Mixed structure beam

Abstract

To provide a mixed-structure beam capable of improving the shear capacity of an end RC at a low cost with good workability without densely arranging shear reinforcing bars.SOLUTION: This structure has a steel frame beam 3 and a reinforced-concrete end RC4 surrounding an end of the steel frame beam 3, the end RC4 is joined to a reinforced-concrete RC column 2, and steel fiber reinforced concrete is used for a concrete part 44 of the end RC4.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hybrid structural beam. [Background technology]

[0002] In a frame consisting of reinforced concrete columns (RC columns) and steel beams, a mixed structural beam construction method has already been proposed in which both ends or one end of the steel beam is wrapped in reinforced concrete to form an end RC, and the end RC is joined to the RC column, thereby joining the steel beam to the RC column (see, for example, Patent Documents 1 to 3). With the mixed structural beam construction method, the RC columns and end RC can be constructed using not only conventional construction methods but also precast construction methods.

[0003] In the case of the mixed beam construction, the shear force input from the steel beam during an earthquake is transferred to the end reinforced concrete and transmitted to the reinforced concrete column. Due to the structure of the mixed beam construction, the end reinforced concrete is subjected to a shear force that is greater than the shear force input to the steel beam. For this reason, the mixed beam construction must be designed so that the end reinforced concrete does not suffer shear failure.

[0004] To prevent shear failure of the end RC, it is necessary to sufficiently reinforce the end RC with shear reinforcement. The length of the end RC is generally designed to be about twice the depth of the steel beams embedded in the end RC. Because the length of the end RC is limited, the shear reinforcement of the end RC may be over-densely arranged. In such cases, there are concerns that the construction period may be affected due to factors such as reduced concrete filling and reinforcement work efficiency in the end RC.

[0005] In response to this, the following measures (1) to (3) are known to eliminate the over-dense arrangement of shear reinforcement bars in RC at the ends. (1) Ensure sufficient length for the end RC. (2) High-strength shear reinforcement is used in the end RC. (3) Tendons are placed in the end RC and tension is applied to the tendons (see, for example, Patent Documents 2 and 3). Tendons include PC steel bars, PC steel strands, and high-strength deformed steel bars. The applied tension ranges from 0 (tension applied by hand tightening) to 100%. The stress at the completion of tension application shall conform to the smaller of 0.75 x (standard tensile strength of PC steel bar) or 0.85 x (standard yield strength of PC steel bar), as specified in the "Design and Construction Standards for Prestressed Concrete and Commentary" compiled by the Architectural Institute of Japan, and this shall be taken as 100%. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-291636 [Patent Document 2] Patent No. 5483055 [Patent Document 3] Japanese Patent Application Publication No. 2023-78732 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above (1) and (2) have the problem of increasing costs. Furthermore, the above (1) does not take advantage of the advantage of the mixed structural beam construction method, which provides reinforced concrete only at the end of a specified area of ​​the steel beam. The above (3) requires the use of a device such as a hydraulic jack to apply the specified tension, which affects workability and construction time, and may not be possible depending on the details of the mixed structural beam.

[0008] Therefore, an object of the present invention is to provide a mixed structure beam that is inexpensive, easy to construct, and can improve the shear strength of the end RC without overly densely arranging shear reinforcement bars. [Means for solving the problem]

[0009] In order to achieve the above-mentioned object, the mixed structural beam of the present invention has a steel beam and an end RC made of reinforced concrete surrounding the end of the steel beam, the end RC being connected to an RC column made of reinforced concrete, and the concrete of the end RC is made of steel fiber reinforced concrete.

[0010] Steel fiber reinforced concrete has high splitting strength due to the bridging effect of steel fibers. By using this in the end RC, the shear strength of the end RC can be improved without over-densifying the shear reinforcement of the end RC. This invention can improve the shear strength of the end RC at a lower cost than increasing the length of the end RC or using high-strength shear reinforcement in the end RC, and is easy to work with because no special equipment is required for construction. Steel fiber reinforced concrete has excellent crack dispersion properties, so during an earthquake, damage is concentrated in the end RC reinforced concrete using steel fiber reinforced concrete, making it possible to design a structure with damage control.

[0011] The shear strength and splitting strength required for the steel fiber reinforced concrete of the end RC of the mixed structural beam according to the present invention may satisfy the following formula (1).

