DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0050] Hereinafter, embodiments of the present invention are described with reference to the accompanying drawings.

[0051] FIGS. 2a, 2b, 2c and 2d are views showing a method for manufacturing preflex beams used as simple beams in accordance with a first embodiment of the present invention in which supporting stands are provided for lower plate girders, in which FIG. 2a is a front view thereof, FIG. 2b is a plan view for the manufacture of straight preflex beams, FIG. 2c is a plan view for the manufacture of curved preflex beams, and FIG. 2d is a side sectional view thereof. Plate girders 10a and 10b of a first set are fixedly connected to each other by a PS steel bar 5 at their both side ends. Two hydraulic jacks 4 are placed between the plate girders 10a and 10b at two positions respectively spaced inwardly apart by about L/5 from both side ends of the plate girders, and apply preflexion loads P to widen the space between the plate girders 10a and 10b. A second set of plate girders 10a′ and 10b′ and a third set of plate girders 10a″ and 10b″ are placed in the same way as that in which the first set of plate girders 10a and 10b are placed. The plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ are sustainably connected to each other by crossbeams 7 at regular intervals so as to prevent the buckling of the plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ that may be caused when the preflexion loads are applied. A plurality of preflex beams used as simple beams are manufactured in such a way that three or more sets of plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ sustained by the crossbeams 7 are placed on the supporting stands 6 in the form of tripods, preflexion loads are applied to the plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″, and the upper flanges of the upper plate girders 10a, 10a′ and 10a″ and the lower flanges of the lower plate girders 10b, 10b′ and 10b″ are covered with concrete.

[0052] FIGS. 3a, 3b, 3c and 3d are views showing a method for manufacturing preflex beams used as the internal beams of continuous beams in accordance with a first embodiment of the present invention in which supporting stands are provided for lower plate girders, in which FIG. 3a is a front view thereof, FIG. 3b is a plan view for the manufacture of straight preflex beams, FIG. 3c is a plan view for the manufacture of curved preflex beams, and FIG. 3d is a side sectional view thereof. The plate girders 10a and 10b of a first set are connected by PS steel bars 5 at two positions respectively spaced inwardly apart by about L/5 from both side ends of the plate girders. Two hydraulic jacks 4 are placed between the plate girders 10a and 10b at two positions respectively spaced inwardly apart by about L/3 from both side ends of the plate girders, and apply preflexion loads P to the girders. A second set of plate girders 10a′ and 10b′ and a third set of plate girders 10a″ and 10b″ are placed in the same way as that in which the first set of plate girders 10a and 10b are placed. The plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ are sustainably connected to each other by crossbeams 7 at regular intervals so as to prevent the buckling of the plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ that may be caused when the preflexion loads are applied. A plurality of preflex beams used as the internal beams of continuous beams are manufactured in such a way that three or more sets of plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ sustained by the crossbeams 7 are placed on the supporting stands 6 in the form of tripods at position respectively spaced inwardly apart from both side ends of the plate girders by L/5, preflexion loads are applied to the plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″, and the upper flanges of the upper plate girders 10a, 10a′ and 10a″ and the lower flanges of the lower plate girders 10b, 10b′ and 10b″ are covered with concrete.

[0053] FIGS. 4a, 4b, 4c and 4d are views showing a method for manufacturing preflex beams used as the external beams of continuous beams in accordance with a first embodiment of the present invention in which supporting stands are provided for lower plate girders, in which FIG. 4a is a front view thereof, FIG. 4b is a plan view for the manufacture of straight preflex beams, FIG. 4c is a plan view for the manufacture of curved preflex beams, and FIG. 4d is a side sectional view thereof. The plate girders 10a and 10b of a first set are connected by PS steel bars 5 at two positions, one of which is situated at one side ends of the plate girders and the other of which is spaced inwardly apart by about L/5 from the other side ends of the plate girders. Two hydraulic jacks 4 are placed between the plate girders 10a and 10b at two positions respectively spaced inwardly apart by about L/3 and about 0.5 L from one side ends of the plate girders, and apply preflexion loads P to the girders. A first set of plate girders 10a′ and 10b′ and a third set of plate girders 10a″ and 10b″ are placed in the same way as that in which the first set of plate girders 10a and 10b are placed. The plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ are sustainably connected to each other by crossbeams 7 at regular intervals so as to prevent the buckling of the plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ that may be caused when the preflexion loads are applied. A plurality of preflex beams used as the external beams of continuous beams are manufactured in such a way that three or more sets of plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″ sustained by the crossbeams 7 are placed on the supporting stands 6 in the form of tripods, preflexion loads are applied to the plate girders 10a, 10b, 10a′, 10b′, 10a″ and 10b″, and the upper flanges of the upper plate girders 10a, 10a′ and 10a″ and the lower flanges of the lower plate girders 10b, 10b′ and 10b″ are covered with concrete.

