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Home , 111 West 57th Street, The Shard, Torre Mayor, Tuned mass damper

WEALTH CREATION THE SKY’S THE LIMIT

Tall buildings are rising in cities all over the world, at a rate and with a variety never seen before. Engineer and writer Hugh Ferguson talked to designer and -based structural engineer Kamran Moazami, about one of New York’s most recent – 432 – and the engineering challenges of designing tall buildings.

432 Park Avenue is one of the most recent additions to the skyline. The building stands at 426.5 metres (86 storeys) high. At about every 12th storey, the building has been left windowless and ‘open’ floors have been created, to reduce in windy conditions © Nicola Evans

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On most tall buildings, extensive wind tunnel testing is an integral part of the structural analysis and design

around five force-balance tests and vibration. This was solved backed up by several all-day by adding mass to the upper workshops to establish the levels, achieved by increasing optimum configuration, followed the typical 250-millimetre by a few aeroelastic tests on the thickness of the concrete floors final design. to 450 millimetres. Two 650 (Left): The square-shaped concrete core of the building is connected by ‘outriggers’ (double-storey concrete frames) to the exterior girder-column grid at roughly every First, the building had to tonne mass-tuned dampers 12th floor so that they act monolithically and increase the overall lateral stiffness. (Right): Two mass-tuned dampers were also added to the top of the building to control its movement. A tuned-mass damper is a mass that is supported by a pendulum arrangement and connected to the structure by means of springs and/or dampers be made as stiff as possible to were also added to the top of resist overall bending, while still the building: these are large In the heart of , World Trade Center twin towers its compact 28.5 metre-square an integral part of the structural leaving large, open areas on the steel weights supported by where land values are among in New York (1973). footprint, it has an aspect analysis and design. This floor plan for attractive living cables as pendulums and the highest in the world, stands Tall buildings continue to ratio (height/width) of 15:1, a may include: high-frequency spaces. This was achieved by connected laterally to the , one of New spring up throughout the world slenderness that would have force balance tests; multi- creating a 9 metre by 9 metre structure by viscous dampers, York City’s newest skyscrapers in response to many factors, been unthinkable a decade ago. degree of freedom aeroelastic concrete shear-wall core around which absorb the energy of the and a towering example of what such as increasing urbanisation, a This has helped the developer to modelling, which explores the the lift shafts, which formed moving weights and transfer new engineering techniques, desire to create iconic buildings, gain more usable space out of a independently acting factors that the building’s central spine. the associated forces into the combined with high-performance and the discovery that people comparatively small plot, and to influence the interaction between Less conventionally, the strong building structure, slowing materials, can achieve. across the world love grand give the residents unrivalled views inertial, elastic, and aerodynamic core was supplemented by a the building’s acceleration to New York was the birthplace views. Now, more than half of the of the Manhattan skyline. It also forces that occur when the framework of concrete columns acceptable limits. of , notably with world’s 20 tallest buildings are presented interesting challenges building is exposed to wind; wind and spandrel beams forming Early wind tunnel testing had the and Empire in China (with four elsewhere in for the building’s engineers. studies done at large scale on just the building envelope and shown that the building exhibited State Building in the early 1930s, the Far East, three in the USA and 432 Park Avenue had to be one section of the building at a avoiding the need for externally significant ‘vortex shedding’, both of which are steel-frame two in the Middle East). Before designed to transfer its own time; cladding pressure and wind mounted cladding. Cast-in-situ which is an oscillating airflow buildings of rectilinear shapes. 2000, two thirds of skyscrapers weight down to the foundations, studies to ensure that cladding floors span between the central phenomenon that can occur In the 1960s, the skyscraper were commercial: since then, to resist seismic and wind loads, panels, windows and fixings core and the perimeter frames. when wind flows past a building had something of a resurgence more than two-thirds have been and – most challenging of all will not be blown off in a gale; To ensure that the concrete at certain velocities. Vortices are with the introduction of tubes residential or mixed-use. with such a slender building – to pedestrian-level comfort testing, perimeter framing and interior formed at the back of the building, (usually in steel) for perimeter manage the movement of the to ensure downdraft wind on core respond monolithically to detaching periodically from either and interior columns and building under wind loading the building and any street-level structural demands, they were side. Each low-pressure vortex perimeter diagonal bracing, SLENDER AND so that no movement would wind tunneling does not create connected by massive double- tends to pull the building towards which greatly reduced the STABLE be detectable. All tall buildings discomfort for nearby pedestrians; storey concrete frames, known the centre of the vortex, so the weight of steel required and At 426.5 metres, 432 Park Avenue move in the wind, but occupants, and wind-induced aeroacoustic as ‘outriggers’, which were effect can be to induce vibration, hence the cost. Many buildings is one of the world’s 20 tallest particularly residents, do not want studies, to check and ensure that concealed in the plant rooms at particularly if the frequency of over 40 storeys that have been buildings. It is the second tallest to feel the sway. That meant wind will not cause any whistling about every 12th floor. vortex shedding approaches constructed since the 1960s use building in New York City and keeping the lateral acceleration or noise-generated issues. Despite its lateral stiffness, the resonant frequency of the a tube system adapted from the the third tallest in the USA. The of the building (its movement At the time, 432 Park Avenue the tower still tended to move structure. The solution at 432 structural engineering principles building is entirely residential in wind) below around 0.1 m/s2 was one of the most slender unacceptably in varying wind Park Avenue was ingenious and of Bangladeshi-American and currently stands as the (metre per second squared) in a buildings in the world, designed conditions, and the lateral- innovative: by leaving the building (Top): A rendering of 111 West . Its aspect ratio of 24:1 will make structural engineer Fazlur Khan. tallest residential tower in the ‘once-in-a-year’ gale. it the world’s most slender building. (Bottom): Shear walls run the full length by engineering consultant WSP. load-carrying system had windowless at precise levels of the east and west exteriors and have been thickened to as much as one This included the John Hancock Western Hemisphere. However, On most tall buildings, Therefore, wind-related testing to be ‘tuned’ to minimise and creating ‘open’ floors – a metre. Like 432 Park Avenue, it also has open floors that the wind will pass Center in , and the much more significantly, with extensive wind tunnel testing is was particularly rigorous, with displacement, acceleration technique comparable to making through to reduce vibration

