JP7680657B2 - Reinforced Concrete Beam - Google Patents

Description

Article 30 Paragraph 2 of the Patent Act applies (Publication 1) ▲1▼ Date held February 4th to 5th, 2021 (Announcement date: February 5th, 2021) ▲2▼ Name of the meeting and venue: "Master's Thesis Presentations, Department of Architecture, School of Environment and Society, Tokyo Institute of Technology, National University Corporation, Finished in March 2020" Midorigaoka Hall, Ookayama Campus, Tokyo Institute of Technology (2-12-1 Ookayama, Meguro-ku, Tokyo) ▲3▼ Distributor: Kazuki Masuda ▲4▼ Content of the publication: Research into the effect of bond removal of main reinforcement on the mechanical behavior of hinge-position-guaranteed RC beams subjected to reverse symmetric bending

The present invention relates to beam structures, and in particular to reinforced concrete beam structures.

Conventionally, the beams in reinforced concrete structures (hereinafter referred to as RC structures) can be damaged, such as by cracks, by external forces such as earthquakes. When a beam is damaged, it is necessary to carry out repair work to ensure the continued use of the building, but when the damage occurs over the entire length of the beam, this leads to prolonged repair work and increased costs, and ultimately makes it necessary to rebuild the building itself.
Patent Document 1 discloses a configuration in which, in order to suppress and control damage to the joint between a column and a beam (hereinafter referred to as the column-beam joint), which is difficult to repair, a first main bar is arranged at the center of the beam and a second main bar that is larger in diameter or stronger than the first main bar and is arranged at the end of the beam, and the first and second main bars are connected to each other with a reinforcing bar joint, thereby achieving so-called hinge relocation toward the center of the beam, thereby preventing cracks near the bending yield hinge from reaching the column-beam joint.

JP 2020-143566 A

However, as is clear from the drawings in the above patent documents, while the above beam structure can suppress damage at the beam-column joint, the damage caused by cracks extends over almost the entire beam, and therefore, although the possibility of continued use is improved, repair work after damage remains difficult. Furthermore, since cracks in beams are generally caused by the stress transfer from the reinforcing bars placed in the beam to the concrete, measures can be considered to reduce the range of cracks or control the range of cracks, for example by not adhering part of the reinforcing bars to the concrete and weakening their anchorage. However, due to the reduced anchorage, the hysteresis characteristics become closer to the so-called slip type, which has poor energy absorption, and there is a concern that the structural performance against external forces such as earthquakes will be reduced compared to ordinary beams.

The present invention was made to solve the above problems, and provides a beam structure that can suppress and control damage caused by external forces while ensuring the structural performance generally required of beams.

In order to solve the above-mentioned problems, the present invention is configured to provide a reinforced concrete beam having a plurality of main bars joined between columns, at least a part of which has a central fixing portion where the main bars are attached to the concrete in a predetermined range from the longitudinal center of the beam toward the columns, and a non-fixed portion where the attachment between the main bars and the concrete in a predetermined range other than the central fixing portion has been removed via an attachment removal means, and an anchoring plate is provided at the boundary between the central fixing portion and the non-fixed portion , and the anchoring plate is a plate-like body having openings through which at least two main bars can be inserted above and below when viewed in longitudinal section of the beam .
According to this configuration, the occurrence of cracks can be suppressed and controlled by the non-anchored portion while ensuring the structural performance of the beam. In addition, the anchor plate's bearing resistance can improve the anchoring properties.
The anchor plate may also be a plate-like body having openings through which all main reinforcements having non-anchored portions can be inserted .
In addition, the main reinforcement having a central fixing portion and a non-fixing portion may be configured to be located at least at a corner portion when viewed in vertical cross section of the beam.
According to this configuration, cracks occurring on the concrete surface can be more effectively suppressed and controlled.
In addition, a position a predetermined distance away from the joint between the pillar and the beam toward the center of the beam may be set as an assumed hinge, and the pillar side end of the non-fixed portion may be set as the position of the assumed hinge.
With this configuration, damage to the joints between the columns and beams can be suppressed.
The adhesion removal means may also be a sheath tube attached to the outer periphery of the main reinforcement.

