If you put aside cost, planning and sanity how high can skyscrapers go? As the piling work for the tallest tower in the world, the 1,000-metre-plus Kingdom Tower in Jeddah, approaches completion, the world’s appetite for super-tall structures shows no signs of abating.
John Parker, Technical Director at WSP Group, the engineers behind The Shard in London, says that in reality there is no limit. While there are practical impediments, the technical ability to go beyond Saudi Arabia’s latest endeavour already exists.
Parker pointed to the feasibility of incredible proposals by NASA for a space elevator as evidence that former limits placed on engineering seem quaint nowadays.
“It is essentially a large weight way out in space, connected to the earth by a super-high-tensile cable,” said Parker of NASA’s concept. “The centrifugal effect of the weight keeps the cable tight, and lift cars climb into space without the need for rockets.”
He admitted that there are certain details that still need to be addressed, such as developing a material for the cable that is light and strong enough (even carbon fibre is not up to the task), protecting the cable from meteor damage, and working out how to get the repair man up there when the lift gets stuck. Nonetheless, the principles for such an undertaking are already in place.
“For down-to-earth structures the limits are set by more mundane things,” Parker said. “Taller buildings have to be wider at the base and it gets more and more difficult to be a reasonable distance from a window.”
This will effectively limit the height of skyscrapers in dense, urban centres, which will be constrained by tight, compact sites. In the vast expanse of Middle Eastern deserts, this is not an issue.
Wind, of course, causes buildings to sway. And the higher a building goes, the greater the effect. The accelerations that occur during this movement can become the governing structural design criterion for the lateral stability system if they are expected to be high enough to cause concern for the occupants of the building.
“Estimating the magnitude of the accelerations is complex,” said Meinhardt national leader – structures Rennie Darmanin. “The main factors that the design team needs to consider are the external shape of the building (i.e. it’s aerodynamic characteristics in the wind), its stiffness, mass and damping.”
The shape of the building has a significant impact on the loads that will be generated by wind. A shape with sharp edges causes more turbulence than a more streamlined form. On a narrow site, however, flexibility in the form of the building is more limited than on a larger site, so the design team has less scope to reduce acceleration by changing the building’s shape.
The stiffness of the building is defined primarily by the type of structural system chosen by the structural engineer and the materials used (i.e. concrete or steel).
“Mass and stiffness, however, can both be predicted reasonably accurately in the design phase, especially with the sophisticated building analysis computer programs available,” said Darmanin.
Taking people up a really tall building also needs lots of lifts.
Advances in electrical supply, reticulation and electric motor technology have enabled lifts to develop from speeds of around 0.5 metres per second 100 years ago to the modern elevators of today which, in the case of the Burj Khalifa in Dubai, can travel at 17 metres per second (or over 60km/h).
With increased lift speed comes improved ability to transport more people higher within buildings. But human physiology means lift speeds cannot continue to increase indefinitely.
Despite NASA’s space elevator concept, elevators in buildings have been limited to a height of approximately 500 metres, requiring transfers to reach the top of the current crop of supertalls (buildings standing 600 metres or higher) due to the weight of a conventional steel rope, its limited ability to bend and the amount of energy required to lift a car as the height of the building increases.
Kone, a leader in this field, earlier this year announced UltraRope – a combination of carbon fibre core and unique high-friction coating – which breaks these industry limits and enables future elevator travel heights of one kilometre – twice the distance currently feasible.
Evacuation in the case of a fire is another consideration.
Statistically, a significant proportion of fire fatalities and injuries occur while occupants are attempting to get out of a building, generally within paths of travel to the exit. When it comes to evacuating a tall building, events like the World Trade Centre have had an impact on the design for fire, although not always as expected. The question which has since arisen is: do you really need to get people out?
“Most people can evacuate moderate distances with relative ease. As the difficulty of egress increases, the effective mobility of occupants decreases,” said Meinhardt leader of fire engineering Denny Verghese. “Evacuation in tall buildings should therefore consider the limitations on the physical ability of its human occupants. It is well known these limitations are only increasing due to age and health related factors that affect the population of most large cities.”
A “defend in place” strategy is a possible solution. This provides a means to safely remain within the building during a fire. Defend in place strategies are common in most healthcare buildings, as they contain a large proportion of occupants having some level of mobility impairment.
Whatever the solution, options to evacuate or defend in place are essential for tall buildings and can aid in providing a sound basis for the performance of a building in the event of a fire.
For those wanting to go one storey higher, although there are significant challenges, the possibilities are there.