Patent Publication Number: US-11639601-B2

Title: System and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of parent application Ser. No. 16/986,249, filed Aug. 5, 2020, pending, the priority of which is claimed. 
    
    
     BACKGROUND 
     1. Field 
     The disclosure of the present patent application relates to construction of buildings, bridges, and similar structures having columns of tubular steel filled with concrete, and particularly to a system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing. 
     2. Description of the Related Art 
     There is an increasing trend in using concrete-filled steel tubular (CFST) columns in recent decades, such as in industrial and high-rise buildings, structural frames, and bridges. CFST columns promote economical and rapid construction. They offer increased strength and stiffness relative to structural steel and reinforced concrete columns. The steel tubes serve as a formwork and reinforcement for the concrete fill, thereby reducing the labor requirements. CFST columns encourage the optimal use of the two materials (concrete and steel), while providing a symbiotic relationship between the two to mitigate undesirable failure modes. The concrete fill increases the compressive strength and stiffness, delays and restrains local buckling of the steel tube, and enhances ductility and resistance. Both rectangular and circular CFSTs have been employed. A missing component for CFST construction is the reliable and ductile column-to-foundation connections under seismic or cyclic lateral loading. 
     Recently, the present inventors have developed an efficient CFST column-to-foundation connection for circular columns. See U.S. Pat. No. 10,563,402, issued Feb. 18, 2020. However, there is no efficient and effective connection available for the rectangular/square columns. There is a need for such CFST column-to-foundation connection for rectangular/square columns that can transfer combined bending and axial loads and have sufficient deformability to sustain multiple inelastic deformation cycles under extreme seismic loading. 
     Thus, a system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing solving the aforementioned problems is desired. 
     SUMMARY 
     The system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing begins with forming a cavity in the reinforced concrete footing, the cavity having an elliptical opening at the top of the footing and a circular base. A short steel pipe is partially embedded in the footing, the pipe having a top end and a bottom end. At least two flanges extend radially from the top and bottom ends of the pipe, the bottom end being embedded in the footing and the top end extending through the base of the cavity so that the flanges extend above the base of the cavity. An elliptical base plate is welded to the bottom of the tubular steel column, the base plate having a circular opening defined therein and a plurality of spaced flange slots depending therefrom. The bottom end of the column is lowered into the cavity, the elliptical base plate passing through the elliptical opening in the cavity, and the column is rotated 90° to interlock the flanges with the flange slots. The cavity is filled with concrete grout, and the square or rectangular steel tubular column is filled with concrete. 
     The column-footing connection formed in this manner provides improved connection between square CFST columns and RC footings for carrying gravity and lateral loads. It also minimizes the fabrication work after first-stage concreting of RC footing and controls the story drift in high-rise buildings in which CFST columns are becoming more popular. The system and method enhance the connection response and construction ease while maintaining the benefits of precast construction. 
     These and other features of the present disclosure will become readily apparent upon further review of the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a square steel tubular column with attached base plate as seen from below in a system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing. 
         FIG.  2    is a perspective view of the square steel tubular column with attached base plate of  FIG.  1    as seen from above in a system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing. 
         FIG.  3    is a perspective view of a flange slot shown before attachment to the base plate of  FIG.  1   . 
         FIG.  4    is an exploded perspective view of the flange slots and base plate of  FIG.  1   . 
         FIG.  5    is a perspective view of the assembled base plate of  FIG.  1    as seen from below, shown before attachment to the bottom of the steel tubular column. 
         FIG.  6    is a perspective view of a cavity formed in a reinforced concrete footing in a system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing. 
         FIG.  7    is a steel form used to make the cavity of  FIG.  6   . 
         FIG.  8    is a top view of the elliptical and circular rings used in the steel form of  FIG.  7    to make the cavity of  FIG.  6   . 
         FIG.  9    is a perspective view of a short steel pipe that will be partially embedded in the footing of  FIG.  6   . 
         FIG.  10 A  is a diagrammatic top view of a square steel tubular column after initial placement in the footing cavity of  FIG.  6    and embedding the steel pipe of  FIG.  9   , but before rotation of the column. 
         FIG.  10 B  is a section view drawn along lines  10 B- 10 B of  FIG.  10 A . 
