Floor structure

Abstract

To provide a floor structure suppressed in generation of lateral buckling in a beam.SOLUTION: A floor structure 10 is formed of an H-section steel, and includes a beam 26 to which no lateral stiffener is attached, a pair of girders joined to both ends of the beam 26 with rigid joint or semi-rigid joint, and floor slabs attached to each of the pair of girders and the beam 26. A non-reinforced through hole 29a is formed only in a specific area in an intermediate part in a material axis direction X of the beam 26 in a web 29 that the beam 26 has.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a floor structure. [Background technology]

[0002] Traditionally, structural members of buildings consist of vertical columns and horizontal beams that span between the columns. In steel-structure buildings in Japan, square steel pipes with a rectangular cross section and rounded corners are mainly used as columns. An H-shaped steel 100 with an H-shaped cross section, as shown in Figure 17, is used for a beam. The H-shaped steel 100 has high cross-sectional performance around the x-axis, which is the strong axis, and is an efficient cross-sectional shape. On the other hand, the H-shaped steel 100 has low cross-sectional performance around the y-axis, which is the weak axis. For this reason, when the H-shaped steel 100 is subjected to a bending moment, a phenomenon called lateral buckling occurs, in which the part of the H-shaped steel 100 that is subjected to a compressive force twists and collapses out of plane, reducing the strength and deformation performance of the H-shaped steel 100.

[0003] Furthermore, the beam is provided with through holes for facility piping. In the case of a steel beam (steel beam) using H-shaped steel 100, through holes (not shown) are provided in the web 101 of the H-shaped steel 100. In Japan, through holes are often circular in shape. Beams with through holes not only have reduced strength and deformation performance due to the influence of cross-sectional loss, but are also more susceptible to lateral buckling and the like compared to beams without through holes. Therefore, beams are generally used with reinforcement around the through holes.

[0004] The reinforcing members that reinforce the area around the through-holes and the beams are welded together by hand, which makes installing the reinforcing members time-consuming. Furthermore, there are many through-holes in the beams for equipment piping in a single building, and reinforcing them requires a great deal of time and effort. Therefore, it would be desirable to eliminate the need to reinforce the through-holes in the beams.

[0005] Prior art related to the lateral buckling of steel beams largely relates to reducing the weight of the lateral stiffeners of steel beams, or to steel beams and floor structures that omit lateral stiffeners in consideration of the restraining effect of floor slabs, as well as to their design methods (see, for example, Patent Documents 1 to 8). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2023-113494 [Patent Document 2] Japanese Patent Publication No. 2023-078554 [Patent Document 3] Japanese Patent Application Publication No. 2019-190109 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-161124 [Patent Document 5] Japanese Patent Application Publication No. 2018-172876 [Patent Document 6] Japanese Patent Publication No. 2022-144692 [Patent Document 7] Japanese Patent Application Publication No. 2023-114887 [Patent Document 8] Patent Publication No. 2021-055464 Summary of the Invention [Problem to be solved by the invention]

[0007] Generally, a floor slab exists above the steel beam, and the steel beam and floor slab are fixed together via shear connectors such as headed studs. When the upper flange of the steel beam is fixed by the floor slab and its lateral movement and rotation are restricted, twisting and collapse of the steel beam are suppressed, making it less susceptible to lateral buckling than when the steel beam is a single unit. It should be noted that Patent Documents 1 to 8 are directed to steel beams without through holes, and do not describe steel beams with through holes. Therefore, Patent Documents 1 to 8 cannot be used to determine whether or not lateral stiffeners are required for steel beams with through holes, or to consider the placement area of ​​the through holes. In order to use Patent Documents 1 to 8, it is necessary to reinforce the through holes so that the performance is equivalent to that of steel beams without through holes.

[0008] When the restraining effect of the floor slab is taken into account, it is believed that lateral buckling will be less likely to occur even in steel beams with through holes. Furthermore, when a bending moment is applied to a steel beam with an H-shaped cross section, it will lateral buckle in the area where compressive force is applied. In other words, by taking into account the restraining effect of the floor slab, it is believed that there is an area in steel beams with through holes where the presence or absence of a through hole does not affect the likelihood of lateral buckling, and if this area can be clarified, it will be possible to omit reinforcement of the through holes in the steel beams. Furthermore, among steel beams, sub-beams, which are joined at both ends to a pair of main beams, are subject to load conditions specific to sub-beams.Measures to prevent lateral buckling are also being considered for sub-beams.

