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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

By ROBERT L. REID

Rising high in the skies over New York City, Chicago, Hong Kong, and other great metropolises are tall towers that appear impossibly slender. Fueled mostly by market demand from wealthy clients who desire spectacular views, the design and construction of these superslim, generally residential skyscrapers also depend on engineering advances over recent decades in building materials and damping technologies as well as careful coordination by the design teams.

The skylines of New York City and other major metropolises around the world are being redrawn with some strikingly slender lines—skyscrapers that feature almost impossibly thin forms, some standing as straight and slim as towering pencils, others narrow in one direction but broad as sails in the other. Most are luxury residential buildings, but some are mixed-use projects, offering hotel or office space as well. The one thing that all these giants of slender geometry have in common is the need to resist the wind forces that try to topple such towers.

The designation of a structure as "slender" is highly subjective, ranging widely depending on who is asked. Some engineers—and building codes—consider any building with an aspect ratio of at least 7:1 to be slender, the ratio representing the height of the building divided by the narrowest width of its base. Others see slenderness starting at a greater difference between height and width. Carol Willis, the director and curator of New York City's Skyscraper Museum, pointed Civil Engineering to the museum's 2013-14 exhibition, Sky High & the Logic of Luxury , which stressed a ratio of 10:1 or even 12:1 to consider a building slender.

By the museum's definition, the twin towers of New York's World Trade Center, while undeniably tall at 1,368 and 1,362 ft, were not actually considered slender because the base width of 209 ft per side gave the office buildings an aspect ratio of just 6.5, explained written material from the Sky High exhibition. In contrast, the residential tower at 432 Park Avenue in New York City, completed in December 2015, rises 1,396 ft tall while measuring just 93 ft square at the base—dimensions that generate an aspect ratio of 15:1.

When it opens in 2020, the residential tower at 111 West 57th Street in New York City will likely be the slenderest building in the world, standing 1,435 ft tall with a base that measures just 59 ft wide at its narrowest. That will produce "a staggering aspect ratio of 24:1," explains a press release from the building's structural engineer, WSP, headquartered in Montreal. WSP was also the structural engineer for 432 Park Avenue and numerous other slender towers.

Rafael Viñoly Architects, based in New York City, designed 432 Park Avenue. New York City-based SHoP Architects designed 111 West 57th Street.

New York City has been constructing buildings that were considered skinny for their time since the late 19th century, the Sky High exhibit explained. But the modern trend toward incredibly slender structures is a more recent effort, dating to roughly the early 2000s. And many of the most prominent skinny structures have either been completed within the past few years or are still under construction. A Skyscraper Museum document, New York's Super-Slenders , indicates that as of May 2016 some 18 superslender, ultraluxury residential towers—each 50 to 90-plus stories tall—had either opened to tenants or were in the works in New York alone.

The luxury residential aspect of these slender towers has been one of the most critical factors driving the trend toward taller and slimmer buildings. New York City, especially, but also Hong Kong, some cities in Japan, and certain other major metropolitan areas are basically built out, with limited land available for new construction, explains Silvian Marcus, P.E., F.ASCE, WSP's director of building structures, U.S.A., headquartered in the firm's New York City office. Developers in those locations "have no space to go laterally, so they go high!" Marcus says.

Moreover, the limited space for new buildings in places like New York City generally involves small parcels of land, which means that these new structures tend to be both tall and slender—two related factors, Marcus explains. And given the high cost of real estate in Manhattan, these tall, slender buildings almost have to cater to the luxury market—often with just a single apartment or condominium per floor—because only the wealthiest tenants can afford to pay the equally towering costs to live there. Some penthouses have reportedly sold for as much as $90 million, the Sky High exhibit noted.

In return for astronomical prices, the tenants receive astonishing views. Renderings of 111 West 57th Street, for example, show the skinny tower rising high above its neighbors, providing tenants unobstructed views of New York's Central Park to the north. Residents of 432 Park Avenue enjoy similar views along a northwest angle.

Ironically, not everyone who owns such a view will necessarily see it on any regular basis. At the Council on Tall Buildings and Urban Habitats' annual conference in 2014, held in Shanghai, Willis presented a paper titled, The Logic of Luxury: New York's Super-Slender Towers. In her paper, she noted that many of these ultraluxury apartments were being purchased as investments by wealthy individuals and corporations, rather than as residences—investments dubbed by one real estate appraiser as "strong-boxes in the sky."