[0012]

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[0013] By adopting such a configuration, the end RC can be easily designed. [Effects of the Invention]

[0014] According to the present invention, the shear strength of end RC can be improved inexpensively, easily constructed, and without overly densely arranging shear reinforcement bars. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a perspective view of a joint between a mixed structural beam and an RC column according to an embodiment of the present invention. [Figure 2] 1A and 1B are a side view and a cross-sectional view of a component to be analyzed; [Figure 3] 1 is a table showing specifications of components to be analyzed. [Figure 4] (a) is a table showing the physical properties of concrete, and (b) is a table showing the physical properties of steel. [Figure 5] This is a diagram of a model of reinforcing bars. [Figure 6] FIG. 10 is a diagram showing mesh division, support conditions, and load conditions. [Figure 7] 1 is a table showing analysis parameters. [Figure 8] 1 is a graph and a diagram showing the relationship between load and the deformation angle of the entire mixed structure beam (hereinafter simply referred to as the deformation angle). [Figure 9] 1 is a graph showing the relationship between the shear strength ratio and the splitting strength ratio of concrete. [Figure 10] FIG. 4 is a graph showing the relationship between tensile strength and steel fiber mixing ratio. [Figure 11] 10 is a table showing the concrete splitting strength σt, steel fiber mixing ratio Vf, and beam shear force Qb at shear capacity of the mixed structure beam for Case 1 to Case 3. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a mixed structural beam according to an embodiment of the present invention will be described with reference to FIGS. As shown in Fig. 1, a mixed structural beam 1 according to this embodiment is a beam joined to a reinforced concrete column 2. The construction method in which the mixed structural beam 1 and the RC column 2 are joined is referred to as a mixed structural beam construction method. The mixed structural beam 1 has a steel beam 3 and an end RC4. The steel beam 3 is, for example, a steel beam such as an H-shaped steel or an I-shaped steel. The end RC4 is made of reinforced concrete and surrounds the end of the steel beam 3 in the longitudinal direction. The end RC4 is joined to the RC column 2.

[0017] The end RC4 has beam main reinforcement bars 41, shear reinforcement bars 42, concentrated reinforcement bars 43, and a concrete section 44. The beam main reinforcement bars 41 extend in the longitudinal direction of the steel beam 3 and are provided in multiple numbers to surround the steel beam 3. The beam main reinforcement bars 41 protrude outward in the longitudinal direction (toward the RC column 2) from the longitudinal end face of the steel beam 3. The shear reinforcement bars 42 are reinforcing bars that surround the multiple beam main reinforcement bars 41 and are provided in multiple numbers at intervals in the longitudinal direction. The concentrated reinforcement bars 43 are provided near end 4b of the end RC4, which is opposite end 4a that is connected to the RC column 2. The concentrated reinforcement bars 43 are provided more densely than the shear reinforcement bars 42. The steel beam 3, beam main reinforcement bars 41, shear reinforcement bars 42, and concentrated reinforcement bars 43 are embedded in the concrete section 44.

[0018] The longitudinal end face of the steel beam 3 is arranged at approximately the same position as the side face of the RC column 2 so as to abut against it. The longitudinal end face of the steel beam 3 may be arranged at a position that is more inward of the end RC4 than the side face of the RC column 2. The steel beam 3 is not inserted inside the RC column 2 (beam-column joint). The portion of the main beam reinforcement 41 that protrudes outward in the longitudinal direction beyond the steel beam 3 is inserted into the RC column 2 and embedded in the concrete of the RC column 2. As shown in Figure 2, when a mixed structural beam 1 is joined to both sides of the RC column 2, the main beam reinforcement 41 is arranged so as to span the end RC4 of one mixed structural beam 1, the RC column 2, and the end RC4 of the other mixed structural beam 1.

[0019] Steel fiber reinforced concrete is used for the concrete portion 44 of the end RC4. Hereinafter, steel fiber reinforced concrete may be referred to as SFRC. SFRC is ordinary concrete mixed with steel fibers, and has a higher splitting strength than ordinary concrete due to the bridging effect of the steel fibers. By using SFRC for the concrete portion 44 of the end RC4, the shear strength of the end RC4 can be improved compared to when ordinary concrete is used, and over-dense arrangement of shear reinforcement bars 42 can be eliminated. The splitting strength of SFRC depends on the amount of steel fiber in the concrete. The amount of steel fiber is the amount of steel fiber relative to the concrete, and is expressed as a volume ratio. The unit of steel fiber amount is [vol.%]. Below, the amount of steel fiber may be referred to as the steel fiber mixing ratio.

