Abstract:
The present invention relates to a reinforced-concrete foundation system for erecting superstructures that transmit high axial loads, shearing forces and/or flexural moments at individual points, such as, for example, wind turbines. The foundation that is the subject matter of the present invention is formed by an upper reinforced-concrete slab poured “in situ”, of polygonal or circular footprint, and which is made rigid at the bottom by means of reinforced-concrete ribs of rectangular or trapezial cross section which are arranged radially. The method makes provision for said ribs to be produced from concrete “in situ” or, alternatively, by means of prefabricated elements, always working, jointly with the upper slab. This new foundation considerably reduces the costs of forming traditional foundations for this type of superstructure, by considerably improving completion deadlines.

Description:
OBJECT OF THE INVENTION 
       [0001]    The present invention relates to a reinforced-concrete foundation system for erecting superstructures that transmit high axial loads, shearing forces and/or bending moments at individual points, such as wind turbines. 
         [0002]    The foundation object of the present invention is formed by an upper reinforced-concrete slab poured “in situ”, having a polygonal or circular footprint, and which is made rigid at the bottom by means of reinforced-concrete ribs of a rectangular or trapezial cross-section which are arranged radially. The method envisages said ribs being made with concrete “in situ”, or alternatively by means of prefabricated elements, always working integrally with the upper slab. 
         [0003]    This new foundation seeks to considerably reduce costs for building conventional foundations for such superstructures by considerably improving completion deadlines. 
       BACKGROUND OF THE INVENTION 
       [0004]    The type of foundation most frequently used today for superstructures such as wind turbines is a foundation slab having different footprint configurations: they can be square, circular, hexagonal, octagonal, etc. The edge of the slab can be constant or variable for the purpose of optimizing the use of concrete. 
         [0005]    The applicant is aware of the existence of other foundation systems which attempt to minimize the volume of steel and concrete, such as in the following publications: 
         [0006]    European patent application EP1074663 is known, for example, which discloses an example of a foundation with three stabilizing members arranged symmetrically around a central support, with the drawback of having very little surface for contacting with the terrain, with the subsequent increase in stress applied on the terrain and in settlements. 
         [0007]    Patent application PCT WO20041101898 describes a circular foundation based on prefabricated triangular sections. This solution requires completely emptying out the excavated cavity, resulting in an inverted T-shaped concrete section that does not involve any structural advantage because the width of the compressed concrete head is very small. This means that when calculating bending, the neutral axis will be lower and there will be a smaller mechanical arm, the need for reinforcement increasing and sectional ductility considerably dropping. 
         [0008]    Finally, Spanish patent application ES-2347742 describes a cone-shaped foundation together with a lower planar ring-shaped slab. It is a very complex solution to implement and presents serious questions concerning its structural performance. 
         [0009]    The type of foundation for these superstructures is a very well-known and easy to calculate and design technical solution, and the simplicity thereof further simplifies the formwork and construction. However, such foundations have the drawback of being quite large, so the use of steel and concrete as well as the volume of earth that is removed is extraordinarily high, all of which noticeably increases the economic cost of the structure. The impact of the cost of the foundation on the cost of the structure is higher the larger the superstructure is; for example, in the case of wind turbines, an increase in shaft height from 80 m to 120 m (50%) produces a 300% foundation cost increment. This problem is very real due to the tendency to building more and more high-powered wind turbines with a shaft height of 120 m, which produces a cost increment for producing the foundation, making it a weak competitor for all these conventional systems of producing reinforced-concrete foundations. 
       DESCRIPTION OF THE INVENTION 
       [0010]    In an attempt to overcome the mentioned problems in the state of the art, the present invention presents a solution formed by an upper reinforced-concrete slab poured “in situ” having a polygonal or circular footprint and which is made rigid at the bottom by means of reinforced-concrete ribs of a rectangular or trapezial cross-section which are arranged radially stemming from a central core. The concrete for the ribs is poured directly on the previously excavated terrain, whereas the slab rests on the terrain that has not been removed, acting as permanent formwork. 
         [0011]    The solution consisting of an upper slab and ribs making the slab rigid at the bottom refers to the arrangement of both elements with respect to the surface of the ground where the foundation is built, where the slab is arranged first and the stiffening ribs would be arranged below the slab. 
         [0012]    The advantages of this foundation are
       The volume of concrete used is much less than in the conventional solutions described above.   The excavation cost is reduced, with the subsequent reducing of hauling to a dump.   Formwork is reduced and simplified.   Unmatched T-shaped resistant section efficiency is obtained because compressions are absorbed by the slab.       
 