1 is a schematic cross-sectional view of a beam-column structure according to one embodiment of the present invention; 2A to 2C are cross-sectional views of the beam-column structure of FIG. 1 along lines A-A, B-B, and C-C. FIG. 2 is a partially enlarged view of a reinforced concrete beam. FIG. 2 is a cross-sectional view showing an outline of the fixing steel plate. FIG. 2 is a diagram showing details of an analysis target and an analysis model. FIG. 2 is a diagram showing specifications of a model to be analyzed and analysis parameters. 1 is a schematic diagram showing the boundaries and loading conditions. This is a diagram showing the shear force-member angle relationship up to zero displacement after unloading R = ±1/67 rad. FIG. 13 is a diagram showing the analytical value of maximum strength and the calculated value of shear force at ultimate bending strength. FIG. 1 is a schematic diagram showing the cracking state of each model. FIG. 1 is a diagram showing material properties of concrete and reinforcing steel. FIG. 1 is a schematic diagram showing a structural testing setup. FIG. 13 is a diagram showing the shear force-member angle relationship. FIG. 1 is a diagram for explaining division into regions for observation. FIG. 1 is a diagram for explaining a method for measuring cracks. FIG. 1 is a diagram illustrating modeling of a crack. FIG. 1 is a schematic diagram showing a crack state. FIG. 13 is a diagram showing the change in the number of cracks. FIG. 13 is a diagram showing the transition of maximum crack width. FIG. 13 is a diagram showing the distribution of residual crack area ratio.

[Overall structure]
Hereinafter, a column-beam structure 1 having a reinforced concrete beam 10 will be described with reference to Figs. 1 and 2. As shown in Figs. 1 and 2, the column-beam structure 1 includes columns 3A and 3B of reinforced concrete, and a beam 10 of reinforced concrete joined to the columns 3A and 3B and erected between the columns 3A and 3B. The beam 10 includes upper end main reinforcement 13 and lower end main reinforcement 15 extending between the columns 3A and 3B. Both ends of the upper end main reinforcement 13 and the lower end main reinforcement 15 extend through the columns 3A and 3B, respectively, to the other beam 12 side shown by the chain line. Depending on the position of the entire structure of the beam 10, it may be bent and fixed in the columns 3A and 3B. Throughout this specification, each main reinforcement arranged in the beam 10 is assumed to be a deformed reinforcement having a rib formed around it.

As shown in FIG. 2(a), the upper end main reinforcement 13 is composed of a plurality of upper end main reinforcement 13A to 13D arranged on the uppermost end surface side of the beam 10. The lower end main reinforcement 15 is composed of a plurality of lower end main reinforcement 15A to 15D arranged on the lowermost end surface side of the beam 10.

In addition to the upper end main bars 13 and the lower end main bars 15, the beam 10 also has upper end main bars 17 and lower end main bars 19 that extend toward the columns 3A and 3B, respectively. As shown in FIG. 2(b), the upper end main bars 17 are composed of upper end main bars 17A to 17D arranged directly below the positions of the upper end main bars 13A to 13D. The lower end main bars 19 are composed of lower end main bars 19A to 19D arranged directly above the positions of the lower end main bars 15A to 15D. In addition, one end of the upper end main reinforcement 17 and the lower end main reinforcement 19 on the column 3A; 3B side, like the upper end main reinforcement 13 and the lower end main reinforcement 15, either passes through the column 3A; 3B and extends to the other beam 12 side shown by the dashed line, or is bent and fixed within the column 3A; 3B.

The other ends of the upper end main reinforcement 17 and the lower end main reinforcement 19 on the beam 10 side extend toward the center of the beam 10 and terminate at a position a predetermined distance away from the joint surface (joint surface 10A; 10B) between the column 3A; 3B and the beam 10. The other ends of the upper end main reinforcement 17 and the lower end main reinforcement 19 that terminate within the beam 10 are fastened with mechanical fasteners, for example, screw-type or grout injection type, to fix them to the beam 10 side. As shown in FIG. 2, the upper end main reinforcement 13 and the lower end main reinforcement 15, the upper end main reinforcement 17 and the lower end main reinforcement 19 arranged on the beam 10 are wrapped around ribs (stilts S) not shown in FIG. 1 arranged at predetermined intervals over the entire length of the beam 10.

