Sky the limit for tall buildings
Although concrete’s pre-eminence in tall structures will not be threatened for some time, that isn’t constraining research into new materials. Guy Richards reports.
When complete, perhaps as soon as 2019, the Kingdom Tower in Jeddah, Saudi Arabia, will be the world’s first kilometre-high skyscraper. It will beat the current record holder, Dubai’s Burj Khalifa, which opened in early 2010, by about 180m, and will mark a momentous threshold in a global construction boom that has seen the number of supertall buildings (300m or higher) triple since 2007.
Using the right structural materials is key to building such tall structures, and the science behind them is constantly advancing. But according to industry experts, it is not so much a case of developing entirely new and innovative materials, more one of tailoring the properties of those already in use to address the specific challenges. The materials of choice – concrete and, to a lesser extent, steel – are therefore unlikely to be superseded for the foreseeable future.
Concrete’s low cost, high elastic modulus and high damping capabilities make it the material of choice for supertall buildings, says Peter Irwin, Senior Executive Consultant at engineering firm RWDI.
‘When designing a tall building, exotic materials with a high strength-to-weight ratio don’t necessarily have advantages over traditional materials such as concrete or steel,’ he says. ‘Concrete structures tend to be relatively heavy, and this helps reduce the building’s dynamic response to wind.’ This greater mass also means a tall concrete building can be thinner than a steel one, still have the same resistance to wind forces and, unlike steel, doesn’t need fireproofing.
Modern concrete, however, bears little resemblance to the traditional mixture of cement, stone and water – it is produced using various recipes of chemicals and other ingredients. As Leonard Joseph, Principal at engineering design consultancy Thornton Tomasetti, explains, ‘[Modern concrete is based on] admixtures – chemicals or materials that are not cement themselves – and pozzolans, [silicatebased] materials that are cement-like or cement enhancers but not cement themselves.
‘Admixtures known as superplasticisers have been key to providing high-strength concrete that is still economical and workable,’ he continues. ‘They make the water in the concrete act wetter by separating cement particles, so the mixture flows more freely.’ This is a crucial property when pumping concrete to a height of several hundred metres.
There are also admixtures for reducing shrinkage, he says, to lessen the amount concrete shortens over time. ‘All concrete shrinks, but this can be a particular concern on supertall towers because a small amount of shrinkage per foot can add up to 1,000 feet or more, affecting floor elevations, floor levelness, mechanical systems, cladding panels and more,’ he explains.
Pozzolans, meanwhile, address several high-strength concrete issues – in particular fly ash and microsilica. ‘First, fly ash does this by reacting with compounds in conventional cement to create more glue than may otherwise develop,’ Joseph says. ‘It is also a partial substitute for cement, lowering cost, while the chemical reaction between the ash and the cement occurs far more slowly than the initial hydration of conventional cement, reducing the concrete’s peak temperature. The ash consists of tiny glassy spheres, which act like ball bearings when the concrete is initially mixed, helping to make the wet concrete easier to move into place.’
Microsilica pozzolans have a similar effect on strength, he says, though unlike fly ash, their chemical reaction occurs quickly, so heat gain is a concern.
Ultimately, as buildings become taller, concrete will have to make way for other materials. ‘Concrete as we know it will reach a practical height limit,’ says Joseph. ‘But ‘as we know it’ is the key qualifier. In the future, what’s called concrete could be quite different from today’s version – the binders, aggregates and reinforcement could all change.
‘It is also quite possible that developments out of left field will change everything. For example, there is the experimental work on self-healing concrete based on specialised bacteria’, he says, referring to research carried out at various academic institutions around the world, where bacteria embedded in concrete responded to water entering any cracks by producing limestone to seal those cracks.
Irwin agrees with Joseph’s implication that such changes are still a long way off. ‘Since people expect tall buildings to stand up for 100 years or more and put their trust in the ability of the structural engineer to provide a durable structure, with absolutely unquestionable levels of safety, there is strong impetus to continue to use the tried and true,’ he says.
Nonetheless, research into new materials is being carried out. For example, Bath University’s BRE Centre for Innovative Construction Materials is looking at composites such as glass fibre, natural fibre, carbon and aramid, which researcher Dr Mark Evernden says could have applications in tall buildings. His focus is on identifying and evaluating the degradation mechanisms of composites when exposed to aggressive environments. He explains, ‘There is a lack of confidence in the actual design life of composites when specifying them for structural applications, which limits their application by riskaverse engineers.’
He says composites will also need higher strengthand stiffness-to-weight ratios for them to be used widely in supertall buildings, but adds, ‘Material properties are only half the solution, fire resistance is a huge issue here as well. Most composites have a low operating temperature, so their structural use in a very tall building would require either an improvement in fire resistance or protection to mitigate the effects of fire.’
Overcoming these drawbacks will take time, but the structural use of composites in supertall buildings looks inevitable. First, though, as Joseph explains, ‘several changes in construction methods will be needed for advanced composites to be considered for typical building projects. There is their cost, the wider use of prefabrication, the need for advanced fire safety strategies and the application of integrated strategies for controlling dynamic behaviour. Four major paradigm shifts may seem like a lot to change, but the introduction of iron- and steel-framed construction successfully went through a similar process.
60% Increase in height of the world’s current tallest building, Dubai’s Burj Khalifa (828m), over its predecessor, Taipei 101 (509m) in Taiwan
3 Number of buildings to hold the title of the world’s tallest building since 1998 (all in Asia). Height increased by 386m during this period
830 Difference in height in metres between Philadelphia City Hall, the world’s tallest building at the start of the 20th Century, and Kingdom Tower (1km) due for completion in 2019
101 Number of supertall buildings in the world in 2014., up threefold since 2007
100% Increase in height of the world’s tallest building in the past 10 years when Kingdom Tower is completed
9 Number of buildings to hold the title of world’s tallest building between 1901-1998 – all in the USA. During this time, the height increased by 275m