobs, by and large, remained in lower Manhattan, although there was a growing presence in midtown. Specifically, note the largest presence of midtown employment locations were in the Madison Square and Washington Square areas (about halfway between the southern border of Central Park and the southern tip of Manhattan and well south of the Grand Central neighborhood). This suggests that this area was the center of midtown commerce at this time.

Manhattan Moves North. The four maps show the northward residential movement of finance, insurance and real estate workers (FIRE) and corporate officers over the 19th century. These people continued to move north, but their employment mostly remained Downtown. Even in 1905/6, much of the Midtown employment was south of the Grand Central Terminal neighborhood.

Business Clusters

We can also look at the locations of specific companies. The top graph below shows the concentration of bankers and advertising agents relative to the lower tip of the island. For example, the graph shows that in the three periods surveyed—1866, 1882 and 1898—about 90% of bankers in Manhattan remained in the Wall Street area. However, over the same period we see something quite different for newspaper publishers and architects.

In 1866 and 1888, they were still strongly clustered downtown. But then note the “jump” in 1898. During the period from 1888 to 1892, a large fraction of companies in these industries moved from lower Manhattan to the area three miles north of the tip. This was around Madison Square, far south of Grand Central Terminal (another mile to the north). The key thing to note is that certain types of firms left midtown—those that were dependent on residential consumers for their businesses.

Job Concentration versus Southern Tip of Manhattan. The graphs show the relative employment concentration for bankers, advertising agents, architects and newspaper publishers, respectively, over the course of the 19th century. It shows that bankers and advertising agents remained Downtown (about one mile or less from the tip). Architects and newspaper publishers “jumped” to the Madison Square area after 1882 to be close to their residential clients, who were moving north since the 1830s.

Newspaper Publishers at the Turn of the 20th Century

Newspaper publishers present an interesting example as illustrated by the movements of two of New York’s most famous dailies: the New York Times and the New York Herald. Originally, they both resided in lower Manhattan at Newspaper Row near City Hall. In 1890, the New York Herald left lower Manhattan to move to 34th Street and 6th Avenue. At the time, the Real Estate Record and Builders Guide wrote,

As regards the gathering of local news we should judge the Herald will have no little advantage over its contemporaries…The upper part of the city is alive and active till past midnight…The reporters will be able to undertake more assignments, and those which are given to them can be covered more quickly, and, in case time is pressing, with greater thoroughness.

Similarly, in 1904, the New York Times left its headquarters near City Hall to move to 42nd  Street and Broadway. Its movement prompted the renaming of that area from Longacre Square to Times Square. In both cases, these newspapers felt that being close to the action was paramount—here the “action” was local news and events taking place in the city’s social, retail, and entertainment center.

The Rise of Midtown

To summarize, Midtown’s birth is due to the growth of consumer-oriented businesses around Madison Square after the Civil War. By the 1880s, a critical threshold had been reached where particular office-based industries found it profitable to move there to be closer to consumers. Once those firms moved, Midtown emerged as an office-based district. By the turn of the 20th century, land values had risen high enough so that skyscrapers were then profitable investments. This process unfolded at least two decades before Grand Central Terminal was completed in 1913. And, of course, access to bedrock played no role. During the Roaring Twenties, Midtown moved northward.

Myth Busting

The next time someone tells you that Manhattan has two skyscraper districts because of either access to the bedrock or Grand Central Terminal, please tell them these are just myths and misconceptions. And then you can share with them the real reasons presented in this blog series. While stories about the mighty forces of nature, or the ego-driven construction schemes by Gilded Age Railroad Titans, may help to make New York’s history more exciting, the truth about its growth is equally dramatic. New York City has always welcomed the strivers, who left their native countries to seek their fortunes. The skyline is the product of countless people—rich and poor—driven to build a home in the New World. Manhattan Island beckoned and they it made it so.

 

Continue reading Part I of this series here.