[0009] The present invention has been made in consideration of such problems, and has an object to provide a floor structure that suppresses the occurrence of lateral buckling in the sub-beams. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention proposes the following means. (1) Aspect 1 of the present invention is a floor structure comprising a sub-beam formed of H-shaped steel and having no lateral stiffeners attached thereto, a pair of main girders joined to both ends of the sub-beam by rigid or semi-rigid joints, and floor slabs attached to the pair of main girders and the sub-beam, respectively, in which unreinforced through holes are formed in the web of the sub-beam only in a specific region in the middle of the sub-beam in the material axis direction.

[0011] In this invention, the inventors have conducted extensive research into lateral buckling that occurs in a floor structure that includes a sub-beam made of H-shaped steel and not fitted with lateral stiffeners, a pair of main girders joined to both ends of the sub-beam by rigid or semi-rigid joints, and floor slabs attached to the pair of main girders and sub-beam, and in which unreinforced through-holes to which no reinforcing members are attached are formed in the webs of the sub-beam.As a result, they have found that lateral buckling in the sub-beam can be suppressed when through-holes are formed only in specific regions in the middle of the sub-beam in the material axis direction. Therefore, by configuring in this way, it is possible to suppress the occurrence of lateral buckling in the sub-beam.

[0012] (2) Aspect 2 of the present invention is a floor structure comprising a sub-beam formed of H-shaped steel and having no lateral stiffener attached thereto, a first girder joined to a first end of the sub-beam by a rigid or semi-rigid joint, a second girder joined to a second end of the sub-beam by a pin joint, and floor slabs attached to the first girder, the second girder, and the sub-beam, respectively, in which an unreinforced through hole is formed in the web of the sub-beam only in a specific region in the middle of the sub-beam in the material axis direction.

[0013] In this invention, the inventors have conducted extensive research into lateral buckling that occurs in a floor structure that includes a sub-beam made of H-shaped steel and not equipped with a lateral stiffener, a first girder connected to a first end of the sub-beam with a rigid or semi-rigid joint, a second girder connected to a second end of the sub-beam with a pin joint, and a floor slab attached to each of the first girder, second girder, and sub-beam, and in which the sub-beam has an unreinforced through-hole formed in its web, to which no reinforcing member is attached.As a result, they have found that lateral buckling in the sub-beam is suppressed when the through-hole is formed only in a specific region in the middle of the sub-beam in the material axis direction. Therefore, by configuring in this way, it is possible to suppress the occurrence of lateral buckling in the sub-beam.

[0014] (3) Aspect 3 of the present invention may be a floor structure described in (1) or (2), in which the specific area is an area of ​​0.6L in the material axis direction, with the center of the material axis direction of the sub-beam being the center of the specific area, when the length of the sub-beam in the material axis direction is defined as L. In this invention, it is possible to suppress lateral buckling of the sub-beam in a specific region that is a region of 0.6L in the axial direction, with the center of the specific region being the center of the axial direction of the sub-beam.

[0015] (4) A fourth aspect of the present invention may be any one of the floor structures described in (1) to (3), in which all of the through holes formed in the specific region are unreinforced. In this invention, all of the unreinforced through holes can more reliably prevent lateral buckling of the sub-beams. [Effects of the Invention]