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SAFER SKYSCRAPERS Skyscrapers are now among some of the safest buildings in the world, if only because they must be thoroughly engineered to THE ENGINEERING CHALLENGES OF TALL BUILDINGS the finest detail, and not just for structural safety. For example, the fixings of every external cladding panel should allow for Torre Mayor sufficient movement to accommodate the building’s sway under Completed in 2012, the Shard is London and the UK’s tallest The 225-metre high, 55-storey Torre Mayor in was anticipated conditions. building at 310 metres and 95 storeys (‘Building the Shard’ Ingenia Latin America’s tallest building when it opened in 2003 and is now Most modern tall buildings have a fire strategy, which 52). It is shaped as an irregular pyramid with highly complex Mexico’s fourth tallest. The design priority for this building was covers such issues as means of escape, detection and alarm, geometry. The Shard is a modern mixed-use tall building with retail seismic design, as it stands in the lakebed area where the heaviest holes in the sail of a boat – the maintenance-free, the exterior of fire suppression, smoke control, firefighter access and facilities, at the base, offices in the lower levels, a hotel in the middle and damage from the catastrophic Michoacán 1985 was wind loading was reduced and the building required access for compartmentation, and internal and external spread of fire. Most residential near the top. A strong central concrete core and a hat recorded. Designed to resist an 8.5 Richter earthquake, Torre Mayor so was the vortex shedding. glass cleaning and replacement, high-rises now include dedicated fire stairs and some have lifts truss that engages all perimeter columns form the spine of the has an arrangement of 96 viscous fluid dampers (with technology Conveniently, these levels could which will be done using a that can be used for fire escape, which require special details such building, and the remainder varies with use: structural steel was the borrowed from the shock-absorbers of a car) attached to the be designed to coincide with the telescopic building maintenance as pressurised lobbies where people can wait for the lifts and an optimum solution for the offices, where deep steel beams could diagonal cross-bracing on the perimeter of the building, which plant rooms, and to some extent unit (BMU) mounted on the roof. independent cooling system for the lift motor room. The main fire allow plenty of space for the extensive services required. For the safely absorb the earthquake’s energy. A 7.6 Richter earthquake in the outriggers, at about every element of tall building design is ensuring that compartmentation hotel and flats, where acoustic separation was more important and 2003 did no damage: indeed, the occupants did not even notice 12th floor. is maintained until the fire is exhausted or controlled. Spread where spans were shorter owing to the tapering of the building, that a tremor had taken place. The structure also responded as Although these ‘open’ floors EVEN TALLER of fire on the exterior of a building is usually mitigated by concrete floors were preferred, which were thinner so that two expected when the 7.1-magnitude earthquake of Axochiapan take up valuable floor space, this TOWERS choosing insulation materials of limited combustibility, provision extra storeys could be added within the same overall building occured in September 2017. is more than compensated for Now, just three blocks away, of adequate fire-stopping, cavity barriers around openings, and height. The tower then reverts to steel for the viewing gallery and by the additional stability that 432 Park Avenue’s successor is main compartmentation lines. For example, in August 2016, a the spire. 22 allowed the height of the building nearing completion. Not content fire burned dramatically for two hours on the outside of ’s When it is completed in 2019, will be London’s to be increased to 86 storeys. with an aspect ratio of 15:1, 84-storey , where some of the cladding systems were Newfoundland second tallest building at 278 metres with 62 floors. This will be even The concrete required for has a ratio not of limited combustibility and fire-stopping was inadequate. The 220-metre high, 60-floor Newfoundland residential tower in more of a ’vertical village’ including retail, restaurants, an auditorium, the building was no ordinary of 24:1, which will make it the Despite this, other elements of the fire strategy worked well, , London, straddles the Jubilee Line tube tunnels, various leisure facilities, including a climbing wall on the 25th floor, concrete. For the lower sections world’s most slender building. particularly suppression that ensured safe escape for occupants, which are close to the surface at the tower’s location. This required and offices. It will be the first building in the UK to be accredited for carrying larger vertical and lateral At 438 metres, it is slightly taller and consequently there were no casualties. careful piling between and beside the tunnels, as well as a major standards to improve building users’ health and wellbeing. The