Patent Publication Number: US-11661742-B2

Title: Steel reinforced concrete column

Description:
TECHNICAL FIELD 
     The present invention generally relates to a steel reinforced concrete column for a high rise building. It further relates to a steel structure for such a steel reinforced concrete column and a high-rise building comprising such a steel reinforced concrete column. 
     BACKGROUND ART 
     Steel reinforced concrete columns are composite columns comprising structural steel sections encased in reinforced concrete. They are widely used in high-rise buildings and, due to their sizes, are also referred to as “mega-columns”. Taking advantage of the composite action between the concrete and the steel sections, the bearing capacity of the composite column is normally larger than the sum of the bearing capacities of the isolated concrete and steel sections. 
     A first type of steel reinforced concrete columns has a welded steel skeleton that consists of heavy steel plates assembled on site by welding. Such a column is for example disclosed in Chinese utility model CN 204919988 U. The steel skeleton of this column comprises a cross-shaped section that is centred on the longitudinal central axis of the column. The section of the column itself is square-shaped, wherein cages of rebars reinforce the four corners of the column. It is also known to design the steel skeleton as a huge steel caisson consisting of heavy steel plates assembled on site by welding. This steel caisson is filled with concrete and encased in concrete reinforced with longitudinal and transversal rebars. 
     It is further known to combine open steel sections with closed steel sections in a steel reinforced concrete column. Such a column is for example disclosed in Chinese utility model CN 104405082 U. This column has a cross-shaped cross-section. Each arm of the cross includes a welded T-shaped steel section having a web pointing to the centre of the cross. In the centre of the column, a tubular steel section is embedded in the concrete and filled with concrete. 
     In steel reinforced concrete columns of this first type the design of the steel skeleton can be freely designed so that the concrete and the steel efficiently cooperate. However, building such a steel skeleton generally requires a lot of onsite welding work on heavy structural steel, which is costly, time consuming and may result in quality problems. 
     A second type of steel reinforced concrete columns includes isolated hot-rolled steel sections. Such a column is for example disclosed in Chinese utility model CN 203113624 U. The steel reinforced concrete column disclosed therein has a square-shaped or rectangular cross-section, wherein an I-section steel beam is arranged in each of the corners of the column. The webs of these I-section steel beams are arranged along two opposite sides of a concrete core that is reinforced with longitudinal and transversal rebars. In case of a rectangular cross-section of the column, the webs of the four I-section beams are located along the small sides of the column. Rebar rings surround pairs of I-section beams and the whole arrangement of I-sections. 
     Steel reinforced concrete columns of this second type do not require a lot of onsite welding work on heavy structural steel, but they are generally less efficient as regards the cooperation between the concrete and the steel sections for warranting a high bearing capacity. 
     It is an object of the present invention to propose a steel reinforced concrete column that is easy to build on site and in which the concrete and the steel nevertheless efficiently cooperate to warrant a high bearing capacity. 
     SUMMARY OF INVENTION 
     A steel reinforced concrete column for a high rise building in accordance with the invention comprises a plurality of hot-rolled steel sections extending longitudinally through the concrete column, wherein each of these steel sections has an outward flange with an outer surface turned outwards in the concrete column, an opposite inward flange with an outer surface turned inwards in the concrete column, and a central web connecting the outward flange to the inward flange. Preferred hot rolled steel sections are, for example, H-shaped steel sections with wide flanges, such as European HEA, HEB or HEM beams according to prEN16828-2015, EN 10025-2:2004, 10025-4:2004, or American wide flange or W-beams according to ASTM A6/A6M-14, or other hot-rolled steel section having two flanges and a central web similar to or in line with the aforementioned beams. The steel reinforced concrete column has a longitudinal axis along which the steel sections extend, preferably so that the longitudinal axis of each steel section is parallel to the longitudinal axis of the steel reinforced concrete column. 
     According to a first aspect of the invention, the steel sections are arranged in the concrete column so that the outer surfaces of their inward flanges delimit therein a central concrete core with n lateral sides and a transversal cross-section that forms an n-sided polygon, n being at least equal to three, wherein each of the n lateral sides of the central concrete core is coplanar with the outer surface of the inward flange of at least one steel section. It will be understood that “coplanar” here means that the respective lateral side of the central concrete core and the outer surface of the inward flange lie in a same plane, of course, within the bounds of flatness tolerances of the outer surface of the inward flange. What matters is that the outer surface of the inward flange forms an outward boundary for the central concrete core. It follows that confinement of the central concrete core—which is usually solely ensured by external reinforced concrete layers—is improved by a specific arrangement of the inward flanges of the steel sections. “Confinement” here means a blocking of transversal expansion of the concrete under compression forces. As a result of the improved confinement of the concrete core, a 3D stress state is developed in the concrete core which increases the bearing capacity and ductility of the steel reinforced concrete column. Crack expansion and growth are minimized in the axially compressed concrete core. It remains to be noted that the confinement effect is not (yet) taken into consideration in the design codes, but it surely provides extra safety to the user. In summary, the present invention proposes a steel reinforced concrete column that can be easily built on site with hot-rolled steel sections, wherein these sections do not only provide a high bearing capacity but also increase the bearing capacity of the central concrete core. 