 

I would like to thank Maria Thompson for valuable editorial assistance.

by Jason Barr 6 Comments

The Bedrock Myth and the Rise of Midtown Manhattan (Part I)

Jason M. Barr          July 29, 2019

Note: This two-part series is adapted from chapters seven and eight of my book, Building the Skyline: The Birth and Growth of Manhattan’s Skyscrapers (OUP, 2016), which is based on research (here and here) performed with Troy Tassier of Fordham University.

The Bedrock Myth

There’s a popular historical anecdote about Manhattan that’s frequently bandied about at cocktail parties and on hop-on-hop-off bus tours. It has to do with why skyscrapers are “missing” between Downtown and Midtown. In particular, the story says that Manhattan’s skyline is the way it is because of geology.

Since skyscrapers are so tall and heavy, they require proper foundations to prevent them from leaning, sinking, or cracking. One possibility is to anchor them directly to the bedrock. Manhattan Island—really just one big 13 mile (21 km) long boulder—would be ideally suited to support millions of humans and their structures.

But the bedrock is not flat. Starting at the southern tip of Manhattan, it is relatively close to the surface. Going north, say along Broadway, it begins to dip, and by the time you get to City Hall, a mile (1.6 km) to the north, the rock floor gives way to a valley, far below the streets. Then at about 14th Street the bedrock approaches the surface again, where it remains relatively close for the rest of the island. In parts of Central Park, the bedrock crops out above the ground, making for fun summertime climbing.

Correlation not Causation

The skyline tale says that developers naturally gravitated to those parts of the island—Downtown and Midtown—where bedrock was accessible. And, they avoided places in between where the rock was, presumably, too far down. Let me be clear: this notion is false–a case of confusing correlation with causation. It is true that skyscrapers more frequently appear where the bedrock is relatively close to the surface, but it is incorrect to conclude that access to bedrock was the reason. This is a myth. It perpetuates itself because it is constantly repeated by seemingly authoritative people—geologists and tour guides, in particular. It is appealing because, like all founding myths, it helps connect the dots between the great powers of heaven and earth and our humble lives.

Why are skyscrapers “missing” from between Downtown and Midtown Manhattan? Contrary to what many believe, accessibility to the bedrock did NOT shape the skyline. The island’s two skyscraper districts were a result of Manhattan’s economic and demographic history.

Origins

But how did the myth begin? It seems it was first written down by a well-known New York geologist, Christopher Schuberth, who, in his 1968 book, The Geology of New York and Its Environs, took some liberties by steering away from his expertise to write about urban land use:

South of 30th Street…the mica schist, however, is still accessible in deep foundations or excavations. But when one reaches the north side of Washington Square in Greenwich Village, the situation changes rather abruptly. Here, near New York University, the surface of the bedrock drops appreciably—up to several hundred feet below the street level. Farther south, near Chambers Street, it comes again to within about one hundred feet of the surface. Dividing Manhattan into the “downtown” and “midtown” districts, the low-lying area—the area where the top of the bedrock lies hundreds of feet below the street level—has been filled in and built up almost entirely with glacial deposits and, more recently, artificial land fill. That is to say, the skyscrapers of New York City are clustered together into the midtown group, where the bedrock is within several feet of the surface, and the downtown group, where the bedrock again reappears to within forty feet of the surface near Wall Street…..In any event, it is readily seen how clearly the accessibility of the bedrock has, to some degree, controlled the architectural planning of the city.

From there, the great aha! went viral as the truth (to him) was “readily seen.” But to see two things together does not mean there is a connection between them. When one takes the time to review the evidence, the “facts,” like touching gossamer, dissipate in one’s hands.

Economics: The Real Story

The real story is not geology, but economics. As I have documented in another post, the key to understanding why skyscrapers are built begins with a comparison of the costs and benefits that arise from their construction. The benefit is the value created. Corporations, for example, pay generous rents for the ability to shelter their employees. Today, office rents in Manhattan average $77 per square foot ($829 per square meter).[1] For a one million square foot (92,903 square meter) skyscraper, well, that’s a lot of moolah. But to get it, the developer must first pay for the land and the cost of construction. Thus, the location and height of a skyscraper are determined, fundamentally, by the income relative to the cost of producing it, or, in real estate parlance, the return on investment.