[0054] FIGS. 5a, 5b, 5c and 5d are views showing a method for manufacturing preflex beams used as simple beams in accordance with a first embodiment of the present invention in which supporting stands are provided for upper and lower plate girders, in which FIG. 5a is a front view thereof, FIG. 5b is a plan view for the manufacture of straight preflex beams, FIG. 5c is a plan view for the manufacture of curved preflex beams, and FIG. 5d is a side sectional view thereof. In order to minimize the effect of the empty weight of plate girders and eliminate the danger of accidents, three rigid frame type supporting stands 3 for supporting upper plate girders 10a and 10a′ and three tripod type supporting stands 6 for supporting lower plate girders 10b and 10b′ are placed at three positions, one of which is spaced inwardly apart by 0.5 L from both side ends of the plate girders and the others of which are respectively spaced inwardly apart by 0.1 L from both side ends of the plate girders, with each rigid frame type supporting stand 3 and each tripod type supporting stand 6 placed together at each position. In such a case, the supporting stands 3 and 6 may be placed at two positions if the preflex beams are relatively short, whereas the supporting stands 3 and 6 may be placed at four positions if the preflex beams are relatively long. The upper plate girders 10a and 10a′ connected to each other by crossbeams 7 are placed on the rigid frame type supporting stands, while the lower plate girders 10b and 10b′ connected to each other by crossbeams 7 are placed on the tripod type supporting stands 6 (refer to FIG. 5d). Thereafter, each set of upper and lower plate girders are connected by PS steel bars 5 at both side ends of the plate girders. Two hydraulic jacks 4 are placed between each set of upper and lower plate girders at two positions respectively spaced inwardly apart by about L/5 from one side ends of the plate girders, and apply preflexion loads P to widen the space between the upper and lower plate girders of each set. Finally, the upper flanges of the upper plate girders 10a and 10a′ and the lower flanges of the lower plate girders 10b and 10b′ are covered with concrete, thus completing the preflex beams used as simple beams.

[0055] FIGS. 6a, 6b, 6c and 6d are views showing a method for manufacturing preflex beams used as the internal beams of continuous beams in accordance with a first embodiment of the present invention in which supporting stands are provided for upper and lower plate girders, in which FIG. 6a is a front view thereof, FIG. 6b is a plan view for the manufacture of straight preflex beams, FIG. 6c is a plan view for the manufacture of curved preflex beams, and FIG. 6d is a side sectional view thereof. In order to minimize the effect of the empty weight of plate girders and eliminate the danger of accidents, three rigid frame type supporting stands 3 for supporting upper plate girders 10a and 10a′ and three tripod type supporting stands 6 for supporting lower plate girders 10b and 10b′ are placed at three positions, one of which is spaced inwardly apart by 0.5 L from both side ends of the plate girders and the others of which are respectively spaced inwardly apart by 0.1 L from both side ends of the plate girders, with each rigid frame type supporting stand 3 and each tripod type supporting stand 6 placed together at each position. In such a case, the supporting stands 3 and 6 may be placed at two positions if the preflex beams are relatively short, whereas the supporting stands 3 and 6 may be placed at four positions if the preflex beams are relatively long. The upper plate girders 10a and 10a′ connected to each other by crossbeams 7 are placed on the rigid frame type supporting stands, while the lower plate girders 10b and 10b′ connected to each other by crossbeams 7 are placed on the tripod type supporting stands 6 (refer to FIG. 6d). Thereafter, each set of upper and lower plate girders are connected by PS steel bars 5 at two positions respectively spaced inwardly apart by about L/5 from both side ends of the plate girders. Two hydraulic jacks 4 are placed between each set of upper and lower plate girders at two positions respectively spaced inwardly apart by about L/3 from both side ends of the plate girders, and apply preflexion loads P to widen the space between the upper and lower plate girders of each set. Finally, the upper flanges of the upper plate girders 10a and 10a′ and the lower flanges of the lower plate girders 10b and 10b′ are covered with concrete, thus completing the manufacture of the preflex beams used as the external beams of continuous beams.