     To improve the confinement of the central concrete core by the inward flanges, preferably at least 30% and more preferably at least 40% and most preferably at least 50% of the surface of each of the n lateral sides of the concrete core shall be limited by the outer surface of the inward flange of one or more steel sections. 
     Furthermore, the horizontal distance between two adjacent steel sections in the column shall at least be several centimetres, so that each of the individual steel sections is sufficiently embedded in concrete. It follows that at maximum 98% of the surface of each of the n lateral sides of the concrete core will normally be limited by the outer surface of the inward flange of one or more steel sections. In preferred embodiments, the percentage of the surface of each of the n lateral sides of the concrete core that is limited by the outer surface of the inward flange of one or more steel sections will be in the range of 30% to 98%, and more preferably in the range of 30% to 80% or 40% to 80%. 
     If a side of the central concrete core is coplanar with the outer surface of the inward flange of a single steel section, then this inward flange is preferably centred relative to the width of this side of the central concrete core. Such a centred arrangement of the inward flange provides a good confinement of the central concrete core and good possibilities of connecting a bearing beam to the column. 
     It will be appreciated that the cross-section of a proposed steel reinforced concrete column—and thereby its bearing capacity—may be easily increased without degrading the confinement of the central concrete core, if there are sides of the central concrete core that are coplanar with the outer surfaces of the inward flanges of more than one steel section. 
     To improve the confinement of the central concrete core, if a side of the central concrete core is coplanar with the outer surfaces of the inward flanges of m steel sections, wherein m is at least equal to two, the distance between two consecutive inward flanges arranged along this side of the central concrete core, as well as the distance between a corner laterally delimiting this side of the central concrete core and the inward flange closest to this corner, shall preferably not be greater than 0.8·w/(m+1), preferably not greater than 0.7·w/(m+1), where w is the width of this side and m is the number of steel sections arranged along this side. 
     Usually, all the inward flanges will have the same width. In special cases, the inward flanges may however have different widths. 
     Usually, the inward flange of a steel section will have the same width as its outward flange. In special cases, the inward flange may however be wider than the outward flange. 
     Usually, all steel sections will have the same dimensions. In special cases, the steel sections of different dimensions may however be used in the same column. 
     An excellent confinement of the central concrete core can be easily achieved, if the latter has a transversal cross-section that forms an n-sided convex polygon. However, as long as it is possible to arrange at least one steel section along each side of the central concrete core, it is not excluded that the latter may have transversal cross-section forming an n-sided concave polygon, such as e.g. a star. (A convex polygon is defined as a polygon with all its interior angles less than 180°. A concave polygon has at least one angle greater than 180°.) 
     In many cases, the n sides of the central concrete core will all have a same width. However, it is not excluded that the n sides of the central concrete core may have different widths. This is for example the case if the central concrete core has a transversal cross-section that is a rectangle. 
     It will be appreciated that excellent confinement of the central concrete core can be achieved, if this central core has a transversal cross-section that forms a regular polygon, i.e. a polygon that is equiangular (all angles are equal in measure) and equilateral (all sides have the same length). However, architectural and/or structural constraints (e.g. bearing directions of beams connected to the column) may imply to confer to the central concrete core a transversal cross-section that forms a polygon that is not equiangular and/or not equilateral. 
     Similarly, to improve confinement of the central concrete core, it is of advantage if the steel sections form an arrangement of which the longitudinal central axis of the column is an axis of rotation symmetry of 360°/n, wherein n is the number of sides of the central concrete core. 
     If a side of the central concrete core is coplanar to the outer surface of the inward flange of a single steel section, confinement of the central concrete core is also improved if the web of this steel section has a midplane containing, with the usual tolerances for such a structural steel application, the longitudinal axis of the column. 
     Each inward flange preferably comprises a multitude of shear connectors penetrating into the central concrete core. These shear connectors provide the advantage that the arrangement of steel sections and the central concrete core behave more effectively as a composite body, whereby the ability of the steel reinforced concrete column to withstand bending stresses induced by eccentric column loads is strongly improved. 
     Each of the steel sections may additionally or alternatively comprise a multitude of shear connectors penetrating into the concrete between its outward and inward flanges and/or into the concrete surrounding the outer surface of its outward flange. These shear connectors provide the advantage that the steel sections and the concrete enveloping the steel sections behave more effectively as a composite body. 