The column-beam structure 1 having the above basic structure is a so-called hinge relocation structure in which the position where the yield hinge occurs (assumed hinge position) is located toward the center of the beam 10 from the joint surface 10A; 10B by mainly cutting off the end upper main reinforcement 17 and the end lower main reinforcement 19. This structure reduces the possibility that damage will occur to the joint between the columns 3A; 3B and the beam 10 due to external forces such as earthquakes, making repairs difficult.

[Regarding fixed and non-fixed parts]
Of the eight main bars consisting of upper end side main bars 13A to 13D and lower end side main bars 15A to 15D extending over the entire length of the beam 10, the upper end side main bars 13A;D and the lower end side main bars 15A;15D closest to the corners (four corners in this example) of the beam 10, which is formed into a vertically elongated rectangular cross section, are provided with fixed parts P1;P2;P3 and non-fixed parts Q1;Q2 along their extension direction. The fixed parts P and non-fixed parts Q will be described in detail below.

As shown in FIG. 1, the fixing portion P1 is a central section (central fixing portion) that includes the center of the longitudinal direction of the beam 10 (between columns 3A and 3B) and extends in the column 3A; 3B direction with a predetermined dimension. The non-fixing portions Q1 and Q2 are sections that sandwich the fixing portion P1 and extend in the column 3A; 3B direction with a predetermined dimension from the end of the fixing portion P1. The fixing portions P2 and P3 are sections (column-side fixing portions) that are adjacent to the non-fixing portions Q1 and Q2, respectively, and extend in the column 3A; 3B direction.

2(a) and (b), the peripheral surfaces of the upper end main reinforcement 13A;D and the lower end main reinforcement 15A;15D at the anchoring parts P1, P2, P3 are in direct contact with the surrounding concrete, and adhesion between the surface of each main reinforcement and the concrete is ensured. On the other hand, as shown in FIG. 2(c), the peripheral surfaces of the upper end main reinforcement 13A;D and the lower end main reinforcement 15A;15D at the non-anchored parts Q1;Q2 adjacent to the anchoring part P are covered in a sheath tube 20 as an adhesion removal means along the length. In addition, the end of the sheath tube 20 on the column 3;3B side coincides with the assumed hinge position described above, and the non-anchored parts Q1;Q2 are sections that extend continuously from the end of the anchoring part P1 to the assumed hinge position. By attaching the sheath pipe 20 to the circumferential surfaces of the upper end main reinforcement 13A;D and the lower end main reinforcement 15A;15D, the sheath pipe 20 removes adhesion between the surface of each main reinforcement and the concrete in the non-fixed portions Q1;Q2.

In other words, of the eight main bars in this example, some of the main bars located at the corners (upper end main bars 13A; 13D and lower end main bars 15A; 15D) have sections in their length direction where adhesion to the concrete is ensured (anchored sections) and sections where adhesion has been removed (non-anchored sections). With this configuration, the beam 10 has anchored sections P1, P2, P3 in its length direction where the main bars are anchored to the concrete (large), and non-anchored sections Q1; Q2 that are less anchored to the concrete than these anchored sections P (smaller).

[Other forms]
Next, another embodiment of the beam 10 having a fixed portion and a non-fixed portion will be described with reference to Figures 3 and 4. In these figures, the same components as those in the above embodiment are designated by the same reference numerals, and the description thereof will be omitted.

FIG. 3 is an enlarged cross-sectional view showing the boundary between the fixed portion P1 and the non-fixed portions Q1 and Q2 of the beam 10. As shown in FIG.
As shown in the figure, an anchoring steel plate 25 is provided at each of the boundaries. As shown in Fig. 4, the anchoring steel plate 25 is composed of a steel plate 25A having a vertically elongated rectangular shape (strip shape) extending in the vertical direction of the cross section of the beam 10 and having circular holes through which the upper end side main reinforcement 13A and the lower end side main reinforcement 15A corresponding to the top and bottom can be inserted, and a steel plate 25B also having a strip shape and having circular holes through which the upper end side main reinforcement 13D and the lower end side main reinforcement 15D corresponding to the top and bottom can be inserted, respectively. As shown in FIG. 3, each steel plate 25A; 25B is positioned at the boundary between the fixed portion P1 and the non-fixed portion Q1; Q2 with the main reinforcements inserted therethrough, and is firmly fixed between the upper main reinforcement 13A and the lower main reinforcement 15A, and between the upper main reinforcement 13D and the lower main reinforcement 15D by means of a fixing nut 26 screwed in from the fixed portion P1 side and a fixing nut 27 screwed in from the non-fixed portion Q1; Q2 side.