[0016] The floor structure of the present invention can prevent lateral buckling of the sub-beams. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram showing a schematic configuration of a floor structure according to an embodiment of the present invention. FIG. [Figure 2] FIG. [Figure 3] FIG. 10 is a diagram showing the bending moment acting on a sub-beam when a uniformly distributed load is applied. [Figure 4] FIG. 10 is a diagram showing the shear force acting on a sub-beam when a uniformly distributed load is applied. [Figure 5] FIG. 10 is a diagram showing the change in the load capacity of a sub-beam due to the deformation of the sub-beam, depending on whether or not lateral buckling occurs. [Figure 6] FIG. 10 is a diagram showing the change in elastic buckling strength with respect to shear span ratio due to the opening area. [Figure 7] This is a contour diagram showing the buckling mode of the girder when the shear span ratio is 6. [Figure 8] This is a contour diagram showing the buckling mode of the girder when the shear span ratio is 20. [Figure 9] This is a contour diagram showing the buckling mode of the girder when the shear span ratio is 30. [Figure 10] FIG. 10 is a graph showing the change in yield strength reduction rate with respect to shear span ratio in the case of H-500×200. [Figure 11] FIG. 10 is a graph showing the change in yield strength reduction rate with respect to shear span ratio in the case of H-700×200. [Figure 12] FIG. 10 is a graph showing the change in yield strength reduction rate with respect to shear span ratio in the case of H-900×300. [Figure 13] FIG. 10 is a graph showing the change in yield strength reduction rate with respect to shear span ratio. [Figure 14] FIG. 10 is a diagram showing the change in the opening area relative to the aspect ratio. [Figure 15] FIG. 10 is a diagram showing the change in elastic buckling strength with respect to shear span ratio due to the opening area. [Figure 16] FIG. 10 is a diagram showing the change in yield strength reduction rate with respect to shear span ratio due to the opening area. [Figure 17] FIG. 1 is a front view of a conventional H-beam. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of a floor structure according to the present invention will be described with reference to FIGS.

[0019] [1. Floor structure] As shown in Fig. 1, a floor structure 10 of this embodiment is provided in a building 1. The floor structure 10 has a plurality of columns (not shown), a pair of girders 21, a minor beam 26, and a floor slab (not shown). The structure of the pillars is not limited. For example, the pillars may be made of square steel pipes. The pillars extend in the vertical direction Z from a supporting member (not shown) such as a foundation. The columns are spaced apart from one another along a horizontal plane. The columns may be made of H-beams, steel-reinforced concrete, concrete-filled steel pipes, reinforced concrete, etc.

[0020] For example, the girder 21 is formed of an H-shaped steel. The girder 21 extends along a horizontal plane and spans between a pair of columns. The girder 21 here refers to a beam whose both ends are joined to a pair of columns. The girder may be made of steel reinforced concrete, reinforced concrete, or the like.

[0021] As shown in FIG. 2, the sub-beam 26 is formed of an H-shaped steel. The sub-beam 26 extends in the material axis direction (longitudinal direction) X along a horizontal plane. Specifically, the sub-beam 26 has an upper flange 27, a lower flange 28, and a web 29. The upper flange 27, the lower flange 28, and the web 29 are each formed into a flat plate shape from steel. The upper flange 27 is positioned above the lower flange 28. The web 29 is positioned between the upper flange 27 and the lower flange 28. The web 29 is joined to the center of the upper flange 27 in the width direction and the center of the lower flange 28 in the width direction, respectively.

[0022] A plurality of through holes 29a are formed in the web 29. For example, when viewed in the thickness direction of the web 29, each of the through holes 29a has a circular shape. The number of through holes 29a formed in web 29 may be one. No reinforcing member that reinforces the periphery of each through hole 29a is provided in each through hole 29a. In other words, each through hole 29a is unreinforced. The reinforcing member referred to here means, for example, a member fixed to a portion of web 29 that is located within the radius of through hole 29a from the periphery of through hole 29a when viewed in the thickness direction of web 29. A shear connector, such as a headed stud, is preferably fixed to the upper flange 27. The shear connector protrudes upward from the upper flange 27.

[0023] As shown in Fig. 1, both ends of the sub-beam 26 in the material axis direction X are rigidly or semi-rigidly joined to a pair of main girders 21. The rigid and semi-rigid joints referred to here are joints specified by, for example, "Eurocode 3: Design of steel structures - Part 1-8: Design of joints", 2004 (hereinafter simply referred to as Eurocode 3). The sub-beam 26 referred to here means a beam whose both ends are joined to a pair of main girders 21.