Although market demand has played a leading role in the development of tall, slender structures, advances in engineering and improvements in the strengths of building materials have also been critical. Not long ago, a typical material strength for concrete might have been 10,000 or 12,000 psi, notes Charles Besjak, P.E., S.E., F.AIA, M.ASCE, the director of structural engineering in the New York City office of Skidmore, Owings & Merrill (SOM). "But now we can get up to 18,000 or even 20,000 psi," Besjak explains, which is a substantial change because it enables engineers to minimize the thickness of concrete walls and thus keep the structural systems from intruding too much within the building's interior spaces.

Likewise, higher-strength reinforcing steel with yield strengths of 100 ksi has become more available over the past decade or so, while on the analytical side "we understand how buildings behave [in the wind] a lot better than we did even ten years ago," notes Stephen DeSimone, P.E., LEED AP, M.ASCE, the president and chief executive of DeSimone Consulting Engineers, of New York City.

Although "our industry is very slow to change," DeSimone adds, the technology necessary for such slender towers "is keeping up…because people seem willing to pay a premium to live in these really tall buildings with unobstructed views."

"These towers clearly allowed for new thinking and innovations," concludes Besjak.

Controlling for wind loads is perhaps the most critical engineering concern in the design of today's tall, slender buildings. Such structures "are so thin that when the wind blows, they want to tip over," explains Donald W. Hamlin, S.E., a vice president in the Chicago office of Thornton Tomasetti. Thus, ensuring a safe and sufficiently stable structure is paramount.

But even when the building is not in danger of falling, engineers also work to minimize the amount of overall movement, or building acceleration, that tenants might feel. This can be especially challenging in tall, slender buildings because "the more slender it is, the faster it is moving," Marcus notes. Tenants do not want to feel that motion because it feels unsafe, he adds.

To address wind-related challenges, engineers rely on a variety of tools and techniques, ranging from wind tunnel tests to deep foundations, from reshaping a building's exterior geometry to reinforcing its interior structure or installing special damping systems, and sometimes even poking holes right through the structure.

The use of wind tunnels helps engineers better understand how these tall, slender structures will behave under different wind conditions—and can lead to changes in a building's design. Such tests "validate certain assumptions made in engineering," Marcus notes. For example, WSP initially designed one tall, slender building without certain features—such as openings in the facade that allow wind to pass through the building, thus reducing the overall wind load. But after wind tunnel tests confirmed that the building's motion would be higher than desired, openings were added to the design in the form of an open mechanical systems floor—reducing motion by anywhere from 10 to 15 percent, Marcus explains. The tests also indicated that the wind would flow best if the open floor—on which the mechanical equipment was concealed within the core—featured rounded surfaces on the exposed core walls, he adds.

Other aspects of a tall, slender building's design that can prove beneficial to reducing wind loads include the use of setbacks, chamfered corners, louvered openings, and "anything you can do to disrupt the airflow," notes DeSimone, who adds: "It's counterintuitive, but buildings don't want to be aerodynamically shaped."

At SOM, the firm turns to professional wind consultants to help confirm and fine-tune the design of new structures. But SOM's Chicago office also operates its own wind tunnel as a tool for use during the early stages of projects. "Having the ability to understand [wind issues] much earlier in the design process is important," says Besjak. "Before, you'd wait for the [consultant's] wind tunnel test and then react. Now, we can test more ideas much quicker."

Deep foundation systems are often essential to the stability of tall, slender towers. Using an analogy of trees, Marcus compares and contrasts the foundations of slender buildings to the root systems that support trees. But root systems typically spread horizontally, whereas the small footprints of land on which the slender towers are generally constructed make it impossible for the foundations to extend to the sides of a project site without intruding on adjoining properties, Marcus notes. So the tower foundations need to go further down.

The foundations for 432 Park Avenue, for instance, feature spread footings with approximately 60 rock anchors that extend down 60 to 70 ft to secure the tower in Manhattan bedrock, Marcus says. Because the 111 West 57th Street building will be much slenderer than 432 Park Avenue, the foundations for 111 West 57th feature more than three times as many rock anchors—approximately 200 in all—that extend as deep as 100 ft into Manhattan's bedrock, Marcus adds.

Elsewhere, in locations without available rock, even deeper foundations can be required. DeSimone Consulting Engineers is working on a tall, slender tower in Bangkok that will exceed 1,300 ft in height and be supported on caissons 150 ft deep, explains Stephen DeSimone.

The efforts to accommodate wind loads on slender buildings can reach inside the structure as well. One Bennett Park, for instance, is a tall and slender residential building that Thornton Tomasetti engineered in Chicago. Opened this year, the building was designed by New York City's Robert A.M. Stern Architects LLP. Nearing completion at press time, One Bennett Park measures 840 ft tall and just under 73 ft wide at its narrowest point on the east and west sides of the structure, for an aspect ratio of roughly 11.5:1. Unlike the pencil-thin slender towers, however, One Bennett Park presents a much wider face on its north and south sides, almost 171 ft wide per side—which in turn picks up higher wind loads along those sail-like faces.