In other words, in comparison with the above-mentioned embodiment, the anchoring steel plate 25 is an anchoring member that is arranged to correspond to the upper end main reinforcement 13A; 13D and the lower end main reinforcement 15A; 15D which have sections where adhesion to the concrete has been removed, and the anchoring steel plate 25 ensures adhesion to the concrete.
In other words, the anchoring portion P1 in this example is the section from one anchoring steel plate 25 to the other anchoring steel plate 25, and due to the main reinforcement between the anchoring steel plates 25;25 and the anchoring of both anchoring steel plates 25;25 to the concrete, the anchoring portion P1 has a greater anchorage to the concrete than the beam 10 in the previously described embodiment.

Next, we will explain the analysis results showing the usefulness of the beam 10 according to each of the above embodiments. The analysis is a nonlinear three-dimensional finite element analysis, and Figures 5(a) and (b) show the details of the analysis target and the analysis model, and Figures 6(a) and (b) show the specifications of the model to be analyzed and the analysis parameters.

As shown in Figure 5, this analysis model is a super high-rise RC structure, and is assumed to be half the actual size. It is composed of one span of RC beam members and stubs with an inside span of 3050 mm. As shown in Figure 6 (a), the cross section of the beam is 275 mm wide and 450 mm high. The assumed hinge position is 450 mm from the beam edge (joint surface), and the main reinforcement up to that position is 4+4-D19. In addition, the second-stage reinforcement (corresponding to the end upper end side main reinforcement 17 and end lower end side main reinforcement 19 in Figure 1) is cut off at the assumed hinge position and mechanically fixed. The hinge position is intentionally moved to the center of the beam by making the central part of the beam 4-D19. The above design concept is intended to have the same strength as a beam with main reinforcement of 4+2-D19 over the entire length. In addition, two sets of ribs are placed at the cut-off position of the second-stage reinforcement. The slab is 350mm wide on each side and is attached on both sides.

As shown in Figure 6(b), a total of eight models were analyzed using the analysis parameters of the rebars to be removed ("none", "four corners", "all"), the anchoring type ("straight line", "steel plate") at the center of the span (corresponding to anchoring part P1 in Figure 1), and the presence or absence of a slab. The rebars to be removed were set to three parameters: B model with normal bond properties, in which the bond was not removed, DBC model, in which the bond was removed from the main rebars at the four corners on the side with the smaller moment than the assumed hinge position, and DBA model, in which the bond was removed from all main rebars in the same section as the DBC model.
The anchorage type at the center of the span was set to two parameters: the SA model, which has a straight anchorage (main reinforcement only) with bond characteristics, and the PA model, which has a straight anchorage and an anchoring steel plate (main reinforcement + steel plate). Here, the required anchorage length was calculated with reference to the RC standard of the Architectural Institute of Japan13), and the section (anchored section) where the bond is not removed in the SA model was set to 300 mm on both sides (total 600 mm) from the beam inflection point position. In the PA model, the section (anchored section) where the bond is not removed was set to 200 mm (total 400 mm), and anchoring steel plates (PL12) were inserted at both ends to anchor the main reinforcement near the inflection point. Regarding the presence or absence of a slab, the model name was given with an N if the slab was not attached, and an S if it was attached.

Analysis was performed on each of the above models under the boundary and loading conditions shown in Figure 7. As shown in the figure, one side of the beam was the force application side stub, and the other side was the vertical displacement restraint side stub. In the force application side stub, rotation was restrained to generate antisymmetric bending in the beam, and a forced displacement δ was applied to the node on the axis in the central depth direction shown by the imaginary line. In addition, in the vertical displacement restraint side stub, vertical displacement was restrained while horizontal displacement was allowed. In addition, the force application was performed by displacement control based on the member angle R calculated from the inside span of the beam (3050 mm), and was performed by alternating positive and negative. In detail, two cycles of forced displacements of R = ±1/1600, 1/800, 1/400, 1/200, and 1/100 rad were performed, and one cycle of forced displacement of R = ±1/67 rad was performed.

