METHOD OF MANUFACTURING PREFLEX BEAMS

Abstract: Disclosed herein is a method for manufacturing preflex beams. The method includes the step of connecting the upper and lower plate girders (10a and 10b) of a first set to each other. Thereafter, the upper and lower plate girders (10a' and 10b') of a second set are connected to each other. The first set and the second set are arranged in parallel. The upper plate girders (10a and 10a') are connected to each other, and the lower plate girders (10b and 10b') are connected to each other. A plurality of lower supporting stands (6) are placed under the lower crossbeams (7). A plurality of hydraulic jacks (4) are placed between the upper and lower plate girders (10a and 10b, 10a' and 10b'). 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.

Description: METHOD OF MANUFACTURING PREFLEX BEAMS Technical Field The present invention relates generally to a method of manufacturing preflex beams, and particularly to a method for simultaneously manufacturing a plurality of preflex beams used as simple beams or continuous beams, using cambered plate girders or non-cambered rolled shape steels. Background Art A conventional art is described with reference to preflex beams used as simple beams. Figs, la and lb are views showing a conventional method for manufacturing preflex beams used as straight simple beams, which has been utilized for about forty years. In the drawings, Fig. l a is a front view, and Fig lb is a plan view. Plate girders 1 are conveyed to a working field, two plate girders 1 are placed on two supporting stands 2, and two props 9 are fixedly placed between the plate girders 1 at two positions respectively spaced inwardly apart by about L/5 from both side ends of the plate girders 1 where preflexion loads are applied. Two hydraulic jacks 4 are respectively placed on both side ends of the plate girders 1 , and apply preflexion loads to the ends. Thereafter, the manufacture of the straight preflex beams is completed by covering the lower flanges of the plate girders 1 with concrete while the preflexion loads continue to be applied to the plate girders using PS steel bars 5. In the conventional method for manufacturing preflex beams, in order to prevent the plate girders from being horizontally buckled, there should be installed a plurality of horizontal buckling preventing devices that are spaced at regular intervals along the lengths of the plate girders and forcibly hold the plate girders. For the application of the conventional method, two supporting stands and a plurality of horizontal buckling preventing devices are necessary for a set of plate girders, and a concrete base having a thickness of more than 250 mm is necessary to resist a horizontal buckling force that may be generated when preflexion loads are applied to the plate girders. Accordingly, excessive working area is needed. Only two preflex beams can be manufactured by the one time application of preflexion loads, so that excessive manufacturing time is required. In particular, the conventional method can only be applied to the manufacture of straight preflex beams. Disclosure of the Invention Accordingly, the present invention has been made to overcome the above problems occurring in the prior art, and an object of the present invention is to provide a method for manufacturing preflex ' beams, which is .capable of manufacturing two or more straight or curved preflex beams by means of one time load applying work. Another object of the present invention is to provide a method for manufacturing preflex beams, in which non-cambered rolled shape steels are cambered by plastic deformation. A further object of the present invention is to provide a method for manufacturing preflex beams used as piles or pillars, in which compressive stress is introduced into concrete with which both flanges of the rolled shape steels are covered. In order to accomplish the above object, the present invention provides a method for manufacturing preflex beams using cambered plate girders, comprising the steps of preparing a first set of cambered upper and lower plate girders, and connecting the upper and lower plate girders of the first set to each other by a plurality of PS steel bars at fulcrum positions; preparing a second set of cambered upper and lower plate girders, and connecting the upper and lower plate girders of the second set to each other by a plurality of PS steel bars at predetermined positions; arranging the first set of the upper and lower plate girders and the second set of the upper and lower plate girders in parallel while being spaced apart from each other by a predetermined interval; connecting the upper plate girders of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower plate girders of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower crossbeams, respectively; placing a plurality of hydraulic jacks between the upper and lower plate girders of each set at load applying positions, and applying preflexion loads to the upper and lower plate girders using the hydraulic jacks; and covering the upper flanges of the upper plate girders and the lower flanges of the lower plate girders with concrete. In order to manufacture the preflex beams used as simple beams, the fulcrum positions are respectively situated on both side ends of the plate girders, and the load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/5. In order to manufacture the preflex beams used as the internal beams of continuous beams, the fulcrum positions are respectively spaced apart from both side ends of the plate girders by about L/5, and the load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/3. In order to manufacture the preflex beams used as the external beams of continuous beams, the fulcrum positions are respectively spaced inwardly apart from both side ends of the plate girders by about L/5, and the load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/4 and 0.5 L. so as to use the preflex beams as the external beams of