JPH06146472A - Precast ferro-concrete beam - Google Patents

Abstract

PURPOSE:To prevent a concrete beam from deflection or cracking due to a long period load and reduce the dead load of the beam. CONSTITUTION:A precast concrete beam A is so structured that a pair of steel pipes 1 are installed confronting up and down as stretching in the longitudinal direction in the central part of the beam section to be manufactured and that a plurality of beam main rods 2 are arranged at the periphery of the steel pipes. At certain intervals, rib rods 3 are arranged for a plurality of beam main rods 2, anti concrete 4 is placed in the condition that the two steel pipes 1 are tightened. When curing of the placed concrete attains a certain specified strength, the tension on the steel pipes 1 is released, and thus manufacture is completed. The beam A thus manufactured transported to the site, lifted, and set over the columns 5 to be erected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precast reinforced concrete beam which is prestressed and mainly used for a long span beam.

[0002]

2. Description of the Related Art Reinforced concrete structures are used in many building structures because of their excellent economic efficiency. However, due to the material property that concrete is weak in bending tensile stress, the allowable span when it is laid horizontally as a beam is usually It is up to about 5 to 8 m. Therefore, when a long span of 10 m or more is required for a reinforced concrete beam, a plurality of PCs are used as a tension material in order to make up for the lack of tensile strength of the concrete beam.
A steel material is embedded in concrete to introduce a predetermined prestress to the concrete beam.

The PC steel material comprises a steel material bundle in which a plurality of PC steel wires are bundled or a PC steel rod. And that PC
In the state where tension is applied to the steel material, concrete is placed around it, and after the concrete is hardened, the tension applied to the PC steel material is relaxed and prestress is introduced into the reinforced concrete beam, or the concrete is hardened. Later, the PC steel material was tensioned and fixed by a fixing device to introduce prestress to the reinforced concrete beam.

[0004]

However, when the span is increased, the beam is inevitably increased and the weight is increased, which complicates the transportation and hoisting work of the beam manufactured in the factory by the precast method to the site. Become. In addition to the increase in the self-weight, the relaxation of the PC steel material that occurs with the passage of material age may cause bending and cracking of the beam due to long-term load.

The present invention has been made in view of the above problems, and it is an object of the present invention to prevent bending and cracking due to long-term load and to reduce the weight of a beam.

[0006]

In order to achieve the above-mentioned object, the precast reinforced concrete beam of the present invention comprises a steel pipe tensed by a pretensioning method and embedded in the center of the cross section of the reinforced concrete beam. It is characterized by the introduction of pre-stress.

[0007]

[Function] Since the hollow part formed inside the steel pipe is not filled with concrete, the self-weight of the reinforced concrete beam is reduced, which makes transportation and lifting work easier than before, and
This will reduce the shearing force input during earthquakes. Furthermore, the steel pipe of the present invention has a larger surface area in contact with the concrete, as compared with the conventional tendon materials such as PC steel material and PC steel rod, thereby improving the adhesion to the concrete, thereby making it integral with the concrete. Turn into. In addition, since the steel pipe itself can support the load in the vertical direction, the shear rigidity of the beam is increased, so that the bending and cracking of the beam due to long-term load are prevented. In addition, the steel pipe is P
It is much lower cost than C steel.

When a steel pipe is used as a tension member as in the present invention, the shear rigidity increases as described above, and the tension force applied to the steel pipe can be smaller than when PC steel is used. The wall thickness of the steel pipe may be reduced according to the strength of the beam to reduce the weight of the steel pipe itself. Regarding the effect of cross-section loss due to the hollow portion of the steel pipe, shear stress is not so problematic because the stress level is low when the span is long, and bending stress is not a problem. Since it is provided in the central portion of, the effect is small as it deviates from the compression area. Further, the decrease in bending rigidity due to the decrease in the second moment of area can be minimized by arranging the steel pipe in the center of the cross section.

