Why skyscrapers do not fall
When they talk about another record high building, they usually talk about what rises above the ground. Of course, about the height, number of floors and elevators, viewing platforms from which you can see half the world, and about, for example, how to deliver water to a hundred or so floors so that the water supply does not burst from the huge pressure in the pipes. They say less about the underground part, although the question of how giant, almost kilometer-long “needles”, such as those built by Burj Khalifa or the Kingdom Tower under construction, are kept in the ground is quite interesting. Why don't they fall? Why don't they fall into the ground and how do they withstand colossal wind loads?
To understand the technology of constructing the foundations for skyscrapers, PM applied to the Gorproekt Institute in Moscow, which was involved, in particular, in the design of high-rise buildings. Our consultant graciously agreed to act as the head of the design department of ZAO Gorproekt, candidate of technical sciences Elena Zaitseva.
Here you are not Manhattan
“The main thing when designing the foundation of a high-rise building is, of course, the high load transferred by the structure to the foundation, ” says Elena Zaitseva. - It is necessary to distinguish between the concepts of "foundation" and "foundation of the building." Under the foundation is understood part of the building (lower structures - plate, pile grillage, piles, etc.), which transfers the load from the structure to the ground. And, accordingly, under the foundation is understood an array of soil in which additional stresses and precipitation arise as a result of the building being exposed to it through its foundation. The challenge is to properly design both the foundation and the foundation. The main difficulty arises due to the fact that the height of the building is large, and the area of transfer of load to the base with respect to the height of the structure is small. This leads to high stresses both in the foundation structure itself (large bending moments and significant forcing load from walls and columns) and in the foundation (foundation ground). "
Thus, the foundation design directly depends on the characteristics of the soil. It is known that in the most famous park of skyscrapers - on the island of Manhattan - rocky soil is located at the surface, which greatly facilitates the work of designers. It is enough to clear a flat area - and on it you can put the foundation in the form of a thick slab of reinforced concrete. However, today the championship on super-high construction is taking place in another corner of the world - on the Arabian Peninsula. It is there that the highest skyscraper Burj Khalifa (828 m, UAE) is standing and preparing the construction of another monster 1007 m high - Kingdom Tower (Saudi Arabia). They wanted to make the latter a mile high (1609 m), but geologists said a decisive “no” - the soil could not stand it. Arabia is a desert land formed by bottom sediments of the ancient ocean, that is, consisting mainly of sandy rocks. Relatively hard rocks such as calcareous rocky soil are found only at depths. This factor had to be taken into account by the Chicago architect Adrian Smith, the main creator of Arabian miracles, and other authors of skyscraper projects in the sand.
Holding on to the bowels
The foundation of Burj Khalifa was designed as a pile-plate. The 3.7 m thick slab is something like a flower with three petals, which reflects the general structure of the building, consisting of a central hexagonal core and three wings, which serve as buttresses (vertical supporting structures). This gives the building greater rigidity for lateral load and torsion. It was decided to lean the slab on 192 piles with a diameter of 1.5 and a length of 43 m. Piles for skyscrapers are in most cases bored, that is, they are made by drilling wells of the required diameter and depth and then filling them with reinforcement elements and concrete mortar.
Sometimes piles penetrate layers of soft soil and reach hard rock at a certain depth, giving solid support to the foundation. But in Arabia, even at a depth of 50 m, the rocks are soft, with a low degree of cementation. The piles supporting the foundation slab are essentially “hanging”, that is, the load from the building is transmitted to the upper layers of the soil through the plate and the lower ones - mainly through the friction of the surfaces of the piles and soil. An interesting engineering problem had to be solved in the construction of the Kuala Lumpur twin towers - Petronas Towers. Under the site of their future foundation, solid rocky soil was present, but in the form of a rather steep slope. It was possible to choose the option with piles resting on a rock, but then some of them would be very short, while others would be much longer. Designers feared that under the weight of buildings longer piles would shrink over time and their length would be significantly reduced, resulting in a roll. In the end, it was decided to move the construction to where the rocky soil did not come close to the surface, and put the skyscrapers on the “hanging piles”.
Concrete works great for compression, but not so good for tensile and bending. “When building foundations, reinforced concrete is used, which includes steel reinforcement and heavy concrete, ” explains Elena Zaitseva. - The slabs are reinforced with horizontal grids that perceive bending, and the concrete takes up the compression loads. The diameter of steel reinforcement in slabs reaches 40 mm, but they can use special reinforcement of larger diameter in piles. ” Thus, an ultra-tall building transfers vertical load and bending moments to the foundation through a slab or slab-pile foundation. But how is the building attached to the foundation?
One of the design features of high-rise buildings in Moscow can be called the lack of solid rocky soils and, in some places, a rather high level of groundwater. The soil stratum in Moscow is represented by interbedded layers of sand and clay soils of various consistencies. In principle, this is a pretty good foundation for ordinary buildings, but given that the pressure under the sole of the foundation of a high-rise building is on average in the range of 7–11 kg / cm2 this is not enough. True, in Moscow almost everywhere on the accessible (for buildings with a large underground part) and in the presence of a pile base lies a layer of limestone. They are trying to lean on the foundations of skyscrapers. However, limestone is a material, firstly, significantly less durable than, for example, the same granite, and secondly, they are prone to destruction under the influence of acids. Given that human waste products slowly but surely pollute the horizons of groundwater, it is necessary to keep this in mind in the long run of a skyscraper. But we were lucky with the absence of hurricanes and earthquakes, which would have a frequent and catastrophic nature. The issues of protecting the pit from underflooding with groundwater during the construction period are solved either by deep water reduction with the help of needle-filtering units pumping water from depths below the bottom of the pit, or by creating a waterproof "wall in the ground", the lower end of which is lowered into clay soil, which is water-resistant (ie non-permeable water). Protection of the underground part of the building from water is carried out either using different waterproofing systems, or using the so-called “white bath”. This is a special concrete with reduced water permeability, and in the places where the expansion and technological joints are installed, elastic keys are installed that prevent water from seeping through the joints. Of course, these works require good qualifications of builders, as mistakes made when installing the underground part of the building are very difficult and very expensive to fix.
“At present, when it comes to high-rise buildings, the direct connection of the building structures with a slab or grillage (a beam that distributes the load on piles) is carried out according to a rigid scheme, ” says Elena Zaitseva. - Rebar outlets are made from the plate in the places where vertical structures are supported on it in such a way that they coincide with the reinforcement of these structures. Subsequently, when concreting walls and columns, the reinforcement of the slab and structures is connected, forming a continuous connection. This allows the skyscraper to have a reliable "anchor", where the horizontal load arising from gusts of wind or seismic shocks that have a shear effect will be transmitted. As for the connection of piles with the grillage, it is possible to swivel when the reinforcement of the pile does not start in the grillage plate, or rigid when not only the reinforcement, but also part of the head of the pile is brought into the plate. In the first case, only vertical loads on the piles are transferred from the building, in the second - also the bending moment. ”
If you approach the construction site, which is just starting to build a skyscraper, we will not see either piles or slabs. Most likely, a huge pit will gap in front of us: underground floors are designed in any, even the highest skyscraper, and therefore construction begins with digging a foundation pit. So that the pit, the slopes of which can be 5-10 meters or more, does not collapse, walling is made of sheet piles (usually they are made of metal) or in the form of a “wall in the ground”. And only at the bottom of the pit will wells be drilled under bored piles, and then a plate will be cast there, which will become the main invisible support of the skyscraper from the outside.The article “Why Skyscrapers Don't Fall” was published in the magazine Popular Mechanics (No. 3, March 2013).