[0056] FIGS. 7a, 7b, 7c and 7d are views showing a method for manufacturing preflex beams used as the external beams of continuous beams in accordance with a first embodiment of the present invention in which supporting stands are provided for upper and lower plate girders, in which FIG. 7a is a front view thereof, FIG. 7b is a plan view for the manufacture of straight preflex beams, FIG. 7c is a plan view for the manufacture of curved preflex beams, and FIG. 7d is a side sectional view thereof. In order to minimize the effect of the empty weight of plate girders and eliminate the danger of accidents, three rigid frame type supporting stands 3 for supporting upper plate girders 10a and 10a′ and three tripod type supporting stands 6 for supporting lower plate girders 10b and 10b′ are placed at three positions, one of which is spaced inwardly apart by 0.5 L from both side ends of the plate girders and the others of which are respectively spaced inwardly apart by 0.1 L from both side ends of the plate girders, with each rigid frame type supporting stand 3 and each tripod type supporting stand 6 placed together at each position. In such a case, the supporting stands 3 and 6 may be placed at two positions if the preflex beams are relatively short, whereas the supporting stands 3 and 6 may be placed at four positions if the preflex beams are relatively long.

[0057] The upper plate girders 10a and 10a′ connected to each other by crossbeams 7 are placed on the rigid frame type supporting stands, while the lower plate girders 10b and 10b′ connected to each other by crossbeams 7 are placed on the tripod type supporting stands 6 (refer to FIG. 7d). Thereafter, each set of upper and lower plate girders is connected by PS steel bars 5 at two positions respectively spaced inwardly apart by about L/5 from both side ends of the plate girders. Two hydraulic jacks 4 are placed between each set of upper and lower plate girders at two positions respectively spaced inwardly apart by about L/4 and 0.5 L from one side ends of the plate girders, and apply preflexion loads P to widen the space between the upper and lower plate girders of each set. Finally, the upper flanges of the upper plate girders 10a and 10a′ and the lower flanges of the lower plate girders 10b and 10b′ are covered with concrete, thus completing the manufacture of the preflex beams used as the external beams of continuous beams.

[0058] In the methods shown in FIGS. 2a to 7d, if horizontal buckling preventing devices are employed instead of the crossbeams, only a set of preflex beams can be manufactured. Four or more sets of straight or curved preflex beams can be simultaneously manufactured by laterally connecting a plurality of plate girders to each other by means of crossbeams.

[0059] Hereinafter, there is described a method for manufacturing preflex beams using non-cambered rolled shape steels and preflex beams used as piles and pillars in which both side flange concretes are compressively prestressed.

[0060] A conventional art causes considerable inconvenience in working because second load applying work have to be performed while rolled shape steels are turned upside down and placed on standing stands after first load application work is performed while rolled shape steels are placed on supporting stands so as to camber non-cambered rolled shape steels by plastic deformation or to give compressive stress to both flange portions of concrete. Accordingly, the below-described load applying method is to eliminate such inconvenience.