[0009]

Embodiments of the present invention will be described with reference to the drawings.
The precast reinforced concrete beam A of this embodiment is shown in FIG.
A pair of steel pipes 1 are vertically opposed to each other along the longitudinal direction at the center of the cross section of the beam to be manufactured, and further a plurality of beam main bars 2 are arranged on the outer periphery thereof. At the same time, after arranging the ribs 3 at predetermined intervals with respect to the plurality of beam main bars 2, concrete 4 is placed in a state in which the pair of steel pipes 1 is tensioned, and the placed concrete 4 is predetermined. When the strength is reached, the tension force applied to the steel pipe 1 is released to manufacture the beam A.

The beam A manufactured as described above
Is transported to the site and hoisted, and is erected between the columns 5 to be erected, as shown in FIG. Further, an omnier plate 6 is bridged between the bridges A, and slab reinforcements 7 are arranged on the bridges, and the beam top main reinforcements 2a are arranged along the ribs 3 projecting upward from the above-mentioned beams A. Is installed on site.

After that, the cast-in-place concrete 8 is poured to construct the beam A of the building and the slab to be mounted on the beam A. The precast reinforced concrete beam A manufactured as described above and installed between the pillars 5 is the embedded steel pipe 1
Since the hollow 4a is not filled with the concrete 4, the self-weight of the beam A is reduced, and the work of transporting and lifting the beam A to the site is easier than before, and it is input during an earthquake. The shearing force can be made smaller than before.

At this time, in order to prevent bending and cracking of the beam due to long-term load, the magnitude of prestress introduced in the steel pipe 1 is 5 to 10 kgf /
A cm ² is sufficient. Therefore, the tension force to be applied to the entire steel pipe 1 may be small, and thus the thickness of the steel pipe 1 may be thin. In addition, since the steel pipe has a larger surface area than PC steel rods or the like, which are conventional tension materials, the adhesive force to concrete is improved, and the steel pipe itself can resist vertical load, so that Shear stiffness is increased.

In addition to the increase in shear rigidity, the weight of the beam itself is reduced as described above, so that the beam is prevented from being bent or cracked more than allowable due to long-term load. In addition,
The steel pipes 1 are individually arranged in the respective beams A, and the force transmission with the columns 5 and other members is not performed. Further, since the adhesion of the steel pipe 1 to the concrete 4 is large as described above, it is not necessary to provide the fixing portions at both ends of the beam, but the fixing portions may be provided.

Further, in the above embodiment, the circular steel pipe 1 is embedded in the concrete 4 as a tension material, but as shown in FIG. 3, the square steel pipe 1 is used as a tension material, and the sectional shape of the steel pipe is However, the shape is not limited to a circle. Further, as shown in FIG. 4, the steel pipe 1 may be embedded as a tension material and a conventional PC steel rod or steel wire 9 may be arranged.

Further, the steel frame 11 may be arranged in the beam A. In this case, for example, as shown in FIG. 5, an H-shaped steel 11 is arranged at the center of the beam cross section, and a pair of square steel pipes 1 are arranged so as to abut on both sides of the web, respectively. 11 and the square steel pipe 1 are both arranged at the center of the beam cross section.

[0016]

As described above, the precast reinforced concrete beam of the present invention is more effective than the conventional one in that it is possible to prevent bending and cracking due to long-term load, and to reduce the weight of the beam due to the hollow section.

[Brief description of drawings]

FIG. 1 is a sectional view of a precast reinforced concrete beam according to an embodiment of the present invention.

FIG. 2 is a side view showing a erected state of the beam in the embodiment according to the present invention.

FIG. 3 is a sectional view of a precast reinforced concrete beam according to a second embodiment of the present invention.

FIG. 4 is a sectional view of a precast reinforced concrete beam according to a third embodiment of the present invention.

FIG. 5 is a sectional view of a precast reinforced concrete beam according to a fourth embodiment of the present invention.

[Explanation of symbols]

 1 Steel pipe 1a Hollow part 2 Beam main bar A Precast reinforced concrete beam

Claims (1)

[Claims] 1. A precast reinforced concrete beam, characterized in that a steel pipe is embedded in a central portion of a cross section of reinforced concrete forming the beam in a state in which a predetermined tension is applied.