Hybrid beam design method

Abstract

To provide a design method of a hybrid beam capable of taking into consideration the influence of a steel frame embedded in the beam ends of an SRC structure on the bond stress of the beam main reinforcement.SOLUTION: In an SRC beam structure 15 constructed by embedding the beam end of a steel beam 14 made of steel frame 12 in an RC section, when calculating the adhesion stress of the beam main reinforcement 16 with the fixing piece 18 installed at the tip, when the main beam reinforcement 16 is on the tension side, the length reduced from the actual length L of the main beam reinforcement 16 is the attachment length of the main beam reinforcement 16. When the main beam reinforcement 16 is on the compression side, the actual length L of the main beam reinforcement 16 is used as the attachment length.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for designing a hybrid beam. [Background technology]

[0002] In recent years, there has been an increase in buildings that use composite beams (hybrid beams) with a steel frame (S) in the center and steel frames covered with reinforced concrete (RC) at both ends as the beam skeleton (see, for example, Patent Documents 1 to 3). Because the center of a hybrid beam is made of steel, it is possible to reduce the beam's own weight and beam depth compared to RC beams, which has the advantages of allowing for longer beam spans, cost reductions, and increased freedom in floor plan design. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-113464 [Patent Document 2] Patent Publication No. 2021-113465 [Patent Document 3] Japanese Patent Publication No. 2021-113466 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional hybrid beams, the bond length of the beam reinforcement at the beam end of SRC construction was set to the same as that of RC construction to calculate the bond stress of the beam reinforcement. However, this does not take into account the effect of the steel frame embedded in the beam end of SRC construction on the bond stress of the beam reinforcement.

[0005] In view of the above, the present invention aims to provide a design method for hybrid beams that can take into account the effect of steel frames embedded in the ends of SRC beams on the bond stress of the main beam reinforcement. [Means for solving the problem]

[0006] The hybrid beam design method of the present invention is characterized in that when calculating the bond stress of the main beam reinforcement with anchors at the tip of a steel-reinforced concrete beam section formed by embedding the beam end of a steel beam made of steel in a reinforced concrete section, if the main beam reinforcement is on the tension side, the bond length of the main beam reinforcement is taken to be a length subtracted from the actual length of the main beam reinforcement, and if the main beam reinforcement is on the compression side, the actual length of the main beam reinforcement is taken to be the bond length.

[0007] According to the present invention, when calculating the bond stress of the beam reinforcement in a steel-reinforced concrete beam section where the end of a steel beam is embedded and anchored at its tip, the experimental results described below show that when the beam reinforcement is on the tension side, the bond length of the beam reinforcement is determined to be a length less than the actual length of the beam reinforcement. This makes it possible to calculate the allowable shear stress in the steel-reinforced concrete beam section, taking into account the crack width. It also makes it possible to determine the bond length of the beam reinforcement, taking into account the influence of the steel beam embedded in the steel-reinforced concrete beam section.

[0008] In the design method for hybrid beams of the present invention, from the test results described below, for example, when the main beam reinforcement is on the tension side, it is preferable to calculate the design bond stress τf of the main beam reinforcement as τf=dh·Δσ / {4(L-0.5d)}, where dh is the diameter of the main beam reinforcement, Δσ is the difference in stress levels of the main beam reinforcement at the ultimate limit state, L is the actual bond length of the main beam reinforcement within the steel-reinforced concrete beam part, and d is the effective depth of the steel-reinforced concrete beam part.

[0009] Furthermore, in the design method for hybrid beams of the present invention, from the test results described below, for example, when the main beam reinforcement is on the compression side, it is preferable to calculate the design bond stress τf of the main beam reinforcement as τf=dh·Δσ / 4L, where dh is the diameter of the main beam reinforcement, Δσ is the difference in stress levels of the main beam reinforcement at the ultimate limit state, L is the actual bond length of the main beam reinforcement within the steel-reinforced concrete beam part, and d is the effective length of the steel-reinforced concrete beam part. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic front view showing an example of a hybrid beam to which a hybrid beam design method according to an embodiment of the present invention is applied; [Figure 2] Schematic longitudinal cross-section of the test specimen. [Figure 3] 3 is a schematic cross-sectional view taken along line III-III in FIG. 2. [Figure 4] A partial schematic front view of a hybrid beam and a graph showing the relationship between tensile stress and bending stress and the distance from the column of the SRC beam. [Figure 5] Graph showing the relationship between tensile strain and its measurement location in specimen No. 4-5. [Figure 6] Graph showing the relationship between tensile strain and its measurement location in specimens No. 4-6. [Figure 7] Graph showing the relationship between compressive strain and its measurement location for specimen No. 4-5. DETAILED DESCRIPTION OF THE INVENTION

[0011] An example of a hybrid beam 10 to which the hybrid beam design method according to the embodiment of the present invention is applied will be described with reference to the drawings. Hybrid beams to which this design method is applied are, for example, those described in Patent Documents 1 to 3. Note that Figures 1 to 4 are diagrams for schematic explanation, and the dimensions are exaggerated.

