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buildings, structures & design; foundations; piles & piling
High-rise buildings are usually founded on some form of piled foundation subject to a combination of vertical, lateral and overturning forces. However, conventional methods for assessing stability may not be adequate when designing such foundations because they tend to focus on resistance under vertical loading. This paper sets out an ultimate-limit-state approach for computer-based design of pile foundation systems for high-rise buildings and provides an example application on a 151-storey tower in South Korea.
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Conventional methods of assessing foundation stability tend to focus primarily on foundation resistance under vertical loading but, for tall buildings, the resistance to combined vertical, lateral and moment loadings must be considered. This paper sets out a limit-state design approach for tall-building foundation systems, with attention being focused on piled and piled-raft foundation systems - the predominant types currently used.
The key characteristics of piled rafts are outlined and then the principles of the design approach are set out. An example of the application of this approach is described for a 151-storey tower on reclaimed land in Incheon, South Korea (Figure 1).
Piled-raft foundation systems
In a piled-raft foundation system, the piles provide most of the stiffness for controlling settlements at serviceability loads and the raft element provides additional capacity at ultimate loading. A geotechnical assessment for designing such a foundation system therefore needs to consider not only the capacity of the pile and raft elements, but their combined capacity and interaction under serviceability loading.
The most effective application of piled rafts occurs when the raft can provide adequate load capacity, but the settlement and/or differential settlements of the raft alone exceed the allowable values. Poulos (2001) has examined a number of idealised soil profiles and found that the following situations may be favourable
* soil profiles consisting of relatively stiff clays
* soil profiles consisting of relatively dense sands.
It has been found that the performance of a piled-raft foundation can be optimised by selecting suitable locations for the piles below the raft. In general, the piles should be concentrated in the most heavily loaded areas, while the number of piles can be reduced, or even eliminated, in less heavily loaded areas (Horikoshi and Randolph,...