         FIG.  10 C  is a section view drawn along lines  10 C- 10 C of  FIG.  10 A . 
         FIG.  11 A  is a diagrammatic top view of a square steel tubular column after initial placement in the footing cavity of  FIG.  6    and embedding the steel pipe of  FIG.  9   , and after 90° rotation of the column to interlock the flanges with the flange slots. 
         FIG.  11 B  is a section view drawn along lines  11 B- 11 B of  FIG.  11 A . 
         FIG.  11 C  is a section view drawn along lines  11 C- 11 C of  FIG.  11 A . 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing begins with forming a cavity in the reinforced concrete footing, the cavity having an elliptical opening at the top of the footing and a circular base. A short steel pipe is partially embedded in the footing, the pipe having a top end and a bottom end. At least two flanges extend radially from the top and bottom ends of the pipe, the bottom end being embedded in the footing and the top end extending through the base of the cavity so that the flanges extend above the base of the cavity. An elliptical base plate is welded to the bottom of the tubular steel column, the base plate having a circular opening defined therein and a plurality of spaced flange slots depending therefrom. The bottom end of the column is lowered into the cavity, the elliptical base plate passing through the elliptical opening in the cavity, and the column is rotated 90° to interlock the flanges with the flange slots. The cavity is filled with concrete grout, and the square or rectangular steel column is filled with concrete. 
     As shown in  FIGS.  1 - 5   , an elliptical base plate  10  with a central circular (or square) hole  12  is prepared for attachment to the base of the square steel tubular column  15 . The minor diameter of the base plate  10  is slightly greater than the outer size of the concrete-filled steel tubular (CFST) column  15  and the major diameter is 10% to 40% larger than the minor diameter. The diameter of the circular hole  12  in the base plate  10  is less than or equal to the size of the square column  15  (the size of the hole  12  shown in  FIG.  2    is equal to the size (i.e., the width of one side) of the square column  15 ). By keeping the diameter of the circular hole  12  smaller than the size of the square column  15 , the size of the base plate  10  can be reduced. This also helps in welding the base plate  10  properly to the inner face of the steel tubular column  15 . However, the size of the circular hole should not be less than that required for easy access for welding of the base plate  10  (to the inner face of the steel tubular column  15 ). Also, the system and method can be used for circular CFST columns, in which the diameter of the hole in the elliptical base plate can be less than the diameter of steel pipe of column. Two quadrant slots  14  are cut (these may be in the form of several small size slots at regular spacing, which will require corresponding teeth in the form of vertical circular segmental plate of the flange slots) in the base plate  10 , as shown in  FIG.  4   , for accommodating the arcuate angles forming the flange slots  16  (female). The flange slots  16  are prepared by welding horizontal quadrant arcuate plate  18  with vertical circular segmental plate  20 , as shown in  FIG.  3   , i.e., the slots  16  are arcuate angles having a vertical flange  20  and a horizontal flange  18  defining the slots  16 . The flange slots  16  are fixed in the cut slots  14  of the base plate  10  and welded to form flange slots  16  depending from or extending below the base plate  10 , as shown in  FIGS.  4  and  5   . This method of welding is adopted for avoiding difficulty in welding the inner edges of flange slots to the base plate without cut slots. This base plate assembly is then welded to the column base. Although  FIGS.  1 - 5    show two diametrically opposed 90° flange slots  16 , it will be understood that in some embodiments, the base plate  10  may have more than two flanges slots  16 . 
