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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates, in general, to a modular column system constructed by stacking at least one precast unit between a foundation section and a coping section, as well as a method of constructing the same, in which the precast unit makes use of an internally confined hollow column unit fabricated in advance, and in which a joint section between the precast units is firmly formed. As a result, the modular column system realizes a short construction period and economy because reinforcement bars and forms are not used, and also realizes high resistance to bending moment and a reduction in cross section and self-weight of the precast unit, so that the modular column system enables easier and more economical assembly, and prevents brittle fracture of the joint section between the precast units. 
         [0003]    2. Description of the Related Art 
         [0004]    Generally, piers constructed in bridge work are cast-in-place concrete structures, in which, after foundation pit excavation work is carried out at the site, concrete is poured into formwork, in which reinforcement bars are arranged, and is cured for a predetermined period of time, and thereby a foundation section is formed. 
         [0005]    After the foundation section is cured, the reinforcement bars and forms are arranged on the cured foundation section, and the concrete is poured into and cured in the formwork to form a pier. An upper portion of the pier, i.e. a coping section supporting a bridge deck, is formed by pouring concrete in a primary or secondary process after the forms are arranged in the state in which a staging is supported. Thereby, the pier is finished. 
         [0006]    For this reason, a long construction period and great expense are required for the work of setting up the formwork and dismantling the formwork after curing the concrete. Further, problems with the pier can occur depending on the construction location. For example, in the case of constructing an elevated roadway, the pier creates a traffic jam around the construction site. Further, in the situation where the work environment is unfavorable, as in underwater work, the construction and management of the pier are difficult, and the possibility of faulty construction is increased. 
         [0007]    In consideration of these aspects, piers have been constructed at sites in a method of constructing the foundation section including the foundation pit excavation under the ground, and then fabricating and assembling the pier as a unit structure. 
         [0008]    To realize the advantages of construction of the modular pier, because a structure fabricated in a precast way in respective units, namely precast units, is fabricated in a factory, it is advantageous to control the quality of the concrete. Further, because the precast units are continuously fabricated, it is advantageous to manage manpower and the forms. In addition, because the precast units can be fabricated together with the construction of the foundation section, it is possible to reduce the construction period in comparison with the cast-in-place method. 
         [0009]    In this manner, the modular pier constructed using the precast unit is disclosed in Korean Patent No. 10-99113 (titled “Modular Pier and Column Structure and Method of Constructing the Same”). 
         [0010]    In the document, a plurality of precast units having a cross section of a spherical shape, a circular shape, or an oval shape is fabricated according to the height of a pier. Among the precast units, an upper precast unit has a convex shape at a lower end thereof, whereas a lower precast unit corresponding to the upper precast unit has a concave shape at an upper end thereof. Then, the upper and lower precast units are assembled. The convex and concave portions are provided with shear keys that are adapted to transmit compressive force, tensile force, axial force, and shear force when a bending moment is applied to the upper precast unit at five different positions. The shear keys are assembled in a construction method of injecting grout into shear key recesses of the middle thereof through an injection hole from the outside. 
         [0011]    The above-described construction of the modular pier is characterized by directly assembling and constructing the pier structure as a unit structure at a site, and by the unit stably withstanding the fractional force transmitted thereto via the shear keys, installed in different directions, in conjunction with the injection of the grout. 
         [0012]    However, when constructing this pier, it is difficult to set up the formwork for fabricating units having convex and concave shapes. Practically, due to its self-weight, the precast unit made of concrete has nothing but to be lifted by a crane, so that, when constructing a high pier, the number of precast units to be fabricated is excessively increased. 
         [0013]    Meanwhile, the pier of the bridge serves to accept the load of its upper structure as the force to transmit it in a downward direction, and acts as a main member for resisting transverse loads, such as that of an earthquake. Hence, the pier should be designed such that it can resist vertical loading, transverse loading, bending moment, and so forth. 
         [0014]    In consideration thereof, the design of the pier is based on the concept of a plastic hinge enabling core concrete to resist great compressive deformation and thus have the capability to dissipate energy. This means that the pier is endowed with ductile capability capable of causing plastic deformation against repeated load without remarkable reduction of stress resistance or rigidity. 
         [0015]    The response modification factor taken into consideration in an earthquake-proof design is greatly influenced by this ductile capability. In the bridge, the ductile capability of the pier accounts for most of the ductile capability of the entire bridge. 
