Composite beam and floor structure

Abstract

To provide a composite beam, which is equipped with a beam having H-shaped and biaxially symmetrical cross section and a floor slab, and makes rigidity as well as bearing force of the beam improved.SOLUTION: A composite beam 3 comprises: a steel beam 10 having H-shaped and biaxially symmetrical cross section in its axis line O1 direction; a floor slab 15 that is supported by the beam and has a reinforcing bar 17 within its effective width; and a shear connector 25 connected to the beam and the floor slab, respectively. The beam is formed of a steel material having yield strength of 325 N / mm2 or more. Yield strength of the reinforcing bar is 295 N / mm2. Sum rA (mm2) of cross sectional area sA (mm2) of the beam and cross sectional area of the reinforcing bar satisfies a numerical expression, where h is distance (mm) from a top face of an upper flange 11 of the beam to center of gravity of the reinforcing bar; H is beam depth (mm) of the beam; sft is allowable stress (N / mm2) of the beam; rft is allowable stress (N / mm2) of the reinforcing bar; and sAw is cross sectional area (mm2) of a web 13 of the beam.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to composite beams and floor structures. [Background technology]

[0002] Traditionally, in steel-framed buildings, steel H-section members have generally been used for the beams that support the floor slab. When a floor slab is attached to a beam, the beam and floor slab are integrated using shear connectors such as headed studs to form a composite beam, and the beam is sometimes designed to withstand external forces such as live loads, wind loads, and earthquake loads as a composite beam. When treating a beam as a composite beam, taking into account the contribution of the floor slab can be expected to improve rigidity and strength compared to a single beam. The design of composite beams can be broadly divided into two types: when they are subjected to a positive bending moment, where a compressive force acts on the upper flange where the floor slab is attached, and when they are subjected to a negative bending moment, where a compressive force acts on the lower flange where the floor slab is not attached.When subjected to positive bending, the concrete part of the floor slab bears the compressive force, but when subjected to negative bending, the reinforcing bars in the floor slab bear the tensile force.

[0003] In the case of composite beams subjected to positive bending, since the concrete bears the compressive force, it is known that a more rational structure can be achieved by making the cross-sectional area of ​​the upper flange of the H-shaped beam asymmetrical from top to bottom, with the cross-sectional area being reduced compared to the cross-sectional area of ​​the lower flange, and related patent applications have also been filed. For example, Patent Document 1 proposes an optimum range for the cross-sectional area ratio of the upper and lower flanges as the optimum cross section for a composite beam subjected to positive bending. Also, as in Patent Document 2, by attaching a steel plate to the lower flange of an H-shaped cross section beam, the cross-sectional area of ​​the lower flange and the steel plate as a whole is increased, making it possible to rationally design the beam using a small amount of steel material.

[0004] In the case of composite beams subjected to negative bending, there have been fewer proposals compared to those subjected to positive bending, but rational structures have been proposed by taking into account the contribution of the reinforcing bars in the floor slab. For example, Patent Document 3 proposes the use of an asymmetric cross section in the top and bottom of an H-shaped cross section beam, and the optimal relationship between the cross-sectional areas of the top flange, bottom flange, and reinforcing bars. In this way, rational structures that take into account the composite effect of floor slabs attached to beams have been proposed in the past, regardless of whether they are for positive or negative bending, but most of them are characterized by the cross-sectional dimensions of the asymmetric cross sections at the top and bottom.

[0005] Regarding the design of composite beams, design guidelines are described in Non-Patent Document 1, and it is common to design based on these guidelines. When rigidity is calculated based on the guidelines, it is found that the bending rigidity of a composite beam subjected to negative bending is always higher than the rigidity of a single beam, regardless of the specifications of the floor slab and beam. On the other hand, when performing allowable stress design, which is a strength study based on the guidelines, there are two ways to calculate the cross section of a composite beam that will be subjected to a negative bending moment: it can be determined by the allowable tensile stress of the reinforcing bars in the floor slab, or it can be determined by the allowable compressive stress of the bottom flange of an H-shaped cross section beam. When it is determined by the allowable tensile stress of the reinforcing bars, the strength of the composite beam may be lower than the strength of the beam itself.