continuous beams. In the manufacture of preflex beams used as simple beams and the internal and external beams of continuous beams, in a case where the length of the plate girders is 20 m or less, the lower supporting stands are placed at the applying positions of preflexion loads, and the method further comprises the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. In a case where the length of the plate girders is 20 to 30 m, the lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower plate girders by about 0.1 L and 0.5 L, and the method further comprises the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. In a case where the length of the plate girders is 30 m or more, the lower supporting stands are placed at four positions respectively spaced inwardly apart from both side ends of the lower plate girders by about 0.1 L and 0.3 L, and the method further comprises the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. Another method for manufacturing preflex beams, comprises the steps of preparing a first set of cambered upper and lower plate girders; preparing a second set of cambered upper and lower plate girders; connecting the upper plate girders of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower plate girders of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower plate girders, respectively; placing a plurality of props on the lower crossbeams at predetermined positions, respectively; placing a plurality of hydraulic jacks between the upper and lower plate girders of each set at load applying positions; enclosing the upper and lower plate girders of all sets and the hydraulic jacks with a square frame; applying preflexion loads to the upper and lower plate girders using the hydraulic jacks; and covering the upper flanges of the upper plate girders and the lower flanges of the lower plate girders with concrete. In the same manner as that of the previous method in which a square frame is not employed, this method for manufacturing preflex beams employs the positions of PS steel bars and the applying positions of preflexion loads determined depending upon the use of beams and the length of plate girders, and further comprises the step of placing a plurality of upper supporting stands. When the second set of upper and lower plate girders arranged in parallel with the first set of upper and lower plate girders is plural, a plurality of preflex beams can be manufactured by one time application of preflexion loads. In accordance with a second embodiment of the present invention, a method for manufacturing preflex beams using non-cambered rolled shape steels comprises the steps of preparing a first set of non-cambered straight upper and lower rolled shape steels, and connecting the upper and lower rolled shape steels of the first set to each other by a plurality of PS steel bars at fulcrum positions; preparing a second set of non-cambered straight upper and lower rolled shape steels, and connecting the upper and lower rolled shape steels of the second set to each other by a plurality of PS steel bars at fulcrum positions; arranging the first set of the upper and lower rolled shape steels and the second set of the upper and lower rolled shape steels in parallel while being spaced apart from each other by a predetermined interval; connecting the upper rolled shape steels of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower rolled shape steels of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower crossbeams, respectively; placing a plurality of hydraulic jacks between the upper and lower rolled shape steels of each set at load applying positions, and applying preflexion loads to the upper and lower rolled shape steels using the hydraulic jacks so as to camber the upper and lower rolled shape steels by plastic deformation; placing a plurality of props on the lower crossbeams at predetermined positions, respectively; removing the PS steel bars placed at fulcrum positions and the hydraulic jacks placed at load applying positions; placing a plurality of hydraulic jacks on the upper rolled shape steels of each set or under the lower rolled shape steels of each set at load applying positions; enclosing the upper and lower rolled shape steels of all sets and the hydraulic jacks with a square frame; applying preflexion loads to the upper and lower rolled shape steels of all sets using the hydraulic jacks; and covering the upper flanges of the upper rolled shape steels and the lower flanges of the lower rolled shape steels with concrete. In the same manner as that of the method of the first embodiment in which a square frame is not employed, the method for manufacturing preflex beams according to the second embodiment employs the positions of PS steel bars and the applying positions of the preflexion loads determined depending upon the use of beams and the length of the rolled shape steels, and further comprises the step of placing a plurality of upper supporting stands. When the second set of upper and lower plate girders arranged in parallel with the first set of upper and lower rolled shape steels is plural, a plurality of preflex beams can be manufactured by one time application of preflexion loads. In accordance with a third embodiment of the present invention, a method for manufacturing preflex beams, utilizing the way of connecting two dummy rolled shape steels to both ends of a non-cambered straight rolled shape steel, comprises the steps of connecting two dummy rolled shape steels to both ends of a non-cambered straight rolled shape steel by means of bolts into a rolled shape steel assembly; preparing a first set of non-cambered straight upper and lower rolled shape steel assemblies, and connecting the upper and lower rolled shape steel assembles of the first set to each other by a plurality of PS steel bars at both side ends of the rolled shape steels; preparing a second set of non-cambered straight upper and lower rolled shape steel assemblies, and connecting the upper and lower rolled