         [0016]    Currently, the design criteria of roadway and railway bridges prescribe a transverse reinforcement ratio in order to secure the ductile capability of the plastic hinge section of the pier with respect to the earthquake and the transverse load. A reinforced concrete pier having a solid cross section is widely constructed on the basis of the transverse reinforcement ratio, and has good load supporting capacity. 
         [0017]    However, the solid cross-section reinforced concrete pier has various disadvantages in that it is difficult to apply to a place where the foundation section encounters a structural problem due to the self-weight thereof, that it is economically unfavorable due to the increase in concrete material cost, and in that it encounters a chance of cracks occurring due to the generation of the heat of hydration when concrete is poured. 
         [0018]    For this reason, attention is paid to a steel pier having advantages of good ductile capacity and a reduced construction period in spite of a certain disadvantage with respect to construction expenses. 
         [0019]    However, the steel pier generally has a relatively greater width over the thickness of a deck constituting the pier, and thus has a problem in that it is vulnerable to local buckling when earthquakes occur. 
         [0020]    Conventionally, in order to address this problem, a concrete filled steel tube (CFT) is used. CFT refers to a structure in which a steel tube having a circular or angular cross section is filled with concrete. Because the concrete is confined in the steel tube, CFT has advantages in that it is excellent in preventing the local buckling, has good ductile capability, and is reduced in cross section and self-weight due to an increase in resistance to the bending moment compared to the existing concrete pier. 
         [0021]    However, CFT has a problem in that the material expense thereof is very high. Further, CFT still has a problem in that, when constructed as a high pier, it is increased in cross section and self-weight and thus is unsuitable for on-site conditions having restrictions on the ratio of width to thickness. In addition, CFT encounters a problem of maintenance for preventing corrosion thereof in on-site conditions, in which the corrosion becomes an issue, as in the underwater pier. 
         [0022]    Further, in the case in which the pier of the bridge or the column of a building has a structural problem due to the excessive self-weight of concrete, or the material expense of the concrete is relatively high, a pier or a column having a hollow cross section is used, instead of one having a solid cross section. 
         [0023]    Such a hollow cross-section pier or column is estimated to have high applicability because the resistance to the bending moment is not dynamically weaker compared to a usual pier or column. 
         [0024]    Especially, as earthquake-proof design is currently becoming a hot issue, there is a necessity to design a pier or column capable of withstanding a greater bending moment, and furthermore, transverse displacement is required. Thus, when designing a pier or column capable of withstanding a great bending moment, a hollow cross-section pier or column, the self-weight of which is reduced because the inside thereof is hollow, can be more favorable according to the circumstances. 
         [0025]    However, the ductile capacity of the hollow cross-section pier or column is doubtful, because it is difficult to expect a concrete confining effect therefrom. In other words, due to brittle fracture behavior caused in the inside of the hollow cross section because there is no concrete confining effect, the pier or column having a hollow cross-section evidences poor ductile capability in practice. 
         [0026]    Nevertheless, in the case of modular piers proposed to date, no special study has been made of excessive self-weight in the case of using concrete for the precast unit, high material expense in the case of using CFT as a material for replacing concrete, whether to introduce the hollow cross section as an approach to resolve the increase in cross section and self-weight when constructing a high pier, the brittle fracture of the hollow cross section in the case of using the hollow cross section, and maintenance in the case of using CFT for, for example, an underwater pier. 
       SUMMARY OF THE INVENTION 
       [0027]    Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a modular column system using at least one internally confined hollow column unit, in which the internally confined hollow column unit includes an outer pipe section formed of steel or fiber reinforced plastic (FRP), a concrete section filled with concrete and forming a hollow section inside the outer pipe section, and an inner pipe section installed in the hollow section of the concrete section and formed of steel or FRP, which confines the concrete section, thereby providing easy assembly of the units as well as economical construction due to the advantages derived from construction using a general modular column system, due to the hollow section, and due to a reduction in self-weight caused by reduction of a cross section, and improving corrosion resistance by means of the inner and outer pipe sections formed of FRP in the case of, for example, an underwater pier. 
         [0028]    In order to achieve the above object, according to one aspect of the present invention, there is provided a modular column system using at least one internally confined hollow column unit. The modular column system includes at least one precast unit installed between a foundation section and a coping section, and means for fastening the precast unit to the foundation section, fastening the precast units to each other, and fastening the coping section to the precast unit. The precast unit is an internally confined hollow column unit which includes an outer pipe section formed of steel or fiber reinforced plastic (FRP), a concrete section filled with concrete and forming a hollow section inside the outer pipe section, and an inner pipe section installed in the hollow section of the concrete section and formed of steel or FRP confining the concrete section. 
