Super-slender towers stretch structural engineering innovation to new heights.
By Gustavo J. Oliveira, P.E.
We are at the crossroads of two significant trends that are driving skyscrapers to a new golden age — urbanization and climate change. Driven by rapid population growth in cities worldwide and a need to build more sustainably, cities such as New York are increasingly relying on super-tall, super-slender towers to provide luxury living and working space. These super-slender skyscrapers — with a base-to-height slenderness ratio of 1:6 or more — stretch world-class architects’ imaginations and, in turn, structural engineers’ capabilities for innovation.
These structures require significant innovation in structural engineering in order to mitigate the impact of wind pressure and turbulence, ensure stability, and enable development of new construction techniques to streamline construction and minimize costs. Successful project delivery also requires collaboration with mechanical, electrical, and plumbing (MEP) engineers in the delivery of services that enable these commercial/residential towers to function efficiently and sustainably.
Life at the top
At building heights of 1,000 feet and greater, the wind flow is similar to that experienced by an aircraft. Structural engineers’ challenge is to reduce the building’s movement to a level that is tolerable and, in most cases, imperceptible to its occupants. This challenge increases in proportion to the height of the building. This is particularly true of a super-slender tower with a luxury residential component, where the criteria for occupant comfort are more stringent than for a commercial building.
In a building with a residential component, structural engineers must reduce acceleration without many of the traditional tools at their disposal. An office tower’s concrete core is a traditional source of stiffness, but this core is much smaller in a residential building. Maximizing usable space is even more important in such slender towers, and unobstructed views are a key selling point for residential units.
Solutions include adjusting the shape of a building to make it more aerodynamic, for example, by introducing openings to allow the wind to pass through or by adding curves or angles at critical locations along the façades to minimize the vortex shedding response that may cause high accelerations. Structural engineers must work closely with architects to refine the shape of the building based on a detailed wind load analysis. In addition, the use of high-strength concrete can improve the building’s stiffness without obstructing the layout.
Very tall and/or slender buildings also may require additional mechanisms such as a damper in the upper floors of the building to mitigate the discomfort that acceleration, vibration, and sway may cause on its occupants, as well as to improve structural performance. Dampers comprising a large mass of steel or concrete are called Tuned Mass Dampers (TMD), while those consisting of contained water or other fluids are called Liquid Sloshing Dampers (LSD). Structural engineers seek to minimize material to reduce both the cost and impact of the dampers on leasable/sellable space. High-rise buildings also have the tendency to channel strong winds down the length of their façades to the ground, potentially affecting the comfort and safety of people on sidewalks and in plazas.
Early in the design process, structural engineers normally use wind modeling to identify all of these potential issues. By combining 3D computation fluid dynamics modeling to predict the air flows around a building with long-term historical wind data, followed by statistical analysis to determine whether future conditions will exceed accepted thresholds, structural engineers can collaborate with architects in refining the building’s design to improve the structural performance against wind loads.
Depending on the location of the building, wind loads and/or seismic studies are critically important to examine the complex load effects on a super-slender tower. Components of a wind-load study, for example, may include a wind-induced motion and structural load review to provide structural engineers with preliminary estimates of the overall structural wind loads. A building motion analysis may be required to assure that the structural design provides for human comfort on the highest occupied floor. Using a physical model of the proposed building and the area surrounding the site, wind-tunnel testing can be performed to assess the effects of proximity buildings and terrain.
As construction material technology has improved throughout the years and construction techniques have also been enhanced, the industry has seen slenderness ratios exceeding 1:15 or perhaps 1:20. High-strength materials, such as f ‘c = 14 ksi concrete and Grade 97 steel reinforcement, also allow for smaller sizes on structural elements, capable of spanning larger dimensions and of supporting greater loads.
Collaboration with MEP engineers
Although the focus of this article is on the structural engineering challenges of super-slender towers, one cannot underestimate the role of MEP engineering in the delivery of services that enable these commercial/residential towers to function efficiently and sustainably. Through collaboration and implementation of an integrated approach, structural and MEP engineers can minimize the impact of these projects on the environment while optimizing the overall design and ultimate efficiency to maximize the economic benefits.