[0024] The floor slab is formed in a flat plate shape. For example, the floor slab is formed of a reinforced concrete slab with reinforcing bars installed in the concrete. The floor slab is supported from below by the upper flanges 27 of the main beams 21 and the secondary beams 26. The shear connectors of the girders 21 are embedded in the concrete of the floor slab. The floor slab is attached to the pair of girders 21 and sub-girders 26 via the shear connectors or the like. The floor slab may be formed of a composite slab having a deck plate and concrete placed on the deck plate.

[0025] No horizontal stiffeners are attached to the minor beams 26. The horizontal stiffeners referred to here refer to components that connect the component to be stiffened by the horizontal stiffener to other main beams, minor beams, columns, etc.

[0026] [2. Study of the effect of through holes on lateral buckling] As shown in Fig. 1, it is considered that a uniformly distributed load w due to, for example, a static load acts mainly on the sub-beam 26. Here, the length of the sub-beam 26 in the material axis direction X is defined as L. As shown in Fig. 17, the beam depth (height) of the sub-beam 26 is defined as H. The width of the sub-beam 26 is defined as B. This horizontal load P causes a bending moment shown in FIG. 3 and a shear force shown in FIG. Figure 5 shows the change in the load capacity of the sub-beam 26 depending on whether or not lateral buckling occurs in the sub-beam 26. It can be seen that when lateral buckling occurs in the sub-beam 26, the load capacity of the sub-beam 26 decreases compared to when lateral buckling does not occur in the sub-beam 26.

[0027] First, we used finite element numerical analysis to study the effect of the opening region (arrangement region) where the through-holes 29a are formed in the material axis direction X of the sub-beam 26 on the strength of the sub-beam 26. Figure 2 shows an example of an analytical model of the sub-beam 26. The xyz coordinates were defined for the sub-beam 26 as shown in Figure 2. The center of the opening region was set to the center of the material axis direction X of the sub-beam 26. The opening region was then expanded in the material axis direction X from 0 times (no through-holes) to 1 time (through-holes along the entire length) of the length L in increments of 0.1 times the length L. Note that only the results for increments of 0.2 times are shown in Fig. 6 and other figures described later. When arranging the through holes 29a in the opening area, the through holes 29a with a diameter of H / 2 were provided to fill the opening area to the fullest extent, with the beam depth H as the reference, and at an interval of the center pitch H between the through holes 29a. In other words, in each opening area, a through hole with the maximum opening area actually assumed for the sub-beam was formed.

[0028] A shell was used to model the sub-beam 26. At the first end 26a and second end 26b in the material axis direction X of the sub-beam 26, movement in the x-axis, y-axis, and z-axis directions and rotation around the x-axis were fixed, and a uniformly distributed load w was applied to the entire upper flange 27, thereby reproducing the stress state shown in Figures 3 and 4. In addition, to reproduce the restraining effect of the floor slab, the upper flange 27 restrains movement in the x-axis direction and rotation around the z-axis over the entire length of the sub-beam 26. The analysis variable was the shear span ratio (L / H), which is the length L divided by the beam depth H. The shear span ratio was varied in increments of 2 within the range of 6 to 50.

[0029] The analysis results are shown in Figures 6 to 9. In Figure 6, the vertical axis shows the elastic buckling strength of each analysis model obtained from the analysis, and the horizontal axis shows the shear span ratio. Figure 6 shows that the elastic buckling strength decreases as the opening area of ​​the through-hole 29a increases. Furthermore, regardless of the opening area of ​​the through-hole 29a, the buckling mode changes as the shear span ratio increases, so the elastic buckling strength shows a mountain-shaped distribution with a maximum value as the shear span ratio increases. The reason for this is that in the region where the shear span ratio is small, shear buckling is dominant in the sub-beam 26 as shown in Figure 7, and the smaller the shear span ratio, the more rapidly the elastic buckling strength decreases. Note that in Figures 7 to 9, the shades of gray represent the out-of-plane deformation of the flanges 27, 28 and web 29, non-dimensionalized by the maximum out-of-plane deformation of the sub-beam 26. In each figure, the darker the gray in a certain area, the greater the out-of-plane deformation in that area.

[0030] On the other hand, in the region where the shear span ratio is large, lateral buckling becomes the dominant mode in the sub-beam 26, and the elastic buckling strength decreases gradually as the shear span ratio increases, as shown in Figure 9. In the region where the shear span ratio is intermediate, the buckling mode in the sub-beam 26 is a region where shear buckling and lateral buckling are coupled (see Figure 8).