         [0029]    Therefore, the modular column system of the present invention is constructed in a manner such that the internally confined hollow column unit is attached to the upper portion of the foundation section by means of the fastening means, such that the internally confined hollow column units are stacked on and attached to the attached hollow column unit up to the designed height of the modular column system, and such that the coping section is attached to an upper portion of the uppermost one of the internally confined hollow column units. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
           [0031]      FIG. 1  is a perspective view illustrating an internally confined hollow column unit in a modular column system using at least one internally confined hollow column unit in accordance with the present invention; 
           [0032]      FIG. 2  is a partial cross sectional view illustrating an internally confined hollow column unit as a component of the present invention, wherein an inner pipe section of the internally confined hollow column unit is formed as a cylindrical pipe; 
           [0033]      FIG. 3  is a partial cross sectional view illustrating an internally confined hollow column unit as a component of the present invention, wherein an inner pipe section of the internally confined hollow column unit is formed as a corrugated pipe; 
           [0034]      FIG. 4  is a perspective view illustrating a shear connector that is additionally installed to an internally confined hollow column unit as a component of the present invention; 
           [0035]      FIG. 5  is a cutaway view illustrating an internally confined hollow column unit as a component of the present invention, wherein shear connectors are attached to inner and outer pipe sections of the internally confined hollow column unit; 
           [0036]      FIG. 6  is a side cross sectional view illustrating the state in which internally confined hollow column units are stacked between a foundation section and a coping section, and are attached to each other at joint sections therebetween by fastening means in accordance with the present invention; 
           [0037]      FIG. 7  is an exploded perspective view illustrating the state in which internally confined hollow column units are stacked between a foundation section and a coping section in accordance with the present invention; 
           [0038]      FIG. 8  is a perspective view illustrating a bar-like member that includes a body section and a head section in accordance with an embodiment of the present invention; 
           [0039]      FIG. 9  is an exploded perspective view illustrating the state in which a foundation section and a lowermost internally confined hollow column unit, two adjacent internally confined hollow column units, an uppermost internally confined hollow column unit, and a coping section are fastened by fastening means in accordance with an embodiment of the present invention; 
           [0040]      FIG. 10  is an exploded perspective view illustrating the state in which a common joint section between internally confined hollow column units is attached by supports in accordance with the present invention; 
           [0041]      FIG. 11  schematically illustrates a process of constructing a modular column system using at least one internally confined hollow column unit in accordance with the present invention; and 
           [0042]      FIG. 12  is a flowchart illustrating a method of constructing a modular column system using at least one internally confined hollow column unit in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]    Reference will now be made in greater detail to an exemplary embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
         [0044]    As illustrated in  FIG. 1 , an inner pipe section  13  is installed in the hollow section  14  of a concrete section  12 , and thus it confines the concrete section  12 . Thus, the concrete section  12  is under a triaxial compression load. Thereby, when a precast unit is fabricated, the hollow section  14  of the concrete section  12  is formed so as to reduce the self-weight of the precast unit to facilitate assembly, thereby preventing incidental brittle fracture in the cross section of the hollow section  14  of the concrete section  12 . Further, the inner pipe section  13  is inserted to reinforce the resistance to bending moment as well as to reduce the cross section and self-weight of the precast unit, and thus an internally confined hollow column unit  10  is preferably used as the precast unit for the assembly of a pier. 
         [0045]    When constructed for a general structure, the inner pipe section  13  and an outer pipe section  11  are preferably made of steel, thereby securing the resistance to bending moment. 
         [0046]    However, when being constructed for use in a corrosive environment, such as for an underwater pier, the inner and outer pipe sections  13  and  11  can be made of fiber reinforced plastic (FRP) having corrosion resistance and ductility. More preferably, the FRP is selected from materials suitable for the conditions of on-site construction, for instance FRPs to which reinforcing materials have been added, such as carbon FRP (CFRP), aramid FRP (AFRP), glass FRP (GFRP), and so on. Further, the use of such an FRP incidentally reduces the self-weight of the structure, thereby serving to make assembly easy. 
         [0047]    As illustrated in  FIG. 1 , the diameter D 1  of the hollow section  14  can be adjusted in a factory so as to meet the conditions of on-site construction in consideration of self-weight, costs of concrete materials, etc. at the time of assembly. 