[0012] As shown in Fig. 1, the hybrid beam 10 is a beam in which both ends of a steel frame 12 made of steel such as H-shaped steel or I-shaped steel that is spanned between opposing columns 11 are embedded in a reinforced concrete (RC) structure 13. Although not shown, the hybrid beam 10 may also be one in which the ends of the steel frame 12 are embedded in an RC structure that is integrated with the RC foundation.

[0013] The hybrid beam 10 has a steel beam section (S beam section) 14 in the center with the steel frame 12 exposed, and both ends are SRC beam sections 15 with the steel frame 12 covered by a reinforced concrete structure 13.

[0014] The column 11 is made of reinforced concrete, and although not shown in detail, a plurality of main column reinforcements and shear reinforcement bars surrounding the main column reinforcements are arranged inside.

[0015] 2 and 3, the SRC beam section 15 includes a steel frame 12, multiple beam reinforcement bars 16 arranged above and below the steel frame 12 and extending along the longitudinal direction of the hybrid beam 10, and multiple shear reinforcement bars 17 surrounding the beam reinforcement bars 16. The beam reinforcement bars 16 extend into the column 11. The beam reinforcement bars 16 are fixed to the beam-column joint using anchoring hardware or bent anchoring. An anchoring piece 18 is attached to the tip of the beam reinforcement bars 16. The anchoring piece 18 consists of a nut portion that screws into the threaded portion at the tip of the beam reinforcement bars 16 and a steel plate fixed to the nut portion. The anchoring piece 18 corresponds to the anchoring device of the present invention. Instead of the anchoring piece 18, a 180-degree hook or a mechanical anchoring device can be used as the anchoring device of the present invention.

[0016] At the base end (the end on the column 11 side) and tip end (the end on the steel beam 14 side) of the SRC beam 15, concentrated reinforcement (shear reinforcement) 19 is arranged at closer intervals and more densely than in the middle area where shear reinforcement 17 is arranged. Furthermore, core reinforcement 20 is arranged in each shear reinforcement 17 and concentrated reinforcement 19.

[0017] Furthermore, if necessary, steel rib plates (closing plates) 21 that connect the upper and lower flanges on the left and right sides may be provided by fillet welding at the end (the end on the column 11 side) and the start (the end on the steel beam 14 side) of the steel frame 12 inside the SRC beam 15. The thickness of the rib plates 21 is preferably equal to or greater than the thickness of the web of the steel frame 12.

[0018] The SRC beams 15 are made of cast-in-place concrete. The concrete may be ordinary concrete or fiber-reinforced concrete.

[0019] When designing a building equipped with a hybrid beam 10, it is assumed that the design bond stress τf of the beam main reinforcement 16 at the ultimate limit state in the SRC beam section 15, which is the beam end, is below the bond reliability strength τhu. Therefore, it is necessary to calculate the design bond stress τf of the beam main reinforcement 16. In members with multiple beam main reinforcement 16 arranged in stages, the design bond stress τf must be confirmed for all beam main reinforcement 16 arranged within 1 / 4 of the tensile and compression sides of the RC member depth D within the cross section. Note that if it is confirmed that the design acting shear force is below the shear reliability strength Vhu, which takes into account the effect of bond failure, it can be considered that the bond strength is secured.

[0020] The SRC beam section 15 is located at the end of the hybrid beam 10, and an anchoring piece 18 is provided at the tip of the main beam reinforcement 16, so the attachment length is considered as being divided into the tension side and the compression side.

[0021] In order to determine the bond lengths on the tension and compression sides of the main beam reinforcement 16 in the SRC beam section 15, the test specimens described below were prepared.

[0022] A total of six specimens, No. 4-1 to No. 4-6, were prepared. The specifications of each specimen are summarized in Table 1. Specimens No. 4-1 to No. 4-4 were of the bending yield type, while specimens No. 4-5 and No. 4-6 were of the shear failure type.