main loads, it had to have a specified than its neighbour and shares load-transfer structure to connect the building’s loads with the challenge here was to address what had gone before: a previous compressive strength of more many features: both are high- giant cantilevered concrete outstanding views, and locally piles. However, to avoid disturbance to the tunnels, it was necessary scheme was abandoned in 2012 because of the recession, with than 100 MPa (‘high strength’ end residential buildings, both I-beam standing on its end. available high-strength coarse to keep the weight of the building to a minimum. The solution was just the foundations and seven storeys of the core constructed. The concrete is considered to have a use high-strength concrete, and The tower is slightly tapered, aggregates needed for making a ‘diagrid’ of diagonally placed steel members on the outside of the core was deconstructed, but for the new tower to be economical, a compressive strength of 50 MPa or both have a and the external walls will be ultra-high-strength concrete (a building creating a diamond pattern, and the use of post-tensioned way had to be found to place an entirely different building on the more). All structural concrete was on the top and gaps allowing clad in terracotta and bronze similar compressive strength concrete floor slabs connecting the diagrid to the concrete existing foundations (including London’s deepest piles). This required designed for enhanced durability the wind to pass through to rather than architecturally in London, for example, would core. The diagrid was so light and strong that the thickness (and elaborate analysis and design at the base of the building to transfer by minimising air content and the minimise vortex shedding. exposed concrete. Even more require imported aggregate, therefore the weight) of the shear walls in the concrete core could the loads to the foundations such that the full strength of the water/cementitious materials ratio, The main difference is that slender buildings are now being and is therefore a much be reduced to just 300 millimetres. existing piles was mobilised, but no individual pile was overloaded. and with a higher-than-normal the architect and developer planned, although an aspect more expensive concrete). In modulus of elasticity to minimise for the latter wanted clear, ratio of 24:1 is approaching the different parts of the globe, the becoming self-contained cities in constraints of providing fast and deformation. To aid placing and to unobstructed views northwards limit of what is currently feasible. parameters or constraints are the sky with a mix of residences, efficient vertical transportation, BIOGRAPHY improve the finished appearance, over and south likely to be different and so too offices, hotels, restaurants and and economic viability: larger Kamran Moazami is Managing Director, Property and Buildings, it had to be able to be pumped to towards . are the solutions. public areas. buildings cost more, take longer at WSP. He studied structural engineering at Columbia University great heights. Since there was to The solution was a concrete EVOLUTION OF The original rectilinear steel As ever-stronger materials to build, and take longer to in New York. After obtaining his master’s degree, he joined New be no cladding to cover potential core, linked to two concrete SKYSCRAPERS buildings most usually associated are developed, there is no let, so developers need even York skyscraper firm, Cantor Seinuk, where he worked on several blemishes on the exterior of the shear walls (a structural system The buildings at 432 Park with skyscrapers have largely structural limit to how high deeper pockets. However, with high-rise buildings. In 1996, he relocated to the UK to head building, architectural exposed composed of braced panels) Avenue and 111 West 57th been succeeded by concrete or skyscrapers could be built. increased urbanisation, a public up the company’s London office. Later on, Cantor Seinuk was concrete was specified in an running the height of the Street are responding to the composite concrete-and-steel, The only foreseeable limits are appetite for tall buildings and the acquired by London-based firm WSP. attractive white colour and building on the east and west specific parameters of the New and the equally ubiquitous likely to be the larger footprints ingenuity of engineers, the surge Hugh Ferguson also talked to Nick Offer, Head of London Building was placed with near-perfect sides and allowing clear views York City market, including office buildings have given required at ground level and in skyscrapers worldwide is set to Services for WSP. workmanship. Although the north and south, so that the strong demand for high-end way to varieties of mixed-use the transport links to service the increase still further – and some exterior concrete should be overall structure resembles a residential properties with towers: modern skyscrapers are many occupants, for example the of them will be even taller. All projects mentioned in this article have been structurally designed by WSP

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