[0061] The above-mentioned method is described with reference to simple beams. FIGS. 8a, 8b, 8c and 8d are views showing a method for manufacturing preflex beams used as simple beams in accordance with a first embodiment of the present invention in which a square frame is employed, in which FIG. 8a is a front view thereof, FIG. 8b is a plan view for the manufacture of straight preflex beams, FIG. 8c is a plan view for the manufacture of curved preflex beams, and FIG. 8d is a side sectional view thereof. In order to minimize the effect of the empty weight of rolled shape steels and eliminate the danger of accidents, three rigid frame type supporting stands 3 for supporting upper rolled shape steels 10a and 10a′ and three tripod type supporting stands 6 for supporting lower rolled shape steels 10b and 10b′ are placed at three positions, one of which is spaced inwardly apart by 0.5 L from both side ends of the rolled shape steels and the others of which are respectively spaced inwardly apart by 0.1 L from both side ends of the rolled shape steels, with each rigid frame type supporting stand 3 and each tripod type supporting stand 6 placed together at each position. In such a case, the supporting stands 3 and 6 may be placed at two positions if the preflex beams are relatively short, whereas the supporting stands 3 and 6 may be placed at four positions if the preflex beams are relatively long. The upper rolled shape steels 10a and 10a′ connected to each other by crossbeams 7 are placed on the rigid frame type supporting stands, while the lower rolled shape steels 10b and 10b′ connected to each other by crossbeams 7 are placed on the tripod type supporting stands 6 (refer to FIG. 8d). Thereafter, in each set of upper and lower rolled shape steels, two props 9 are placed between the upper and lower rolled shape steels on the both side ends of the upper and lower rolled shape steels. Two hydraulic jacks 4 are placed on the upper rolled shape steels 10a and 10a′ at two positions respectively spaced inwardly apart by about L/5 from one side ends of the rolled shape steels, and a square frame 8 is placed to surround the hydraulic jacks 4, the upper rolled shape steels 10a and 10a′ and the lower rolled shape steels 10b and 10b′. Thereafter, preflexion loads P are applied to the rolled shape steels 10a, 10a′, 10b and 10b′ to widen the space between the upper and lower rolled shape steels of each set. In such a case, the hydraulic jacks 4 may be placed beneath the lower rolled shape steels 10b and 10b′ as occasion demands.

[0062] A plastic deformation theory that is concerned with the manufacture of non-cambered rolled shape steels is described with reference to the graph of FIGS. 9a and 9b.

[0063] FIG. 9a is a graph showing a stress-strain curve for a typical structural steel that is subjected to tension. As shown in the graph, the portion of the curve is a straight line in a region ranging from O to A, stress and strain are directly proportional to each other and the behavior of the structural steel is said to be linear. Beyond point A the linear relationship between the stress and the strain no longer exists, so that the stress at A is called proportional limit. With an increase in loading, the strain increases more rapidly than the stress, until at point B a considerable elongation begins to occur with no appreciable increase in the tensile force. This phenomenon is known as the yielding of the material, and the stress at point B is called the yield point or yield stress. In the region BC the material becomes perfectly plastic, so that the material is elongated without the increase of the stress. After a large strain is experienced during a yielding process in a region ranging from B to C, the material begins to strain harden. At this time, the material experiences changes in its atomic and crystal structures and simultaneously begins to offer additional resistance to increase in load. As a result, the elongation of the material occurs only when the tension is increased, and the corresponding portion of the stress-strain curve has a positive slope in a region ranging from C to D. Thus, with further elongation the stress increases, and it reaches its maximum value, or ultimate stress. Beyond this point further stretching of the material is accompanied by reduction in the load, and the fracture of the material finally occurs at point E on the diagram.

[0064] FIG. 9b is a graph schematically showing the stress-strain curve of FIG. 9a. When deformation generated in the material by the action of the given load is eliminated during unloading, the material is said to behave elastically and linearly. However, when the stress of the material reaches its yield point, the material is yielded. The stress-strain is linearly decreased along line C-D parallel to line A-B during unloading. The reason why the strain does not return to zero is that the material is deformed permanently or plastically. In FIG. 9b, line A-D represents residual strains due to the plastic deformation of the material.

[0065] Accordingly, the method for manufacturing preflex beams according to the present invention is to camber non-cambered rolled shape steels by plastic deformation through a manufacturing process, differently from a conventional art in which there occurs the difficulty and inconvenience of previously cambering steel girders to correspond to deflection due to dead loads.

[0066] Hereinafter, there is described a method for manufacturing preflex beams using non-cambered rolled shape steels with reference to the accompanying drawings.