[0031] In this embodiment, in order to clarify the opening area of ​​the through hole 29a that does not affect lateral buckling, the minimum value of the shear span ratio at which only lateral buckling affects the elastic buckling strength was determined to be 30 based on the distribution of elastic buckling strength in Figure 6 and the buckling modes shown in Figures 7 to 9. Hereafter, a design method for the opening area of ​​the through hole 29a that does not affect lateral buckling will be considered, based on a shear span ratio of 30.

[0032] As can be seen from Figure 6, even when the shear span ratio is 30 or more, the provision of through holes 29a reduces the elastic buckling strength of sub-beam 26 (steel beam), albeit slightly. In other words, to clarify the opening area of ​​through holes 29a that does not affect lateral buckling, it is necessary to determine the allowable value for the rate of reduction in elastic buckling strength (strength reduction rate).

[0033] [2.2. Consideration of the allowable value of the rate of decrease in elastic buckling strength] The cross-sectional size and steel type of the sub-beams of a building are selected to ensure safety against the loads acting on them. H-shaped steel is generally used for the sub-beams. The height, width, and thickness of the H-shaped steel vary in stages. Therefore, when designing the sub-beam 26 using H-shaped steel, the most efficient cross-sectional size that meets or exceeds the performance required for the sub-beam 26 is selected from the standardized H-shaped steel cross-section list shown in Table 1. Therefore, the sub-beam 26 is designed with a cross-sectional size that has a slightly larger margin of performance than the performance required for the sub-beam 26.

[0034] [Table 1]

[0035] Table 1 is an excerpt from the table of cross-sectional sizes for fixed outer dimensions H-shaped steel in the H-shaped steel catalog (Nippon Steel Corporation. https: / / www.nipponsteel.com / product / catalog_download / pdf / K004.pdf). For example, if calculations show that a cross-sectional size of H-700 x 200 x 12 x 17 is sufficient, then select H-700 x 200 x 12 x 19 from the cross-section list. In other words, select an H-shaped steel beam with a beam depth H of 700 mm, width B of 200 mm, web thickness t1 of 12 mm, and flange thickness t2 of 19 mm from the cross-section list. Therefore, even if the elastic buckling strength is slightly reduced by providing through holes 29a in a limited area of ​​sub-beam 26, it was thought that this could be absorbed by the margin of strength of sub-beam 26. It was also thought that this margin of strength would be the allowable value for the rate of reduction in elastic buckling strength. For example, let us consider a case where a beam A, which is part of a building, has a cross-sectional size of H-700 x 200 x 12 x 19, designed based on the load acting on the beam A. In this case, providing through holes in a wide area in the material axis direction of the beam A reduces the elastic buckling strength of the beam A, and it was thought that it would be impossible to provide through holes 29a in an area where the performance of the beam A would be reduced to the same level as the elastic buckling strength of a beam with a cross section of H-700 x 200 x 12 x 16.

[0036] In other words, we thought that the rate of decrease in elastic buckling strength when the cross-sectional size changed from H-700×200×12×19 to H-700×200×12×16 would be the acceptable rate of decrease in elastic buckling strength due to a through hole.

[0037] Additionally, sub-beams are placed between the main girders, and because the distance between the girders is equal to the spacing between the columns, the length of the sub-beams in the axial direction is also long. As shown in Figures 3 and 4, bending moment and shear force are generated in the sub-beams, but generally the bending moment is more prevalent than the shear force. In the case of H-shaped cross-section members, the bending moment (= bending capacity) that can be borne can be efficiently increased by increasing the cross-sectional area at a position away from the neutral axis (x-axis in Figure 17). In other words, when the height and width of the H-shaped steel are fixed, the flange thickness (thickness) has a significant effect on the bending performance of the H-shaped steel. Therefore, in order to determine the allowable value for the rate of decrease in elastic buckling strength, analytical studies were carried out for H-shaped steels of various heights, with the flange thickness as a variable, and the allowable value for the rate of decrease in elastic buckling strength was confirmed.