[0006] The reason for this will be explained using a conventional composite beam 6 shown in FIG. As shown in FIG. 9(A), the composite beam 6 includes a beam 10, a floor slab 15, and a shear connector (shear force transmission member) 25. The beam 10 is made of a steel frame and has an upper flange 11, a lower flange 12, and a web 13. The upper flange 11, the lower flange 12, and the web 13 are each formed in a flat plate shape. The upper flange 11 is positioned higher than the lower flange 12. The web 13 is positioned between the upper flange 11 and the lower flange 12, and is joined to the middle portion of the upper flange 11 in the width direction and the middle portion of the lower flange 12 in the width direction. For example, the beam 10 is a sub-beam whose end in the direction of its axis O1 is joined to a main girder (not shown, a support member). The end of the main girder is joined to a column (not shown). The beam may be a main girder whose end is joined to a pillar, in which case the pillar serves as the support member.

[0007] In this example, the floor slab 15 is a concrete slab. The floor slab 15 includes concrete 16 and a plurality of reinforcing bars 17. The concrete 16 is formed in a flat plate shape and is arranged so that the thickness direction of the concrete 16 is along the vertical direction. Here, a first direction X and a second direction Y are defined that run along the top surface of the concrete 16 and intersect with each other. The first direction X is the direction of the axis O1.

[0008] The plurality of reinforcing bars 17 extend along the first direction X and are arranged in the second direction Y at intervals from one another. A plurality of reinforcing bars 17 are embedded in the concrete 16 . The floor slab 15 is supported from below by beams 10 . In this example, the shear connectors 25 are headed studs. The shear connectors 25 protrude upward from the upper flange 11 and are embedded in the concrete 16 of the floor slab 15. The shear connectors 25 are joined to the beam 10 and the floor slab 15, respectively.

[0009] 9(B) shows stress etc. in relation to the vertical position in FIG. 9(A). In FIG. 9(B), the yield strength σ y10 , yield strength σ of rebar 17 y17 , the stress distribution σ6 at the time of the yield strength of the composite beam 6, and the stress distribution σ at the time of the yield strength of the beam 10 10 Shows. The reason for this is that the position of the neutral axis L6 of the composite beam 6 does not move significantly toward (upward from) the floor slab 15 relative to the position of the neutral axis L10 of the beam 10 alone, and the bending moment when the tensile stress of the reinforcing bar 17 reaches the allowable stress becomes the allowable strength of the composite beam 6, and the compressive stress of the beam 10 has a margin above the allowable stress. This phenomenon occurs when the commonly used yield strength is 295N / mm 2 When the reinforcing bar 17 is used, the yield strength of the beam 10 is 295N / mm 2 or more (for example, yield strength of 325N / mm 2 , 355N / mm 2 , 385N / mm 2 This is particularly likely to occur when a reinforcing bar 17 is used. It is also likely to occur when the height of the beam 10 is low, that is, when the distance between the upper flange 11 and the reinforcing bar 17 is relatively large compared to the height of the beam 10 (beam depth).

[0010] This phenomenon can be prevented by increasing the cross-sectional area of ​​the reinforcing bars 17 in the floor slab 15. However, increasing the diameter of the reinforcing bars 17 to increase their cross-sectional area poses problems such as interference with other reinforcing bars in the floor slab 15 and difficulty in ensuring an appropriate concrete cover thickness. Increasing the number of reinforcing bars 17 to be placed is also not desirable, as it increases the amount of work at the construction site and reduces workability.

[0011] In other words, when a design is made to take into account the composite effect of the floor slab 15 for the beam 10 subjected to a negative bending moment, the rigidity increases compared to the case of the beam 10 alone, but the bearing capacity may decrease compared to the case of the beam 10 alone. This is not a rational design. In addition, Patent Document 3 states that these problems can be solved by using a cross section that is asymmetrical in the vertical direction, and that a rational composite beam can be created. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] International Publication No. 2021 / 059410 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-280687 [Patent Document 3] Japanese Patent Publication No. 2022-156476 [Non-patent literature]

[0013] [Non-Patent Document 1] "Design Guidelines and Commentary for Composite Structures," edited by the Architectural Institute of Japan, Maruzen Publishing Co., Ltd., August 2023 Summary of the Invention [Problem to be solved by the invention]

[0014] However, beams with asymmetric cross sections have issues such as increased stress and deflection during construction before the floor slab is attached, increased management effort required for top and bottom during processing, and poor transportability when multiple beams are bundled together. In addition, because it is difficult to manufacture a vertically asymmetric H-shaped beam using rolled H-shaped steel, it is necessary to manufacture it by welding and assembling steel plates or by processing rolled H-shaped steel. However, this increases the amount of processing required and makes processing more difficult due to the vertical asymmetry, which tends to increase manufacturing costs.