     The concrete will generally comprise longitudinal and/or transversal rebars, wherein “rebar” is a shortened form for “reinforcing bar” and designates a steel bar used as a tension device to strengthen and hold the concrete in tension, the surface of the rebar being often patterned to form a better bond with the concrete. 
     In a preferred embodiment, the concrete comprises an outer reinforcement cage formed of longitudinal and transversal rebars and enclosing the arrangement of steel sections. This outer concrete reinforcement cage allows in particular an outer confinement of a peripheral concrete layer encaging the steel sections. It opposes in particular a bulging of this peripheral concrete layer under axial compression forces, so that this peripheral concrete layer may contribute up to higher loads to the bearing capacity of the steel reinforced concrete column. 
     The outer reinforcement cage advantageously comprises multitude of closed circular rebar rings connected to the longitudinal rebars. It will be appreciated that these closed circular rebar rings efficiently oppose a transversal pressure generated in the axially compressed concrete, by being capable of absorbing important circumferential tension stresses (similar to a cylindrical wall of a pressure vessel). 
     The concrete may also advantageously comprise an inner reinforcement cage formed of longitudinal and transversal rebars, which is arranged between the outer flanges and the inward flanges so as to enclose the central concrete core. This inner concrete reinforcement cage provides in particular a confinement of an intermediate concrete layer immediately surrounding the central concrete core. It thereby opposes a transversal pressure generated in this intermediate concrete layer under axial compression forces, so that this intermediate concrete layer may contribute up to higher loads to the bearing capacity of the steel reinforced concrete column. 
     The inner reinforcement cage preferably comprises closed circular rebar rings passing through holes in the webs of the steel sections. It follows that these rings are structurally independent from the arrangement of steel sections, which is of advantage when the steel sections are exposed to deformations. Alternatively, the inner reinforcement cage comprises arc-shaped segments of rebar rings welded with their ends to the webs of the steel sections. While being less advantageous from the structural point of view, this alternative embodiment has however the non-negligible advantage that it is not necessary to drill holes into the webs of the steel sections. 
     In a preferred embodiment, the steel reinforced concrete column comprises at least two longitudinally spaced beam-to-column connection nodes. Such a “beam-to-column connection node” is a specific section of the steel reinforced concrete column that is specifically equipped for connecting thereto load bearing beams supporting for example a floor in a high rise building. It will be appreciated that between two successive beam-to-column connection nodes, there is advantageously no structural steel interconnecting the steel sections. In other words, between two successive beam-to-column connection nodes, the bearing steel structure of the steel reinforced concrete column just consists of isolated steel sections extending in parallel through the column. At the beam-to-column connection nodes, the steel sections may however be structurally interconnected by means of structural steel. The term “structural steel” herein designates a variety of heavy steel shapes, such as H-beams, I-beams, T-beams, heavy U- or L-sections and heavy steel plates, used as load bearing or load transferring members in a steel structure. Rebars are, in this context, not considered as structural steel. Thanks to the absence of structural steel interconnecting the steel sections between two successive beam-to-column connection nodes, onsite welding work on structural steel is strongly limited which improves notably the quality of the column and makes the latter easier to build. 
     In a preferred embodiment, the steel reinforced concrete column comprises at least one beam-to-column connection element on the outward flange of at least one steel section for connecting to this outward flange a load bearing beam. Such a beam-to-column connection element may for example comprise a structural steel element, such as for example: L-sections rigidly affixed to the outward flange, for welding or bolting thereto the web of the beam; bolt holes in the outward flange, for fixing an end plate of beam to the outward flange, so as to achieve a bolted end plate beam-to-column connection etc. The beam-to-column connection shall preferably be a rigid beam-to-column connection. 
     The steel reinforced concrete column may have a round or oval or another curvilinear cross-section, but it may also have a polygonal cross-section. The present invention consequently offers considerable architectural freedom for designing the cross-section of the column. It will however be appreciated that a very interesting embodiment comprises a polygonal cross-section with 2n sides, if the central concrete core has n sides. Behind every second of these 2n sides will then be arranged the outer surface of the outward flange of at least one of the steel sections. It will be appreciated that such an embodiment allows, amongst others, to efficiently avoid protruding concrete corners that do not comprise a steel section. 