[0038] 2 shows an analytical model of the sub-beam 26A. The sub-beam 26A does not have a through-hole 29a formed in each configuration of the sub-beam 26. A list of analytical models is shown in Table 2.

[0039] [Table 2]

[0040] For three cross sections of H-shaped steel with heights and widths of H-500 x 200, 700 x 200, and 900 x 300, the plate thickness of flanges 27 and 28 was changed to three levels. The length of the analytical model in the material axis direction was also changed so that the shear span ratio changed in increments of 2 in the range of 6≦L / H≦50.

[0041] 10 to 12 show the rate of decrease in elastic buckling strength when the plate thickness of the flanges 27, 28 is reduced. Note that tf in Figs. 10 to 12 means the plate thickness of the flanges 27, 28. Although the magnitude of the strength reduction rate differs depending on the beam height, the change in the strength reduction rate is large in the region where shear span ratio is small and shear buckling is dominant. On the other hand, in the region where shear span ratio is 30 or more and lateral buckling is dominant, the strength reduction rate converges to a constant value, with the minimum convergence value being around 7%. Therefore, the allowable rate of decrease in elastic buckling strength when only lateral buckling affects the elastic buckling strength is set at 5%, which is a safer value than 7%.

[0042] [2.3. Consideration of the opening area of ​​the through-hole] Next, a design method for the opening area of ​​the through-hole 29a will be analyzed and examined. As mentioned above, lateral buckling occurs in the part of the sub-beam that receives compressive force when subjected to a bending moment due to low cross-sectional performance around the weak axis (y-axis in Figure 17). Therefore, we focused on the aspect ratio H / B, which is the ratio of the height H of the sub-beam to its width B, as a sub-beam characteristic that improves performance around the weak axis. Note that the aspect ratio is one example of a variable that organizes the cross-sectional shape of the sub-beam, and the cross-sectional shape of the sub-beam may also be organized using variables other than the aspect ratio. Table 3 shows a list of analytical models in which the aspect ratio of the sub-beam is used as a variable and the aspect ratio is changed in increments of 0.5 within the range of 2≦H / B≦4.

[0043] [Table 3]

[0044] For the sub-beams shown in Table 3, the shear span ratio was changed in increments of 2 within the range of 6≦L / H≦50. The center of the opening area of ​​the through-hole 29a was set to the center of the material axis direction X of the sub-beam 26. The opening area was changed in increments of 0.1 times the length L in the material axis direction X within the range of 0 times (no through-hole) to 1 time (through-hole present over the entire length). The analysis conditions were the same as in [2.1].

[0045] The elastic buckling strength obtained from the analysis when no through holes 29a are formed is compared with the elastic buckling strength when through holes 29a are formed in each region, and an example of the relationship between the strength reduction rate of the elastic buckling strength when through holes 29a are formed and the shear span ratio (L / H) when through holes 29a are formed is shown in Figure 13. Figure 13 shows the case of a cross section of H-700 x 200 x 12 x 19. In Figure 13, the state where the strength reduction rate is 5% is indicated by line L1, and the state where the shear span ratio is 30 is indicated by line L2.

[0046] The rate of decrease in elastic buckling strength increases as the opening area of ​​the through hole 29a is widened. Also, in the range of this study where the rate of decrease in strength is 5% or less and the shear span ratio is 30 or more, the rate of decrease in strength is greatest when the shear span ratio is 30. In other words, to design an opening area of ​​the through hole 29a that does not affect lateral buckling, it is sufficient to find the largest opening area where the rate of decrease in elastic buckling strength is 5% or less when the shear span ratio is 30. More specifically, among the plots on line L2 and below line L1 in Fig. 13, the largest opening area is when the opening area is 0.7L. Therefore, the opening area in which through-hole 29a can be provided is the area of ​​0.7L in the material axis direction X, centered at the center of sub-beam 26 in the material axis direction X.