shape steel assemblies of the second set to each other by a plurality of PS steel bars at both side ends of the rolled shape steel assemblies; arranging the first set of the upper and lower rolled shape steel assemblies and the second set of the upper and lower rolled shape steel assemblies in parallel while being spaced apart from each other by a predetermined interval; connecting the upper rolled shape steel assemblies of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower rolled shape steel assemblies of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower crossbeams, respectively, placing a hydraulic jack between each connection portion of the upper rolled shape steel assembly of each set and each connection portion of the lower rolled shape steel assembly of each set at each load applying position, and applying preflexion loads to the upper and lower rolled shape steel assemblies using hydraulic jacks by plastic deformation; covering the upper flanges of the upper rolled shape steels except for the dummy rolled shape steels and the lower flanges of the lower rolled shape steels except for the dummy rolled shape steels, with concrete; placing a plurality of props on the lower rolled shape steel assemblies at predetermined positions; removing the PS steel bars and the hydraulic jacks; placing hydraulic jacks on the connection portions of the upper rolled shape steel assemblies of all sets or under the connection portions of the lower rolled shape steel assemblies of all sets; enclosing the upper and lower rolled shape steels of all sets and the hydraulic jacks with a square frame; applying preflexion loads to the rolled shape steel assemblies of all sets using the hydraulic jacks; covering the lower flanges of the upper rolled shape steels except for the dummy rolled shape steels and the upper flanges of the lower rolled shape steels except for the dummy rolled shape steels, with concrete; and dismounting the dummy rolled shape steels from the rolled shape steels covered with concrete. When it is necessary to introduce compressive stress into the web portions of the rolled shape steels, the method further comprises the step of providing compressive stress to the rolled shape steels using a pre-tension method after the web portions of the rolled shape steels are covered with concrete and PC steel wires are inserted into the web portions of the rolled shape steels. When each of the manufactured preflex beams should have a relatively great length, the method further comprises the steps of enlarging the cross- sectional areas of the connection portions of the rolled shape steels so as to improve the strength of the connection portions; forming grooves for receiving male rolled shape steels on the female rolled shape steels, and connecting each of the male rolled shape steels and each of the female rolled shape steels to each other; and injecting a grouting agent into the grooves. " In this embodiment, in a case where the length of the rolled shape steels is 20 m or less, the lower supporting stands arc placed at the applying positions of preflexion loads, and the method further comprises the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands In a case where the length of the rolled shape steels is 20 to 30 m, the lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower rolled shape steels by about 0.1 L and 0.5 L, and the method further comprises the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. In a case where the length of the rolled shape steels is 30 m or more, the lower supporting stands are placed at four positions respectively spaced inwardly apart from both side ends of the lower rolled shape steels by about 0.1 L and 0.3 L, and the method further comprises the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. In this embodiment, when the second set of upper and lower plate girders arranged in parallel with the first set of upper and lower rolled shape steels is plural, a plurality of preflex beams can be manufactured by one time application of preflexion loads. Brief Description of the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. la and l b are views showing a conventional method for manufacturing preflex beams used as straight simple beams, in which Fig. 1 a is a front view thereof and Fig. lb is a plan view thereof; 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; 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; 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; 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; 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; 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; 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; Fig. 9a is a graph showing a stress-strain curve for a typical structural steel that is subjected to tension, and Fig. 9b is a graph schematically showing the stress-strain curve of Fig. 9a; Figs. 10a, 10b and 10c are views showing a process in which a non- cambered rolled shape steel used as a simple beam is plastically deformed, in which Fig. 10a is a view of the non-cambered rolled shape steel, Fig. 10b is a view showing the step of applying loads to the rolled shape steel, and Fig. 10c is a view showing a state in which the rolled shape steel is deformed in the form of a cubic parabola; Figs. 1 1 a, l i b and 1 lc are views showing the process of manufacturing a preflex beam using the rolled shape steel of Fig. 10c, in which Fig. 1 l a is a view of the rolled shape steel used as a simple beam deformed plastically, Fig. l ib is a view showing the step of applying loads to the rolled shape steel, Fig. 10c is the step of covering the upper flange of the rolled shape steel of Fig. 1 l c, and Fig. 1 I d is a view showing the state in which the rolled shape steel is transformed into the preflex beam; Figs. 12a. 12b and 12c are views showing a process in which a non- cambered rolled shape steel used as the external beam of a continuous beam structure is plastically deformed, in which Fig. 12a