[0015] The present invention has been made in consideration of these problems, and aims to provide a composite beam comprising a beam and a floor slab that has an H-shaped cross section and is biaxially symmetrical, in which not only the rigidity but also the strength of the beam is improved, and a floor structure comprising this composite beam. [Means for solving the problem]

[0016] In order to solve the above problems, the present invention proposes the following means. (1) Aspect 1 of the present invention is a composite beam comprising a steel beam whose cross section in its axial direction is H-shaped and biaxially symmetric, a floor slab supported by the beam and having reinforcing bars within its effective width, and shear connectors joined to the beam and the floor slab, respectively, wherein the beam has a yield strength of 325 N / mm 2 The steel bar is made of the above steel material, and the yield strength of the steel bar is 295N / mm 2 and the cross-sectional area of ​​the beam s A (mm 2 ) and the cross-sectional area of ​​the reinforcing bar r A (mm 2 ) is a composite beam that satisfies equation (1). where h is the distance (mm) from the top surface of the upper flange of the beam to the center of gravity of the reinforcing bar, H is the beam depth (mm), s f t is the allowable stress of the beam (N / mm 2 ), r f t is the allowable stress of the reinforcing bar (N / mm 2 ), s A w is the cross-sectional area of ​​the beam web (mm 2 )

[0017]

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[0018] In this invention, the inventors have conducted extensive research and have found that in a composite beam in which a beam and a floor slab are connected by shear connectors, the beam has a yield strength of 325 N / mm 2 It is made of steel with a yield strength of 295N / mm 2 and the cross-sectional area of ​​the beam s Sum of A and the cross-sectional area of ​​the rebar r It was found that when A satisfies equation (1), not only the rigidity but also the strength of the composite beam is improved. Therefore, in a composite beam having a beam and a floor slab with an H-shaped cross section and biaxial symmetry, not only the rigidity but also the strength of the beam can be improved.

[0019] (2) A second aspect of the present invention may be a composite beam as described in (1), in which at least one end of the beam is rigidly or semi-rigidly joined to a support member. In the present invention, at least one end of the beam can be rigidly or semi-rigidly joined to the support member, thereby creating a negative bending region at at least one end of the beam.

[0020] (3) A third aspect of the present invention may be the composite beam according to (1) or (2), in which the width-to-thickness ratio of the web exceeds 100. In this invention, it is possible to construct beams with a width-to-thickness ratio of the web exceeding 100, which are relatively efficient in cross section.

[0021] (4) A fourth aspect of the present invention is a floor structure comprising the composite beam according to (2) or (3) and the support member. In this invention, a floor structure can be constructed using composite beams that have an H-shaped cross section and biaxial symmetry, and a floor slab, and that have improved not only rigidity but also strength compared to the beams. [Effects of the Invention]

[0022] In the composite beam and floor structure of the present invention, in a composite beam having a beam and floor slab with an H-shaped cross section and biaxial symmetry, not only the rigidity but also the bearing capacity of the beam can be improved. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a cross-sectional view of a floor structure according to an embodiment of the present invention, viewed from the front. [Figure 2] FIG. 10 is a diagram showing the relationship between the bending strength ratio and the area second moment ratio in Case 1 when a standard cross section is used. [Figure 3] FIG. 10 is a diagram showing the relationship between the bending strength ratio and the area second moment ratio when a standard cross section is used in Case 2. [Figure 4] FIG. 10 is a diagram showing the relationship between the bending strength ratio and the area second moment ratio in Case 1 when the cross section shown in Table 1 is used. [Figure 5] FIG. 10 is a diagram showing the relationship between the bending strength ratio and the area second moment ratio in Case 2 when the cross section shown in Table 1 is used. [Figure 6] FIG. 10 is a diagram showing the relationship between sA / sAmax and bending strength ratio in Case 1. [Figure 7] FIG. 10 is a diagram showing the relationship between sA / sAmax and bending strength ratio in Case 2. [Figure 8] FIG. 10 is a diagram showing the relationship between the width-thickness ratio of the web and the width-thickness ratio of the flange. [Figure 9] 1A and 1B are diagrams illustrating a conventional composite beam, in which (A) is a cross-sectional view of the composite beam as viewed from the front, and (B) is a diagram illustrating stresses acting on the composite beam. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, an embodiment of a composite beam and floor structure according to the present invention will be described with reference to FIGS.

