Standing Tall

March 11, 2018

undefinedWhat are some of the technical challenges presented by constructing super tall buildings, especially in the GCC and the wider Middle East?

The structural challenges of “super-tall” buildings in excess of 300 metres in height relate primarily to the large forces that have to be designed for as the building height increases. Firstly, the weight of the building increases non-linearly as the height increases, due largely to the increasing size of the structural members to resist the gravity stresses.

Secondly, the lateral forces due to wind increase substantially as the building height increases. These forces tend to cause increasing movements of the building, especially at the upper floors, and so the building has to be sufficiently stiff to limit these movements such that human comfort is not compromised.

Thirdly, the requirements for vertical transportation become increasingly complex as the building height increases, and this is turn requires careful planning of the structural details to enable multi-stage transportation systems to function effectively.

From the geotechnical viewpoint, the foundation requirements become more demanding as the vertical and lateral forces from the structure increase. This is turn generally requires that the elements of the foundation become larger in size and extend deeper into the ground.

In the GCC and the Middle East generally, the geotechnical challenges can be greater than in some other parts of the world because of the geological characteristics of the supporting ground. In many places, the ground consists of weak rock, which has significantly lower bearing capacity than rocks in some other parts of the world. There are also many places where karstic conditions prevail, that is, where there are cavities within the limestone rock. Such ground conditions make the design and construction of foundations particularly challenging, and will generally require more detailed ground investigations to try and identify the location and size of cavities and their influence on the strength and stiffness of the foundation.

There are additional challenges in foundation design due to the need to design against earthquakes. Foundation systems must be able to resist the forces that are applied by the building moving sideways, and also the forces applied to the foundation system by the ground moving as a consequence of the earthquake. Recent experience indicates that no place on earth is immune from earthquake effects, even Dubai, and that in some countries further east, such as Iran and Turkey, earthquakes pose a major problem in both structural and geotechnical design.

What makes a strong foundation? What design principles are used for the foundation of high-rise buildings in Dubai?

A “strong foundation” is one which provides an adequate margin of safety against collapse and in which the settlement into the supporting ground is limited to an acceptable amount such that the building can function effectively. The design principles that we use to develop such a foundation address both the overall stability and load capacity of the foundation under extreme loading conditions, and the so-called “serviceability” or tolerable movements of the foundation under normal operating loads. These principles are common to any region in the world, and require that we have a good understanding of the strength and deformation properties of the various underlying soil and rock strata.

What kinds of foundations are used in this region and what factors affect the selection process?

For the taller buildings in the UAE and the Middle East, it is common for deep foundations, such as piles or barrettes, to be used as the primary elements within the foundation system. Many of the earlier buildings employed bored piles that were constructed by drilling supported holes into the ground and filling the resulting cavity with reinforced concrete. Typical bore pile diametres have ranged between about 1.2 and 2.5 metres.

In addition, there is almost inevitably a ground or basement slab below the structure and this slab (generally referred to as a mat or a raft) can also contribute to the strength of the foundation together with the piles. This combination of piles with a raft is termed a “piled raft” or a “combined pile-raft foundation” (CPRF), and has been used for buildings such as the Emirates Towers and the Burj Khalifa in Dubai.

In recent years, increasing use has been made of barrettes, which are, in effect, large rectangular bored piles that can typically be 2.8 metres by 1.5 metres in plan, and extend to depths in excess of 70 metres. By judicious arrangement of these barrettes, both the vertical and lateral load-carrying capacity of the foundation system can be enhanced.  

The factors that influence the selection process are primarily the vertical and lateral loads that the foundation has to resist. The larger these loads are, the larger and deeper the foundations may have to be. Other factors that can influence the choice of foundation type are material and equipment availability, local construction practices, site access and local code requirements.

What are some of the latest state-of-the-art design methods being adopted by geotechnical engineers?

Older and more traditional methods of design generally relied on considerations of the vertical load-carrying capacity of the foundation system, and this was often assessed by very simplified or empirical methods. In recent years, with the recognition that the loads on tall buildings are both vertical and lateral, more refined methods of analysis and design have been developed and employed.

These methods rely on advanced numerical analyses, primarily the finite element method, and are used in conjunction with ground properties estimated on the basis of improved methods of in-situ testing. These methods include such devices as the pressuremeter and the dilatometer, together with the use of geophysical methods that enable tomographic images of the subsurface to be generated. Such methods not only provide a means of identifying anomalies such as cavities in the ground, but also provide a quantitative measure of the stiffness of the ground, which can be used for the estimation of foundation movements.

Has technology made the job of designing tall buildings easier over the years?

Over the past two decades, very powerful commercial software packages have been developed for the analysis and design of foundation systems. Such packages now allow for three-dimensional modelling to be carried out and for relatively complex soil models to be incorporated. Such software now makes it possible to model the ground and foundation system more accurately, but at the same time, presents hidden dangers in that inexperienced users of the software may make inappropriate modelling assumptions or unwittingly accept inappropriate default values for some of the unfamiliar geotechnical parameters used in the program. For this reason, it is always imperative that the results of a complex computer analysis be checked to ensure that it is sensible.

In our practice, we always use simple methods to gain an initial estimate of the behaviour of the foundation, for example, the long-term settlement of the foundation. In our subsequent detailed design process, if the software we employ gives dramatically different values, then we investigate further the source of disagreement. Almost always, it turns out that the mistake is in the application of the software, either via poor modelling techniques or by an inappropriate choice of geotechnical or loading parameters.

In cases involving third party peer review, we routinely employ different software to that of the designer, and a completely independent approach, to check the designer’s outcomes. For example, for the Burj Khalifa, we employed an alternative finite element program and our own geotechnical model, and came up with a settlement that was within a few percent of that obtained by the designer. Such agreement helps to increase the confidence in the design outcome.

What are your thoughts on building tall structures using 3D printing?

3D printing is still in its infancy, at least for applications to civil engineering, and to tall buildings in particular. I am aware that a 5 storey building in China has been constructed using 3D printing, and that a much taller building is mooted for Dubai. Having seen so many remarkable innovations during my lifetime, I am hesitant to suggest that 3D printing of tall buildings will remain a novelty, but as the old saying goes, “the proof of the pudding is in the eating”. In other words, we will need to see how such buildings perform and obtain more experience with the pros and cons of this type of application of 3D printing before we can judge whether it will supersede more traditional methods of construction.

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