     The invention also proposes a steel structure for a steel reinforced concrete column for a high rise building comprising a plurality of hot-rolled steel sections arranged so as to extend longitudinally through the concrete column. Each of these steel sections has an outward flange with an outer surface turned outwards in the concrete column, an opposite inward flange with an outer surface turned inwards of the concrete column, and a web connecting the outward flange to the inward flange. The steel sections are arranged so that the outer surfaces of their inward flanges delimit a central core volume with n lateral sides and a transversal cross-section that forms a n-sided polygon, n being at least equal to three; each of the n lateral sides of the central core volume being coplanar to the outer surface of the inward flange of at least one steel section. As soon as such steel structure is encased in concrete, the central concrete core is confined or limited by the inward flanges of the steel sections. As explained hereinbefore, with the improved confinement of the concrete core, a 3D stress state is developed in the concrete core which increases the bearing capacity and ductility of the steel reinforced concrete column. Crack expansion and growth are minimized in the axially compressed concrete core. 
     Such a steel structure normally also comprises at least two longitudinally spaced beam-to-column connection nodes for connecting thereto load bearing beams; wherein between two successive beam-to-column connection nodes, there is no structural steel interconnecting the steel sections. At the beam-to-column connection nodes, the steel sections may be structurally interconnected by means of structural steel. Thanks to the absence of structural steel interconnecting the steel sections between two successive beam-to-column connection nodes, onsite welding work on structural steel is strongly limited which improves notably the quality of the steel structure and makes the latter easier to build. 
     The invention further proposes a high-rise building comprising at least one steel reinforced concrete column as described hereinbefore. 
     This high rise building usually comprises at least two successive floors supported by the steel reinforced concrete column at two successive beam-to-column connection nodes of the steel reinforced concrete column, wherein between two successive connection nodes, there is no structural steel interconnecting the steel sections. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The afore-described and other features, aspects and advantages of the invention will be better understood with regard to the following description of several embodiments of the invention and upon reference to the attached drawings, wherein: 
         FIG.  1   : is a cross-section of a first embodiment of a steel reinforced concrete column in accordance with the invention; 
         FIG.  2   : is a cross-section of a second embodiment of a steel reinforced concrete column in accordance with the invention; 
         FIG.  3 A : is an elevation view of a first embodiment of a steel concrete reinforcement cage to be used in a steel reinforced concrete column in accordance with the invention; 
         FIG.  3 B : is a cross-section of the steel concrete reinforcement cage of  FIG.  3 A ; 
         FIG.  4 A : is an elevation view of a second embodiment of a steel concrete reinforcement cage to be used in a steel reinforced concrete column in accordance with the invention; 
         FIG.  4 B : is a cross-section of the steel concrete reinforcement cage of  FIG.  4 A ; 
         FIG.  5   : is a cross-section of a steel section to be used in a steel reinforced concrete column in accordance with the invention; 
         FIG.  6   : is a cross-section of a third embodiment of a steel reinforced concrete column in accordance with the invention; 
         FIG.  7   : is a cross-section of a fourth embodiment of a steel reinforced concrete column in accordance with the invention; 
         FIG.  8   : is a cross-section of a fifth embodiment of a steel reinforced concrete column in accordance with the invention; 
         FIG.  9   : is a cross-section of a sixth embodiment of a steel reinforced concrete column in accordance with the invention; 
         FIG.  10   : is a cross-section of a steel reinforced concrete column as shown in  FIG.  2   , showing a beam-to-column connection, in which horizontal bearing beams are affixed to the steel reinforced concrete column; and 
         FIG.  11   : is an elevation view of a column as shown in  FIG.  1 ,  2  or  6   , wherein concrete and concrete reinforcement bars are not shown. 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
     It will be understood that the following description and drawings describe embodiments of the invention by way of example and for illustration purposes. They shall not limit the scope, nature or spirit of the claimed subject matter. In the drawings, equivalent elements in different embodiments bear the same reference numbers. 
       FIG.  1    schematically shows a cross-section of a first embodiment of a steel reinforced concrete column  10  in accordance with the invention (also designated in a shortened form as “the column  10 ”). The column  10  comprises a longitudinal central axis  12  and a shell surface (or outer envelope)  14 . The longitudinal central axis  12  is perpendicular to the drawing plane. In the column of  FIG.  1   , the shell surface  14  is a right circular cylindrical surface having the longitudinal central axis  12  as cylinder axis. It follows that the column of  FIG.  1    has a circular cross-section. 
     Four hot-rolled steel sections  16   1 ,  16   2 ,  16   3 ,  16   4  with an H-shaped section (hereinafter also designated in a shortened form as “steel sections  16   i ”, where i=1, 2, 3, 4) extend longitudinally along the longitudinal central axis  12  of the column  10 . Each of these column beams  16   i  has an inward flange  18   i  with a substantially planar outer surface  20   i  turned inwards (i.e. turned to the longitudinal central axis  12 ), an opposite outward flange  22   i  with a substantially planar outer surface  24   i  turned outwards (i.e. turned to the shell surface  14  of the column  10 ), and a central web  26   i  connecting the inward flange  18   i  to the outward flange  20   i . The midplane of the web  26   i  of each steel section  16   i  contains hereby the longitudinal central axis  12  of the column  10 . 