[0025] [1. Floor structure] As shown in Fig. 1, a floor structure 2 of this embodiment is used in, for example, a building 1. The floor structure 2 includes columns (not shown), girders (support members) 5, and composite beams 3. In Fig. 1, the outline of the girders 5 is indicated by a two-dot chain line. The composite beam 3 includes a beam 10, a floor slab 15, and a shear connector 25. The cross section of the beam 10 in the direction of the axis O1 is H-shaped and biaxially symmetric. The biaxially symmetric cross section of the beam 10 means that the cross section is symmetric with respect to a first axis of symmetry (weak axis) that extends along the main surface of the web 13 and passes through the center of the web 13 in the thickness direction, and a second axis of symmetry (strong axis) that passes through the center of the web 13 in the width direction and is perpendicular to the thickness direction of the web 13.

[0026] The beam 10 to be considered below has a thinner cross section and a narrower width than the conventional beam 10 described in FIG. 9(A), but will be described using the same reference numerals as the conventional beam 10. The beam 10 may be formed from rolled H-section steel or may be formed from a welded and assembled H-section.

[0027] It is preferable that at least one end of the beam 10 in the direction of the axis O1 is rigidly or semi-rigidly connected to the main girder 5. The definitions of rigid and semi-rigid connections may comply with the European design standard (Eurocode 3 Part 1-8). The width-to-thickness ratio of the web 13 is preferably greater than 100. In the following, the configuration of the composite beam 3 for solving the problem of improving not only the rigidity but also the bearing strength of the composite beam 3 relative to the beam 10 will be considered.

[0028] [2. Conventional Design] Regarding the design of composite beams, a design method is described in the aforementioned Non-Patent Document 1 (hereinafter referred to as the "Guidelines"), and designs are generally made based on these guidelines. Based on the guidelines, the moment of inertia of the composite beam 3 subjected to a negative bending moment as shown in Figure 1 is c I n , section modulus of the lower end of beam 10 c Z c , section modulus of steel bar 17 c Z r can be calculated from equations (11) to (13).

[0029]

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[0030] The bending strength determined by the beam 10 is calculated by the section modulus of the lower end of the beam 10. c Z c Allowable stress of beam 10 s f t The bending strength determined by the reinforcing bar 17 is obtained by multiplying the section modulus of the reinforcing bar 17 c Z r Allowable stress of rebar 17 r f t The smaller of these two bending strengths is the bending strength of the composite beam 3.

[0031] where: sI is the moment of inertia of beam 10 (mm 4 ), s A is the cross-sectional area of ​​the beam 10 (cross-sectional area of ​​the plane perpendicular to the axis O1 of the beam 10) (mm 2 ), r A is the total cross-sectional area of ​​the multiple reinforcing bars 17 within the effective width B of the floor slab 15 (mm 2 ), x n is the distance (mm) from the bottom end of the beam 10 to the neutral axis of the composite beam 3, as shown in Equation (14), s x n is the distance from the top end of the beam 10 to the position of the neutral axis of the beam 10 (mm; see Figure 1), r d represents the distance from the bottom end of the beam 10 to the center of gravity of the reinforcing bar 17 (mm, see Figure 1), and H represents the depth of the beam 10 (mm, see Figure 1). The effective width B (mm, see Figure 1) of the floor slab 15 is calculated by equation (15), where b is the width (mm) of the beam 10 and l is the length (mm) of the beam 10. The effective width B is the length of the floor slab 15 in the second direction Y.

[0032]

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[0033] The allowable stress of the beam 10 and the reinforcing bar 17 is the strength of the beam 10 and the reinforcing bar 17 calculated based on the standard strength or yield strength of the allowable stress determined by the Building Standards Act or certification by the Minister of Land, Infrastructure, Transport and Tourism. Generally, the short-term allowable stress is determined by the reference strength or yield strength of the allowable stress, and the long-term allowable stress is determined by dividing the reference strength or yield strength of the allowable stress by a safety factor such as 1.5. In this embodiment, an example using the short-term allowable stress is shown, but the long-term allowable stress may also be used in various calculations.

