London's Hybrid Skyscraper

Upon its completion in May 2012, the 1,017 foot tall skyscraper Shard London Bridge will be the tallest building in the European Union—but its unmissable presence on the London skyline has been felt for over a year. Since the completion of its 804 foot, 72 story concrete core in early 2011, the Shard has been the tallest building in London. In a city, country, and continent not famed for skyscrapers, the Shard more than stands out.

Observant Londoners have watched as glass facades have crept up around the core over the past weeks and months. It’s impossible to look at the Shard without extrapolating its lines upwards to a point, completing the pyramidal form in the mind’s eye (crick in the neck notwithstanding). But mock-ups of the completed tower show a pinnacle characterized by a fragmented crown of glassy splinters, not a neat pyramid. Architect Renzo Piano, who conceived the Shard, has compared its shape to “a 16th century pinnacle or the mast of a very tall ship.” But “Shard” is the name that stuck, a name reportedly coined by Piano after criticism from the group English Heritage that his design resembled a “shard of glass.”

With pressure on designers to prove the environmental credentials of their high-rise buildings and to address the safety concerns of the post-9/11 era, we asked the engineers behind the Shard to tell us how they plan to keep the Shard lean, green, and above all vertical—and why skyscrapers are needed in the 21st century.

A skeleton of concrete and steel

Kamran Moazami, Head of Structures at WSP Group, describes the Shard as being a hybrid structure, necessitated by its various intended uses. Unlike most skyscrapers, the Shard is no mere corporate HQ or office block. Certainly, above the ground floor lobby sit 27 floors of premium office space. But above that come three floors of restaurants and a 19 story hotel (with spa to boot). Then there are 13 stories of high-spec residential apartments with views of the city only available to people wondering how to dispose of an extra £20 million or so. Uppermost are the observation decks and the spire.

On top of a reinforced concrete base come the steel-frame office levels. Steel-frame maximizes the possible spans that can be achieved, minimizing supporting columns which eat up valuable space (a problem that multiplies in a high-rise building).

For the hotel bedrooms and residential areas above, floor plates are formed from post-tensioned concrete, which is a means of overcoming concrete’s inherent weakness in tension by using steel tendons made tense with hydraulic jacks. It’s an extremely space-efficient technology that allows the maximum number of floors to be packed, sardine-style, into the available space. And concrete absorbs noise, which is handy if you have Patrick Bateman upstairs blasting Huey Lewis and the News at all hours.

At the uppermost levels, the construction is all steel, incorporating what is called a “hat truss” system that binds together the building’s perimeter columns like a rope binding the top of a teepee.

All of this sits upon 164 ft-deep foundations comprised of 3 ft-wide piles. That’s no mean feat when you consider the builders had to get through the foundations of Southwark Towers (the site’s previous, now long-demolished occupant) and numerous abandoned stairwells, vents, and shafts of the London Underground—all while keeping vibrations to absolute minimum in an otherwise fully functioning city hub.

But if it only had the support systems described above, the Shard would fold over in the first brisk westerly wind. As Moazami puts it, “Every building is a cantilever.” In the tallest high-rise, the lateral forces exerted by wind (and sometimes by seismic activity) can eclipse the vertical load of the building’s own weight.

It’s the Shard’s mighty concrete core that withstands these lateral loads, not to mention the torsional (twisting) loads that the building will also undergo. This concrete spinal column houses a number of essential systems such as elevators, electrical cables, water mains, janitors’ cupboards, and often (though tragically not in the case of the former World Trade Center) escape stairs.

In essence, Moazami says, good structural design is about “placing material at optimum locations. Every shape has its own opportunities. You need to find those opportunities.” For the Shard, that meant discovering at which floor supporting columns could be discontinued because of the reduced area of the floor plate above them. The materials may not all be cutting edge, but the increasingly sophisticated computer software used to test the limits of those materials allows audacious designs and configurations of materials that not so long ago might have been inconceivable.