Figure 8 shows the shear force-member angle relationship up to zero displacement after unloading R=+1/67rad, which was common to all models, and Figure 9 shows the analytical value of maximum strength and the calculated value of shear force at ultimate bending strength. The dashed line in Figure 8 is the calculated value of shear force at ultimate bending strength, Qu_cal. The ultimate bending strength Mu was calculated using the simplified formula (Formula (1)) shown in the RC Standard of the Architectural Institute of Japan13). In the model with slab, the calculation was performed by taking into account the reinforcing bars of the slab when calculating the cross-sectional area a _t of the tensile reinforcing bars at the top end, when the slab is the tension side. For the model without slab, Qu_cal=190kN, and for the model with slab, Qu_cal=205kN.

Formula (1) Mu=0.9a _t σ _y d

a _t : Cross-sectional area of tensile steel bar [mm ² ],
σ _y : Yield strength of tensile rebar [N/mm ² ],
d: Effective depth [mm].

In all models, the hysteresis loop did not become slip-type up to R = 1/100 rad cycle, but drew a stable spindle-type loop. Also, as shown in Figure 8, in the DBC-SA-N model (Figure 8 (b)), which has bond removal sections for the main reinforcement bars at the four corners and has a straight line fixed at the center, the fixing effect was not achieved and a slight decrease in both stiffness and strength was observed. However, the DBC-PA-N (Figure 8 (c)), which has a steel plate fixed at the center, and the DBC-SA-N model (Figure 8 (c)), which has a straight line fixed at the center, showed a slight decrease in stiffness and strength.
The maximum strength of the DBC-PA-S model (same as (g)) significantly exceeded the calculated value.

In addition, when comparing the maximum positive strength of the N model without a slab, compared to 218kN for the B-N model with normal bond properties, the DBA-SA-N model (d) which has bond removal sections for all main bars and has a straight-line fixed center section, had a strength of 158kN, approximately 28% lower, and the DBC-SA-N model (b) which has bond removal sections for the main bars at the four corners and has a straight-line fixed center section had a strength of 193kN, approximately 11% lower.

The difference in the rate of strength reduction between DBA, which has bond-removed sections in all main bars, and DBC, which has bond-removed sections in the main bars at the four corners, is thought to be because in DBA, the tensile strength of all tensile rebars is smaller than in B-N, but in DBC, the tensile rebars of the inner main bars other than those at the four corners, which have bond, yield and bear the tensile force, just like the B-N model.

Compared to the B-N model, the DBA-PA-N model (e), which has adhesion removal sections for all main reinforcements and is fixed at the center with steel plates, had a strength reduction of 204 kN, a drop of about 6%, while the DBC-PA-N model (c), which has adhesion removal sections for the main reinforcements at the four corners and is fixed at the center with steel plates, had a strength reduction of 209 kN, a drop of about 4%, and the drop in strength was significantly suppressed. This is thought to be because the steel plates increase the fixing effect at the center of the span, ensuring the switching of the tensile and compressive loads on the main reinforcements. Also, when comparing the S model with a slab, almost no effect from adhesion removal was observed, and both models had almost the same strength. This is thought to be because the attachment of the slab reduces the compressive force borne by the upper end reinforcements.

Next, the crack range in each model will be described with reference to FIG. 10. This figure shows the cracking conditions of each model when R = +1/100 rad. As shown in the figure, in all models with hinge relocation structure, large cracks have occurred in the section from the column edge to the assumed hinge position. In addition, in comparison with the B model (B-N, B-S) with normal adhesion properties, in each model with an adhesion removal section, there are fewer cracks in the range on the side where the moment is smaller than the assumed hinge position (towards the center of the span), and it can be seen that the damage range is effectively suppressed. In addition, in comparison with the DBC model and the DBA model, that is, depending on whether the target of the main reinforcement having the adhesion removal section is the four corners or all, it can be seen that the influence on the damage range of the cracks is small.
In other words, the concrete cover thickness of the main reinforcement bars located at the corners of the beam cross section is smaller than that of the main reinforcement bars on the inside, and considering that cracks in the concrete are caused by stress transmission from the main reinforcement bars, it is thought that providing adhesion removal sections in the main reinforcement bars at the four corners close to the concrete surface is extremely effective in reducing damage to the beam.