         [0048]    The hollow section  14  is formed so as to run through the concrete section  12 . Thus, as illustrated in  FIG. 2 , in the case in which the inner pipe section  13  is formed as a cylindrical pipe  13   a , the hollow section  14  has the shape of a cylindrical column, thus being integrally formed with the inner pipe  13   a . As illustrated in  FIG. 3 , in the case in which the inner pipe section  13  is formed as a corrugated pipe  13   b , the hollow section  14  has the shape of a cylindrical corrugated column, and is thus integrally formed with the corrugated pipe  13   b.    
         [0049]    As illustrated in  FIGS. 2 and 3 , the inner pipe section  13  can employ the cylindrical pipe  13   a  having smooth surfaces, or a corrugated pipe  13   b  alternating convexity with concavity. 
         [0050]    In the case of using the cylindrical pipe  13   a , there is an advantage in that it can exert resistance to axial compressive force and bending moment. In the case of using the corrugated pipe  13   b , it has a good effect of confining the concrete section  12 , and thus is suitable for preventing local buckling of the inner pipe section  13  as well as increasing the ductility of the precast unit, so that it can be properly selected according to the conditions at a construction site. 
         [0051]    As illustrated in  FIG. 5 , the outer pipe section  11  is provided with a plurality of shear connectors  16 , which serve to secure unified behavior with the concrete section  12 , attached to the inner surface thereof, to which the concrete section  12  is attached. 
         [0052]    Similarly, as illustrated in  FIG. 5 , the inner pipe section  13  is also provided with a plurality of shear connectors  16 , which serve to secure unified behavior with the concrete section  12 , attached to the outer surface thereof, to which the concrete section  12  is attached. 
         [0053]    The shear connectors  16  are formed of steel in the case in which the inner and outer pipe sections  13  and  11  are formed of steel. Thereby, the shear connectors  16  are preferably welded to the outer and inner surfaces of the inner and outer pipe sections  13  and  11 , to which the concrete section  12  is attached. At this time, a weld zone between the inner or outer pipe section  13  or  11  and each shear connector  16  is preferably prevented from brittle fracture by maintaining strength greater than that of each of the inner and outer pipe sections  13  and  11  and the shear connectors  16 . 
         [0054]    Meanwhile, the shear connectors  16  are formed of FRP in the case in which the inner and outer pipe sections  13  and  11  are formed of FRP. Thereby, the shear connectors  16  are preferably bonded to the outer and inner surfaces of the inner and outer pipe sections  13  and  11 , to which the concrete section  12  is attached. At this time, the bonding zone between the inner or outer pipe section  13  or  11  and each shear connector  16  is preferably prevented from brittle fracture by maintaining strength greater than that of each of the inner and outer pipe sections  13  and  11  and the shear connectors  16 . 
         [0055]    As illustrated in  FIG. 4 , each shear connector  16  includes a stiffener portion  16   a  that has plate shape and is attached in an axial direction of the unit and functions to resist the buckling of the unit, and a stud portion  16   b  that is integrally formed with the stiffener portion  16   a , is attached in a plate shape in a radial direction of the unit, and secures unified behavior of the inner and outer pipe sections  13  and  11  with the concrete section  12 . Thereby, each shear connector  16  can be constructed to secure unified behavior of the inner and outer pipe sections  13  and  11  with the concrete section  12 , as well as exert a function of preventing the precast unit from buckling. 
         [0056]    Meanwhile, as illustrated in  FIGS. 6 and 7 , means for fastening a foundation section  20  and the uppermost one of the internally confined hollow column units  10  includes a plurality of foundation section anchoring holes  21  that are formed in the upper portion of the foundation section  20 , a plurality of lower anchoring holes  15   b  that are formed in the lower portion of the concrete section  12  and are opposite the foundation section anchoring holes  21 , a plurality of bar-like members  40  that are inserted between the lower anchoring holes  15   b  and the foundation section anchoring holes  21 , and grout  50  that fixes the bar-like members  40 . 
         [0057]    Further, means for fastening a coping section  30  and the lowermost one of the internally confined hollow column units  10  includes a plurality of coping section anchoring holes  31  formed in the lower portion of the coping section  30 , a plurality of upper anchoring holes  15   a  that are formed in the upper portion of the concrete section  12  and are opposite the coping section anchoring holes  31 , a plurality of bar-like members  40  that are inserted between the upper anchoring holes  15   a  and the coping section anchoring holes  31 , and grout  50  that fixes the bar-like members  40 . 