To accommodate those wind loads, Thornton Tomasetti designed four internal "outrigger walls" in the lower portion of the building—from level four up to the underside of a mechanical floor at level 40. The outrigger walls are located within the building's floor plate, perpendicular to the structure's wider sides. They engage columns along the perimeter of the building and work together with a series of belt walls on those wider sides, north and south, within the mechanical floor, says Hamlin. The system stiffens the overall structure and distributes the overturning forces to all the building's bell caisson foundations, rather than just the four caissons directly associated with each outrigger wall, Hamlin explains.

Varying in thickness as they rise up the structure, the outrigger walls measure 32 in. at the base and decrease to about 18 in. at the mechanical level, Hamlin notes.

To accommodate the wind loads imposed on the thin, towering structure of 111 West 57th Street, WSP designed two enormous concrete shear walls that rise up the east and west facades of the building, leaving the north and south facades open to spectacular views, says Marcus. Interior structural walls will connect the giant shear walls across the floor plate of the building, encapsulating the core, which contains the elevators and stairs, and even incorporating a kitchen wall in the residential units, Marcus explains.

"In this way, we satisfied the structural constraint for this very slender building … [with] a robust system that does not intrude on the floor space for residents," Marcus notes.

The east and west shear walls will vary in thickness from approximately 30 to 36 in. at the base to roughly 16 in. at the top of the building and are recessed from the northern edges of the building by about 10 to 15 ft to create corner window views. A series of terra-cotta pilasters will also be attached to the east and west shear walls to help improve the building's stability and increase the structure's mass and weight, which will also help minimize movement.

"The more weight we have on the building, the better," Marcus says.

SOM served as architect and engineer on One Manhattan West, a 995 ft office tower in New York City. Because this building is being erected above existing railroad tracks for Penn Station, the perimeter columns do not come down to ground. Instead, they slope back on a 60-degree angle into the building's concrete core, which must accommodate all gravity and lateral loads. This resulted in a structural aspect ratio of 16:1 that has to resist the wind loads of a much wider building above, explains Besjak.

Damping systems also play critical roles in controlling movement of these tall, slender towers. These include various types of tuned mass dampers, a liquid version of the same system, and new technologies as well.

For 432 Park Avenue, a traditional tuned mass damper system features two massive, suspended boxes made from steel and concrete, each weighing approximately 650 tons, says Marcus. The boxes move with the building but are restrained by shock-absorber systems. Because the dampers move "a little slower than the building is moving, that helps keep the building from moving too fast," Marcus explains.

One of the earliest uses of a liquid damping system for a superslender structure was in Highcliff, a residential tower constructed in Hong Kong in 2003 that featured an aspect ratio of roughly 17:1 or even 20:1, depending on where exactly the base measurement is taken, notes Ron Klemencic, P.E., S.E., F.ASCE, the chairman and chief executive officer of Magnusson Klemencic Associates (MKA), of Seattle. MKA was the structural engineer of Highcliff, which was erected on a steep hillside, a fact that explains the varying aspect ratios, Klemencic explains. The tower was designed by Hong Kong-based Dennis Lau & Ng Chun Man Architects & Engineers (HK) Ltd.

In addition to its high aspect ratio, Highcliff also featured one of the first uses of a perforated tube design made from concrete rather than steel as well as its tuned liquid mass damper—or sloshing damper—to help keep the building "from swaying too much in the wind," Klemencic explains. The building's overall rounded form—resembling two intersecting ellipsoids in plan—also helped reduce the effects of wind loads on the tall, slender structure, Klemencic adds.

Another MKA project, Chicago's soon-to-be-completed Vista Tower, will feature two liquid sloshing dampers, Klemencic says. The hotel/residential building project—designed by Chicago-based Studio Gang—involves a series of three progressively taller slender structures with a maximum height of 1,100 ft and a minimum width at the base of approximately 85 ft, leading to an aspect ratio of about 13:1, Klemencic notes. The dampers will be located at the roof of the tallest tower and at a level that is partially in the tallest tower and partially in the adjoining, next-tallest structure, Klemencic says.

Although the liquid damping system for Vista Tower will use technology similar to that from Highcliff, Vista will be a larger building and thus the damping system must be larger-perhaps three times larger, Klemencic estimates. "The amount of water you need to have these dampers work is a proportion of the weight of the building, so as the building gets bigger and weighs more, so goes the amount of water you need to control it," Klemencic explains.