With this in mind, let’s go back in time, to about 1885, when the notion of how to build skyscrapers was just dawning on the real estate community. Lower Manhattan was, arguably, the densest business district in the world. This clustering was bidding up land values, which was signalling to landowners that the demand for those locations was rising fast. This, in turn, incentivized developers to find innovative solutions to “make the land pay,” and get access to the revenues.

But, of course, they had to contend with the geology. The myth states that since the bedrock is just below the surface in lower Manhattan, skyscrapers were simply thrust down upon it like an empty beer can on a table. But this is not true. In fact, Downtown, from Wall Street to City Hall, has some of the worst geological conditions on the island. Builders in lower Manhattan built skyscrapers not because of the geological conditions, but despite them!

Watch Out for the Quicksand!

Just below the ground level, atop the bedrock, sit uneven layers of sand, gravel, and clay, with large boulders scattered throughout. Beneath the waterline, the sand morphs into a wet, viscous goo, known as quicksand. Anyone who’s watched the 1960’s television comedy, Gilligan’s Island knows that quicksand is a trap, instantly enveloping whatever falls into it. This was a major problem for developers in lower Manhattan. They couldn’t just dig their way to bedrock because once they hit the quicksand, it would flow back into the construction site (also dooming the foundations of surrounding buildings.)

The solution was the caisson—a large cube with no bottom. Workers inside the cube (“sandhogs”) would dig up the soil and pass it up through chutes to the surface. The box would slowly sink. Once it hit the quicksand, compressed air was pumped in to keep the soil out. Eventually the box would reach the bedrock. It was then filled with cement, and piers were built atop it up to the basement level to hold up the structure.

The First Caissons. The first structure in Manhattan to use caissons was the Manhattan Life Insurance Building at 66 Broadway (near Wall Street). Completed in 1894, it rose 18 floors and was estimated to weigh 30,000 tons. One major concern during construction was to not disturb the foundation of the six-story Consolidated Exchange building next door, which was placed on piles. Source: Engineering Record.

Men of Robust Physique

But make no mistake: using caissons was expensive and dangerous. In 1912, The New York Record and Builder’s Guide commented:

Builders sometimes ask why foundation work costs so much in New York. The answer is: The risk is so great no matter how well trained the units of a foundation company’s working organization may be, mistakes are sure to happen at times. No matter how carefully the site may have been bored, boulders or quicksand are liable to appear….All perilous work is expensive, because men of robust physique and of sufficient bravery hourly to risk death are hard to find.

Skyscrapers and Bedrock in Lower Manhattan

But, upon further inspection, the “perilous work” to get to the bedrock in the worst areas was not a barrier. How do we know? Because, using geological maps, we can ascertain the bedrock depths below the earliest skyscrapers. And what do we see? A few of the city’s tallest skyscrapers, in fact, were built over some of the deepest bedrock; suggesting that skyscraper builders were able to overcome the geological obstacles put before them, and were not scared off, as the myth implies.

The figure below shows the depth to bedrock below a sample of buildings in Manhattan as of 1915 (the vertical axis) versus its location relative to the southern tip of the island. The triangles are skyscrapers (80 meters/242 feet or taller) and the circles are (randomly chosen) low-rise buildings. The hump shows that as we go from south to north the bedrock depths increase into a valley. Then, continuing north, the rock moves closer to the surface.

We can also see that a few skyscrapers, most notably the Woolworth Building (1913, 55 stories) and the Municipal Building (1914, 40 stories)–the highest triangles in the figure just above the City Hall indicator–were constructed over some of the worst geological conditions. The graph shows that as lower Manhattan’s bedrock got deeper, high-rise builders were not deterred. But, just as importantly, on the north side of the valley (between Canal and 14th Streets), there are no skyscrapers where the bedrock is relatively accessible. If bedrock depths were key, we would likely see them where it was easy to build. Rather, we see none. The reason was the lack of demand for such projects (discussed below).