         [0058]    Also, means for fastening the two adjacent internally confined hollow column units  10  includes a plurality of upper anchoring holes  15   a  that are formed in the upper portion of the lower one of the internally confined hollow column units  10 , a plurality of lower anchoring holes  15   b  that are formed in the lower portion of the upper one of the internally confined hollow column units  10  and are opposite the upper anchoring holes  15   a , a plurality of bar-like members  40  that are inserted between the upper anchoring holes  15   a  and the lower anchoring holes  15   b , and grout  50  that fixes the bar-like members  40 . 
         [0059]    These fastening means are constructed to attach the lowermost one of the internally confined hollow column units  10  to the foundation section  20 , and between the two adjacent internally confined hollow column units  10 , and between the uppermost one of the internally confined hollow column units  10  and the coping section  30  by inserting the bar-like members  40  into the anchoring holes  15   a ,  15   b ,  21  and  31 , and then curing the grout  50  between the bar-like members  40  and the anchoring holes  15   a ,  15   b ,  21  and  31 . 
         [0060]    In order to provide stronger attachment, as illustrated in  FIG. 8 , the bar-like members  40  include a body section  41  and head sections  42  that are located at opposite ends of the body section  41  and are thicker than the body section  41 . Preferably, when the bar-like members  40  are inserted into the anchoring holes  15   a ,  15   b ,  21  and  31 , and then are attached by injecting of the grout  50 , each bar-like member  40  is more strongly attached due to the step in each head section  42 . Each bar-like member  40  is preferably made of a steel bar, etc, and the dimensions thereof, such as length, are adjusted according to the bending moment generated from the joint sections  80  of the internally confined hollow column units  10 . 
         [0061]    The numbers of the bar-like members  40  and the anchoring holes  15   a ,  15   b ,  21  and  31  are preferably adjusted depending on the bending moment generated in the joint sections  80  of the internally confined hollow column units  10 . 
         [0062]    The grout  50  is used not only for attaching the bar-like members  40  to the anchoring holes  15   a ,  15   b ,  21  and  31 , but also for preventing the bar-like members  40  from corroding in the case in which each bar-like member  40  is made of a mixture of a steel bar and cement paste or mortar. The grout  50  preferably has fluidity and expansibility suitable to compactly fill the anchoring holes  15   a ,  15   b ,  21  and  31 . 
         [0063]    As illustrated in  FIGS. 6 and 7 , the foundation section  20  is provided with a column unit insertion recess  22  into which the lowermost internally confined hollow column unit  10  is inserted. Thereby, the lowermost internally confined hollow column unit  10  can be firmed stacked on the foundation section  20 . The foundation section  20  can be formed in the shape of a circle, a tetragon or the like depending on the shape of the lowermost internally confined hollow column unit  10 . 
         [0064]    According to one embodiment of the present invention, as illustrated in  FIG. 9 , the means for fastening the foundation section  20  and the lowermost internally confined hollow column unit  10  includes a lower outer flange  62   a  that is attached to the lower outer circumference of the outer pipe section  11  of the lowermost internally confined hollow column unit  10  and is provided with a plurality of lower outer fastening holes  63   a , and a plurality of fasteners  101 , such as bolts, that pass through the lower outer fastening holes  63   a , are inserted into a plurality of foundation section outer fastening holes  23   a  at the upper portion of the foundation section  20 , and fasten the lower outer flange  62   a  to the foundation section  20 . 
         [0065]    A lower inner flange  62   b  is attached to the lower inner circumference of the inner pipe section  13  of the lowermost internally confined hollow column unit  10  and is provided with a plurality of lower inner fastening holes  63   b . A plurality of fasteners  101 , such as bolts, passes through the lower inner fastening holes  63   b , is inserted into a plurality of foundation section inner fastening holes  23   b  at the upper portion of the foundation section  20 , and fastens the lower inner flange  62   b  to the foundation section  20 . Thereby, the means for fastening the foundation section  20  and the lowermost internally confined hollow column unit  10  preferably prevents brittleness of the joint section  80  between the foundation section  20  and the lowermost internally confined hollow column unit  10 . 