[0047] Figure 14 shows the relationship between the opening area and aspect ratio of through-hole 29a, which has little effect on lateral buckling for the H-shaped cross-sectional sizes shown in Table 3. Figure 14 shows that the opening area of ​​through-hole 29a, which has little effect on lateral buckling, changes little with the aspect ratio. In other words, there is no clear correlation between the opening area of ​​through-hole 29a and the aspect ratio. Based on the above considerations, in this embodiment, the opening region of the through hole 29a is set to a region (specific region) having a length of 0.6L in the material axis direction X and centered at the center of the material axis direction X of the sub-beam 26. The through hole 29a is formed only in a region having a length of 0.6L at a middle portion of the sub-beam 26 in the material axis direction X. The specific region is a region between positions of length 0.3L on both sides in the material axis direction X, with the center of the region being the center of the region in the material axis direction X of the sub-beam 26.

[0048] [2.4. When the shear span ratio is less than 30] In this study, the analysis results were used for shear span ratios (L / H) of 30 or more, so that lateral buckling would be the dominant mode. However, it was confirmed that the same tendency occurs even in areas where the shear span ratio is less than 30, when lateral buckling is the dominant mode. As an example, for a cross section of H-500x50x9x16, the shear span ratio was changed in increments of 2 within the range of 6≦L / H≦50, as before, and the opening area of ​​the through-hole 29a was changed in increments of 0.1 from the center of the sub-beam within the range of 0 times the length L (no through-hole) to 1 time (through-hole over the entire length). The relationship between the elastic buckling strength and the shear span ratio is shown in Figure 15, and the relationship between the strength reduction rate of the elastic buckling strength and the shear span ratio is shown in Figure 16. In Figure 16, the state where the strength reduction rate is 5% is also shown with line L5.

[0049] In a cross section prone to lateral buckling, such as the H-500 × 50 × 9 × 16 cross section, lateral buckling is dominant even in the region where the shear span ratio (L / H) is less than 30, as shown in Fig. 15. Also, Fig. 16 confirms that the range where the shear span ratio is less than 30 can be evaluated in the opening region of the through hole 29a obtained from the result where the shear span ratio is 30. In addition, the strength reduction rate exceeds 5% when the shear span ratio is less than 12, but this is because even with a cross section of H-500 x 50 x 9 x 16, the shear span ratio range of less than 12 is a range in which lateral buckling and shear buckling are coupled, and this is the reason why the strength reduction rate exceeds 5%. However, in cross sections where lateral buckling is more likely to occur, lateral buckling can become dominant even when the shear span ratio is less than 12, so the method (configuration) of this embodiment can also be applied to steel beams with a shear span ratio of less than 12.

[0050] 3. Effects of this embodiment As explained above, in the floor structure 10 of this embodiment, the inventors have conducted extensive research into lateral buckling that occurs in a floor structure 10 that includes a sub-beam 26 formed of an H-shaped steel and to which no lateral stiffener is attached, a pair of main girders 21 joined by rigid or semi-rigid joints to both ends of the sub-beam 26, and floor slabs attached to the pair of main girders 21 and the sub-beam 26, and in which unreinforced through-holes 29a to which no reinforcing member is attached are formed in the webs 29 of the sub-beam 26. As a result, it has been found that lateral buckling of the sub-beam 26 is suppressed when the through-holes 29a are formed only in a specific region in the middle of the sub-beam 26 in the material axis direction X. Therefore, with this configuration, the occurrence of lateral buckling in the sub-beam 26 can be suppressed.

[0051] The specific region is a region of 0.6L in the material axis direction X, with the center of the specific region being the center of the material axis direction X of the sub-beam 26. Therefore, it is possible to suppress lateral buckling of the sub-beam 26 in the specific region, which is a region of 0.6L in the material axis direction, with the center of the specific region being the center of the material axis direction X of the sub-beam 26.

[0052] [4. Notes] In the floor structure of this embodiment, each component of the floor structure 10 may have a first girder and a second girder instead of the pair of girders 21. The first girder is joined to a first end portion of the sub-beam 26 in the material axis direction X by a rigid or semi-rigid joint. The second girder is joined by a pin joint to a second end portion of the secondary beam 26 opposite to the first end portion in the material axis direction X. The pin joint referred to here is a joint defined by, for example, Eurocode.

