A tall order
Laying a firm foundation for high-rise buildings. By Barry Mellor
More high-rise buildings are now being constructed than at any other time in Britain’s history. There are currently more than 270 existing tall buildings and structures in the UK, 70 per cent of which are in London1. Despite only 17 of this number being over 150m (492ft) in height and just one building – The Shard in London – over 300m, this overall number is likely to sky-rocket in the coming years as around 500 tall buildings are in the pipeline2. Of these projects, more than 85 per cent are planned in London, while the rest are clustered in key cities such as Birmingham, Liverpool, Manchester and Salford3.
As such, this high-rise boom is presenting new challenges for engineers, particularly in relation to the structural design of below ground foundations. Tasked with bringing ever-daring architectural feats to life that often tower above a bustling metropolis, many structural engineers are finding that traditional design methods are frequently difficult to apply with any confidence when limited by time, space and budget constraints.
The result is a clear imperative is to utilise more innovative and sophisticated methods, where to taking a more ‘ground up’ approach during the design and planning stages is key. More and more, this starts with making the most of innovative construction materials where pioneering solutions are not only helping to strengthen and minimise the need for piling in foundations but can also meet the complex structural requirements of making a modern skyscraper a reality.
Structural considerations
When constructing a tall building, there are a number of characteristics that can have a significant influence on foundation design, including the building weight and the corresponding vertical load to be supported by the foundation, which can be substantial.
For many skyscrapers, the building weight increases non-linearly with height, so both ultimate bearing capacity and settlement need to be considered carefully. What’s more, high-rise buildings are often surrounded by low-rise podium structures which are subjected to much smaller loadings – meaning differential settlements between the high- and low-rise portions need to be controlled.
This is certainly the case for high-rise construction in London where the presence of dense suburban structures below ground such as the London Underground’s network of train tunnels can make piling very difficult.
To overcome this challenge, more and more structural engineers are finding that the answer lies with changing their approach to concrete specification. One such way is the use of concrete made from secondary aggregate – materials which would otherwise become landfill – instead of conventional concrete solutions.
For instance, when Lytag, a lightweight aggregate made from fly ash generated by coal fired power stations, is used in the concrete mix for laying foundations, it has been proven to reduce incidence of piling for foundations by up to 25-30 per cent.
Consequently, the smaller foundations needed make it much easier to construct tall buildings in and around densely populated city centres such as London, with the added benefits of smaller and cheaper piling rigs and savings in vibration, shuttering, blinding, placing and hook time.
Lightening the load
Due to its unique composition, Lytag also affords contractors and structural engineers a raft of other invaluable benefits, such as reducing the number and size of columns, in order to create bigger structures with greater spans, whilst at the same time being able to reduce the total load of their builds – which essentially works hand in hand with the smaller foundations the secondary aggregate can help to create.
Crucially, it’s important to note that although lightweight material can effectively reduce the composite deck concrete slab load by approximately 25 per cent over normal weight control, it offers the same high level of structural performance as standard weight concrete.
Overall, replacing natural aggregate with secondary aggregates for the composition of lightweight concrete mixes significantly reduces the total volumes of concrete and reinforcement steel needed in the construction of tall buildings.
Design framework
For structural engineers looking to utilise lightweight concrete solutions and achieve the associated benefits for both foundations and the structure overall, the need to plan early cannot be overstated.
This begins from the early stages of design and modelling of a high-rise building on prominent design software suites such as SCIA, where the drop-down option for lightweight concrete must be activated to formulate the correct calculations. In addition, all modelling must also 7account for the parameters of using lightweight concrete, which includes adjustments needed to meet current design codes.
In some instances, concrete mixes using secondary aggregates, although lighter, maybe weaker due to host of other factors that need to be accounted for.
It’s also worth remembering that in replacing the need for quarried natural aggregate, this approach reduces CO2 emissions in buildings by up to ten per cent This is because by being up to 50 per cent lighter than natural aggregate, it means fewer truck journeys are needed to transport construction materials – making it the ideal ‘green’ solution for builds looking to achieve a high BREAM status.
A lighter solution
With the UK amidst a high-rise revolution, now more than ever before structural engineers are having to adopt new ways of working that push the boundaries of architectural design. For many, taking a ‘ground up’ approach is key, where making the most of innovative construction materials such as Lytag can not only help to strengthen and minimise a building’s foundations but can, ultimately, ensure the long-term success of modern skyscrapers.
1, 2, 3: AMA Research, Construction in the High-Rise Buildings Market Report – UK 2018-2025
Barry Mellor is Lytag’s commercial manager. Lytag is an Aggregate Industries business and benefits from the resources and capabilities of a highly skilled research and development team, as well as the financial and global business strength of its holding company Holcim.
For more information, please see
www.lytag.com
www.aggregate.com
www.lafargeholcim.com