is a view of the non-cambered rolled shape steel, Fig. 12b is a view showing the step of applying loads to the rolled shape steel, and Fig. 12c is a view showing a state in which the rolled shape steel is deformed in the form of a cubic parabola; Figs. 13a, 13b and 13c are views showing the process of manufacturing a preflex beam using the rolled shape steel of Fig. 12c, in which Fig. 13a is a view of the rolled shape steel used as the external beam of a continuous beam structure deformed plastically. Fig. 13b is a view showing the step of applying loads to the rolled shape steel, Fig. 1 3c is the step of covering the upper flange of the rolled shape steel, and Fig. 13d is a view showing the state in which the rolled shape steel is transformed into the preflex beam; Figs. 14a, 14b and 14c are views showing a process in which a non- cambered rolled shape steel used as the internal beam of a continuous beam structure or the shape steel of a construction structure is plastically deformed, in which Fig. 14a is a view of the non-cambered rolled shape steel, Fig. 14b is a view showing the step of applying loads to the rolled shape steel, and Fig. 14c is a view showing a state in which the rolled shape steel is deformed in the form of a cubic parabola; Figs. 15a, 15b and 15c are views showing the process of manufacturing a preflex beam using the rolled shape steel of Fig. 14c, in which Fig. 15a is a view of the rolled shape steel used as the internal beam of a continuous beam structure or the rolled shape steel of a construction structure deformed plastically, Fig. 15b is a view showing the step of applying loads to the rolled shape steel, Fig. 15c is the step of covering the upper flange of the rolled shape steel, and Fig. 15d is a view showing the state in which the rolled shape steel is transformed into the preflex beam; Figs. 16a, 16b. 16c. 16d, 16e, 16f and 16g are views showing the process of manufacturing a preflex beam used as a pile or pillar in accordance with the present invention, in which Fig. 16a is a view showing the step of connecting dummy rolled shape steels to both ends of a rolled shape steel, Fig. 16b is a view showing the step of applying loads to the upper or lower surface of the rolled shape steel assembly, Fig. 16c is a view showing the step of covering the lower flange of the rolled shape steel assembly with concrete, Fig. 16d is a view showing the step of removing the loads from the rolled shaped steel assembly, Fig. 16e is a view showing the step of applying loads to the opposite surface of the rolled shape steel assembly, Fig. 16f is a view showing the step of covering the upper flange of the rolled shape steel assembly with concrete, and Fig. 16g is a view showing the step of removing the loads from the rolled shape steel assembly and dismounting the dummy rolled shape steels from the rolled shape steel. Fig. 17 is a view showing a method in which compressive stress is introduced into the web portions of the concrete of a rolled shape steel; Fig. 18a is a perspective view showing a preflex beam used as a pile according to the present invention, and Fig. 18b is a perspective view showing a preflex beam used as a pillar according to the present invention; and Fig. 19 is a detailed view showing the connection portions of preflex beams used as a pile or pillar. Best Mode for Carrying Out the Invention Hereinafter, embodiments of the present invention a.re described with reference to the accompanying drawings. 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. The plate girders 10a and 10b of a first set are fixedly connected to each other by PS steel bars 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. 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. 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 1 Ob" are covered with concrete. 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 1 0a' 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, the upper and lower plate girders of each set are connected by PS steel bars 5 at both side ends of the plate girders. Two hydraulic jacks 4 are placed between the upper and lower plate girders of each set 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 manufacture of the preflex beams used as simple beams. 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! Ob 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! 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, the upper and lower plate girders of each 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 upper and lower plate girders of each set 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. 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. 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, the upper and lower plate girders of each 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 upper and lower plate girders of each set 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. . 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 the crossbeams. Hereinafter, there is described a method for manufacturing preflex beams using non-cambered rolled shape steels, or preflex beams used as piles and pillars in which compressive stress is introduced into concretes covering both flanges of each beam. A conventional art causes considerable inconvenience in work 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 introduce compressive stress into both flange portions of concrete. Accordingly, the below- described load applying method is to eliminate such inconvenience. 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 I L from both side ends of the rolled shape steels, with each rigid frame type supporting stand 3 and each tripod t> pe 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 under the lower rolled shape steels 10b and 10b' as occasion demands. A plastic deformation theory that is concerned with the manufacture of non-cambered rolled shape steels is described with reference to the graphs of Figs. 9a and 9b. 