     Preferred hot rolled steel sections are H-shaped steel sections with wide flanges, such as European HEA, HEB or HEM beams according to prEN16828-2015, EN 10025-2:2004, 10025-4:2004, or American wide flange or W-beams according to ASTM A6/A6M-14, or other hot-rolled H-shaped steel section similar to or in line with the aforementioned beams. Relevant mechanical parameters and steel grades of suitable steel sections are for example listed in European standard EN 1993-1-1:2005, Table 3.1 and clause 3.2.6. 
     The four steel sections  16   i  are arranged in the column  10  so that the outer surfaces  20   i  of their inward flanges  18   i  delimit therein a central core volume  28  with four lateral sides and a transversal cross-section that forms a four-sided polygon. Reference number  30  identifies the outer limit of this central core volume  28  in the plane of the drawing, which outer limit has the form of a square in  FIG.  1   . In space, the outer limit (i.e. the enveloping surface) of the central core volume  28  is defined by four virtual planes, each of these four virtual planes being coplanar with the outer surfaces  20   i  of one of the four inward flanges  18   i . The longitudinal central axis  12  of the column  10  is also the central axis of the central core volume  28 . 
     Concrete  32  (schematically represented by a dotted pattern fill) encases the four steel sections  16   i  and also fills the central core volume  28  delimited by the outer surfaces  20   i  of the inward flanges  18   i  of the four steel sections  16   i . Consequently, the column  10  comprises a central concrete core  28 ′ with four lateral sides and a transversal cross-section that forms a four-sided polygon, more particularly a square, wherein each of the four lateral sides of the central concrete core  28 ′ is coplanar with the outer surface  20   i  of the inward flange of one of the steel section  16   i . 
     It follows that confinement of the central concrete core  28 ′, which is usually solely provided by external reinforced concrete layers, is improved by a specific arrangement of the inward flanges  18   i  of the steel sections  16   i . This confinement very efficiently blocks a transversal expansion of the concrete under compression forces. As a result of the improved confinement of the concrete core  28 ′, a 3D stress state is developed in the concrete core which increases the bearing capacity and ductility of the steel reinforced concrete column  10 . Crack expansion and growth are minimized in the axially compressed concrete core. It remains to be noted that the confinement effect is not (yet) taken into consideration in the design codes, but it surely gives an extra safety to the user. 
     Suitable concrete to be used for encasing the hot-rolled steel sections and filling the central core volume  28  is for example in accordance with European standard EN 1992-1-1:2004 Table 3.1 or with equivalent other standards. If high strength steel material is used for the steel sections, then it is recommended to have high strength concrete material too. 
     To achieve a sufficient confinement of the central concrete core  28 ′, at least 30% of the surface of each of the four lateral sides of the concrete core  28 ′ shall be limited by the outer surface  20   i  of the inward flange  18   i  of the respective steel section  16   i . In  FIG.  1   , each of the inward flanges  18   i  is centrally located on the respective side of the central concrete core  28 ′ and limits about 78% of the surface of this side. In other words, the central concrete core  28 ′ is limited by the inward flanges  18   i  over about 78% of its perimeter surface  30 . 
     Combining  FIG.  5    with  FIG.  1   , it will be understood that each inward flange  18   i  preferably comprises a multitude of shear connectors  34  protruding from its outer surface  20   i . These shear connectors  34  deeply penetrate into the central concrete core  28 ′. As a consequence, the central concrete core  28 ′ is fully bonded to the four inward flanges  18   i  of the steel sections  16   i , i.e. the connectors fully transfer shear stresses at the flange-concrete core interfaces. It follows that a composite steel concrete column  10  is formed that takes full advantage of the high compressive strength of the confined central concrete core  28 ′ and of the high tensile and compressive strength of the steel sections  16   i . 
     As solely illustrated in  FIG.  5   , each of the steel sections  16   i  may further comprise shear connectors  36  penetrating into the concrete  32  between its outward flange  22   i  and its inward flange  18   i  and/or shear connectors  38  penetrating into the concrete  32  surrounding the outer surface  24   i  of its outward flange  22   1 . All the shear connectors  34 ,  36 ,  38  shown in the drawings are headed shear studs, but it is not excluded to use other types of shear connectors, as long as they are capable of properly transferring the shear stresses at the respective concrete-steel interfaces. 
     In  FIG.  1   , reference number  40  identifies an outer reinforcement cage surrounding the four steel sections  16   i  in the concrete  32 . A preferred embodiment of such a concrete reinforcement cage  40  is illustrated by  FIGS.  4 A and  4 B , wherein a side view thereof is shown in  FIG.  4 A  and a cross-section thereof is shown in  FIG.  4 B . In this preferred embodiment, the concrete reinforcement cage  40  comprises reinforcement bars  42  longitudinally extending through the column  10  (also called longitudinal rebars  42 ) and closed circular reinforcement rings  44  (also called closed circular rebar rings). The closed circular reinforcement rings  44  are manufactured from at least one rebar, which is bent to have the shape of a circular ring, which ring is then closed by welding together the two ends of the rebar. The closed circular reinforcement rings  44 , which are in the column  10  preferably parallel to a horizontal plane and have their centre located on the longitudinal central axis  12 , are secured to all or some of the longitudinal rebars  42  preferably by welding, or alternatively by mechanical connections, such as e.g. tying steel wire or mechanical couplers. Geometrical and material characteristics of the steel rebars are defined for example in EN 1992-1-1:2004, EN 10080, table 6, and EN 1992-1-1:2004, section 3.2.2. (3). It will be appreciated that the closed circular rebar rings  44  efficiently oppose a bursting of the axially compressed concrete  32  by being capable of absorbing substantial circumferential tension stresses (similar to a cylindrical wall of a pressure vessel).  