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"  The 2010 completion of the absurdly tall Burj Khalifa in Dubai has not curbed humanity&amp;#039;s urge to reach even higher. This January, construction began in Jeddah, Saudi Arabia, on the next building likely to wear the crown of world&amp;#039;s tallest. Known as the Kingdom Tower, it will soar past the 828-meter-tall Burj all the way to the full kilometer mark (3281 feet).  Even though Kingdom Tower is not scheduled to open until 2017, we already know its reign probably won&amp;#039;t last long. An Azerbaijani businessman, for instance, recently announced his goal of erecting a 1050-meter-tall Azerbaijan Tower later this decade. Other, even loftier projects will emerge. Adrian Smith + Gordon Gill, the firm behind the Kingdom Tower, has already begun modeling a prospective mile-high tower.  It takes some seriously smart engineering to build so high. Here are the key challenges for the Kingdom Tower as its builders go for the record.  Gobs and Gobs of Money Antony Wood, executive director of the Council on Tall Buildings and Urban Habitat, says it&amp;#039;s really all about the cash. &amp;quot;I think the only barrier to building a mile-high tower and beyond\u2014we&amp;#039;re not talking to the moon or anything, but a mile or mile-and-a-half high\u2014[is] financial. There are many technical considerations in building buildings a mile high and beyond, but I believe we can overcome them if the checkbook is deep enough.&amp;quot; Yes, it takes a pile of money to build a modern super-skyscraper. But the total cost is not even as steep as, say, a giant new football stadium. The Burj Khalifa, for instance, cost $1.5 billion, the Kingdom Tower will cost $1.2 billion and the drawing-board Azerbaijan Tower has a cool $2 billion price tag. Those tabs are on par with the new Dallas Cowboys stadium ($1.3 billion) and the new home of the New York Giants and Jets, MetLife Stadium ($1.6 billion). Notably, however, all these skyscraper projects serve as a sort of &amp;quot;jewel in the crown&amp;quot; for broader development projects costing many billions more. The amount of capital investment required can put a real strain on clients. The Burj Khalifa, for its part, struggled initially to find buyers for its 900 apartments, though property values have risen this year.  Going Up Although we all want the view from the mountaintop, actually getting to the peak can be a real pain. The same goes for skyscrapers reaching higher and higher into the sky. Beyond finances, Wood says, the biggest barrier to building taller structures is elevator tech. &amp;quot;These buildings are getting so tall and travel distances and travel times are becoming so great that they&amp;#039;re off-putting for people to live or work in the building,&amp;quot; he says. The stumbling blocks are elevator-shaft height and speed limits. Today, elevator shafts cannot exceed 600 meters or so because the elevator car&amp;#039;s cable becomes too heavy. To get around this issue, the Burj Khalifa uses, and the Kingdom Tower will use, &amp;quot;sky lobbies&amp;quot;\u2014elevator transfer junctions where people board additional cars to reach the uppermost floors. Mile-high towers, however, would require at least three transfers\u2014not an ideal solution.      Speed is capped for the sake of passenger comfort. Dedicated observatory-level cars in the Kingdom Tower will cruise upward at a maximum speed of 10 meters per second, same as in the Burj Khalifa, for a short in-elevator commute of under 2 minutes.  Faster cars are coming: In 2014, Mitsubishi plans to deploy in China&amp;#039;s Shanghai Tower the fastest elevators in the world, which will zip along at 18 meters per second. An air-pressure control system will prevent riders&amp;#039; ears from popping during trips, while an &amp;quot;active roller guide&amp;quot; will dampen vibrations. Nevertheless, the penthouse suites in a futuristic mile-high tower would still take considerable time to reach. &amp;quot;People are not going to want to travel five minutes to get to their front door,&amp;quot; Wood says.  A smart though far-off solution, Wood says, would be to replace cabled elevators with electromagnetically propelled cars. Occupants could be strapped in, rather like on theme park rides, and whisked to their in-building destinations through shafts running horizontally as well as vertically. &amp;quot;That would be the single biggest breakthrough in tall buildings,&amp;quot; Wood says. &amp;quot;The minute you have elevators go horizontal, you can reduce the number of shafts in the building. You could design one with just two shafts and have 80 cars working in a system where they pass each other.&amp;quot; Stable and Aerodynamic Record-setting tall buildings must possess enough lateral stiffness to withstand the powerful winds at thousands of feet in altitude and to ride out earthquakes as well.  So, to an extent, building higher means building wider. Robert Sinn of Thornton Tomasetti, the structural engineering consultant on the Kingdom Tower project, says the footprint of the building helps determine how far the structure can extend vertically. &amp;quot;The so-called aspect ratio, slenderness ratio, or height-to-base dimension ratio is a simple way to gauge potential for resistance to overturning forces from lateral wind and seismic loads,&amp;quot; he says. In general, he says, the wider the base, the more resilient the building will be. But thanks to modern engineering, remarkable thinness can be safe. The Burj Khalifa&amp;#039;s aspect ratio is close to 9:1; Kingdom Tower&amp;#039;s will exceed 10:1.  