Steel beam

Abstract

To provide a steel frame beam capable of reducing influence on a steel frame column when local buckling occurs by moving a position of the local buckling to a beam central part side.SOLUTION: A steel frame beam 1 has an H-shaped cross section and is joined to a steel frame column 2. Three stiffeners 20, which are plate materials extending in a direction orthogonal to the axial direction of a steel frame girder 1, are provided along the axial direction of the steel frame girder 1 on the side face of the web 10 of the steel frame girder 1, and the stiffeners 20 are a first stiffener 20A and a second stiffener 20B in order from the girder end side of the steel frame girder 1 toward the girder center side. The horizontal-distance 20A between the first stiffener 20B and the second stiffener S1 is larger than the horizontal-distance 20A between the steel column 2 and the first stiffener S2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel beam joined to a steel column. [Background technology]

[0002] Traditionally, when steel beams made of H-shaped steel are subjected to bending moments and shear forces during an earthquake, out-of-plane deformation of the constituent plate elements of the H-shaped cross section, known as local buckling, can occur at the ends of the beam where the bending moment is large, resulting in an early loss of strength of the member. Therefore, a reinforcement method in which a plurality of steel plates called stiffeners are arranged perpendicular to the flanges and webs has been widely used (see Patent Documents 1 to 3).

[0003] Patent Document 1 shows that the spacing between adjacent stiffeners is roughly equal. With this configuration, the position at which local buckling occurs at the end of the beam where the bending moment is greatest is between the stiffener and the column, or at the end of the unstiffened region. Patent Document 2 shows a configuration in which a horizontal haunch (widening of the flange width) is provided at the end of a beam, and further reinforced with a stiffener. With this configuration, by setting the member dimensions so that the plasticized region of the member remains around the widening start position, it is possible to maintain the elasticity of the beam-end weld and guide the position where local buckling occurs toward the center of the beam's axial direction. However, this method has the disadvantage that the width of the column to which the beam is joined must be large in order to provide the horizontal haunch.

[0004] Patent Document 3 describes a configuration in which the thickness of the web and flanges at the center of a beam is made thinner than that at the end of the beam and further reinforced with stiffeners. This configuration maintains the elasticity of the beam-end welds by setting the member dimensions so that the plasticized region of the member remains within the reduced thickness range, thereby guiding the local buckling location toward the center of the beam. However, this method does not allow yielding at the beam end, and the inherent cross-sectional performance cannot be fully realized. This is because if both the beam end and the reduced thickness range are plasticized, local buckling may occur in the unstiffened region of the beam end. Simply plasticizing the reduced thickness range does not control the local buckling location.

[0005] In steel beam-column structures, which consist of steel columns and steel beams, the steel columns and steel beams are firmly joined by welding. The joint surface that is integrated by this welding is the column-beam welded joint, which plays an important role in ensuring the strength and earthquake resistance of the building. Therefore, in steel beam-column structures, the column-beam welded joint is required to maintain its integrity even when earthquake loads occur. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-156473 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-220873 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-145593 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention aims to provide a steel beam that can reduce the impact on steel columns and beam-to-column welded joints even if local buckling occurs in the steel beam by moving the position of local buckling toward the center of the beam. [Means for solving the problem]

[0008] The inventors have developed a steel beam structure that can reduce the impact of local buckling on steel columns and beam-to-column welded joints. This structure includes a first vertical stiffener and a second vertical stiffener along the axial direction of the steel beam, and the horizontal distance between the first vertical stiffener and the second vertical stiffener is set greater than the horizontal distance between the steel column and the first vertical stiffener. Although a large bending moment acts on the end of the steel beam, the short horizontal distance between the steel column and the first vertical stiffener prevents local buckling from occurring in this area. This reduces the impact of local buckling on steel columns and beam-to-column welded joints, improving the strength and seismic performance of the steel beam.

[0009] The steel beam of the first invention (for example, steel beam 1 described later) is a steel beam with an H-shaped cross section or an I-shaped cross section that is joined to a steel column (for example, steel column 2 described later), and at least two vertical stiffeners (for example, stiffeners 20 described later) that are plate members extending in a direction perpendicular to the axial direction of the steel beam are provided on the side of the web (for example, web 10 described later) of the steel beam along the axial direction of the steel beam, and the vertical stiffeners are, in order from the beam end side of the steel beam toward the beam center side, a first vertical stiffener (e.g., first stiffener 20A described below) and a second vertical stiffener (e.g., second stiffener 20B described below), and the horizontal distance between the first vertical stiffener and the second vertical stiffener (e.g., horizontal distance S1 described below) is greater than the horizontal distance between the steel column and the first vertical stiffener (e.g., horizontal distance S2 described below).

[0010] According to this invention, a first vertical stiffener and a second vertical stiffener are provided in this order on a steel beam from the beam end toward the beam center. Furthermore, the horizontal distance between the first vertical stiffener and the second vertical stiffener is set to be greater than the horizontal distance between the steel column and the first vertical stiffener. As a result, when an external load exceeding the yield strength acts on a steel beam, local buckling occurs not in the web or flange between the steel column and the first vertical stiffener, but in the web or flange between the first vertical stiffener and the second vertical stiffener. In other words, compared to conventional methods, the location of local buckling is shifted toward the center of the beam, reducing the impact of local buckling on steel columns and beam-to-column welded joints. This makes it possible to suppress local buckling in steel beams while improving the deformation capacity of the steel beams compared to conventional methods.