FIGS.  3 A and  3 B  show an alternative embodiment of the outer reinforcement cage  40 . In this embodiment, a continuous rebar  48  is wound in a helical form around the longitudinal rebars  42 . The helically wound continuous rebar  48  is secured to all or some of the longitudinal rebars  42  preferably by welding, or alternatively by mechanical connections, such as e.g. tying steel wire or mechanical couplers. It remains to be noted that the outer concrete reinforcement cage  40  warrants an outer confinement of a peripheral concrete layer encaging the steel sections  16   i . It opposes in particular a bulging of this peripheral concrete layer under axial compression forces, so that this peripheral concrete layer may contribute up to higher loads to the bearing capacity of the steel reinforced concrete column  10 . 
     Reference number  50  identifies an inner concrete reinforcement cage arranged between the outer flanges  22   i  and the inward flanges  18   i  so as to enclose the central concrete core  28 ′. Preferred embodiments of this inner concrete reinforcement cage  50  are also illustrated by  FIG.  3 A,  3 B  and  FIG.  4 A,  4 B . Just as the outer reinforcement cage  40 , the inner reinforcement cage  50  advantageously comprises vertical reinforcement bars  52  (also called longitudinal rebars  52 ) and closed circular reinforcement rings  54  as shown in  FIG.  4 A  and  FIG.  4 B  or a continuous rebar  58  that is wound in a helical form around the longitudinal rebars  52  as shown in  FIG.  3 A  and  FIG.  3 B . The closed circular reinforcement rings  54  and the helically wound continuous rebar  58  advantageously pass through small holes drilled into the webs  26   i . Alternatively, to avoid drilling of holes into the webs  26   i , a closed circular reinforcement ring  54  may be replaced by four arcs of a circle, wherein the ends of each of these arcs are welded to two adjacent webs  26   i . It will be appreciated that the inner concrete reinforcement cage  50  warrants in particular a confinement of an intermediate concrete layer immediately surrounding the central concrete core  28 ′. It thereby blocks a transversal expansion of the concrete under compression forces, so that this intermediate concrete layer may contribute up to higher loads to the bearing capacity of the steel reinforced concrete column  10 . 
     It remains to be noted that an embodiment with four steel sections  16   i  in a cross-shaped arrangement as shown  FIG.  1   , but also the embodiments of  FIGS.  2  and  6    described hereinafter, are of particular interest, if the column  10  has to support horizontal bearing beams arranged according to two perpendicular directions, which is the most common case. 
     The column  10  of  FIG.  2    distinguishes over the column  10  of  FIG.  1    mainly by the following features. It has a square-shaped cross-section (instead of a circular cross-section), wherein its shell surface comprises four planar side surfaces  14   i , which are basically parallel to the outer surfaces  24   i  of the four outward flanges  22   i . Each of the inward flanges  18   i  limits about 52% of the surface of the respective side of the 4-sided central concrete core  28 ′. In other words, the 4-sided central concrete core  28 ′ is limited by the inward flanges  18   i  over about 52% of its perimeter surface  30 . The outer concrete reinforcement cage  40 ′ and the inner concrete reinforcement cage  50 ′ comprise closed reinforcement rings  44 ′ that are square-shaped. Rebar corner brackets  60  stiffen the square-shaped reinforcement rings  44 ′, so that they are better suited for opposing a bulging of the concrete  32  under axial compression forces. This embodiment with square-shaped reinforcement rings  44 ′ remains however less efficient for reducing a bulging of the concrete  32  than the embodiment with closed circular reinforcement rings  44 . 
     The column  10  of  FIG.  6    distinguishes over the column  10  of  FIG.  1    by mainly the following features. It has an octagonal cross-section, wherein its shell surface comprises eight planar side surfaces  14   i , of which every second side surface is basically parallel to the outer surface  24   i  of one of the four outward flanges  22   i . Each of the inward flanges  18   i  limits about 52% of the surface of the respective side of the central concrete core  28 ′. In other words, the central concrete core  28 ′ is limited by the inward flanges  18   i  over about 52% of its perimeter surface  30 . It is to be noted that closed circular reinforcement rings  44  fit very well in the octagonal section of the column  10 , in which the concrete is much better used than in the column of  FIG.  2   . 
     The column  10  of  FIG.  7    distinguishes over the column  10  of  FIG.  1    by mainly the following features. It only includes three steel sections  16   i  confining a central concrete core  28 ′ that has a triangular cross-section  30 ′. The column  10  as a whole has a hexagonal cross-section, wherein its shell surface comprises three small planar side surfaces  14   1 ,  14   2 ,  14   3 , which are basically parallel to the outer surfaces  24   i  of the three outward flange  22   i , and which alternate with three large planar side surfaces  14   4 ,  14   5 ,  14   6  (“large” and “small” referring here to the width of the side surfaces). Each of the inward flanges  18   i  covers about 75% of the surface of one of the three sides of the central concrete core  28 ′. The outer concrete reinforcement cage  40 ″ comprises hexagonal reinforcement rings  44 ″ having a similar outline as the hexagonal cross-section of the column  10 . Such a column  10  is of particular interest if it has to support three horizontal beams arranged according to three different directions (here three directions mutually separated by angles of) 120°. (It remains to be noted that in  FIG.  7    the longitudinal rebars are not shown.) 