         [0017]    The stiffening ribs can be made with concrete “in situ” or can be prefabricated, always working integrally with the upper slab, and they can have a constant or variable edge, either a stepped edge or an edge having a constant slope, the section thereof decreasing as it moves away from the central core of the slab. 
         [0018]    In the case of using stiffening ribs that are entirely prefabricated, said ribs will be provided with projecting reinforcement for connection with the slab built “in situ”. The stiffening ribs could also be made by means of semi-prefabricated elements, such as double-walled parts, for example, which remain embedded when the concrete is poured to form the foundation. 
         [0019]    The foundation object of the present invention is formed by an upper reinforced-concrete slab poured in “situ” having a circular or polygonal footprint and which is made rigid at the bottom by means of reinforced-concrete ribs of a rectangular or trapezial cross-section which are arranged radially. When the reinforced-concrete ribs are of a trapezial cross-section, the ribs will be wider in the upper part thereof in contact with the slab, in order to make use of the excavation embankments as permanent formwork, increasing the resistant concrete section. 
         [0020]    Said foundation is obtained by pouring the concrete “in situ” directly on the terrain that has not been removed and acts as permanent formwork. All the reinforcement necessary for the slab and ribs is put in place prior to pouring the concrete. 
         [0021]    A T-shaped resistant section having high structural efficiency is generated because compressions are absorbed by the upper slab and tensions are resolved by reinforcements housed in the bottom part of the inside of the stiffening ribs. 
         [0022]    To further optimize the use of concrete and the volume of earth that is excavated without jeopardizing the bearing qualities of the foundation, stepping can be made in the excavation of the ribs, thereby varying the edge thereof, which decreases as it moves away from the center of the slab. 
         [0023]    Said ribs can be made with concrete “in situ”, or they can be entirely or partially prefabricated, always working integrally with the upper slab. 
         [0024]    In terrains having very little bearing capacity, piles can be made under the stiffening ribs, the structural design of the footing therefore being suitable for both superficial and deep foundations. 
         [0025]    The terrain is improved by means of columns of gravel under the stiffening ribs. 
         [0026]    Furthermore, the upper slab does not necessarily have to be formed by a single part entirely covering the stiffening ribs from their inner end to their outer end, rather said upper slab can be formed by several portions, a first portion covering the central core of the stiffening ribs. In this context, it is understood that the term “portion” refers to a part of the central slab which is physically separated from other possible portions of said slab. On the other hand, the central core is where the inner ends of the different stiffening ribs are joined to one another, and it is therefore considered to be an integral part of said stiffening ribs. 
         [0027]    Therefore, the simplest configuration of the upper slab of the invention coincides with a single portion of upper slab covering only the inner ends of the stiffening ribs, i.e., covering only the central core. However, in another preferred embodiment of the invention the central slab is formed by two portions: a first portion covering the central core, and also a perimetral ring-shaped second portion separated from the first portion and connecting the outer ends of the ribs. The amount of concrete used is therefore reduced while at the same time maintaining good foundation performance because the second portion of the slab connecting the outer ends of the ribs makes the assembly rigid. 
         [0028]    In yet another preferred, embodiment of the invention, the central core furthermore has a hollow cylinder shape, thereby saving even more concrete and making the foundation more lightweight. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    To complete the description that is being made and for the purpose of aiding to better understand the features of the invention, a set of drawings is attached to the present specification in which the following is depicted with an illustrative and non-limiting character: 
           [0030]      FIG. 1  shows a perspective view of the system as a whole. 
           [0031]      FIG. 2  shows a perspective view of the foundation with ribs having a stepped variable section. 
           [0032]      FIG. 3  shows a longitudinal section view taken along one of the ribs. 
           [0033]      FIG. 4  shows a cross-section view. 
           [0034]      FIG. 5  shows a perspective view of the foundation with ribs of a trapezial cross-section. 
           [0035]      FIG. 6  shows the T-shaped resistant section object of the foundation of the present invention, as well as the stress-strain diagrams to calculate bending. 
           [0036]      FIG. 7  shows the inverted T-shaped resistant section as well as the corresponding stress-strain diagrams to calculate bending. 
           [0037]      FIG. 8  shows a top perspective view of an improved foundation according to the present invention where the upper slab covers only the central core. 
           [0038]      FIG. 9  shows a top perspective view of another improved foundation according to the present invention where the upper slab comprises a first portion covering the central core and a second portion connecting the outer ends of the ribs. 
           [0039]      FIG. 10  shows a bottom perspective view of another improved foundation according to the present invention where the central core is more lightweight as it has a hollow cylinder shape, 
       