Buildings and Bedrock. The figure shows the depth to bedrock below skyscrapers (triangles) and 99 chosen low-rise buildings (circles) going from south to north on Manhattan, as of 1915. Going north the bedrock depths increase and reach their greatest depth below Canal Street, about 1.5 miles (2.4 km) north of the tip. Notice that a few skyscrapers near City Hall were built over some of the deepest bedrock in the city–demonstrating that bedrock depths were not a barrier to their construction. Source: Barr, Tassier and Trendfilov (2011).

The Construction Costs

The bedrock myth ignores the cost of construction in lower Manhattan. But the terrible conditions below the surface made them high, in both blood and treasure. Then, why did builders persist? Why was the Woolworth Building—the world’s tallest building when completed in 1913—built over some of the deepest bedrock in the city? Because the rewards were so great.

Once the decision to build a skyscraper was made, the foundation costs, as a fraction of the total construction costs, were not that high. The bulk of the caisson costs, for example were fixed and associated with planning, building the boxes, setting up the air compression equipment, hiring medical professionals to help workers from the bends, and so on. Once the box was placed on the ground, the difference in costs for going from say 20 feet (6 meters) to 40 feet (12 meters) down was relatively small.

Costs versus Land Values

Given the low costs of sinking the caissons further down, we can do some back-of-the-envelope calculations to see where a skyscraper developer would have built if he were only concerned about cost minimization. During this period—in 1905, for example—average land values were about $72 per square foot ($775 per square meter) south of City Hall, and $23 ($248 per square meter) just north of it. Say we take a plot of land suitable for a skyscraper of 250 feet by 100 feet (2,323 square meters). Using these average prices, would yield a land cost of $1.8 million downtown, but only $575,000 north of City Hall. This means that a developer could have saved a bundle on land costs by moving his project northward. The question then becomes: do the extra costs of getting to bedrock in the north exceed the savings in land costs? The answer is a definite no.

A statistical analysis reveals that once a developer decided to use caissons, the additional cost of sinking the boxes another foot was about $3,000 (about $9900 per meter). So, let’s say, in an extreme case, the difference between the bedrock depths on the southern and northern lots was 50 feet (15 meters), this would yield an extra cost of about $150,000—far below the difference in land costs. An enterprising developer could have saved substantial sums by buying a plot along the bedrock valley at a much lower price and paying the additional costs of digging to the bedrock. But none did so. Why not?

The Densest Place on Planet Earth

Just north of City Hall were the densest immigrant neighborhoods on Planet Earth. In 1900, one block held 1200 people per acre. These residents were mostly newly arrived European immigrants, who were unwilling and unable to pay for high-quality space. The majority of them could only afford a tiny tenement apartment with few amenities. For the landlord, the most profitable form of real estate was the five-story walk up tenement. They were easy to construct from off-the-shelf blueprints. The sand below the ground was strong enough to hold up a low-rise, without creating additional expenses.

But, most importantly, the tenement was the structure with the most demand, and thus had the highest return on investment. A skyscraper developer could have put up a 20-story high-rise for a lower total cost there, as compared to Wall Street. But who would pay the rents needed to make it profitable? Nobody. Immigrants couldn’t afford them, and banks and brokerage houses would not move to a crowded residential district far away from the business action.

Manhattan Population Density in 1900. This map shows a 3D rendering of population density across Manhattan north of about Chamber Street and south of 155th Street. The greatest concentration of population was in the Lower East Side and the areas just north of City Hall, where coincidentally the bedrock was furthest down. For sources, see Barr (2016), chapter 4.