     The column  10  of  FIG.  8    distinguishes over the column  10  of  FIG.  6    by mainly the following features. It includes five steel sections  16   i  that confine a central concrete core  28 ′ having a pentagonal cross-section  30 ″. The column  10  as a whole has a decagonal cross-section, wherein its shell surface comprises ten planar side surfaces  14   i , of which every second surface is basically parallel to the outer surface  24   i  of one of the five outward flange  22   i . Each of the inward flanges  18   i  covers about 93% of the surface of the respective side of the central concrete core  28 ′. In other words, the central concrete core  28 ′ is limited by the inward flanges  18   i  over about 93% of its perimeter surface  30 ″. Such an embodiment is of particular interest, if the column  10  has to support five horizontal beams arranged according to five different directions (here five directions separated by angles of 72°). (It remains to be noted that in  FIG.  8    the longitudinal rebars are not shown.) 
     The column  10  of  FIG.  9    distinguishes over the column  10  of  FIG.  2    by mainly the following features. Along each side of the central concrete core  28 ′, which also has a square-shaped cross-section  30 , are arranged the inward flanges  18   i ,  18 ′ i  of a pair of steel sections  16   i ,  16 ′ i . The two inward flanges  18   i ,  18 ′ i  limit about 85% of the surface of the respective side of the central concrete core  28 ′. Such an embodiment is of particular interest, if the column  10  has to support two parallel horizontal bearing beams on each of its four sides or if a particularly strong steel reinforced concrete column is required. Arranging the inward flanges  18   i  of more than one steel sections  16   i  along a side of the central concrete core  28 ′ allows to design larger concrete cores  28 ′ and, consequently, larger columns despite a limitation of the flange width of the commercially available steel sections. 
     In a further embodiment of the column (not shown), which comprises six steel sections and in which the central concrete core has a rectangular cross-section with two long sides and two short sides, the inward flanges of two steel sections are arranged along each of the two long sides and the inward flange of one steel section is arranged along each of the two short sides. Such an embodiment is of particular interest, if the column has to support two parallel horizontal bearing beams along a first direction and single (or no) horizontal bearing beams according to a second direction. 
     In all embodiments shown in the drawings, all the steel sections  16   i  have the same dimensions and have inward flanges, respectively outward flanges having the same width. However, it is not excluded to have in the same steel reinforced concrete column: smaller and larger steel sections  16   i ; steel sections  16   i  having inward flanges, respectively outward flanges with different widths. 
     In all embodiments shown in the drawings, the n sides of the central concrete core  28 ′ all have the same width. However, it is not excluded to have a central concrete core whose sides have different widths. This would e.g. be the case for a central concrete core having a rectangular cross-section or a cross-section that is an irregular polygon. 
     In the embodiments of  FIGS.  1 ,  2 ,  6 ,  7  and  8   , the web of each of the steel sections  16   i  has a midplane containing the longitudinal central axis  12  of the column  10 . As shown e.g. by  FIG.  9   , this is however not necessarily the case. 
     While the columns shown in the drawings either have a circular, square-shaped, hexagonal, octagonal or decagonal cross-section, it will be understood that a column in accordance with the invention may have any kind of cross-section, including, for example: rectangular, cross-shaped and oval cross-sections, cross-sections that are regular or irregular polygons, cross-sections composed of curved lines etc. 
     It will further be understood that the cross-section of the column may decrease with the height. In such a case, the cross-section of the central concrete core may also decrease in the same proportion, so that the inward flanges of the steel sections may not be parallel to the longitudinal central axis of the column. 
       FIG.  10    is cross-section of a column  10  as shown in  FIG.  2   , more particular at a so-called beam-to-column connection node  70 , where—at a specific vertical location or level along the column  10 —a horizontal bearing beam  72   i  is secured to each of the outward flanges  22   i  of the vertical column  10 . Such horizontal bearing beams  72   i  support e.g. a floor in a high rise building. Arrow  74  points to optional transversal structural steel advantageously interconnecting the inward flanges  18   i  at the connection node  70 , at the same level where the horizontal bearing beams  72   i  are connected to the outward flanges  22   i  of the column  10 . 
       FIG.  11    is an elevation view of a column as shown in  FIG.  1 ,  2  or  6   , wherein concrete and concrete reinforcement steel are not shown. This column  10  comprises at least two longitudinally spaced beam-to-column connection nodes  70 ,  70 ′ as shown in  FIG.  10   , for supporting two successive floors. It will be noted that between the two longitudinally spaced beam-to-column connection nodes  70 ,  70 ′ there is no structural steel interconnecting the steel sections  16   i . In other words, between the two longitudinally spaced connection nodes  70 ,  70 ′ of the column  10 , the steel sections  16   i  are structurally interconnected exclusively by the steel reinforced concrete  32 . 
     While the present invention has been described more specifically with regard to a steel reinforced concrete column for a high rise building, it will be understood that a steel reinforced concrete column in accordance with the invention may also be used in nonbuilding structures such as e.g. huge halls, platforms, bridges, pylons etc. 
     