     As shown in  FIGS.  6 - 8   , during the casting of the reinforced concrete (RC) footing  22 , a cavity  24  is created for accommodating the CFST column base. The shape of the cavity  24  is such that it transforms from an elliptical shape in plan at the top  26  of the RC footing  22  to a circular shape at the base  28  of the cavity  24 , as shown in  FIG.  6   . The diameter of the base  28  of the cavity  24  is equal to the major diameter of the elliptical opening. The major axis of the elliptical cavity is aligned with the axis of maximum column moment. The rebars on the cavity surface should be in the shape of the cavity  24 , which can be easily achieved by leaving a uniform clear cover on the surface of the cavity  24 . The cavity  24  is formed by using a demountable cavity form  30 , shown in  FIG.  7   . The cavity form  30  is fabricated using an upper elliptical ring  32  and a bottom circular ring  34 , shown in  FIG.  8   , which are connected through slanting steel strips  36  with the help of screws or other fasteners, as shown in  FIG.  7   . The two rings  32 ,  34  and the strips  36  have screw holes at regular intervals, which are used for connecting wooden battens (not shown in  FIGS.  7  and  8   ) for closing the openings. The smooth transition from elliptical at the top  26  to circular at the base  28  of the cavity  24  is not required. The shape of the cavity  24  at the top  26  and the base  28 , however, is significant. For demounting the form  30 , the wooden battens can be easily removed by unscrewing the screws. The steel cage can either be left in place or extracted by unscrewing the screws connecting the strips  36 . In case the steel cage is be extracted, it should be lubricated or covered with plastic sheet before concreting. The bottom circular steel ring  34  can either be left in place, or if this is to be extracted, it should be fabricated by screwing two or more semicircular segments together. 
     The depth of the cavity  24  in the RC footing  22  may vary from 20% to 100% of the outer size of the square CFST column  15 , depending upon the connection design. As shown in  FIG.  9   , a small length of the steel pipe  40  with two opposite flanges  42  (or collars) welded at its top  44  as well as at the bottom  46  of the pipe  40  at vertically the same alignment is partially embedded in the RC footing  22 , as shown in  FIGS.  10 A- 11 C . The top flanges  42  can be welded on the top edge  44  of the pipe  40  (as shown in  FIG.  9   ) or on the outside face of the pipe  40  and flush with the top edge  44  of the pipe  40 . The flanges  42  may be diametrically opposite each other and extend radially outward from the pipe  40  in a 90° arc. The welding on the outside face of the pipe  40  will make the top edge  44  of the pipe assembly flat, thus making the column base plate  10  to rest on it without any gap between the two, as seen in  FIG.  11 B . The use of flanges  42  at the bottom  46  of the pipe  40  helps in improving the anchorage of the steel pipe  40  in the concrete footing  22 , and hence reducing the length of the pipe  40 , which is desired when sufficient depth is not available for accommodating the pipe  40  in the concrete footing  22 . The bottom flanges  42  will also help in keeping the small embedded steel tube  40  in position before the first-stage concreting of the RC footing  22 . Other means of better anchoring of the small embedded steel pipe  40  may alternatively or additionally be adopted. These may include the use of shear studs welded to the inner/outer or both surfaces of the embedded steel pipe or making perforations in the embedded length of the steel pipe. The height of the pipe  40  projecting through the base  28  into the cavity  24  is such that there is a gap equal to the thickness of steel plate under the upper flanges  42 . The width of all flanges  42  is the same and may vary from 10% to 25% of the outer size of the steel tube, but not less than the thickness of pipe. Each flange  42  subtends an angle of 90° at the center (axis of column). These flanges  42  are located symmetrically opposite to the major axis of the elliptical cavity opening, as shown in  FIGS.  10 A- 11 C . The outer diameter of the flanges  42  is equal to the minor diameter of the ellipse at the top  26  of the cavity  24  minus the thickness of the steel plates used for making the flanges  42 . The longitudinal axis of the small pipe  40  embedded in the first-stage concreting of the RC footing  22  is aligned with the longitudinal axis of the square CFST column  15 . The length of this small embedded steel pipe  40  is such that it can be accommodated in the RC footing  22  under the cavity  24 . 
     After hardening of the first-stage concrete of the RC footing  22 , the square steel tubular column  15  with welded base plate  10  assembly is lowered into the cavity  24  of the RC footing  22 . The shape of both the top  26  of the cavity  24  as well as the base plate  10  of the column  15  being elliptical, the column  15  will be required to be aligned so that the elliptical base plate  10  of the steel column  15  may be lowered vertically into the cavity  24 . After the initial lowering of the column  15  to the base  28  of the cavity  24  (shown in  FIGS.  10 A- 10 C ), the steel tubular column  15  is rotated by 90°, thereby making an interlock between the flanges  42  of the steel pipe  40  embedded in the first-stage concrete of the RC footing  22  and the corresponding flange slots  16  at the column base  10 , as shown in  FIGS.  11 A- 11 C . The thickness of the flanges  42  (male) and matching slots  16  (female) should be equal to or greater than the thickness of the steel tube of the CFST column  15 . 