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. Fig. 9b is a graph schematically showing the stress-strain curve of Fig. 9a. When deformation caused 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. 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. Hereinafter, there is described a method for manufacturing preflex beams using non-cambered rolled shape steels with reference to the accompanying drawings. 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 dead loads. In this case, the method shown in Fig. 2 or 5 is employed as a load applying method for cambering the rolled shape steel. 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 dead loads, 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 dead loads and live loads 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. Figs. 1 1a, l ib, l ie and l id are views showing a process of introducing compressive stress into 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. Fig. 1 1 b 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 introduce compressive stress into the rolled shape steel by covering the upper flange of the rolled shape steel with concrete. Fig. 1 l c 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. Fig. l id shows a state in which the compressive stress, contrary to the stress that can be generated by dead loads and live loads, is introduced to a preflex beam by removing the applied loads P after the curing of the concrete. 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 inwardly apart by about 1/5 L from the left end of the shape steel. In this process, a non- cambered rolled shape steel is cambered to correspond to the deflection of the shape steel due to the dead loads of the external beam of a continuous beam structure 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. 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 inwardly apart by about 1/5 L from the right end of the shape steel. 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 dead loads, 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 dead loads and live loads and the use of a structure. Figs. 13a, 1 3b, 1 3c and 13d are views showing a process of introducing compressive stress into 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. 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 introduce compressive stress by covering the upper flange of the rolled shape steel with concrete. 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. Fig. 13d shows a state in which the compressive stress, contrary to the stress that can be generated by dead loads and live loads, is introduced into a preflex beam 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. 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 inwardly apart by about 1/5 L from both ends of the shape steel. In this process, a non-cambered rolled shape steel is cambered to correspond to the deflection of the shape steel due to the dead loads of the internal beam of a continuous beam structure 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. 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 inwardly apart by about 1/5 L from both ends of the shape steel In the case of a continuous beam structure, the internal beam may be made longer than the external beam by about 25%, thereby improving its economical efficiency. 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 dead loads and live loads 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. Figs. 15a, 15b, 15c and 15d are views showing a process of manufacturing a preflex beam using the interior beam of a continuous beam structure or a rolled shape steel connecting pillars in an architectural structure. 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 introduce compressive stress into 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. 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. Fig. 15d shows a state in which the compressive stress, contrary to the stress that can be generated by dead loads and live loads, is introduced into a preflex beam 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 Hereinafter, there is described a process of manufacturing a preflex used as a pile or pillar in which compressive stress is introduced into concrete that covers its both flanges. 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 introduced into 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. Fig. 16a is a view showing a state in which a rolled shape steel 10 is connected to two dummy rolled shape steels 1 1 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 in which compressive stress is uniformly introduced into the total cross sections. After loads P are applied to the rolled shaped steel assembly at positions spaced inwardly 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). Fig. 16d is a view showing a process of providing compressive stress to concrete by removing the loads P. 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. 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. Fig. 16g is a view showing a process in which compressive stress is introduced into 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 compressive stress is introduced into the web portions of the concrete, the compressive stress is introduced into 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 introduced into its total cross sections. 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. Figs. 1 8a and 18b are perspective views respectively showing one preflex beam used as a pile and another preflex beam used as a pillar, in compressive stress is introduced into 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). 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. Industrial Applicability . 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. 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 dead loads. In addition, the preflex beam used as a pile or pillar in which compressive stress is introduced into 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. 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.