[0067] FIGS. 10a, 10b and 10c are views showing a first process of manufacturing preflex beams using rolled shape steels used as simple beams, in which a non-cambered rolled shape steel is cambered to compensate for the deflection of the steel due to a dead load. In this case, the method shown in FIG. 2 or 5 is employed as a load applying method for cambering the rolled shape steel.

[0068] FIG. 10b shows a process in which there occurs plastic deformation in the form of a cubic parabola that has an apex at a position where the maximum bending moment occurs during the application of a dead load, that is, the center of the rolled shape steel, by applying two loads P to the non-cambered rolled shape steel (refer to FIG. 10a), which is supported at its both ends by fulcrums, at positions respectively spaced inwardly apart by about 1/5 L from both ends of the shape steel so as to allow the shape steel to exceed the boundary of the elastic region. In such a case, the shape of the cubic parabola can be easily obtained from the equation of a cubic parabola, depending on the sizes of a dead load and an active load and the use of a structure. FIG. 10c is a perspective view showing a rolled shape steel that is deformed in the form of a cubic parabola in the above-described process.

[0069] FIGS. 11a, 11b, 11c and 11d are views showing a process of giving compressive stress to the concrete that covers a rolled shape steel downwardly deformed as shown in FIG. 10, by a load applying method in which a square frame is employed.

[0070] FIG. 11b shows a state, in which two loads P are applied to a cambered rolled shape steel at positions respectively spaced inwardly apart by about 1/5 L from both ends of the rolled shape steel, so as to give compressive stress to the rolled shape steel by covering the upper flange of the rolled shape steel with concrete.

[0071] FIG. 11c shows a state in which the upper flange of the rolled shape steel is covered with concrete while two loads are applied to the rolled shape steel.

[0072] FIG. 11d shows a state in which a preflex beam is given the compressive stress contrary to the stress that can be generated by a dead load and an active load, by removing the applied loads P after the curing of the concrete.

[0073] FIGS. 12a, 12b and 12c are views showing a first process of manufacturing a preflex beam using a rolled shape steel, which is used as a beam placed in the left or right marginal portion of a continuous structure and is supported by a right fulcrum at the right end of the shape steel and a left fulcrum spaced apart by about 1/5 L from the left end of the shape steel. In the process, a non-cambered rolled shape steel is cambered to correspond to the deflection of the shape steel due to the dead load of the external beam of a continuous beam structure. In such a case, the method shown in FIG. 4 or 7 is employed as a load applying method for cambering the non-cambered rolled shape steel.

[0074] FIG. 12a is a view showing the non-cambered rolled shape steel that is supported by a left fulcrum positioned at the left end of the shape steel and a right fulcrum spaced apart by about 1/5 L from the right end of the shape steel.

[0075] FIG. 12b shows a process in which there occurs plastic deformation in the form of a cubic parabola that has an apex at a position where the maximum bending moment occurs during the application of a dead load, that is, the center of the rolled shape steel, by applying two loads P to the non-cambered rolled shape steel at positions respectively spaced inwardly apart by about 1/4 L from both ends of the shape steel so as to allow the shape steel to exceed the boundary of the elastic region. In such a case, the shape of the cubic parabola can be easily obtained from the equation of a cubic parabola, depending on the sizes of a dead load and an active load and the use of a structure.

[0076] FIGS. 13a, 13b, 13c and 13d are views showing a process of giving compressive stress to the concrete that covers a rolled shape steel downwardly deformed as shown in FIG. 12, by a load applying method in which a square frame is employed.

[0077] FIG. 13b shows a state, in which two loads P are applied to a cambered rolled shape steel at positions respectively spaced inwardly apart by about 1/4 L and about 2/4 L from the left end of the rolled shape steel, so as to give compressive stress to the shape steel by covering the upper flange of the rolled shape steel with concrete.

[0078] FIG. 13c shows a state in which the upper flange of the rolled shape steel is covered with concrete while two loads P are applied to the rolled shape steel.

[0079] FIG. 13d shows a state in which a preflex beam is given the compressive stress contrary to the stress that can be generated by a dead load and an active load, by removing the applied loads P after the curing of the concrete. In this case, it will be understood that the outer upper portions of the right end portions of the concrete are free from stress.