It is critically important for MEP engineers to consider the overall emissions and resource consumption over the whole lifespan of a building, and to prioritize strategies such as designing low-carbon building services — in particular, energy-efficient MEP systems — to optimize building performance and reduce operational costs. Similarly, innovative design of both structural and MEP systems can enable the use of construction techniques that streamline building construction and reduce cost for the owner.
Case study: 53W53 tower
The 53W53 tower (www.53w53.com), which is currently under construction on a narrow, 17,000-square-foot site at 53 West 53rd St. directly west of the Museum of Modern Art and former Folk Art Museum, epitomizes the global aesthetics and market influences that demand high design and super-slender skyscrapers.
With a slenderness ratio of 1:12, the 1,050-foot, 76-story reinforced cast-in-place (CIP) concrete tower combines luxury condominium residences and museum gallery spaces. Architect Jean Nouvel’s vision for 53W53 — a super-slender tower comprising a triangular silhouette, with its north and south façades gradually sloping away from 54th and 53rd streets at different angles — has its major characteristic on the aesthetic elements asymmetrically/randomly expressed on the façades in a diagonal pattern called diagrids.
The architect’s vision drove the WSP | Parsons Brinkerhoff structural engineering team to adapt an innovative structural system that previously had not been used in a structure of this scale. Making use of the aesthetic elements articulated on the exterior envelope, a reinforced concrete diagrid system comprised of vertical columns, diagonal elements, and horizontal spandrel beams was implemented at the perimeter of the building. These elements, coupled with concrete shear wall cores surrounding the elevator shaft and egress stairs, provide the lateral stability system of the building.
Similarly, three floors of concrete belts or outrigger walls were located at mid-height of the structure (levels 35, 36, and 37) to efficiently distribute stresses from the perimeter diagrid system to the internal shear wall cores. These floors house the mechanical spaces normally required on high-rise buildings to allocate and properly distribute MEP services throughout the building.
A series of structural transfers at floors 6 and 7 use steel trusses and concrete girders to pick-up the superstructure interior concrete columns and the east shear wall core around the egress stairs, providing an ample open area free of supporting elements for ±50,000 square feet of additional museum exhibition space on floors 2, 4, and 5 of the new building.
A structural wind load study was performed by Rowan Williams Davies and Irwin Inc. (RWDI) to determine in greater detail the complex wind load effects of 53W53. Components of the study included wind-tunnel testing to assess the effects of proximity buildings and terrain on 53W53 on a physical model of the proposed building in this area of New York City. As a result, structural engineers incorporated thicker concrete floors and a steel TMD at the 74th level to bring acceleration parameters to comfort levels defined by the industry.
From inception, one of the principal structural concerns was to prove the constructability of the perimeter diagrid system comprised of multiple reinforced CIP concrete elements converging into a singular point. The team designed and tested a mockup of a steel core — a steel plate assembly with welded mechanical couplers that attach to the rebar of the concrete members — to accommodate the irregular geometry of the converging diagrids and improve rebar congestion within the concrete node. Prefabrication of the diagrid/steel node system was the key to the feasibility analysis of this high-strength reinforced concrete tower.
Due to the building’s triangular silhouette, each floor of the tower is unique, and extensive offsets of the building services occur at every floor. The geometry of the building required extensive coordination among the design disciplines, which was facilitated with the use of building information modeling.
Each super-slender tower project is unique and associated structural challenges are formidable. Structural engineering innovations help developers make the most of limited real estate in urban areas and capitalize on small and constricted sites to create desirable commercial and residential properties with magnificent views and high market values.
Through innovative structural engineering combined with use of advanced materials, the form and function of super-slender towers can be perfectly adapted to the various needs of the cities in which they stand and the people who use them.
Gustavo J. Oliveira, P.E., is vice president, Building Structures, at WSP | Parsons Brinckerhoff (www.wsp-pb.com) in New York.