Talking Points

So next time someone tells you the skyline is the way it is because of the bedrock, here are the responses you can give. First, depth to bedrock is irrelevant if there’s a bed of quicksand floating on it. Lower Manhattan, below the waterline, is a viscous lake of wet sand, which required expensive foundation technologies—most notably caissons—to anchor the buildings to bedrock. So Downtown was not, in fact, some geological paradise, but rather presented a treacherous journey for the adventurous builder.

Second, a few of the world’s tallest buildings at the time—most notably the Woolworth Building (193, 55 stories)—were built over some of the deepest bedrock in the city. Bedrock depths were not, in fact, a true barrier to construction. The high costs of construction were willingly paid to get the reward of high market rents where businesses were willing to pay them.

Developers avoided building over the deepest bedrock just north of City Hall due to a coincidence. Given Manhattan’s particular economic and demographic history, the area with the deepest rock also just happened to have the most crowded immigrant neighborhoods on Planet Earth, where there was no demand for tall buildings. If there was, a developer could have paid the extra costs of digging the caissons deeper to earn the extra revenue. However, there was no money in skyscrapers, just in five-story tenements.

But Why Midtown?

But we still haven’t addressed the question of why skyscrapers appeared in Midtown at the dawn of the 20th century. The reason, though, has nothing to do with bedrock, and everything to do with economics and demographics. This story will be taken up in the next post.

I would like to thank Maria Thompson for valuable editorial assistance.

Read Part II of “The Bedrock Myth and the Rise of Midtown Manhattan” here.

—

[1] The average apartment rent in New York City is about $3000 per month for a one-bedroom apartment. Assuming the average one bedroom unit is 750 square feet (70 square meters) then the average renter pays $4 per square foot per month (or $43 per square meter).

by Jason Barr 6 Comments

The Technology of Tall (Part I): Skeletons, Outriggers, and Buttresses

Jason M. Barr    July 16, 2019

Note this is Part I of an on-going series on the evolution of skyscraper technology. The rest of the series can be found here.

The Birth of Height

In 1900, the architect Cass Gilbert—who later designed the 55-story Woolworth Building (1913)—wrote an article to justify the existence of tall buildings. At the time, New Yorkers were witnessing their first skyscraper boom. They were curious and puzzled, and unsure of what to make of these new structures. Since the dawn of civilization, buildings for work and living were never more than six stories. Then, in a matter of a few short years, offices of 10, 15, and even 20 stories were appearing with alarming frequency.

In an understated tone, Gilbert wrote that the tall building was “merely a machine that makes the land pay.” He implicitly argued that the tremendous economic forces drawing businesses together into Lower Manhattan were driving land values upward. Finance and insurance companies, transportation and communication firms, importers and exporters, corporate headquarters, publishers, law firms, etc., etc., were tightly clustered in Manhattan’s lower toe in order to sell their products and be close to the port.

Emergence

High land prices meant that if a landowner wanted a reasonable return on his investment, he needed to find a way to generate more revenue from the lot. The solution: build taller. With more floors comes more rent. In the history of cities like New York and Chicago, the skyscraper was a naturally emergent phenomenon. Expensive land values incentivized developers, engineers, and architects to produce cost-effective technologies to create new “land” where it was needed most. Thus, in the 1880s, the skyscraper–land in the sky–was born.

Positive Feedback

Once the technology of tall was possible, it generated a positive feedback loop: highrise buildings created more economic growth, as they allowed for more firms to be at the same place and the same time, which increased land values and incentivized builders to improve the technology that allowed them to go even taller. But skyscrapers, by their very nature, also represent public displays. Businesses and developers realized they could use height to convey additional information, such as corporate advertising or to signal their success.

As a result, between 1890 and 1931, the world’s tallest building grew from 94 meters (350 feet) to 381 meters (1250 feet), for an average annual growth rate of 6.3%. The Great Depression and World War II halted the demand for tall buildings for 15 years. Then, as the world cleaned up its mess, tall buildings began to rise again. It wasn’t until the 21st century, however, that the skyscraper became truly a global phenomenon.

Growth in the Height of the World’s Tallest Buildings from 1885 to the Present. Source: CTBUH.