         [0066]    As illustrated in  FIG. 9 , the means for fastening the coping section  30  and the uppermost internally confined hollow column unit  10  includes an upper outer flange  60   a  that is attached to the upper outer circumference of the outer pipe section  11  of the uppermost internally confined hollow column unit  10  and is provided with a plurality of upper outer fastening holes  61   a , and a plurality of fasteners  101 , such as bolts that pass through the upper outer fastening holes  61   a , are inserted into a plurality of coping section outer fastening holes  32   a  at the lower portion of the coping section  30 , and fasten the upper outer flange  60   a  to the coping section  30 . 
         [0067]    An upper inner flange  60   b  is attached to the upper inner circumference of the inner pipe section  13  of the uppermost internally confined hollow column unit  10  and is provided with a plurality of upper inner fastening holes  61   b . A plurality of fasteners  101 , such as bolts, passes through the upper inner fastening holes  61   b , is inserted into a plurality of coping section inner fastening holes  32   b  at the lower portion of the coping section  30 , and fastens the upper inner flange  60   b  to the coping section  30 . Thereby, the means for fastening the coping section  30  and the uppermost internally confined hollow column unit  10  preferably prevents brittleness of the joint section  80  between the coping section  30  and the uppermost internally confined hollow column unit  10 . 
         [0068]    Further, the means for fastening two adjacent internally confined hollow column units  10  includes an upper outer flange  60   a  that is attached to the upper outer circumference of the outer pipe section  11  of the lower internally confined hollow column unit  10  and is provided with a plurality of upper outer fastening holes  61   a , a lower outer flange  62   a  that is attached to the lower outer circumference of the outer pipe section  11  of the upper internally confined hollow column unit  10  and is provided with a plurality of lower outer fastening holes  63   a  opposite the plurality of upper outer fastening holes  61   a , and a plurality of fastener tools  100 , for example, bolts and nuts, that pass through the upper and lower outer fastening holes  61   a  and  63   a  and fasten the upper and lower outer flanges  60   a  and  62   a  to each other. 
         [0069]    An upper inner flange  60   b  is attached to the upper inner circumference of the inner pipe section  13  of the lower internally confined hollow column unit  10 , and is provided with a plurality of upper inner fastening holes  61   b . A lower inner flange  62   b  is attached to the lower inner circumference of the inner pipe section  13  of the upper internally confined hollow column unit  10 , and is provided with a plurality of lower inner fastening holes  63   b  opposite the plurality of upper inner fastening holes  61   b . A plurality of fastener tools  100 , for example bolts and nuts, passes through the upper and lower inner fastening holes  61   b  and  63   b  and fastens the upper and lower inner flanges  60   b  and  62   b  to each other. Thereby, the means for fastening two adjacent internally confined hollow column units  10  preferably prevents brittleness of the joint section  80  between the two adjacent internally confined hollow column units  10 . 
         [0070]    The flanges  60   a ,  60   b ,  62   a  and  62   b , which are attached to the upper and lower circumferences of the internally confined hollow column unit  10 , have an annular shape, such as a circular shape or a quadrilateral shape, which can be determined to correspond to the shape of the cross section of the internally confined hollow column unit  10 . The hollow inner diameter D 4  of each of the upper and lower outer flanges  60   a  and  62   a  is equal to the outer diameter D 2  of the outer pipe section  11  of the internally confined hollow column unit  10 . Thus, the upper and lower outer flanges  60   a  and  62   a  can be attached to the outer surfaces of the opposite ends of the outer pipe section  11  by means of welding. Alternatively, in the case in which the outer pipe section  11  is formed of plastic, such as FRP, the upper and lower outer flanges  60   a  and  62   a  may be attached to the outer surfaces of the opposite ends of the outer pipe section  11  by means of bonding. 
         [0071]    The outer diameter D 5  of each of the upper and lower inner flanges  60   b  and  62   b  is equal to the inner diameter D 3  of the inner pipe section  13  of the internally confined hollow column unit  10 . Thus, the upper and lower inner flanges  60   b  and  62   b  can be attached to the inner surfaces of the opposite ends of the inner pipe section  13  by means of welding. Alternatively, in the case in which the inner pipe section  13  is formed of plastic, such as FRP, the upper and lower inner flanges  60   b  and  62   b  may be attached to the inner surfaces of the opposite ends of the inner pipe section  13  by means of bonding. 
         [0072]    According to another embodiment of the present invention, as illustrated in  FIG. 10 , the means for fastening two adjacent internally confined hollow column units  10  includes an outer support  70   a  that is attached in a strip shape to the outer circumference of the joint section  80  between the upper and lower internally confined hollow column units  10 . 