Claims: Claims 1. A method for manufacturing preflex beams, comprising the steps of: preparing a first set of cambered upper and lower plate girders, and connecting the upper and lower plate girders of the first set to each other by a plurality of PS steel bars at fulcrum positions; preparing a second set of cambered upper and lower plate girders, and connecting the upper and lower plate girders of the second set to each other by a plurality of PS steel bars at predetermined positions; arranging the first set of the upper and lower plate ' girders and the second set of the upper and lower plate girders in parallel while being spaced apart from each other by a predetermined interval: connecting the upper plate girders of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower plate girders of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower crossbeams, respectively; placing a plurality of hydraulic jacks between the upper and lower plate girders of each set at load applying positions, and applying preflexion loads to the upper and lower plate girders using the hydraulic jacks; and covering the upper flanges of the upper plate girders and the lower flanges of the lower plate girders with concrete. 2. The method according to claim 1 , wherein, in order to manufacture the preflex beams used as simple beams, said fulcrum positions are respectively situated on both side ends of the plate girders, and said load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/5. 3. The method according to claim 1 , wherein, in order to manufacture the preflex beams used as the internal beams of continuous beams, said fulcrum positions are respectively spaced inwardly apart from both side ends of the plate girders by about L/5, and said load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/3. 4. The method according to claim 1 , wherein, in order to manufacture the preflex beams used as the external beams of continuous beams, said fulcrum positions are respectively spaced inwardly apart from both side ends of the plate girders by about L/5, and said load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/4 and 0.5 L, so as to use the preflex beams as the external beams of continuous beams. 5 The method according to claim 2, wherein said lower supporting stands are placed at three positions l espectively spaced inwardly apart from both side ends of the lower plate girders by about 0 1 L and 0 5 L, further comprising the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 6. The method according to claim 3, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower plate girders by about 0! L and L/2, further comprising the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands 7 The method according to claim 4, wherein said lower supporting stands aie placed at three positions lespectively spaced inwardly apart from both side ends of the lower plate girders by about 0.1 L and L/2, further comprising the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 8. A method for manufacturing preflex beams, comprising the steps of: preparing a first set of cambered upper and lower plate girders; • preparing a second set of cambered upper and lower plate girders; connecting the upper plate girders of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower plate girders of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower plate girders, respectively; placing a plurality of props on the lower crossbeams at predetermined positions, respectively; placing a plurality of hydraulic jacks between the upper and lower plate girders of each set at load applying positions; enclosing the upper and lower plate girders of all sets and the hydraulic jacks with a square frame; ' applying preflexion loads to the upper and lower plate girders using the hydraulic jacks; and covering the upper flanges of the upper plate girders and the lower flanges of the lower plate girders with concrete. 9. The method according to claim 8, wherein, in order to manufacture the preflex beams used as simple beams, said fulcrum positions are respectively situated on both side ends of the plate girders, and said load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/5. 10. The method according to claim 8, wherein, in order to manufacture the preflex beams used as the internal beams of continuous beams, said fulcrum positions are respectively spaced inwardly apart from both side ends of the plate girders by about L/5. and said load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/3. 1 1 . The method according to claim 8, wherein, in order to manufacture the preflex beams used as the external beams of continuous beams, said fulcrum positions are respectively spaced inwardly apart from both side ends of the plate girders by about L/5, and said load applying positions are spaced respectively apart from the both side ends of the plate girders by about L/4 and L/2. 