Globalization and the World’s Tallest Buildings

Rapid urbanization, globalization, and economic development created new wealth for formerly low-income countries. This, then, permitted cities and nations around the world to embrace skyscraper construction, not just as a response to economic conditions, but also as an engine for economic growth. That is to say, building innovations combined with rising global incomes meant that skyscrapers could be used as drivers of urban growth and as part of deliberate urban planning strategies to create new places and new industrial clusters.  This is particularly the case in Asia, where skyscrapers have been used to turn the world’s focus eastward.

Rise of the Supertalls

Arguably, the first time any government took an active role in the construction of the world’s tallest building was in New York in the 1960s. Lower Manhattan, an aging business district, had fallen on hard times. The Twin Towers, completed by 1973, became part of a deliberate effort by a local government agency—in this case, the Port Authority of New York and NJ (PANYNJ)—to build and operate the world’s tallest buildings, and to revive what was essentially the world’s first skyscraper district (in parallel with Chicago’s Loop).

Although, architecturally speaking, New Yorkers were ambivalent about the towers, the structures took advantage of new innovations that made this kind of height economically feasible (similar technology was used for the Sears (Willis) Tower). Then, starting in 1998 with the Petronas Towers in Kuala Lumpur, all of the world’s record-breaking buildings were promoted by government officials to boost the visibility of their respective cities—Taiwan (the Taipei 101, 2004), Dubai (the Burj Khalifa, 2010), and, next up, Jeddah (the Jeddah or Kingdom Tower, 2020).

In short, the competitive pressures of economic growth, along with the existence of a large industrial organization of engineers, architects, suppliers, general contractors, and financial institutions, have allowed new cities to get into the height game. These cities have taken advantage of, and have driven, new technological innovations that allow buildings to be both taller and less expensive. We now turn to the evolution of some of this technology.

The Gravity Problem

In the old days, before the advent of skyscrapers, most large buildings had load-bearing masonry walls. That is, the walls held up the building. But over the course of the 19th century, as the demand for taller and bigger structures increased, these walls needed to be proportionally thicker at the base. This presents two problems. First is that thicker walls reduce access to internal light. Second, at some point, especially on narrow lots, the thickness of the walls at the ground floor would remove much of the usable space, and was therefore not an economical use of the land. For example, Chicago’s 16-story Monadnock Building (1893) was the world’s tallest masonry load-bearing commercial structure when completed. Its lower walls are six feet thick. The walls of Joseph Pulitzer’s World Building (1890) in New York City were 11.5 feet thick in the basement and seven feet thick at street level.

Over the second half of the 19th century, architects and engineers experimented with using iron columns and beams to reduce the amount of masonry, until they finally hit upon the idea of using iron–and later steel–to frame the entire structure, eliminating the need for load-bearing masonry altogether. The discovery of the steel skeleton meant that the gravity barrier, so to speak, had finally been broken. This was truly revolutionary, since from the dawn of civilization, some 10,000 years ago, until the 1880s, all of the world’s tallest structures were held up with stone.

Blowing in the Wind

Once the gravity problem was solved, however, a new problem arose. For a building of about 10 stories or higher, wind forces—the so-called lateral loads—start to increase dramatically (ignoring the problem of earthquakes for now. I will return to this in a future post). No one wants to work in a building that sways; and even more so, no one wants a building that might someday topple over in a hurricane.

For about 80 years, the solution to the wind problem was, essentially, to use more steel—either thicker and/or more beams and columns, and/or diagonal bracing elements. The problem though is that as a building gets taller, the amount of required steel rises disproportionately. Furthermore, the additional steel columns needed for rigidity meant that the internal space became increasingly crowded for occupants who, over the course of the 20th century, were demanding larger and more open floor plans.

In 1969, the renowned structural engineer, Fazlur Khan of Skidmore, Owings & Merrill (SOM), estimated the theoretical steel “premium” as building got taller. He demonstrated that if gravity forces were the only concern the amount of steel would need to rise at a relatively slow, linear rate as buildings got taller. But, if the wind problem was factored in, the amount of steel needed would rise at an increasing rate.