[0023] [Table 1]

[0024] The dimensions of the test specimen were determined by assuming it was a half to two-thirds smaller than an actual building. Referring to Figures 2 and 3, the distance L1 to the load point of the cantilevered test specimen was 2,425 mm, and the distance L2 between the inflection points was 2,350 mm.

[0025] For specimen No. 4-1, H-shaped steel beams made of SN490B with a height (depth of steel beam section 14) of 500 mm, a side length (width of steel beam section 14) of 200 mm, a web thickness of 9 mm, and a flange thickness of 16 mm were used as the steel frame 12. The embedded length of this steel frame 12 into the SRC beam section 15 was 1000 mm, and no rib plates 21 were installed.

[0026] In specimen No. 4-1, the SRC beam 15 has a height (beam depth) d of 800 mm, a width (beam width) of 650 mm, and a length of 1075 mm, and the design strength Fc is 36 N / mm 2 It was formed using concrete.

[0027] In specimen No. 4-1, the SRC beam section 15 consisted of eight 19mm diameter SD390 rebars arranged horizontally in the upper and lower sections as the main beam reinforcement 16, with two rebars arranged inside each of them. The intermediate shear reinforcement 17 consisted of 8mm diameter KSS785 rebars arranged at 60mm intervals, surrounding the main beam reinforcement 16. Furthermore, at the beginning of the SRC beam section 15, five sets of 10mm diameter KSS785 rebars were arranged at 30mm intervals, surrounding the main beam reinforcement 16. Finally, at the end of the SRC beam section 15, five sets of 8mm diameter KSS785 rebars were arranged at 30mm intervals, surrounding the main beam reinforcement 16, as the concentrated reinforcement 19.

[0028] In the test specimen No. 4-1, the shear margin of the end of the SRC beam 15 (= shear strength at bending strength) J Q U_vu / Shear strength at shear load J Q U_mu ) is greater than 1, and the failure mode is bending failure mode.

[0029] Specimen No. 4-2 differs from specimen No. 4-1 only in that the beam depth d of the SRC beam section 15 is lower at 670 mm and the arrangement of the concentrated reinforcement bars 19 is reduced to four sets.

[0030] Specimen No. 4-3 differs from Specimen No. 4-2 only in that the beam width of the SRC beam section 15 is narrower at 500 mm and the spacing of the shear reinforcement bars 17 is wider to 75 mm. Specimen No. 4-4 differs from Specimen No. 4-2 only in that rib plates 21 made of steel plates of the same thickness as the webs of the steel frame 12 are fixed to the steel frame 12 by fillet welding at the start and end of the SRC beam section 15.

[0031] Specimen No. 4-5 differs from Specimen No. 4-3 only in that the number of main beam reinforcement bars 16 in the upper and lower sections of the SRC beam section 15 has been reduced to six each. Specimen No. 4-6 differs from Specimen No. 4-5 in that the design strength Fc of the SRC beam section 15 has been reduced to 30 N / mm 2 The only difference is that concrete with reduced strength was used.

[0032] Loading tests were conducted using each of the above-mentioned specimens. In this test, each specimen was fixed at its base end in the form of a cantilever beam, and a load was applied with a jack to the load point on the free end of the steel frame 12. Although not shown, an out-of-plane vibration prevention device (not shown) was attached to the tip of the steel beam 14 to prevent twisting of the steel beam 14 due to deformation caused by the load. In addition, strain gauges were attached to multiple locations on the top and bottom main beam reinforcement 16.

[0033] Loading was carried out in sequence at three levels of deflection angle of ±(1 / 400, 1 / 200, 1 / 100) rad at the tip of the steel beam 14. The direction in which the top end of the steel beam 14 is in tension is the positive direction. The amount of strain was detected using a strain gauge.

[0034] As an example, Figure 5 shows the test results for specimen No. 4-5, and Figure 6 shows the test results for specimen No. 4-6. These are the measurement results using strain gauges attached to five locations on the topmost beam main reinforcement 16. The other specimens No. 4-1 to 4-4 showed similar test results. However, specimens No. 4-1, 4-2, and 4-4 only had strain gauges attached to three locations each.

[0035] From these test results, and also referring to Figure 4, when L is the distance from the surface of the column 11 facing the SRC beam section 15 to the anchoring piece 18 (the length of the main beam reinforcement 16 within the SRC beam section 15) and d is the effective depth of the SRC beam section 15, it can be seen that in the range from the surface of the column 11 facing the SRC beam section 15 to (L-0.5d), the amount of strain is almost constant at each deflection angle, and the amount of strain decreases beyond this range.