       
         
           
               
             
               
                   
               
               
                 Reference signs list 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 10 
                 steel reinforced concrete column 
               
               
                   
                 12 
                 longitudinal central axis of 10 
               
               
                   
                 14 
                 shell surface of 10 
               
               
                   
                 14 i   
                 side surfaces of 14 
               
               
                   
                 16 i   
                 hot-rolled steel section 
               
               
                   
                 18 i   
                 inward flange of 16 i   
               
               
                   
                 20 i   
                 outer surface of 18 i   
               
               
                   
                 22 i   
                 outward flange of 16 i   
               
               
                   
                 24 i   
                 outer surface of 22 i   
               
               
                   
                 26 i   
                 web of 16 i   
               
               
                   
                 28 
                 n-sided central core volume 
               
               
                   
                 28&#39; 
                 n-sided central concrete core (= 28 filled with concrete) 
               
               
                   
                 30 
                 outer limit of 28 (= perimeter surface of 28’) 
               
               
                   
                 32 
                 concrete 
               
               
                   
                 34 
                 shear connector 
               
               
                   
                 36 
                 shear connector 
               
               
                   
                 38 
                 shear connector 
               
               
                   
                 40 
                 outer reinforcement cage 
               
               
                   
                 42 
                 vertical reinforcement bar (vertical rebar) 
               
               
                   
                 44 
                 closed circular reinforcement ring 
               
               
                   
                 44&#39; 
                 closed square-shaped reinforcement ring 
               
               
                   
                 46 
                 mesh of 40 
               
               
                   
                 48 
                 helically wound continuous rebar 
               
               
                   
                 50 
                 inner reinforcement cage 
               
               
                   
                 52 
                 vertical reinforcement bars 
               
               
                   
                 54 
                 closed circular reinforcement ring 
               
               
                   
                 58 
                 helically wound continuous rebar 
               
               
                   
                 60 
                 corner bracket 
               
               
                   
                 70, 70&#39; 
                 beam-to-column connection node of 10 
               
               
                   
                 72 i   
                 horizontal bearing beam 
               
               
                   
                 74 
                 transversal structural steel interconnecting 18