The “Premium for Height.” This graph was produced by the engineer Fazlur Khan. It illustrates how the amount of steel needed to stiffen a building against wind forces rose dramatically with building height. This idea helped spark the search in the 1960s for new engineering solutions for skyscrapers. For source, see Ali and Moon (2007).

A New Hope

In the 1960s, things started to change. Fazlur Khan was one of several engineers who helped trigger a revolution or paradigm shift for engineering tall buildings. His basic argument was that because the braced steel skeleton was inefficient at greater heights, new structural methods were needed to stop the wind. And soon they began to emerge. As the noted SOM engineer, William Baker, writes,

This cloudburst of new systems was made possible by the advent of computers and engineering pioneers such as Fazlur Khan, Hal Iyengar, William LeMessurier, Leslie Robertson and others. Although the conceptual foundations of these systems were straightforward, earlier computational methods were not adequate for use in design. At last, the viability of these systems could be demonstrated by utilizing the mainframe computer. In addition to validating the systems, important parametric studies were done to establish the applicable height range for the various systems.

These new systems allowed for buildings that were not only taller but also were stiffer and used less steel per square foot of building area. The Empire State Building weighed in at 42.2 pounds per square foot, while the much taller Sears (Willis) Tower (1974) was only 33.0 pounds per square foot.

Composite Construction

Besides design innovations, another vital discovery was with materials. No longer are the world’s tallest buildings constructed using only steel. Today, the vast majority of them employ a composite design of both steel and concrete to make an more efficient structure.  William Baker writes,

Steel is excellent for framing long span office floors; it is lightweight, can be easily modified by tenants and can be quickly erected. Concrete, on the other hand, is very cost effective in carrying the weight of the tower, and the mass is beneficial in reducing building motions. New formwork systems and the ability to pump concrete to great heights have also greatly increased the speed of concrete construction. In addition, the inherent damping of a concrete lateral system is generally higher than a steel system.

The resulting merger of steel and concrete into composite buildings has resulted in seemingly endless combinations of the two systems. Perhaps the most common composite systems use reinforced concrete cores and steel floor framing.

Highrise Building Under Construction in London, as of June 2019. This building has a concrete shear wall central core, which helps stiffen it against the wind. Photo: Jason Barr.

The Structural Revolution

Engineers have developed many different structural systems to go taller while neutralizing the wind (and other technologies to be discussed in a future post). In fact, one can say that today there is an abundance of possibilities. Builders, in some sense, can take a one-from-column-A-and-one-from-column-B approach depending on local conditions, architectural styles, and the intended occupants. Here, I will briefly highlight a few (but certainly not all) of the vital innovations in structural systems to give a flavor of how the technology has evolved.

The Tube

The first innovation in the modern technological revolution was the tubular design, developed by Fazlur Khan in 1961, and first used in the 43-story DeWitt-Chestnut Apartment Building (1963) in Chicago. With the tube design, the façade is the structure–a vertical tube–where the perimeter columns are placed relatively close to each other and are held together by beams. Thus, the façade performs triple duty: it holds up the structure, it braces it against the wind, and is part of the architectural style. The Twin Towers represent the early mastery of this form.

The tube design can be further modified. The tube-in-tube design puts an additional tube in the central core. A braced tube adds extra stiffness with braces that run diagonal to the columns. A bundled tube design puts many tubes together (and each tube can be a different height). This design, also by Fazlur Khan, was used to create the Sears (Willis) Tower (1974) in Chicago.

Left: Twin Towers (1972/3). Right: Willis (Sears) Tower (1974). Both projects used a form of tube design for the structure. Each of the Twin Towers was a perimeter framed tube. The Willis (Sears) tower has a bundled tube design–nine perimeter tubed fused together. Sources: Wikipedia, here and here.