[0034] 3. Design of this embodiment Next, calculations are made using equations (11) to (13), and the stiffness and strength are compared when evaluated as a single beam 10 and when evaluated as a composite beam 3. For the beams 10 (cross sections of beams 10) under consideration, a total of 38 types of standard cross sections of H-shaped steel (hereinafter also referred to as JIS standard cross sections) described in JIS G 3192 (2014), with a beam depth of 250 mm or more, which are used as beams supporting floor slabs, were used.

[0035] The length of the beam 10 was assumed to be 12 times the beam depth. Regarding the specifications of the reinforcing bars 17 (floor slab 15), two cases with different specifications were considered, with reference to the conditions of the reinforcing bars 17 used in the actual building. Case 1 assumes a deck composite slab with a thickness of 150 mm, and 10 mm diameter deformed steel bars (nominal diameter D10) are placed at 150 mm intervals at a position 100 mm from the top end of the upper flange 11. Case 2 assumes a concrete slab 200 mm thick, with 13 mm diameter deformed steel bars (nominal diameter D13) placed at 150 mm intervals 150 mm from the top end of the upper flange 11.

[0036] In both cases, the standard yield strength of rebar 17 is 295 N / mm 2 It was decided to use reinforcing bars. In both Case 1 and Case 2, the composite beam 3 and floor slab 15 are shown with the same symbols, and are distinguished by indicating Case 1 or Case 2.

[0037] Generally, in addition to the reinforcing bars that act as bending reinforcement and are expected to bear the bending strength of the composite beams mentioned above, reinforcing bars and welded wire mesh that are typically placed in floor slabs are also embedded within the floor slab. While these can also be expected to bear tensile stress, these reinforcing bars are primarily placed to ensure the load-bearing capacity of the floor slab and prevent cracks. For this reason, it is preferable to consider only bending reinforcement when expecting the strength of the negative bending composite beams. Therefore, in this study, only the above bending reinforcement (reinforcement bar 17) will be considered as an effective reinforcement bar.

[0038] Figure 2 shows the results of Case 1, and Figure 3 shows the results of Case 2. In Figures 2 and 3, the vertical axis represents the ratio of the moment of inertia of composite beam 3 to the moment of inertia of beam 10 (single beam 10), and the horizontal axis represents the ratio of the bending strength of composite beam 3 to the bending strength of beam 10. In other words, the vertical axis represents the stiffness ratio and the horizontal axis represents the strength ratio, and when both exceed 1, this means that performance can be expected to improve by taking the composite effect into consideration. It can be said that it is preferable to plot in region R1, where both the vertical and horizontal axes exceed 1. 2 and 3, the yield strength (sft) of the beam 10 is 235 N / mm 2 , and 325N / mm 2 We are considering changing it to the following two types.

[0039] 2 and 3, the area second moment ratio exceeds 1 in all cases, while the bending strength ratio is a mixture of cases where it exceeds 1 and cases where it does not. In other words, it can be seen that by taking into account the composite effect of the floor slab 15, the rigidity is always increased compared to the beam 10 alone, but the strength may be lower than that of the beam 10 alone. The reason for this is that, as shown in Figure 9, the position of the neutral axis L6 of the composite beam 6 does not move significantly toward the floor slab 15 relative to the position of the neutral axis L10 of the beam 10 alone, and the bending moment when the tensile stress of the steel bar 17 reaches the allowable stress becomes the allowable strength of the composite beam 6, and the compressive stress of the beam 10 has a margin above the allowable stress.

[0040] This phenomenon occurs when the commonly used yield strength is 295N / mm 2 When the reinforcing bar 17 is used, the yield strength of the beam 10 is 295N / mm 2 or more (for example, yield strength of 325N / mm 2 , 355N / mm 2 , 385N / mm 2 This is especially likely when using 2 and 3, the yield strength of the beam 10 is 325 N / mm 2 In the case of standard reinforcing bar 17, all cross sections have values ​​less than 1 on the horizontal axis. In other words, under the condition of standard reinforcing bar 17, if a standard cross section such as the JIS standard cross section is used for beam 10 and the yield strength is 325N / mm 2 If the above steel materials are used, the strength will decrease due to the composite effect, making rational design impossible.

[0041] Therefore, the cross section of the beam 10 is H-shaped and biaxially symmetrical, and the yield strength of the beam 10 is 325 N / mm 2 Even when the beam is made of the above steel materials, we investigated the conditions under which the bending strength can be improved by taking into account the composite effect. s A (mm 2 ) and the sum of the cross-sectional areas of multiple rebars 17 r A (mm 2 ) and we noticed that if equation (21), calculated using equations (22) to (24), is satisfied, the bending strength will improve when the composite effect is taken into account.