Simulations and wind tunnels: the post-9/11 skyscraper

A five-minute Googling on skyscraper design will teach you that engineers can’t be absolutely certain that their building can stand up beneath every load scenario. Failure of the building’s structural integrity is always possible.

“That’s definitely not true,” says Moazami. “You make damn sure that if you design a building, it works. And not only that it works, but at least by a factor of 1.8 to 2 times the loads to eliminate any risk whatsoever.”

Mark O’Connor, head of structural analysis and design at WSP, puts it this way: “There are obviously some areas of the Shard that go outside the envelope of normal design. That means we have to spend far more time looking at those areas and we definitely don’t under-engineer them. They’re over-engineered if anything.”

To make damn sure, detailed computer models are built and simulations run of every conceivable circumstance. The day-to-day structural modeling and analysis is carried out with ETABS and SAP2000, but since 9/11, more complex tools have been employed.

“We are lucky enough to be working on projects like 1 and 7 World Trade Center, the latter being the first project on Ground Zero,” Moazami explains. “Our clients came to us and said, ‘Look, you’ve got to do something special here.’”

To make that happen, O’Connor says, “We use sophisticated finite element analysis and advanced nonlinear dynamic analysis. It’s the same software car designers use except we’re modeling steel and concrete, and how they interact.”

Whereas car designers use non-linear dynamic analysis to test crumple zones, a structural engineer might use such technology to test the performance of their design in the case of “accidental or extreme events,” as O’Connor puts it—which might include anything from earthquakes to car bombs.

Even in the most expensive apartments, people don’t like to feel their homes moving around in the air. The Shard design underwent wind tunnel analysis to understand its likely wind-induced movement. The Shard’s “acceleration” (as structural engineers somewhat unnervingly put it) was managed by placing additional mass at the building’s upper levels. Even so, in high wind the Shard is intended to move by up to 20 inches. But it’s not movement alone that’s problem. Moazami points out that “the biggest issue is making sure the movement can be taken by the components, so the cladding can rock, that the partitions can move.”

O’Connor explains that so-called “1 in 50 year winds” are theoretical possibilities produced by wind-loading specialists and that in reality, such winds have probably never been recorded in the United Kingdom. (Apparently, there are wind-loading specialists.) “We add a factor of safety on to the materials as well,” he adds. “All in all we’re probably talking about withstanding a 1 in 500-year event, really.”

In any case, with high winds it’s not so much about making sure the building doesn’t fall down as making sure that nothing falls off.

In reality, the most likely unwelcome scenario in a tall building is fire, like the one on the 51st floor of the Empire State Building in 1990. The fire compartments formed by that building’s concrete structure prevented the fire from spreading, and it’s an approach still employed in skyscraper design today. “Compartmentation is an important part of the strategy” says O’Connor. “You try to keep the fire on the floor of origin, and once it’s burnt out, it’s burnt out. It’s handled structurally because we design fire protection systems to enable that compartmentation.”

But fire safety design has advanced, partly in response to 9/11. As Moazami puts it, “Before 9/11, all that was done was to make sure every element was 2- or 3-hour rated in case of fire. But designers didn’t really look at the overall behavior of the structure to see, if there’s a fire and you’re putting a 2-hour rating on certain elements, whether those elements restrain the movement of others, which actually causes more damage. You want the structure to breathe.”

Understandably, structural engineers liaise with fire departments to ensure their designs meet with approval. Neither side wants a McQueen-versus-Newman-style confrontation further down the line. In the end, the engineers deeply believe in the safety of their structures.

“If there’s a big event, go to a tall building because they’re safer than any other type of building,” says Moazami. “I tell everybody, if you want to be safe in a hurricane go to a tall building, because it’s designed for it. If you look at Miami, it’s always the two or three-story buildings where the roof comes off. If there is a bomb threat, go to a tall building.”

http://arstechnica.com/gadgets/news/2011/12/the-shards-bleeding-edge-anatomy-of-a-21st-century-skyscraper.ars

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