[0011] The steel beam of the second invention is characterized in that, of the total m vertical stiffeners, the nth one counting from the beam end side is located at a horizontal distance L(n) from the beam end of the steel beam, which satisfies the equations (1) to (8) described below.

[0012] According to this invention, by suppressing local buckling of the steel beam using vertical stiffeners attached to the beam ends and ensuring the plastic deformation performance of the steel beam, while controlling the location where local buckling occurs, it is possible to realize a steel beam that does not experience a sudden decrease in strength due to strain concentration at the welded joints even at the end of its life.

[0013] The steel beam of the third invention is characterized in that when an external load exceeding the yield strength acts on the steel beam, buckling occurs in the portion of the steel beam between the first vertical stiffener and the second vertical stiffener.

[0014] According to this invention, when an external load exceeding the yield strength acts on a steel beam, buckling occurs in the portion of the steel beam between the first vertical stiffener and the second vertical stiffener (web or flange). This reduces the influence of local buckling on steel columns and beam-to-column welded joints. [Effects of the Invention]

[0015] According to the present invention, a steel beam can be provided that, by moving the position of local buckling toward the center of the beam, can reduce the impact on steel columns and beam-to-column welded joints even if local buckling occurs in the steel beam. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a side view of a steel beam according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram showing a state in which local buckling has occurred in a steel beam. [Figure 3] FIG. 10 is a diagram showing the combination of beam depth and beam width of an H-shaped steel in finite element analysis. [Figure 4] FIG. 10 is a diagram showing the web thickness and flange thickness of an H-shaped steel in a finite element analysis. [Figure 5] FIG. 10 is a diagram showing a subelement obtained by extracting an area sandwiched between adjacent stiffeners. [Figure 6] FIG. 10 is a diagram showing the relationship between the calculation results according to equations (1) to (3) and the finite element analysis results. [Figure 7] FIG. 10 is a diagram showing the relationship between the calculation results according to equations (5) and (6) and the finite element analysis results. [Figure 8] FIG. 10 is an explanatory diagram of a member angle and a material end rotation angle. [Figure 9] FIG. 10 is a diagram showing a three-fold linear model that determines the elastic-plastic behavior between stiffeners. [Figure 10] FIG. 10 is a diagram showing the relationship between the end bending moment and the member angle in the examples and the comparative examples. [Figure 11] FIG. 10 is a diagram showing the results of finite element analysis of an example and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention relates to a steel beam having an H-shaped or I-shaped cross section that is joined to a steel column, characterized in that a first vertical stiffener and a second vertical stiffener are provided along the axial direction of the steel beam, and the horizontal distance between the first vertical stiffener and the second vertical stiffener is greater than the horizontal distance between the steel column and the first vertical stiffener. Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a side view of a steel beam 1 according to one embodiment of the present invention. The steel beam 1 is an H-shaped steel beam joined to a steel column 2, and comprises a web 10 extending approximately vertically, a lower flange 11 extending approximately horizontally from the lower end of the web 10, and an upper flange 12 extending approximately horizontally from the upper end of the web 10.

[0018] Three stiffeners 20 serving as vertical stiffeners are provided at predetermined intervals along the axial direction of the steel beam 1. Each stiffener 20 is a plate extending in a direction perpendicular to the axial direction, and is joined to the upper surface of the lower flange 11, the side surface of the web 10, and the lower surface of the upper flange. The stiffeners 20 are, in order from the beam end side of the steel beam 1 toward the beam center side, a first stiffener 20A as a first vertical stiffener, a second stiffener 20B as a second vertical stiffener, and a third stiffener 20C as a third vertical stiffener. The horizontal distance S1 between the first stiffener 20A and the second stiffener 20B is greater than the horizontal distance S2 between the steel column 2 and the first stiffener 20A. Therefore, as shown in Figure 2, when a bending load exceeding the yield strength acts on the steel beam 1, buckling occurs in the web 10 and bottom flange 11 between the first stiffener 20A and the second stiffener 20B of the steel beam 1.

[0019] A finite element analysis was performed on an extremely short span H-shaped steel separated by stiffeners. The parameters set for this H-shaped steel were the beam depth, beam width, web thickness, flange thickness, ratio of stiffener spacing to beam depth, and ratio of bending moments at adjacent stiffener positions. The combinations of beam depth and beam width for H-shaped steel beams are shown in Fig. 3. The web thickness and flange thickness of the H-shaped steel beams used in this study are shown in Fig. 4. The ratio of stiffener spacing to beam depth was set to three types: 1 / 3, 1 / 2, and 2 / 3, and the ratio of bending moments at adjacent stiffener positions was set to four types: 1.0, 0.9, 0.8, and 0.7. Furthermore, the flange thickness was set to be equal to or greater than the web thickness, and cases where the width-thickness ratio of both the web and flange was FA rank, cases where the web width-thickness ratio exceeded 120, and cases where the flange width-thickness ratio exceeded 20 were excluded. From the above, the number of H-shaped steel sections to be analyzed was 2,844.