[0020] Required performance (splitting strength σ) of mixed structural beam 1 when SFRC is used for the concrete part 44 of the end RC4 t , tensile fracture energy G f ) as a parameter, the splitting strength σ of the SFRC at the end RC4 was t and the shear strength Q of mixed structure beam 1 b The relationship between Splitting strength σ of SFRC at end RC4 t and the shear strength Q of the end RC4 b The outline of the FEM analysis carried out to formulate the relationship is shown below.

[0021] (1) Components to be analyzed The components to be analyzed are shown in Fig. 2, the specifications of the components to be analyzed are shown in Fig. 3, and the physical properties of the materials used are shown in Fig. 4. The dimensions of the RC4 end are width b = 430 mm, depth D = 580 mm, length Lc = 850 mm, and the distance L from the fixed end (RC column 2) to the point of shear force application is 2750 mm. The main beam reinforcement 41 of the RC4 end is 6 (1st level reinforcement: 4, 2nd level reinforcement: 2) - D22 (SD390) at both the top and bottom ends. The shear reinforcement 42 of the RC4 end is 2 (tie-hang) - D6 @ 150 (SD295). The concentrated reinforcement 43 is 4 sets x 2 (tie-hang) - D6 (SD295). The steel beam 3 is H-400 x 150 x 8 x 13 (SN490B).

[0022] (2) Analysis model Figures 5 and 6 show the analysis model 11. Taking into consideration the symmetry of the specimen in the width direction, the analysis model 11 analyzed half the cross section of the end RC 4, steel beam 3, and stub 12. As this analysis targets a mixed structure beam, the RC column 2 was modeled as a stub 12, and was treated as an elastic body so as not to break during analysis. The cross section of the stub 12 is 1000 x 1000 mm, and the height of the stub 12 is 1200 mm.

[0023] The concrete section 44 and the steel plates at the loading points were treated as 8-node solid primary elements, while part of the steel flange was treated as 16-node secondary solid elements. The web of the steel beam 3 was treated as laminated shell elements. The main beam reinforcement 41, shear reinforcement 42, concentrated reinforcement 43, and main reinforcement 121 and shear reinforcement 122 of the stub 12 were modeled as wire-embedded discrete reinforcing bars. Contact elements were placed at the interface between the concrete elements and steel elements in the end RC4, and the concrete and reinforcing bars were assumed to be completely bonded. The hysteresis law of the steel material was treated as bilinear, and the stub 12 and the steel plates at the loading points were treated as elastic bodies. The analysis was performed using displacement control, with a forced incremental displacement applied in the -Z direction to the tip of the steel beam 3.

[0024] (3) Analysis specifications The analytical parameters are shown in Figure 7. Analysis was carried out for Case 1 to Case 3 with different parameters. Case 2 was performed using the splitting strength σ t and tensile fracture energy G f Case 3 is three times the splitting strength σ t and tensile fracture energy G f In both cases, the ultimate strain ε u is 0.0% and the crack model is fixed.

[0025] Tensile fracture energy G f is as follows: G f =G fo (f cm / f cmo ) 0.7 G fo: The basic value of the fracture energy depends on the maximum size of the coarse aggregate. The maximum size (maximum particle size) of the coarse aggregate used in this study is 20 mm, and Gfo =0.038N·mm / mm 2 f cm : Compressive strength of concrete (N / mm 2 ) f cmo =10N / mm 2 (Reference value) The physical properties of the concrete and steel elements used in the analysis are the same as those in Figure 4, and the failure criteria for the concrete elements and the contact elements set at the interface between the concrete elements and steel elements are as follows: Failure criteria for concrete elements Compression-compression-compression region: Menetrey-Willam plasticity criterion Tensile-tension-tension region...Rankine standard Other areas: a combination of both criteria Plastic displacement wd: 0.5 mm Contact elements at the interface between concrete elements and steel elements Vertical Spring Spring constant: Ec (N / mm 2 / mm) Tensile strength: 0 Spring constant after reaching tensile strength: Ec / 100 (N / mm 2 / mm) Horizontal Spring Spring constant: Ec (N / mm 2 / mm) Adhesive strength: 5.0N / mm 2 Friction coefficient: 0.2 Spring constant after reaching Mohr-Coulomb failure criterion: Ec / 100 (N / mm 2 / mm) The end faces of the concrete elements and steel frame elements of the stub 12 were not connected.

[0026] (4) Analysis results Figure 8 shows the load-deformation angle relationship obtained from the analysis. Figure 9 shows the relationship between shear strength ratio and concrete splitting strength ratio. The shear force ratio (shear strength ratio) for Case 1, calculated by approximating the analytical results for Cases 1 to 3, is expressed by the following equation (1).