[0042]

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[0043] Here, α is a coefficient (-) relating to the position of the reinforcing bar 17, β is a coefficient (-) relating to the strength of the beam 10 and the reinforcing bar 17, and γ is a coefficient (-) relating to the cross-sectional dimension of the beam 10. where h is the distance (mm) from the top surface of the upper flange 11 of the beam 10 to the center of gravity of the multiple reinforcing bars 17, s f t is the allowable stress of beam 10 (N / mm 2 ), r f t is the allowable stress (N / mm 2 ), s A w is the cross-sectional area (mm 2 ) The plurality of reinforcing bars 17 refer to reinforcing bars arranged within the effective width B of the floor slab 15.

[0044] In other words, the cross-sectional dimensions of the beam 10 may be changed so as to satisfy the relationship of formula (21). Specifically, the cross-sectional dimensions of the beam 10 may be set to a narrow and thin cross section as shown in Table 1.

[0045] [Table 1]

[0046] For example, in No. 1, the beam depth H is 400 mm. The width b of the beam 10 is 100 mm, and the thickness t of the web 13 is 100 mm. w The thickness of the upper flange 11 and the thickness of the lower flange 12 are 2.8 mm. f is 4.5 mm, and the width-thickness ratio (b / (2t f )) is 11.1, and the width-thickness ratio (d / t w ) is 139.6. In addition, d is (H-2t f ).

[0047] Figure 4 shows the results of calculating the stiffness and strength of the composite beam 3 for the JIS standard cross section and the cross section of the beam 10 (section listed in the table) as described above under the condition of the reinforcing bars 17 in Case 1, and Figure 5 shows the results of calculating the stiffness and strength of the composite beam 3 for the JIS standard cross section and the cross section of the beam 10 (section listed in the table) as described above under the condition of the reinforcing bars 17 in Case 2. Here, in Figures 4 and 5, the yield strength of the beam 10 is 325 N / mm 2 It states that: In addition, Fig. 6 and Fig. 7 show the bending strength ratio and s A / s A max Here, s A max is the maximum cross-sectional area of ​​the beam 10, and is expressed by equation (26) from the right side of equation (21).

[0048]

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[0049] That is, the range of 1 or less on the horizontal axis in FIGS. 6 and 7 means that the formula (21) is satisfied. 4 and 5, by making the cross-sectional dimensions of the beam 10 thin and narrow as shown in Table 1, the yield strength of the beam 10 is 325 N / mm 2 Even in this case, it is clear that the bending strength of the composite beam 3 can be made greater than the bending strength of the beam 10 alone. In addition, for some of the cross sections shown in the table in Figure 4, the bending strength of the composite beam 3 is lower than the bending strength of the beam 10 alone. The reason for this is that these cross sections do not satisfy equation (21), as can be seen from Figure 6. As mentioned above, in order to satisfy equation (21), it is necessary to make the beam 10 thin-walled and narrow in width to improve the cross-sectional performance per unit mass. Figure 8 shows the relationship between the width-thickness ratio of the flanges 11, 12 and the width-thickness ratio of the web 13 for the tabulated cross section shown in Table 1 and the JIS standard cross section. The width-thickness ratio of the tabulated cross section is extremely large compared to the JIS standard cross section, and the width-thickness ratio of the web 13 for the tabulated cross section is at most around 60, while for the JIS standard cross section it exceeds 100.

[0050] As a result of the above-mentioned thorough investigations, the inventors have found that by using such a thin web 13 cross section, it is possible to improve rigidity and strength when taking into account the composite effect, thereby enabling a rational design. In addition, a cross section in which the width-thickness ratio of the web 13 exceeds 100 has a better cross-sectional efficiency than the JIS standard cross section even when used alone as a beam 10, but it can be used even more efficiently when used as a composite beam 3 in which a negative bending region occurs.

[0051] 4. Effects of this embodiment As described above, in the composite beam 3 of this embodiment, the inventors have conducted extensive research and have found that in the composite beam 3 in which the beam 10 and the floor slab 15 are connected by the shear connectors 25, the beam 10 has a yield strength of 325 N / mm 2 The steel used is of the above grade, and the yield strength of the reinforcing bar 17 is 295N / mm 2 and the cross-sectional area of ​​the beam 10 s Sum of cross-sectional area of ​​A and rebar 17 rIt was found that when A satisfies equation (21), not only the rigidity but also the strength of the composite beam 3 is improved compared to the beam 10. Therefore, in the composite beam 3 including the beam 10 and the floor slab 15, which have an H-shaped cross section and are biaxially symmetric, not only the rigidity but also the bearing capacity of the beam 10 can be improved.