[0053] In this modification, the floor slab is attached to the first girder, the second girder, and the secondary beam 26. The web 29 has an unreinforced through hole 29a formed only in a specific region in the middle of the secondary beam 26 in the material axis direction X. In other words, the web 29 has an unreinforced through-hole 29a formed only in a specific region in the material axis direction X from the end on the second end side of the sub-beam 26.

[0054] In the floor structure of the modified example configured as described above, the floor structure 10 includes a sub-girder 26 formed of H-shaped steel and not fitted with a lateral stiffener, a pair of first girders joined to a first end of the sub-girder 26 by rigid or semi-rigid joints, a second girder joined to a second end of the sub-girder 26 by pin joints, and a floor slab attached to each of the first girder, second girders, and sub-girder 26, and the sub-girder 26 has unreinforced through-holes 29a formed in its webs 29, to which no reinforcing member is attached. The results showed that lateral buckling of the sub-girder 26 is suppressed when the through-holes 29a are formed only in a specific region in the middle of the sub-girder 26 in the material axis direction X. Therefore, by configuring the floor structure of this modified example, it is possible to prevent lateral buckling of the small beams 26.

[0055] The above describes in detail one embodiment of the present invention and its modified examples with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes modifications, combinations, deletions, etc. of the configuration within the scope that does not deviate from the gist of the present invention. For example, the above-described embodiment and modified examples are not limited to cases where the shear span ratio (L / H) is equal to or greater than 30. Furthermore, in the case of a sub-beam that is reinforced with a stiffener or the like against local buckling or shear buckling, buckling modes other than lateral buckling are unlikely to occur, so in these cases as well, lateral buckling becomes dominant even when the shear span ratio is less than 30, and the method (configuration) of this embodiment can be applied.

[0056] The joists, floor slabs, and shear connectors that make up the floor structure of this embodiment will be described. The sub-beam has an H-shaped cross section and may be H-shaped steel (rolled H-shaped steel) manufactured by rolling, or may be built H-shaped steel (welded and assembled H-shaped steel) made by welding steel plates of different thicknesses together into an H shape. The floor slab may be a composite deck slab using a deck plate. The shear connectors may be burnt plug welds, perforated steel dowels, or the like. The minor beams, major beams, floor slabs, columns, and shear connectors are not limited to those described in this embodiment.

[0057] The through-hole 29a is preferably formed below the center of the web 29 in the up-down direction.

[0058] The number of through-holes 29a formed in a particular region may be one or more. Furthermore, by forming the through holes 29a only in specific areas, lateral buckling of the sub-beams 26 can be more reliably suppressed even if all of the through holes 29a formed in the specific area are unreinforced through holes to which no reinforcing members are attached.

[0059] The shape of the through-hole may be circular, rectangular, square, etc., and the size of the through-hole is not limited. Furthermore, the effect of [3] can be obtained regardless of the shape and size of the through-hole. The height and width of the through-hole 29a may be smaller or larger than half the beam depth. [Explanation of symbols]

[0060] 10 Floor structure 21 Large beam 26 Small beam 29 Web 29a Through hole X Material axis direction

Claims

1. A small beam formed by an H-shaped steel and to which no horizontal stiffener is attached; A pair of main beams joined to both ends of the small beam by rigid or semi-rigid joints; a floor slab attached to the pair of main girders and the sub-girders, respectively; Equipped with A floor structure in which an unreinforced through hole is formed in the web of the sub-beam only in a specific region in the middle of the sub-beam in the material axis direction.

2. A small beam formed by an H-shaped steel and to which no horizontal stiffener is attached; a first main beam connected to a first end of the sub-beam by a rigid or semi-rigid connection; a second girder joined to a second end of the sub-beam by a pin joint; a floor slab attached to each of the first girder, the second girder, and the minor girder; Equipped with A floor structure in which an unreinforced through hole is formed in the web of the sub-beam only in a specific region in the middle of the sub-beam in the material axis direction.

3. 3. The floor structure according to claim 1, wherein the specific area is an area of ​​0.6L in the material axis direction, with the center of the material axis direction of the sub-joist being the center of the specific area, when the length of the sub-joist in the material axis direction is defined as L.

4. The floor structure according to claim 1 or 2, wherein all of the through holes formed in the specific region are unreinforced.

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