         [0073]    An inner support  70   b  is attached in a strip shape to the inner circumference of the joint section  80  between the upper and lower internally confined hollow column units  10 . Thereby, the means for fastening two adjacent internally confined hollow column units  10  preferably prevents brittleness of the joint section  80  between the upper and lower hollow column units  10 . 
         [0074]    The outer and inner supports  70   a  and  70   b  are attached to the outer and inner circumferences of the joint section  80  between the upper and lower hollow column units  10  by means of welding in the case in which the outer and inner pipe sections  11  and  13  are formed of steel, or by means of bonding in the case in which the outer and inner pipe sections  11  and  13  are formed of plastic such as FRP. Thereby, the outer and inner supports  70   a  and  70   b  prevent brittleness of the joint section  80  between the upper and lower hollow column units  10 . 
         [0075]    As described above, when the adjacent hollow column units  10  are mutually attached using the flanges  60   a ,  60   b ,  62   a  and  62   b  or the supports  70   a  and  70   b , they are first attached to each other by means of the bar-like members  40  inserted in the upper and lower anchoring holes  15   a  and  15   b  as well as the grout  50 , and then by means of either the fastener tools, such as bolts and nuts, fastened to the flanges thereof or the supports  70   a  and  70   b  attached to the joint section  80 . Thereby, the brittleness of the joint section  80  in the modular column system can be prevented. 
         [0076]    Further, in the case in which the internally confined hollow column unit  10  is attached to either the upper portion of the foundation section  20  or the lower portion of the coping section  30 , they are first attached to each other by means of the bar-like members  40  inserted between the lower anchoring holes  15   b  and the foundation section anchoring holes  21  and between the upper anchoring holes  15   a  and the coping section anchoring holes  31  as well as the grout  50 , and then by means of the fasteners  101 , such as bolts, that are fastened either to the upper portion of the foundation section  20  through the lower outer and inner flanges  62   a  and  62   b  or the lower portion of the coping section  30  through the upper outer and inner flanges  60   a  and  60   b . Thereby, the brittleness of the joint section  80  in the modular column system can be prevented. 
         [0077]    Meanwhile, as illustrated in  FIGS. 11 and 12 , a method of constructing the modular column system using at least one internally confined hollow column unit in accordance with the present invention includes a step S 1  of constructing the foundation section  20  with the plurality of foundation section anchoring holes  21  after pit excavation work. 
         [0078]    In step S 1  of constructing the foundation section  20 , the column unit insertion recess  22 , into which the lowermost one of the internally confined hollow column units  10  is inserted, can be formed at the upper portion of the foundation section  20 . 
         [0079]    The grout  50  is injected into the foundation section anchoring holes  21  (step S 2 ). 
         [0080]    Before the grout  50  in the foundation section anchoring holes  21  is cured in step S 2 , the bar-like members  40 , which have been inserted into and attached in the lower anchoring holes  15   b  of the lowermost internally confined hollow column unit  10  to be placed on the upper portion of the foundation section  20 , are inserted into the foundation section anchoring holes  21 , into which the grout  50  has been injected, and thus the lowermost internally confined hollow column unit  10  is attached to the foundation section (step S 3 ). 
         [0081]    In the case of the bar-like members  40  inserted into the lower anchoring holes  15   b  of the lowermost internally confined hollow column unit  10 , it does not matter that the bar-like members  40  are inserted into the lower anchoring holes  15   b  at the site before injection of the grout  50  into the lower anchoring holes  15   b . However, in this case, the period for construction is delayed due to the time it takes to cure the grout, so that the internally confined hollow column unit  10  is preferably precast in a factory with the bar-like members  40  inserted into and attached in the lower anchoring holes  15   b  in the interest of reducing the construction period as well as economy. 
         [0082]    In step S 3  of attaching the lowermost internally confined hollow column unit to the foundation section, the lower outer flange  62   a  having the plurality of lower outer fastening holes  63   a  and the lower inner flange  62   b  having the plurality of lower inner fastening holes  63   b  are attached to the lower outer and inner circumferences, respectively, of the lowermost internally confined hollow column unit  10 , placed on the upper portion of the foundation section. Then, the fasteners  101  are fastened in the foundation section outer and inner fastening holes  23   a  and  23   b  of the upper portion of the foundation section  20 , thereby attaching the lower outer and inner flanges  62   a  and  62   b  to the foundation section  20 . 