     The foundation cavity  24  is then filled with second-stage non-shrinkable cement grout. After the hardening of the second-stage cement grout, concreting is done in the steel tubular column  15 , thereby converting it to the CFST column. 
     Enough clearances are to be maintained between the coupling members for their free movement. However, these should not be very loose to avoid large slackness. 
     The circular opening  12  in the base plate  10  may be square and of the same size as the inner size of the tubular column  15  or smaller. The smaller size of the opening, and hence the smaller major diameter of the base plate  10 , will not only reduce the foundation cavity size, but also reduce the bending moment in the overhang portion of the base plate  10  due to the reduction in the overhang. 
     The bending of the column under the action of lateral loads on the column tries to pull the square CFST column out of the cavity. The proposed connection resists this pull out and hence provides moment resisting capacity to the column base by the following mechanisms. 
     In a first mechanism, mechanical interlock between the mating steel flanges of the small embedded steel pipe (male) and the flange slots (female) welded underneath the elliptical base plate of the steel tubular column resists the column moments. This contributes significantly in resisting the column moments. 
     In a second mechanism, even after failure of the mechanical interlock or severe deformation in the interlocking flanges, the elliptical column base plate (which is now embedded in cement grout) cannot come out because the second-stage grout need to be pushed upward, which will be resisted by the negatively sloping interface between the first-stage concrete of the RC footing and the second-stage cement grout. This is because the width of the second-stage grout at the top of the RC footing is equal to the minor diameter of the ellipse. 
     The system and method described above is susceptible to variation in several respects. In a first variation, the elliptical shape of the cavity in the first-stage concrete of the RC footing and the column base plate may be replaced by rectangular shapes with rounded corners. The diameter of the base of the first-stage concrete of the RC footing would be equal to the length of the rectangle. 
     In a second variation, the use of two flanges subtending an angle of 90° is most efficient for resisting column moment (or bending) about the major axis of elliptical cavity. However, for resisting column moment in two transverse directions (biaxial bending), the number of flanges (or collars), n, welded to the small steel pipe embedded in the first stage of concrete of the RC footing and the corresponding n flange slots (female) welded to the elliptical base plate of the steel column may be more than two (preferably four or more, depending on the circumferential length of the flanges, as per design). The angle subtended by these flanges would then be 360/(2n) degrees. The use of more than two flanges reduces rotation of the column for achieving mechanical interlock, which will be 360/(2n) degrees. However, for aligning the major axis of the base plate  10  with the minor axis of the elliptical opening  26 , the column is rotated by 90°. In this position, the connection offers maximum moment of resistance along the major axis of the elliptical cavity. 
     In a third variation, reliance may be placed substantially on the use of mechanical interlock alone, wherein the shape of the cavity in the first-stage concrete is cylindrical. Thus, the column base plate may also be circular instead of elliptical. This simplifies the construction of the cavity in the first-stage concrete of the RC footing. The column moments (bending) in this type of connection is resisted by mechanical interlock and the resistance offered by a cylindrical interface between the first-stage concrete of the RC footing and the cement grout. 
     In a fourth variation, the connection may be made without mechanical interlock, which is same as described, above but without any mechanical interlocking flanges. Thus, there is no requirement of embedding a small steel pipe in the first-stage concrete of the RC footing, and no requirement of flange slots welded to the base plate of the steel tubular column. The surface of the cylindrical cavity can be made corrugated for providing additional moment of resistance. 
     The selection of the type of connection will be based on the moment-resisting requirements, ease of construction, etc. 
     Finally, the proposed connection can be easily extended to rectangular and polygonal CFST column-to-foundation connections. 
     The proposed connection is expected to avoid failure of the square CFST column bases. The enhancement in the moment-resisting capacity of the connection reduces the story drift when the proposed connection is adopted in the CFST columns of high-rise buildings. When these columns are used in bridges, the proposed connection helps in reducing vibrations, and keeps the lateral bridge movements in check. 
     It is to be understood that the system and method for connecting a square concrete-filled steel tubular column to a reinforced concrete footing is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.