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       [0040]      FIG. 1  shows a perspective view of the system as a whole, formed by a foundation ( 3 ) and a shaft ( 4 ) in a construction for a wind turbine. 
         [0041]    In the embodiment shown in  FIGS. 1 to 5 , a foundation can be seen which is formed by an upper reinforced-concrete slab ( 1 ) poured “in situ” having a polygonal footprint, although it could be of any other shape, such as circular, for example, and which is made rigid at the bottom by means of reinforcing ribs ( 2 ) of a rectangular cross-section made by means reinforced-concrete and arranged radially. 
         [0042]      FIGS. 6 and 7  show a calculation example. In the case of  FIG. 6 , the calculations are for a T-shaped section, and an ultimate resisting moment of 33,600 KNm and an x/h ratio of 0.16 are obtained. In the case of  FIG. 7 , the calculations are for an inverted. T-shaped section, and an ultimate moment of 27,900 KNm and an x/d ratio of 0.62 are obtained. 
         [0043]    It can therefore be seen that with the same reinforcement and volume of concrete, the T-shaped section (object of the foundation of the present invention) is structurally more efficient for the type of forces to which the foundations object of this patent will be subjected, providing a 20% higher bending strength. 
         [0044]    Sectional equilibrium requires that the resultant of tensions withstood by the reinforcement to be equal to the volume of compressions withstood by the concrete. The T-shaped section has a much wider compression head, which allows the neutral axis to remain high, and the mechanical arm to be noticeably larger than in the case of the inverted T-shaped section. 
         [0045]    Additionally, as can be determined by the x/h ratio indicating the depth of the neutral axis with respect to the edge of the section, ductility is much greater in the T-shaped section, which allows for the plastic redistribution of forces in plastic regime. 
         [0046]    Despite the fact that the inverted T-shaped section shown has the same amount of reinforcement and concrete, it is much less ductile, so it will not have any capacity for the plastic redistribution of forces, behaving in a fragile manner. 
         [0047]    Therefore, the proposed T-shaped section has a dual advantage:
       Greater bearing capacity and structural efficiency, i.e., greater resistance is obtained with the same reinforcement and volume of concrete (20% higher in the analyzed example, and even higher the higher the levels of requirement).   Greater ductility and capacity for the plastic redistribution of forces, which makes it much more suitable for dynamic requirements, such as those produced by an earthquake.       
 
         [0050]      FIGS. 8-10  show embodiments where the concrete slab ( 1 ) does not cover the stiffening ribs in their entirety, rather only a part of them. Specifically,  FIG. 8  shows an example of a foundation comprising an upper slab ( 1 ) formed by a single portion covering the central core ( 7 ) of the stiffening ribs ( 2 ). 
         [0051]      FIG. 9  shows another example of a foundation where the upper slab ( 1 ) is formed by two portions: a first portion ( 1 ) similar to that shown in  FIG. 8  covering only the central core ( 7 ) of the stiffening ribs ( 2 ), and a perimetral ring-shaped second portion ( 6 ) connecting the outer ends of all the stiffening ribs ( 2 ). This allows reducing the total volume of concrete used while at the same time maintaining assembly rigidity. 
         [0052]      FIG. 10  shows another example of a foundation having an upper slab ( 1 ) similar to the one in  FIG. 8 , covering only the central core ( 7 ) of the stiffening ribs ( 2 ), and where the central core ( 7 ) furthermore has a hollow cylinder shape. The weight of the assembly and the amount of material are doubly reduced in comparison with other embodiments of the invention where the slab ( 1 ) entirely covers the ribs ( 2 ) and the central core ( 7 ) is solid. 
         [0053]    Finally, for a description of the method as the foundation ( 3 ) is produced, the mentioned foundation is obtained by pouring the concrete “in situ” directly on the terrain that has not been removed, acting as permanent formwork. As seen in  FIG. 5 , the natural embankments of the terrain that are generated when excavating the radial ribs are what provide the trapezial section of said ribs, noticeably improving their bearing capacity. 
         [0054]    Therefore, the method for producing the foundation could comprise the following steps:
       Excavating terrain to form a cavity with a shape that is complementary to the foundation element.   Building a reinforcement in the excavated cavity which covers the opening of the slab ( 1 ) and the reinforcing ribs ( 2 ).   Pouring concrete into said excavated cavity in the terrain and in which the reinforcement has been arranged.       
 
         [0058]    To further optimize the use of concrete and the volume of earth that is excavated without jeopardizing the bearing qualities of the foundation, stepping ( 5 ) can be made in the excavation of the ribs, as shown in  FIG. 2 , thereby varying the edge thereof, which decreases as it moves away from the center of the slab. 
         [0059]    Said ribs can be made with concrete “in situ” or can be completely or partially prefabricated (such as with double-walled semi-prefabricated elements, for example), always working integrally with the upper slab.