The Outrigger

Originally, an outrigger was a set of horizontal beams on a sailboat emanating from the ship’s side to add support. The idea for skyscrapers was first used by architect, Luigi Moretti, and his collaborator, engineer, Pierre Luigi Lervi, for the 190-m tall 47-story Place Victoria Building (now the Stock Exchange Tower) in 1964 in Montreal. Recently, it has become one of the most popular designs for supertall structures. In this case, evenly spaced throughout the building are two or three outriggers, which are floor-high “walls” of either concrete or steel trusses that run from the central core to the perimeter. Many of the outrigger buildings also have mega columns—very thick columns—to further stiffen the structure. The 128-story Shanghai Tower (2015) uses this design, as does the 101-story Taipei 101 (2004).

The Structural System for the 128-story Shanghai Tower (2015). The figures show the building’s outriggers–the dark blue trusses emanating from the central core, attached to the mega columns (cyan). The outriggers and mega columns add extra stiffness against the wind forces. Source: Xia, Poon and Mass (2010).

The Buttressed Core

SOM engineer William Baker initiated the idea of buttressed core for supertalls. Originally it was used for the structure of the 73-story Tower Palace Three (2004) in Seoul, though the idea seems to have been first implemented in the CN Tower in Toronto (1976). The success of the design suggested it could be used for an even taller structure, and was eventually chosen as the structural system for the 163-story Burj Khalifa (2010). The key is that the core of the building is shaped like the letter “Y.” The skyscraper has a typical central core, but also has three additional cores attached to it, where each wing “buttresses” the other two. The central core and wings together make the building particularly resistant to the various wind forces. This structural type is also going to be used for the world’s first 1-kilometer skyscraper, the Jeddah Tower in Saudi Arabia, which is under construction and will likely be completed in 2020.

The Burj Khalifa (2010). The right figure shows the buttressed core design for one floor (red), which in three dimensions creates a pyramidal structure (left). Source: Baker, Korista, and Novak (2008).

Toward the Mile-High Building

Given the feverish pace of skyscraper growth around the world, it’s unlikely that the Jeddah Tower will remain the world’s tallest building for very long. Global competition will drive not only innovations but also the demand for greater heights.

So, what kind of structure might we expect for the first mile-high tower? There is no way, of course, to answer this question. The final design depends on many factors, including architectural style, building use, cost of materials, local building regulations, and environmental and geological conditions. But it is reasonable to say that a mile-high skyscraper will contain a mix of structural elements described above.

Inspiration

If we want inspiration, we can look to some of the visionary proposals offered in recent years. Here are two that exploit the idea of joining multiple structures to make one larger, stiffer skyscraper. Tokyo’s Sky Mile Tower calls for multiple sets of three building “legs” interconnected to create a hexagon-shaped structure. Each building leg has “megabracing” combined with concrete shear walls along the sides.

Another design is offered by Yale professor Kyoung Sun Moon and his student Chris Huyan. In this design, the mile-high skyscraper is comprised of four braced tube towers. Each tube is interconnected by horizontal braced tubes of multiple stories.

Two Visions of the First Mile-high Skyscraper. On left is a design for the Sky Mile Tower for Tokyo, which uses mega-bracing with side concrete shear walls. On the right is a design for Chicago using co-joined tubed towers. Source: Sky Mile Tower: Malott, Robertson, Hiei and Werner (2015). Mile-high Superframed Tower: Moon (2018).

The Skyscraper System

But structural form is only one part of the equation. The successful supertall is a system of many interconnected parts and must employ a host of technologies to provide vital services, including fast movement up and down, safety, and comfort. It is to these we will turn in future posts.

The rest of the series on the technology of tall can be found here.

I would like to thank Maria Thompson for valuable editorial assistance.

Buy the Book

Building the Skyline can be purchased through Amazon or Oxford University Press.

Credits

The New Amsterdam map on this page was sourced here, and the atlas image is from the 1891 Atlas of City of New York by GW Bromley & Co.

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