In addition, when we look at the anchorage type in the center, the SA model with linear anchorage has no cracks in the center of the span, but the PA model with steel plate anchorage has small cracks on the surface roughly corresponding to the position of the steel plate. Furthermore, no significant difference was observed with or without a slab, and the effect of suppressing the damage range was also observed on the top surface of the slab.

Based on the above analysis results, it was confirmed that the DBC-SA-N model, which has adhesion removal sections in the main reinforcement bars at the four corners and a straight-line anchor in the center, and the DBC-PA-N model, which has a straight-line anchor and steel plate anchors, have excellent balance in terms of structural performance and damage suppression and control, although there are differences in strength.

Next, the results of the structural experiment of the beam member of the present invention are shown. As shown in Figure 5, the specimen for this structural test is composed of a one-span RC beam member with an inside span of 3050 mm and a stub. As shown in Figure 6 (a), the cross section of the beam is 275 mm wide and 450 mm high. The assumed hinge position is 450 mm from the beam edge (joint surface), and the main reinforcement up to that position is 4+4-D19. In addition, the two-stage reinforcement (corresponding to the end upper end side main reinforcement 17 and the end lower end side main reinforcement 19 in Figure 1) is cut off at the assumed hinge position and mechanically fixed. The hinge position is intentionally moved to the center of the beam by setting the central part of the beam to 4-D19. The above design concept is intended to have the same strength as a beam with main reinforcement of 4+2-D19 over the entire length. In addition, two sets of ribs are placed at the cut-off position of the two-stage reinforcement. The slab is 350 mm wide on each side and is attached on both sides (corresponding to the DBC-PA-S model of analysis parameters).

Figure 11(a) shows the material properties of the concrete used in the test specimens. The concrete had a nominal strength of Fc50 and a compressive strength of 58.6 (N/mm2), splitting strength of 2.8 (N/mm2), and Young's modulus of 32,800 (N/mm2). The material properties of the reinforcing bars are shown in Figure 11(b).

Figure 12 shows the setup for the structural test. The vertical displacement restraint (right) stub of the test specimen was fixed to the reaction floor via a jig. The actuator and the force application (left) stub were connected via a force beam. Out-of-plane restraint was achieved by connecting the force beam to the pantograph. A counterweight was used to offset the weight of the parallel crank connected to the force beam, the force application jig, and the test specimen itself. The actuator was used to apply alternating positive and negative incremental loads in the vertical direction.

The alternating positive and negative incremental loading was displacement controlled by the member angle R calculated from the beam internal span and the vertical displacement of the load-applied (left) side stub, and the load cycle consisted of two cycles at deformation angles of R = ±1/800, 1/400, 1/200, 1/100, 1/67, and 1/50 rad, and one cycle at a deformation angle of R = +1/33 rad.

Figure 13 shows the relationship between shear force and member angle obtained from structural testing. No reduction in strength was observed up to R = 1/33 rad, and the hysteresis curve formed a stable spindle-shaped loop. The maximum load was 250 kN on the positive side and 210 kN on the negative side, and although the negative side was slightly lower, the results were equivalent to the calculated shear force at ultimate strength. From the above, it was found that the structure has sufficient structural performance.

Next, the evaluation of the damage state of the test specimens will be explained using Figures 14 to 20. For the evaluation, the crack occurrence state of the test specimens during the structural experiment was visually observed, and the maximum width of each crack was measured. Based on the results of the observation and measurement, the damage state was evaluated focusing on the crack occurrence position, the number of cracks related to the repair effort, the maximum crack width indicating the degree of damage, and the crack area related to the repair cost.

As shown in Figure 14, in order to confirm the effects of adhesion removal and central anchorage when observing the test specimens, the beam members were divided into eight regions A through H (hereafter referred to as observation regions) by determining a predetermined distance from one end of the beam. Observations were made on two surfaces of the test specimen, the side of the beam and the top surface of the slab, including the crack characteristics and crack width. Observations were made at the first peak of each deformation angle cycle and when the load was removed, and were continued until the cycle in which the cover concrete peeled off and it became impossible to evaluate the cracks.