12. The method according to claim 9, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower plate girders by about 0.1 L and 0.5 L, further comprising the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 13. The method according to claim 10, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower plate girders by about 0! L and 0.5 L and further comprising the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 14. The method according to claim 1 1 , wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower plate girders by about 0.1 L and 0.5 L, further comprising the step of respectively placing a plurality of upper supporting stands under the upper plate girders at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 5 15. The method according to claim 1 , wherein said second set of upper and lower plate girders arranged in parallel with the first set of upper and lower plate girders is one or more 16. The method according to claim 8, wherein said second set of upper and lower plate girders arranged in parallel with the first set of upper and lower 10 plate girders is one or more. 17. A method for manufacturing preflex beams, comprising the steps of: preparing a first set of non-cambered straight upper and lower rolled shape steels, and connecting the upper and lower rolled shape steels of the first set to each other by a plurality of PS steel bars at fulcrum positions; - 5 preparing a second set of non-cambered straight upper and lower rolled shape steels, and connecting the upper and lower rolled shape steels of the second set to each other by a plurality of PS steel bars at fulcrum positions; arranging the first set of the upper and lower rolled shape steels and the second set of the upper and lower rolled shape steels in parallel while being spaced 20 apart from each other by a predetermined interval; connecting the upper rolled shape steels of the first and second sets by a plurality of upper crossbeams spaced apart from one another by predetermined regular intervals, and connecting the lower rolled shape steels of the first and second sets by a plurality of lower crossbeams spaced apart from one another by 25 predetermined regular intervals; placing a plurality of lower supporting stands under the lower crossbeams, respectively: placing a plurality of hydraulic jacks between the upper and lower rolled shape steels of each set at load applying positions, and applying preflexion loads to the upper and lower rolled shape steels using the hydraulic jacks so as to camber the upper and lower rolled shape steels by plastic deformation; placing a plurality of props on the lower crossbeams at predetermined positions, respectively; removing the PS steel bars placed at fulcrum positions and the hydraulic jacks placed at load applying positions; placing a plurality of hydraulic jacks on the upper rolled shape steels of each set or under the lower rolled shape steels of each set at load applying positions; enclosing the upper and lower rolled shape steels of all sets and the hydraulic jacks with a square frame; applying preflexion loads to the upper and lower rolled shape steels of all sets using the hydraulic jacks; and covering the upper flanges of the upper rolled shape steels and the lower flanges of the lower rolled shape steels with concrete. 18. The method according to claim 17. wherein said fulcrum positions are respectively situated on both side ends of the rolled shape steels, and said load applying positions are spaced respectively apart from the both side ends of the rolled shape steels by about L/5, so as to use the preflex beams as simple beams. 19. The method according to claim 17, wherein said fulcrum positions are respectively spaced inwardly apart from both side ends of the rolled shape steels by about L/5, and said load applying positions are spaced respectively apart from the both side ends of the rolled shape steels by about L/3, so as to use the preflex beams as the internal beams of continuous beams. 20. The method according to claim 17, wherein said fulcrum positions are respectively spaced inwardly apart from both side ends of the rolled shape steels by about L/5, and said load applying positions are spaced respectively apart from the both side ends of the rolled shape steels by about L/4 and L/2, so as to use the preflex beams as the external beams of continuous beams. 