[0052] By using a thin-walled, narrow H-shaped cross section that satisfies equation (21) for the beam 10, the stress borne by the beam 10 when the reinforcing bars 17 that receive tensile force reach the allowable stress level becomes larger than when a conventional cross section is used. In other words, while the conventional cross section results in excessive excess stress in the bottom flange 12, the cross section of this embodiment improves the balance of stress burden between the reinforcing bars 17 and the beam 10, eliminating waste. Because the reinforcing bars 17 in the floor slab 15 can remain the same as before, there are no problems with workability or increased material costs due to an increase in the diameter of the reinforcing bars 17 or an increase in the number of reinforcing bars to be placed. As a result, even when a biaxially symmetric beam 10 is subjected to negative bending, evaluating it as a composite beam 3 will result in a higher strength than evaluating it as a single beam 10, making it possible to achieve a rational design that reduces the amount of steel used.

[0053] Since a biaxially symmetrical cross section can be used for the beam 10, problems such as deflection during construction and transportability that arise when a uniaxially symmetrical cross section is used do not occur, and the manufacturing costs of the beam 10 itself can also be reduced. Furthermore, since there is no need to increase the diameter or number of reinforcing bars 17 in the floor slab 15 compared to conventional methods, quality problems such as interference between reinforcing bars 17 or insufficient cover thickness of concrete 16 do not occur, and problems that lead to reduced workability, such as increased work required for reinforcing bar arrangement, do not occur. Since materials of standard strength can be used for the beams 10 and reinforcing bars, they are readily available and the cost of the composite beam 3 can be reduced.

[0054] At least one end of the beam 10 may be rigidly or semi-rigidly joined to the girder 5. In this case, at least one end of the beam 10 can be rigidly or semi-rigidly joined to the girder 5. This allows a negative bending region to be generated in at least one end of the beam 10. The width-thickness ratio of the web 13 may exceed 100. In this case, a beam 10 having a width-thickness ratio of the web 13 exceeding 100 and having a relatively high cross-sectional efficiency can be configured.

[0055] Furthermore, in the floor structure 2 of this embodiment, the floor structure 2 can be constructed using a composite beam 3 that has a beam 10 with an H-shaped cross section and biaxial symmetry and a floor slab 15, and that has improved not only rigidity but also strength compared to the beam 10.

[0056] Although one embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and configuration changes, combinations, deletions, etc. are also included within the scope that does not deviate from the gist of the present invention. For example, in the above embodiment, the floor slab may be a composite deck slab, and the beam may be formed of H-section steel. The number of reinforcing bars 17 provided in the composite beam 3 may be one. [Explanation of symbols]

[0057] 2 floor structure 3 Composite beam 5 Large beam (supporting member) 10 beams 11 Upper flange 13. Web 15 Floor slab 17 Reinforced concrete 25 shear connector O1 axis

Claims

1. A steel beam whose cross section in its axial direction is H-shaped and biaxially symmetric; a floor slab supported by the beams and having reinforcing bars within its effective width; a shear connector connected to the beam and the floor slab, respectively; The beam has a yield strength of 325 N / mm ２ It is made of the above steel materials, The yield strength of the reinforcing bar is 295 N / mm ２ and Cross-sectional area of ​​the beam ｓ A (mm ２ ) and the sum of the cross-sectional area of ​​the reinforcing bar ｒ A (mm ２ ) satisfies equation (1). where h is the distance (mm) from the top surface of the upper flange of the beam to the center of gravity of the reinforcing bar, H is the beam depth (mm), ｓ f ｔ is the allowable stress of the beam (N / mm ２ ), ｒ f ｔ is the allowable stress of the reinforcing bar (N / mm ２ ), ｓ A ｗ is the cross-sectional area of ​​the beam web (mm ２ ) [Equation 1]

2. The composite beam of claim 1 , wherein at least one end of the beam is rigidly or semi-rigidly connected to a support member.

3. The composite beam of claim 1 or 2, wherein the width-to-thickness ratio of the web is greater than 100.

4. The composite beam according to claim 2; The support member; A floor structure comprising:

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