One Bennett Park also features a tuned liquid damper system. Located at the building's roof, the damper consists of three stacked concrete tanks containing water and suspended steel paddles to restrict the flow of water as it sloshes back and forth—all the elements "tuned to certain frequencies to counteract the accelerations of the building under a wind event," notes Hamlin.

The overall damping system measures approximately 40 ft long by 30 ft wide by 30 ft tall and is structurally tied into the building's concrete core. Located atop the dampers is the building maintenance unit—a heavy piece of equipment used for washing the tower's windows. The additional weight of the maintenance unit was included in the mass properties of the building that were used to determine the structure's acceleration and thus the final tuning of the damper, Hamlin says.

A damping system was installed in SOM's 35 Hudson Yards mixed-use tower in New York City in part because the building is part of a larger development surrounded by other tall structures that will potentially affect the wind loads, says Besjak. "You get the winds coming from the river, and because of the flow, it actually caused our tower to increase its wind loads and its dynamic effects because of the configuration of towers around it," he explains.

Although the proximity of other buildings has always been a factor engineers had to consider, it's even more necessary for large, master-planned developments in which multiple buildings might all be completed at roughly the same time, Besjak notes. SOM was both architect and engineer for the 1,006 ft tall 35 Hudson Yards.

DeSimone Consulting Engineers installed a special tuned mass damping system known as a compound action damper at 220 Central Park South, a 950 ft tall tower in New York City. The building was designed by Robert A.M. Stern Architects, with SLCE Architects, of New York City and London, and features an 18:1 aspect ratio. Similar to a regular tuned mass damper, the compound-action system was designed to fit into a restricted space, says Stephen DeSimone. The 1,100-ton system involved two masses that were suspended from the ceiling but also raised on floor-level supports, "almost like two opposing dampers," explains DeSimone. "It was really efficient form a space standpoint."

To maximize the space for the residential units and optimize their views, the design of 220 Central Park South featured some unique structural solutions, including an offset core, megacolumns, and perimeter spandrel beams that obviated the need for internal shear walls in the residential portion of the building, DeSimone explains. All mechanical equipment was restricted to the first 200 ft of the building's height, and that lower portion was stiffened sufficiently so that "we almost created the foundations 200 ft up in the air!" DeSimone says.

To be certain that the solutions for controlling movement in tall, slender towers are actually solving the problem, DeSimone Consulting Engineers has begun monitoring the performance of select buildings. Although Stephen DeSimone would not identify which buildings were being monitored, he did say that the project began about a year ago and currently collects data from nine structures regarding building frequencies, damping, and accelerations.

"We're learning a lot about damping, inherent damping, stiffness, changes in stiffness, and how well did we get it right," DeSimone explains. "We are gleaning real-time performance data that is either confirming the assumptions we made in design or it's disproving some of those assumptions … the knowledge base that we have is really growing and giving us the opportunity to improve the designs as we move forward."

Looking ahead, some engineers see changes in store for the design and construction of tall and slender buildings, ranging from a possible oversaturation of the market to dramatic new technologies, including so-called "active" damping systems that could make it possible to design structures that are even taller and slimmer. Unlike current passive dampers, an active system would rely on a powered, mechanized unit that would respond to building motions and push the mass accordingly. "If the building moves to the left, the [active system] mass would push to the right," explains DeSimone, who adds that such systems exist but are not yet available at a large-enough scale.

A potential game changer is also in the works via a prestandard on performance-based wind design being developed by ASCE through its Structural Engineering Institute (SEI). The document was nearing publication at press time, explains Klemencic, who has worked on the project. When it becomes available, the prestandard will potentially "change everything," Klemencic declares, enabling engineers to design buildings that will "respond nonlinearly to wind demands"—something currently prohibited. "The benefit of that will be that we can design more efficient and more cost-effective buildings, especially in places with very high winds," Klemencic says.

For SOM's Besjak, the most important lesson to take away from the design and construction of tall, slender towers is the need for engineers to be involved in the project as early as possible. "This is not a 'sit back and wait for the architect to design' situation," Besjak explains. "It's paramount that the structural engineers be involved in the design process at an early stage!" That's because the design of tall, slender towers must be highly organized and even somewhat "simple" in terms of structural ideas, Besjak adds. "They have to be thoughtful in terms of what they are, or they won't make economic sense from a structural perspective or a construction perspective."

If the process becomes too undisciplined—Besjak's phrase of choice is "too willy-nilly"—then "these projects tend not to get built," he warns.

Robert L. Reid is senior editor and features manager of Civil Engineering.

© 2019 AMERICAN SOCIETY OF CIVIL ENGINEERS ALL RIGHTS RESERVED

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