[0027]

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[0028] The SFRC applied to the end RC4 is determined by the required performance of SFRC (splitting strength σ t , tensile fracture energy G f The steel fiber content is adjusted to satisfy the above requirements before manufacturing.

[0029] The above formula (1) shows the relationship between the splitting strength at the evaluation point and the shear strength of a composite structural beam, and does not take into account the amount of steel fiber or the effect of mixing steel fiber in reducing the amount of shear reinforcement in the end RC (hereinafter referred to as the effect of mixing steel fiber). The amount of steel fiber and the effect of mixing steel fiber are described below. Below, ordinary concrete without steel fiber will be referred to as plain concrete. The splitting strength of plain concrete will also sometimes be referred to simply as splitting strength.

[0030] (Splitting strength of plain concrete and SFRC) Splitting strength σ of plain concrete and SFRC t For example, the following intensity formula has been proposed for estimating For plain concrete σ t =0.33σ B 0.5 (N / mm 2 ) [36≦σ B ] 1) σ t =0.291aσ B 0.637 (N / mm 2 )[36<σ B <120] 2) In the case of SFRC σ t =0.5983σ B 0.5627 (N / mm 2 ) [20≦σ B <80] 3) Here, σB : Compressive strength of plain concrete and SFRC (N / mm 2 ) a: Coefficient determined by the type of coarse aggregate

[0031] 1) Architectural Institute of Japan: Guidelines for Ductility-Guaranteed Earthquake-Resistant Design of Reinforced Concrete Buildings and Commentary, September 1999 2) Architectural Institute of Japan: Current Status of High-Strength Concrete Technology, October 2009 3) Kiyoshi Ito, Tadashi Abe, Toshiaki Sawano, and Katsuhiko Fukagawa: Study on the evaluation of the characteristic values ​​of SFRC materials and punching shear strength of SFRC slabs, Proceedings of the Japan Concrete Institute, Vol. 40, No. 2, pp. 367-372, July 2018.

[0032] (Relationship between splitting strength of SFRC and steel fiber content) Previous research results 4) From the above, the splitting strength σ of concrete in Case 2 and Case 3 t (N / mm 2 ) (see Figure 7) f Estimate (vol%). The following assumptions (1)-(3) are adopted: (1) For Ex (expansive agent mixing ratio) in Figure 10, the case of 5.8% (Ex: 5.8% solid line with ●) is adopted. (2) Tensile strength was converted to splitting strength. (3) Splitting strength σ t =12.85N / mm 2 In Case 3 (see Figure 7), the tensile strength exceeds the range of Figure 10, so V f The maximum value of the range, 1.5%, was adopted.

[0033] Splitting strength σ of concrete in Case 2 and Case 3 t (N / mm 2 ) steel fiber mixing rate V f (vol%) is estimated as follows: Case 2 σ t =7.71(N / mm 2) equivalent steel fiber mixing rate is V f ≒1.2 (vol%) Case 3 σ t =12.85(N / mm 2 ) equivalent steel fiber mixing rate is V f >1.5(vol%) Figure 11 shows the splitting strength σ of concrete in Case 1-Case 3. t , steel fiber mixing rate V f and beam shear force Q at shear strength of mixed structure beam b Shows.

[0034] 4) Ryosuke Shionaga and Yasuhiko Sato: Early strain behavior and tensile softening properties of high-performance fiber-reinforced mortar with a compressive strength of approximately 100 N / mm2, Journal of Concrete Engineering, Vol. 27, pp. 33-42, 2016.

[0035] (Relationship between the amount of shear reinforcement by shear reinforcement and the shear strength of the RC cross section at the end of the beam in a mixed structure) The shear strength of the RC section at the end of a mixed structure beam when plain concrete is used for the end RC can be calculated using formula (2).

[0036]

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[0037] Here, V u : Shear strength of end RC section [N] μ: Coefficient representing the angle of the truss mechanism formed within the end RC P´ we : Shear reinforcement ratio of the end RC excluding the flange width of the steel beam in the end RC section [%, use decimals when calculating shear strength] σ wy : Yield stress of shear reinforcement [N / mm 2 ] b´ e : Width of end RC section excluding flange width of steel beam in end RC section [mm] j e: Effective diameter of the cross section involved in the truss mechanism formed in the end RC [mm] v: Effective coefficient of plain concrete compressive strength b´: Width of the end RC section before subtracting the flange width of the steel beam in the end RC section [mm] D: End RC cross section depth [mm] θ: angle of compression beam of arch mechanism formed in end RC [radian] λ: Effectiveness factor of the truss mechanism formed in the end RC L: Length of end RC [mm] L je : Effective length of end RC (≒ L - (length from end 4b to end position of concentrated reinforcement area) [mm]