Figure 15 shows the method for measuring crack width. In the case of beams, cracks were checked for cracks that crossed the beam edge, the beam main reinforcement, and a position 85 mm from the beam main reinforcement (see Figure 15(a)), and in the case of the top surface of the slab, cracks that crossed the slab edge, the slab main reinforcement, the beam end, and the beam main reinforcement were checked. In addition, the crack width was measured perpendicular to the crack using a crack scale (minimum scale 0.03 mm) (see Figure 15(b)).

The crack area was calculated by replacing and modeling the actual cracks shown in Figure 16(a) with a parallelogram crack model as shown in Figure 16(b), and the continuously changing cracks were calculated as a parallelogram crack of "crack width at measurement point x measurement interval". In addition, the crack area was totaled for each of the eight areas A to H, and the total was divided by the area of each area to calculate the crack area ratio γAcr.

Figure 17 shows the cracking condition at R = 1/100 rad. For the side of the beam, bending cracks occurred at the expected hinge position (the boundary between AB and GH in the observation area) during the R = 1/800 rad cycle, and then as the deformation increased, the cracking range expanded from the expected hinge position toward the column edge. The crack width of the crack that occurred at the expected hinge position was large, and the cracks that occurred in other areas were fine. At R = 1/50 rad cycle, it was observed that the cover concrete around the expected hinge position was gradually destroyed, peeling occurred, and the range expanded. For the top surface of the slab, bending cracks occurred at the expected hinge position during the 1/800 rad cycle, and then as the deformation increased, the cracking range expanded from the expected hinge position toward the column edge. The majority of cracks penetrated in a straight line. Even during large deformations, cracks were concentrated at the expected hinge locations, with almost no cracks occurring in the adhesion removal areas, and most of the cracks that occurred outside the expected hinge locations were very small and less than 0.1 mm in size.

Figure 18 shows the number of cracks that occurred on the sides of beams and the slab surface. The vertical axis shows the cumulative number of cracks that occurred in each cycle, and the horizontal axis shows each cycle. On the sides of beams, there were 41 cracks when R = -1/100 rad, 46 when R = -1/67 rad, and 47 when R = -1/50 rad. On the top surface of the slab, after 17 cracks occurred by R = -1/100 rad, the number of cracks did not increase until 1/50 rad. On both the sides of beams and the top surface of the slab, the number of cracks gradually increased until the cycle in which the beam main reinforcement bars and slab main reinforcement bars respectively yielded, but after yielding, only the cracks that occurred at the expected hinge positions became larger, and almost no new cracks occurred in other areas.

Figure 19 shows the transition of maximum crack width of beams and slabs at the peak and when unloading for each loading cycle. The residual maximum crack width of beams at unloading was 0.35 mm when R = -1/200 rad and 0.35 mm when R = -1/100 rad, while the residual maximum crack width of the top surface of the slab at unloading was 0.2 mm when R = -1/200 rad and 0.6 mm when R = -1/100 rad.
In addition, the Architectural Institute of Japan's earthquake resistance performance evaluation guidelines indicate Damage Level II: Repair Limit I (crack width in components of 0.2 to 1.0 mm or less in a full-scale building) as an index of the level of damage that can be repaired with small-scale repairs, and taking into account the dimensions of the test specimen, the crack width index is about half the value, or 0.5 mm.
Regarding the maximum remaining crack width of the beam when unloaded, when the general design criteria for level 2 earthquake motion, R = 1/100 rad, is taken into consideration and the size effect of the test specimen is evaluated based on the above guidelines, the crack width of the member in a full-scale building corresponds to Damage Level II: Repair Limit I (minor repairs required), which is 0.2 to 1.0 mm. Therefore, it was found that if the beam member of the present invention is used in a real building, the building can continue to be used with minor repairs even after an earthquake.

Figure 20 shows the residual crack area ratio γAcr for each observation area on the side of the beam and the top of the slab when the positive load is removed. The vertical axis shows the residual crack area ratio for each observation area, and the horizontal axis shows the area shown at the top of the figure. The areas (B and G) that include the assumed hinge positions on both the side of the beam and the top of the slab have a significantly larger residual crack area ratio than other areas, but the residual crack area ratio on the column side is small, and there is almost no residual crack area ratio in the area where the adhesion has been removed. Therefore, with the beam member of the present invention, it is possible to limit the areas that need to be repaired for earthquake damage to the assumed hinge positions, thereby reducing repair costs.