21. The method according to claim 18, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower rolled shape steels by about 0.1 L and 0 5 L. further comprising the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 22 The method according to claim 19, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower rolled shape steels by about 0! L and 0.5 L. further comprising the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 23. The method according to claim 20, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower rolled shape steels by about 0! L and 0.5 L, further comprising the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 24. A method for manufacturing preflex beams used as piles and pillars. comprising the steps of: connecting two dummy rolled shape steels to both ends of a non- cambered straight rolled shape steel by means of bolts into a rolled shape steel assembly; preparing a first set of non-cambered straight upper and lower rolled shape steel assemblies, and connecting the upper and lower rolled shape steel assembles of the first set to each other by a plurality of PS steel bars at both side ends of the rolled shape steels; preparing a second set of non-cambered straight upper and lower rolled shape steel assemblies, and connecting the upper and lower rolled shape steel assemblies of the second set to each other by a plurality of PS steel bars at both side ends of the rolled shape steel assemblies; arranging the first set of the upper and lower rolled shape steel assemblies and the second set of the upper and lower rolled shape steel assemblies in parallel while being spaced apart from each other by a predetermined interval; connecting the upper rolled shape steel assemblies of the first and second sets by a plurality of upper crossbeams spaced inwardly apart from one another by predetermined regular intervals, and connecting the lower rolled shape steel assemblies of the first and second sets by a plurality of lower crossbeams spaced apart from one another by predetermined regular intervals; placing a plurality of lower supporting stands under the lower crossbeams, respectively; placing a hydraulic jack between each connection portion of the upper rolled shape steel assembly of each set and each connection portion of the lower rolled shape steel assembly of each set at each load applying position, and applying preflexion loads to the upper and lower rolled shape steel assemblies using hydraulic jacks by plastic deformation; covering the upper flanges of the upper rolled shape steels except for the dummy rolled shape steels and the lower flanges of the lower rolled shape steels except for the dummy rolled shape steels, with concrete; placing a plurality of props on the lower rolled shape steel assemblies at predetermined positions; removing the PS steel bars and the hydraulic jacks; placing hydraulic jacks on the connection portions of the upper rolled shape steel assemblies of all sets or under the connection portions of the lower rolled shape steel assemblies of all sets; • enclosing the upper and lower rolled shape steels of all sets and the hydraulic jacks with a square frame; applying preflexion loads to the rolled shape steel assemblies of all sets using the hydraulic jacks; covering the lower flanges of the upper rolled shape steels except for the dummy rolled shape steels and the upper flanges of the lower rolled shape steels except for the dummy rolled shape steels, with concrete; and dismounting the dummy rolled shape steels from the rolled shape steels covered with concrete. 25. The method according to claim 24, when it is necessary to introduce compressive stress into the web portions of the rolled shape steels, further comprising the step of providing compressive stress to the rolled shape steels using a pre-tension method after the web portions of the rolled shape steels are covered with concrete and PC steel wires are inserted into the web portions of the rolled shape steels. 26. The method according to claim 24, when each of the manufactured preflex beams should have a relatively great length, further comprising the steps of: enlarging the cross-sectional areas of the connection portions of the rolled shape steels so as to improve the strength of the connection portions; forming grooves for receiving male rolled shape steels on the female rolled shape steels, and connecting each of the male rolled shape steels and each of the female rolled shape steels to each other; and injecting a grouting agent into the grooves. 27. The method according to claim 24, wherein said lower supporting stands are placed at three positions respectively spaced inwardly apart from both side ends of the lower rolled shape steels by about 0! L and 0.5 L. further comprising the step of respectively placing a plurality of upper supporting stands under the upper rolled shape steels at the same positions as those for the lower supporting stands after the step of placing the lower supporting stands. 28. The method according to claim 24, wherein said second set of upper and lower rolled shape steels arranged in parallel with the first set of upper and lower rolled shape steels is one or more.

Inventors: KOO MIN SE, KOO, MIN, SE
Applicants: KOO MIN SE, KOO, MIN, SE
Application year: 2000
Main classification: E04C3
IPC codes: E02D5, E04B1, E04C3