[0038] About Case 1 Beam shear force Q at shear strength of mixed structure beam of analyzed member (edge ​​RC shear reinforcement spacing: 150 mm) b From equation (2), Q b =83.9kN. p w σ y The amount of shear reinforcement of the end RC shown by p w σ y =0.66N / mm 2 is. p w : Shear reinforcement ratio σ y : Yield stress of shear reinforcement (N / mm 2 ) Here, the shear force Q is calculated by FEM analysis separately. b The shear strength obtained from the FEM analysis of Case 1 is Q b = 87.8 kN, which is approximately equal to the shear strength calculated by equation (2).

[0039] For Case 2 and Case 3, the shear strength, which is a known quantity for Cases 2 and 3, was input into Equation (2) and the shear reinforcement spacing was back-calculated to obtain the same shear strength as the results obtained from the FEM analysis of Cases 2 and 3 without adding steel fibers.

[0040] About Case 2 From equation (1), when the splitting strength ratio of concrete is 3, the shear strength ratio is 1.77, so Q b =1.77×83.9=148.5kN. This Q b The equivalent shear reinforcement spacing is 36 mm according to formula (2), p w σ y =2.75N / mm 2 This becomes: About Case 3 From equation (1), when the splitting strength ratio of concrete is 5, the shear strength ratio is 2.32, so Q b =2.32×83.9=194.6kN. This Q b The equivalent shear reinforcement spacing is 22 mm from equation (2), p w σ y =4.50N / mm 2 This becomes:

[0041] From the above, it can be seen that in Case 2 and Case 3, if SFRC is not used in the concrete of the end RC, the shear reinforcement will be arranged too densely. Furthermore, considering the workability of reinforcing bars and the size of the coarse aggregate in the concrete, a minimum spacing of 50 mm is usually required for the shear reinforcement, and Cases 2 and 3 do not satisfy this requirement. As mentioned above, based on formula (1), the steel fiber mixing ratio V required to ensure the specified shear strength for Case 2 and Case 3 is calculated based on a shear reinforcement interval of 150 mm. f can be estimated.

[0042] Next, the functions and effects of the mixed structural beam according to this embodiment will be described. SFRC has high splitting strength due to the bridging effect of steel fibers. By using this for the end RC4, the shear strength of the end RC4 can be improved without over-densifying the shear reinforcement 42 of the end RC4. The hybrid structural beam 1 according to this embodiment can improve the shear strength of the end RC4 at a lower cost than when increasing the length of the end RC or using high-strength shear reinforcement in the end RC, and is easy to install because no special equipment is required for installation. The SFRC at the end RC4 has excellent crack dispersion properties, so during an earthquake, damage is concentrated at the end RC4 using SFRC, making it possible to design the structure with damage control. In the mixed structural beam 1 according to this embodiment, as a result of analytical study using FEM, SFRC is used for the concrete part 44 of the end RC part 4 in order to prevent shear failure of the end RC part 4. By doing so, the shear strength required for the mixed structural beam 1 can be calculated.

[0043] Although the embodiments of the hybrid structural beam according to the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified as appropriate within the scope of the invention. For example, in the above embodiment, the amount of steel fiber in the SFRC used in the concrete portion 44 at the end RC4 is determined based on the required performance (splitting strength σ t , tensile fracture energy G f ) may be appropriately set to satisfy the above.

[0044] The Sustainable Development Goals (SDGs) are 17 international goals adopted at the United Nations Summit in September 2015. The composite structural beam according to this embodiment can contribute to achieving one of the 17 SDGs goals, for example, goal 11: "Sustainable cities and communities." [Explanation of symbols]

[0045] 1 Mixed structural beam 2 RC pillar 3 Steel beams 41 Beam main reinforcement 42 Shear reinforcement 43 Concentrated reinforcement 44 Concrete Section

Claims

1. Steel beams and and a reinforced concrete end portion RC surrounding the end portion of the steel beam, The end RC is connected to a reinforced concrete column, A mixed structural beam in which steel fiber reinforced concrete is used for the concrete at the end RC.

2. 2. A mixed structural beam according to claim 1, wherein the shear strength and splitting strength required for the steel fiber reinforced concrete of the end RC satisfy the following formula (1): [Equation 1]

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