The present invention has been described above through the embodiments, but the technical scope of the present invention is not limited to the above embodiments. It is clear to those skilled in the art that various modifications and improvements can be made to the above embodiments, and it is clear from the claims that forms incorporating such modifications or improvements can also be included in the technical scope of the present invention.

In the above embodiment, the cross-sectional shape of the beam 10 is rectangular, and the reinforcing bars located at the corners (four corners) are provided with adhesion removal sections to form non-anchored sections, but other angular shapes are also acceptable, and in such shapes, damage can be suppressed and controlled by providing adhesion removal sections for the main bars closest to the corners of the angular shape to form non-anchored sections. Also, in the above embodiment, the anchoring steel plates 25 are configured as strips that are hung over the main bars on the top and bottom in cross section, but they may be configured to be individually provided to correspond to each main bar, to be hung over the main bars on the left and right in cross section, or to have a square-shaped anchoring steel plate with holes through which all the main bars with adhesion removal sections can be inserted.

In addition, it is believed that the length of the anchoring section relative to the overall length of the beam 10 in the above embodiment can be set based on the required anchoring length of tensile reinforcement using deformed reinforcement in the RC structural calculation standards.

Where: Strength that is the standard for bond splitting: Stress level of reinforcing bar at the joint surface: Nominal diameter of deformed reinforcing bar: 1.0 when fixed inside the core
: Correction factor for required fixing length (0.7 for mechanical fixings)

However, the above formula relates to fastening to a joint, and although its applicability to fastening inside the beam 10 is uncertain, it is assumed that it may be similar.

In the above embodiment, the length of the adhesion removal section relative to the overall length of the beam 10 may be appropriately determined based on the bending crack strength and the bending moment distribution of the beam when a plastic hinge occurs. The bending crack occurrence moment is given by the following formula.

Where: Compressive strength of concrete: Section modulus taking into account reinforcing bars
N: Axial force
D: Material

It is considered desirable to compare the bending crack strength calculated by the above formula with the bending moment distribution of the beam when plastic hinges occur, and to remove the adhesion of the main reinforcement in the places where the bending moment exceeds the bending crack strength. Also, the adhesion removal section does not have to be a continuous section as shown in Figure 1, and the adhesion removal section and the fixed section may be formed, for example, alternately in shorter sections.
Taking the above into consideration, it is necessary to appropriately evaluate the required anchorage length in the center of the beam 10 and the location where bending cracks will occur, and set the lengths of the adhesion removal section and anchorage section.

1 Column beam structure, 3A; 3B column, 10 beam, 13 Upper end main reinforcement, 15 Lower end main reinforcement,
P1; P2; P3: fixed part, Q1; Q2: non-fixed part, 25: fixed steel plate

Claims (5)

A reinforced concrete beam having a plurality of main bars and connected between columns,
At least a portion of the plurality of main reinforcements,
A central anchorage portion to which the main reinforcement and concrete are attached in a predetermined range from the center of the longitudinal direction of the beam toward the column direction;
a non-anchored portion in which the adhesion between the main reinforcement and the concrete in a predetermined range other than the central anchored portion is removed by an adhesion removing means;
having
A fixing plate is provided at the boundary between the central fixing portion and the non-fixing portion,
A reinforced concrete beam characterized in that the anchor plate is a plate-like body having openings through which at least two main reinforcements can be inserted, above and below when viewed in longitudinal section of the beam. The reinforced concrete beam according to claim 1, characterized in that the anchor plate is a plate-like body having openings through which all of the main reinforcement bars having the non-anchored portions can be inserted. A reinforced concrete beam as described in claim 1 or 2 , characterized in that the main reinforcement having the central fixing portion and the non-fixing portion is located at least at the corner portion when viewed in longitudinal cross section of the beam. A position spaced a predetermined distance from the joint between the column and the beam toward the center of the beam is set as an assumed hinge;
A reinforced concrete beam as described in any one of claims 1 to 3, characterized in that the column side end of the non-fixed portion is set as the position of the assumed hinge. 5. A reinforced concrete beam according to claim 1, wherein the adhesion removal means is a sheath pipe attached to the outer periphery of the main reinforcement.