         [0083]    Then, the grout  50  is injected into the upper anchoring holes  15   a  of the lowermost internally confined hollow column unit  10 , which has been placed on the upper portion of the foundation section  20  in step S 3  (step S 4 ). 
         [0084]    Before the grout  50  in the upper anchoring holes  15   a  is cured in step S 4 , the bar-like members  40 , which have been inserted into and attached in the lower anchoring holes  15   b  of the internally confined hollow column unit  10  to be placed on the lowermost internally confined hollow column unit  10  on the foundation section  20 , are inserted into the upper anchoring holes  15   a  of the lowermost internally confined hollow column unit  10 , into which the grout  50  has been injected, and thus the adjacent hollow column units  10  are attached to each other (step S 5 ). 
         [0085]    According to one embodiment of the present invention, in step S 5  of attaching the internally confined hollow column units  10  to each other, the internally confined hollow column units  10  are each provided with the upper and lower outer flanges  60   a  and  62   a  and the upper and lower inner flanges  60   b  and  62   b , and then are attached to each other using the fastener tools  100 . 
         [0086]    Further, according to another embodiment of the present invention, in step S 5  of attaching the internally confined hollow column units  10  to each other, the joint section  80  between the internally confined hollow column units  10  is attached to the outer support  70   a  on the outer circumference thereof and to the inner support  70   b  on the inner circumference thereof, and thereby the internally confined hollow column units  10  are attached to each other. 
         [0087]    The internally confined hollow column units  10  are stacked and attached up to the designed height of the modular column system by means of repetition of steps S 4  and S 5  (step S 6 ). 
         [0088]    Then, the grout  50  is injected into the upper anchoring holes  15   a  of the uppermost internally confined hollow column unit  10 , which has been placed on the upper portion of the foundation section  20  in step S 6  (step S 7 ). 
         [0089]    Before the grout  50  in the upper anchoring holes  15   a  is cured in step S 7 , the bar-like members  40 , which have been inserted into and attached in the coping section anchoring holes  31  of the lower portion of the coping section  30 , are inserted into the upper anchoring holes  15   a , into which the grout  50  has been injected, and thus the uppermost internally confined hollow column unit  10  is attached to the coping section (step S 8 ). 
         [0090]    In step S 8  of attaching the coping section  30  to the upper portion of the uppermost internally confined hollow column unit  10 , the upper outer flange  60   a  and the upper inner flange  60   b  are attached to the upper outer and inner circumferences, respectively, of the uppermost internally confined hollow column unit  10 , placed under the coping section  30 . Then, the fasteners  101  are fastened in the coping section outer and inner fastening holes  32   a  and  32   b  of the lower portion of the coping section  30 , thereby attaching the upper outer and inner flanges  60   a  and  60   b  to the coping section  30 . 
         [0091]    As apparent from the above description, the modular column system using at least one internally confined hollow column unit and the method of constructing the same in accordance with the present invention provide the following advantages. 
         [0092]    First, the resistance to the bending moment is increased using the internally confined hollow column units, in which the concrete section is confined by the inner pipe section, and thus is under triaxial compression load. As a result, the internally confined hollow column units are each reduced in cross section and self-weight, and thus make assembly easy. 
         [0093]    Second, the hollow sections of the internally confined hollow column units result in the use of less concrete, a reduction in the cross section of each of the internally confined hollow column units, elimination of the need for forms, elimination of the use of steel bars, and so on. As a result, the economically advantageous effect of reducing labor costs for installation can be obtained. 
         [0094]    Third, the internally confined hollow column units are each provided with the inner and outer pipe section formed of plastic, such as FRP, such that they can be used in corrosive environments, such as in an underwater pier, and thus are advantageous for maintenance. In the case of using the FRP, the internally confined hollow column units can be reduced in self-weight. 
         [0095]    Fourth, the brittle fracture of the joint sections between the internally confined hollow column units can be prevented by the strong attachment of the joint sections. 
         [0096]    Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Summary:
A modular column system and a method of constructing the same are constructed by stacking at least one precast unit between a foundation section and a coping section. The precast unit makes use of an internally confined hollow column unit fabricated in advance, and joint sections between the precast units are firmly formed. Thereby, the modular column system can realize a short construction period and economy because reinforcement bars and forms are not used, as well as high resistance to bending moment and reduction in cross section and self-weight of the precast unit, and thus the modular column system can provide easier and more economical assembly, and prevent brittle fracture of the joint section between the precast units.