Most super-tall buildings nowadays use steel\u2013concrete composites to achieve strength while meeting interior spacing demands. The challenge that puts construction firms to the test is getting those heavy materials hundreds of meters into the sky as the building grows, Wood says. The Kingdom Tower&amp;#039;s innovative solution is to use a second set of concrete pumps at around 600 meters, Sinn says. &amp;quot;Forming the upper-level walls, placing reinforcing and concrete in the upper levels, all under high-wind conditions, will be one of the biggest challenges for the construction of Kingdom Tower,&amp;quot; he says.  The Burj Khalifa under construction in 2008. Kingdom Tower is a further refinement on the idea of tapering a building to the top so it acts less like a sail. &amp;quot;The continually tapering vertical profile for Kingdom Tower has proved to be very beneficial in terms of the performance under wind,&amp;quot; Sinn says. In addition, the Saudi tower&amp;#039;s tapering prevents wind vortices from forming and causing that disconcertingly noticeable swaying. Extensive wind tunnel tests of scaled-tower models and analysis of high-altitude wind-speed data from balloons aided in the design process, Sinn says.  Also like the Burj, the Kingdom Tower has a tripedal shape to provide stability along with plenty of coveted window views. &amp;quot;Having the three-part base with wings or legs, as we did with Burj and will have with Kingdom Tower, allows us to spread out the base of the building,&amp;quot; says Adrian Smith, who designed the Burj Khalifa at a previous firm and the Kingdom Tower at the new firm bearing his name. &amp;quot;That does help make the building more stable while maintaining the rather shallow lease span\u2014the distance from the core to the window\u2014that you want in a residential building.&amp;quot; A Deep Foundation You don&amp;#039;t necessarily need perfect geology to build an enormous skyscraper. &amp;quot;With Chicago, they built a high-rise city on shifting mud,&amp;quot; Wood says. &amp;quot;They didn&amp;#039;t have the bedrock of New York, yet they built the world&amp;#039;s tallest building,&amp;quot; he says, a reference to the Sears Tower (now the Willis Tower) that trumped the twin towers of the World Trade Center to become the world&amp;#039;s tallest building in 1973.  But the ground can impose limits. Soil testing in Jiddah showed that the underlying layers are made of weak, porous limestone and sandstone. As a result, the ambition of the Kingdom Tower was scaled back from a mile-high tower to a kilometer-high tower (0.62 miles). To reach even that height, the Kingdom Tower will require a particularly deep foundation with piles sunk down 105 meters. &amp;quot;Very few towers in the Middle East have foundation elements extending to such depths,&amp;quot; Sinn says. A huge slab of concrete called a raft foundation will also undergird the Kingdom Tower. Inevitably, the 900,000-ton behemoth will settle somewhat. But extensive testing and modeling of the piles and raft will ensure that the settling happens evenly, the builders say.  The creation of a skyscraper&amp;#039;s foundation has a major tie-in to economics as well. &amp;quot;Traditionally the way a tall building is built, you build a bloody big hole, sink the caissons [piles], build up the columns, and two years after you&amp;#039;ve started on the site the building starts to appear out of the ground,&amp;quot; Wood says. &amp;quot;The problem with that is, you can&amp;#039;t start the tower until you&amp;#039;ve finished the site.&amp;quot; To shave several months off that delay and save a bunch of money, many builders now go with what is known as top-down construction. &amp;quot;You start to build the tower vertically, and then while doing that you build your basement by excavating,&amp;quot; Wood says.  A Need to Go Higher? With continuing rapid urbanization, there will be an argument for building up. According to the United Nations, around 200,000 people are moving into cities daily. &amp;quot;Our cities need to get denser and taller,&amp;quot; Wood says. And it&amp;#039;s not just a matter of &amp;quot;up.&amp;quot; Wood thinks the cities of the future must become multilevel. Sky bridges could connect towers and provide new living and commercial space while sharing critical services of safety and fire control. &amp;quot;If you look at cities made in the sci-fi realm, they often show multiple connections at height,&amp;quot; Wood says.  Skyscrapers are something more than a business proposition; they&amp;#039;re also landmarks. The Kingdom Tower on its own does not necessarily make commercial sense, but it is intended to have a galvanizing effect on a planned local development complex costing $20 billion. &amp;quot;Did the space program make economic sense?&amp;quot; Wood asks. &amp;quot;No, it was driven by a different set of factors, and a mile-high building would be driven by a different set of factors.&amp;quot; We may not need supertall buildings to accommodate all the new city dwellers, and they might not seem like sound investments\u2014or even practical\u2014on their own. But it&amp;#039;d be a bad bet to say humans won&amp;#039;t keep building higher. Skyscrapers grow taller and taller thanks to the undeniable motivation to be No. 1. Someone, somewhere who has the means will surely want to continue the game of man-made structural leapfrogging. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/technology/design/a7807/taller-how-future-skyscrapers-will-beat-the-burj-khalifa-8893983/>