[0080] FIGS. 14a, 14b and 14c are views showing a first process of manufacturing a preflex beam using a rolled shape steel, which is used as a beam placed in the interior portion of a continuous structure or as a beam connecting pillars and is supported by two fulcrums at a position spaced apart by about 1/5 L from both ends of the shape steel. In the process, a non-cambered rolled shape steel is cambered to correspond to the deflection of the shape steel due to the dead load of the internal beam of a continuous beam structure. In such a case, the method shown in FIG. 3 or 6 is employed as a load applying method for cambering the non-cambered rolled shape steel.

[0081] FIG. 14a is a view of the beam that is used as a beam placed in the interior portion of a continuous structure or as a beam connecting pillars, in which the non-cambered rolled shape steel is supported by two fulcrums spaced apart by about 1/5 L from both ends of the shape steel. In the case of a continuous beam, the internal beam may be made longer than the external beam by about 25%, thereby improving its economical efficiency.

[0082] FIG. 14b shows a process in which there occurs plastic deformation in the form of a cubic parabola by applying two loads P to the non-cambered rolled shape steel at positions respectively spaced inwardly apart by about 1/6 L from the center of the shape steel so as to allow the shape steel to exceed the boundary of the elastic region. In such a case, the shape of the cubic parabola can be easily obtained from the equation of a cubic parabola, depending on the sizes of a dead load and an active load and the use of a structure. FIG. 14c is a perspective view of the rolled shape steel deformed in the form of a cubic parabola by the above-described process.

[0083] FIGS. 15a, 15b, 15c and 15d are views showing a process of manufacturing a preflex beam using the interior beam of a continuous beam or a rolled shape steel connecting pillars in an architectural structure.

[0084] FIG. 15a shows a state in which two loads P are applied to the rolled shape steel at two positions respectively spaced inwardly apart by about 1/6 L from the center of the rolled shape steel so as to give compressive stress to the concrete that covers the upper flange of the rolled shape steel downwardly deformed as shown in FIG. 14c (refer to FIG. 15b). In the load applying method used, a square frame is employed.

[0085] FIG. 15c shows a state in which the upper flange of the rolled shape steel is covered with concrete while two loads P are applied to the rolled shape steel.

[0086] FIG. 15d shows a state in which a preflex beam is given the compressive stress contrary to the stress that can be generated by a dead load and an active load, by removing the applied loads P after the curing of the concrete. In this case, it will be understood that the outer upper portions of both end portions of the concrete are free from stress.

[0087] Hereinafter, there is described a process of manufacturing a preflex used as a pile or pillar in which compressive stress is given concrete that covers its both flanges.

[0088] The pile serves to transmit load, which is applied to upper and lower structures, to the ground. The pile should be dynamically stable and should not be partially displaced detrimentally. However, a conventional pile may be laterally and partially displaced by a horizontal load or an earthquake due to its structure, thus having low bearing capacity. Additionally, a plurality of piles are required to achieve sufficient bearing capacity, so that an excessive construction cost is necessary. Additionally, in a case where a pile is long, the connection portion of pile elements are weak, so that the use of a long pile is limited. In addition, in the case of a steel pile, reduction in strength due to corrosion is a major shortcoming. In the preflex beam manufacturing method of this embodiment of the present invention in which rolled shape steels are covered with concrete and compressive stress is given through the total cross sections, a horizontal load and an earthquake can be resisted sufficiently due to an increase in bending strength, corrosion is prevented, and bearing capacity can be increased by an increase in skin friction force due to the shapes of the cross sections of rolled shape steels. In the case of a pillar, reduction in cross-sectional area can be achieved due to an increase in bending strength for a pillar having great moment. This embodiment is illustrated in FIGS. 16a to 19.

[0089] FIG. 16a is a view showing a state in which a rolled shape steel 10 is connected to two dummy rolled shape steels 11 at its both side ends by means of hinges 20 and bolts so as to manufacture a preflex beam used as a pile or pillar that is uniformly given compressive stress through total cross sections.