[0036] In addition, the deflection angle at which the main beam reinforcement 16 of the SRC beam section 15 yields is approximately 1 / 100 rad, and at that point, the range in which the main beam reinforcement 16 has almost reached the yield strain is within approximately 0.5d from the base end of the SRC beam section 15. This shows that the bond length of the main beam reinforcement 16 on the compression side should be (L-0.5d).

[0037] From this, the design bond stress τf of the main beam reinforcement 16 on the tension side can be calculated by the following formula (1). Τf=dh·Δσ / {4(L-0.5d)} ··· (1) Here, dh is the diameter of the main beam reinforcement 16 of the SRC beam section 15.

[0038] Δσ is the difference in stress levels in the main beam reinforcement 16 of the SRC beam section 15 at the ultimate limit state, and can be found by structural analysis based on the assumption of flatness. Looking at the strain in the main beam reinforcement 16 of the SRC beam section 15 during the experiment, the amount of strain decreases from the base end to the tip. The main beam reinforcement 16 had a strain of approximately 1000 μm at the tip where the anchor piece 18 was attached, rather than at the cut-off rebar. This means that a tensile stress of approximately 0.4 times the yield strength of the SD450 material that makes up the main beam reinforcement 16 had occurred.

[0039] Furthermore, when an experiment was conducted using beam main reinforcement 16 made of SD390, the same level of strain occurred. In this case, a tensile stress of approximately 0.5 times the yield strength of SD390 occurred. Furthermore, no damage that could lead to breakage occurred. Therefore, Δσ can be calculated using formula (2). In the case of SD390, Δσ=σyu-0.5σy In the case of SD490, Δσ=σyu-0.4σy (2) Here, σyu is the strength for calculating the upper limit strength of the main beam reinforcement 16, and σy is the yield strength of the main beam reinforcement 16.

[0040] On the other hand, as an example of the main beam reinforcement 16 on the compression side, the test results for specimen No. 4-5 are shown in Figure 7. The other specimens No. 4-1 to 4-4 and 4-6 also had similar test results. These are the results of measurements using strain gauges attached to five locations on the bottommost main beam reinforcement 16.

[0041] From these test results, it can be seen that the strain of the main beam reinforcement 16 on the compression side is greatest at the base end of the SRC beam section 16, but does not reach the yield strain, and there is almost no strain at the tip. This shows that there is no hinge region in the main beam reinforcement 16 on the compression side, and the bond length can remain the actual length L.

[0042] Therefore, the design bond stress τf of the main beam reinforcement 16 on the compression side may be calculated using the following equations (3) and (4). Τf=dh·Δσ / 4L (3) Δσ=σyu (4)

[0043] The above-mentioned calculation formulas do not depend on the embedding length of the steel frame 12 into the SRC beam section 15 or whether or not a rib plate 21 is provided.

[0044] The design method of the present invention is not limited to the hybrid beam 10 specifically described in the above embodiment, but can be modified as appropriate within the scope of the claims. [Explanation of symbols]

[0045] 10...Hybrid beam, 11...Column, 12...Steel frame, 13...Reinforced concrete (RC) structure, 14...Steel beam section (S beam section), 15...Steel reinforced concrete beam section (SRC beam section), 16...Main beam reinforcement, 17...Shear reinforcement, 18...Anchoring piece (anchoring device), 19...Concentrated reinforcement, 20...Core reinforcement, 21...Rib plate.

Claims

[Claim 1] A design method for hybrid beams, characterized in that when calculating the bond stress of beam main reinforcement with anchors at the tip of a beam end of a steel-framed reinforced concrete structure in which the beam end of a steel beam made of steel is embedded in reinforced concrete, the diameter of the beam main reinforcement is dh, the difference in stress of the beam main reinforcement at the ultimate limit state is Δσ, the actual bond length of the beam main reinforcement within the beam end of the steel-framed reinforced concrete structure is L, and the effective depth of the beam end of the steel-framed reinforced concrete structure is d, when the beam main reinforcement is on the tension side, the design bond stress τf of the beam main reinforcement is calculated as τf = dh Δσ / {4(L-0.5d)}, and when the beam main reinforcement is on the compression side, the design bond stress τf of the beam main reinforcement is calculated as τf = dh Δσ / 4L.

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