[0090] After loads P are applied to the rolled shaped steel assembly at positions spaced apart by about L/5 from both ends of the rolled shape assembly while the rolled shape steel assembly is placed on a support stand as shown in FIG. 2 or 5 (refer to FIG. 16b), the lower flange of the rolled shape steel is covered with concrete (refer to FIG. 16c).

[0091] FIG. 16d is a view showing a process of providing compressive stress to concrete by removing the loads P.

[0092] FIG. 16e is a view showing a state in which loads P are applied in the opposite direction to the rolled shape steel using a square frame as shown in FIG. 8. In this case, the flange of the rolled shape steel is covered with concrete, so that horizontal strength is achieved, thereby reducing the number of crossbeams required.

[0093] FIG. 16f is a view showing a state in which the upper and lower flanges of the rolled shape steel are covered with concrete while loads P are applied to the rolled shape steel.

[0094] FIG. 16g is a view showing a process in which compressive stress is given the secondly formed concrete by removing the loads P after the curing of the concrete and demounting the dummy rolled shape steel 11 and the hinges 20 from the rolled shape steel 10. When the web portions of the concrete are given compressive stress, the compressive stress is given the web portions of the concrete by inserting PC steel wires into the web portions of the concrete, or using a pre-tension method, thereby manufacturing preflex beams used as a pile or pillar in which stress is given through its total cross sections.

[0095] Though the above-mentioned processes, there can be manufactured in large quantities preflex beams used as piles and pillars, which have superior performance and uniform compressive stress through the total cross sections. The length of the preflex beam can be freely adjusted by the connection or cutting of the preflex beam.

[0096] FIGS. 18a and 18b are perspective views respectively showing one preflex beam used as a pile and another preflex beam used as a pillar, which are respectively given compressive stress through the total cross sections of concrete. In the case of the preflex beam used as a pile, a concrete head in the form of a wedge is additionally formed on the front end of the preflex beam so as to prevent the damage of its front portion and friction (refer to FIG. 18a).

[0097] In addition, there is shown in FIG. 19 a method for connecting two preflex beams to each other without reduction in strength, in a case where the connection of the preflex beams is necessary due to the long setting depth of a beam. In the method, the cross sections of the connection portions are enlarged to compensate for reduction in strength, a socket type recess for receiving a rolled shape steel is formed to connect preflex beams used as piles or pillars, and a recess is formed to receive a grouting agent, thereby achieving perfect connection.

[0098] As described above, the present invention provides a method for manufacturing preflex beams in which preflexion loads are applied to plate girders or rolled shape steels while upper plate girders or rolled shape steels are supported by supporting stands in the form of tripods at fulcrum positions, upper and lower plate girders or rolled shape steels are all supported by supporting stands in the form of rigid frames at two, three or four positions, upper plate girders or rolled shape steels are supported by supporting stands in the form of rigid frames, or lower plate girders or rolled shape steels are supported by supporting stands in the form of tripods. In the preflex beam manufacturing method of the present invention, two or more preflex beams can be manufactured, that is, a great quantity of straight and curved preflex beams can be manufactured, by one time preflexion work. Accordingly, the working period for manufacturing beams can be reduced greatly.

[0099] In addition, a preflex beam can be manufactured using non-cambered rolled shape steel by means of a method for applying loads in a way of inwardly pulling upper and lower girders or rolled shape steels by the use of a square frame, so that there can be eliminated difficulty and inconvenience in which a cambering process is previously performed to correspond to deflection due to a dead load.

[0100] In addition, the preflex beam used as a pile or pillar in which compressive stress is given the total cross sections can be manufactured, so that horizontal load and earthquake can be resisted due to an increase in bending strength in the case of a preflex used as a pile, corrosion is prevented and bearing capacity is increased due to an increase in skin frictional force due the shape of the cross section of a rolled shape steel.

[0101] Furthermore, due to the great strength of the preflex beam of the present invention, desired bearing capacity can be obtained by a small number of piles in comparison with a conventional art. Additionally, in the case of the preflex beam used as a pillar, its bending strength is increased, so that the cross-sectional area of a pillar requiring great moment can be reduced.