[0020] FIG. 5 is a diagram showing a subelement obtained by extracting an area sandwiched between adjacent stiffeners. For this partial element, the elastic buckling strength eMcr when the stiffener is considered to be rigid is determined by equations (1) to (3). Note that eMcrf(n) in equation (1) is taken from the approximate evaluation formula (4.4) for flange buckling strength described in Igarashi Kiyoshio et al.: Elastic buckling strength calculation method for H-section plate elements subjected to bending shear force and axial force, Transactions of the Architectural Institute of Japan, Structural Engineering, No. 613, pp. 137-146, March 2007.

[0021]

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[0022] Here, eMcr is the elastic buckling strength, the subscript f is the flange under compressive stress, wB is the web under pure bending stress, wS is the web under pure shear stress, w is the web under combined bending-shear stress, Mcr is the maximum bending strength due to local buckling, Mcr' is the end bending moment, d is the web dimension, tw is the web thickness, b is the flange half-width, tf is the flange thickness, Z is the section modulus, Mp is the full plastic strength, E is the elastic modulus, ν is Poisson's ratio, and L0 is the shear span. Also, let L(0) = 0 and L(m+1) = L0. The validity of defining the elastic buckling strength eMcr using the following equations (1) to (3) has been confirmed by comparison with the results of elastic buckling eigenvalue analysis using the finite element method for a wide range of parameters, as shown in Figure 6.

[0023] In addition, the index R in the following equation (4) is defined as the ratio between the elastic buckling strength when local buckling of the web is the main cause and the elastic buckling strength when local buckling of the flange is the main cause.

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[0024] Next, an elastic-plastic finite element analysis was performed on the same partial elements in Figure 5, taking into account the plasticity of the material, to confirm the maximum bending strength Mcr due to local buckling. As shown in Figure 7, the relationship between eMcr and the value of R was found to be as shown in equations (5) to (6).

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[0025] Based on this, the end bending moment Mcr' when the maximum bending strength due to local buckling is reached in each area sandwiched between stiffeners of the beam member is derived using the following equation (7), and the position where local buckling occurs at the end is controlled by comparing the magnitude relationship.Here, if the following equation (8) is satisfied, the position where local buckling occurs at the end can be guided to between the first and second stiffeners from the end of the beam.

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[0026] In this finite element analysis, the beam model is given either perfectly elastic or elastic-plastic material properties. Therefore, the elastic buckling eigenvalues ​​corresponding to the combinations of each parameter (web width-thickness ratio, flange width-thickness ratio, beam width-depth ratio, stiffener spacing to beam depth ratio, and moment ratio at both ends of the model) are obtained from the elastic-plastic analysis. These elastic buckling eigenvalues ​​are organized as the magnitude of the bending moment acting at the stiffener position on the fixed end side. In the elastic-plastic analysis, three items are obtained: the maximum bending moment acting on the stiffener position on the fixed end side, and the member angle and end rotation angle of the beam model when the maximum bending moment is reached. The definitions of member angle and end rotation angle are shown in Fig. 8. By using the values ​​of these three items, the elastoplastic behavior of an extremely short span separated by stiffeners up to the maximum strength can be expressed as a two-line model, as shown in Fig. 9.

[0027] Examples of the present invention will be described below. In these examples, finite element analysis was performed on H-section steel of 900 x 300 x 9 x 22. Figure 10 shows the relationship between the end bending moment and the member angle for the examples and comparative examples. Case 1 is a case where no stiffener reinforcement is performed (Comparative Example). Case 2 is a case where stiffener reinforcement is performed at equal intervals (Comparative Example). Specifically, four stiffeners are used, and the spacing between the stiffeners is 300 mm. Case 3 is a case where stiffener reinforcement according to the present invention is performed at uneven intervals (Example). Specifically, three stiffeners are used, and the spacing is 300 mm, 400 mm, and 500 mm, respectively, from the material end side toward the center side. In addition, initial imperfections of about 5% of the web thickness were introduced at the material end part in Case 1, and between each stiffener and at the end of the non-stiffened region in Cases 2 and 3.

[0028] In Case 2, Mcr´(1) / Mp = 1.20, Mcr´(2) / Mp = 1.28, Mcr´(3) / Mp = 1.38, and as shown in Figure 11, it can be seen that local buckling occurs at the end of the material. In Case 3, Mcr´(1) / Mp = 1.20, Mcr´(2) / Mp = 1.19, Mcr´(3) / Mp = 1.23, and as shown in Figure 11, it can be seen that local buckling occurs at the center of the material.

[0029] From Figure 10, it can be seen that the reinforcing method of the present invention can guide the location of local buckling toward the center of the material axis within the stiffener stiffening area, alleviating strain concentration at the material end, while maintaining the same member strength as when stiffeners are installed at sufficiently small, even intervals. The design strength according to the present invention, also shown in Figure 10, corresponds well to the results of finite element analysis.

[0030] According to this embodiment, the following effects are obtained. (1) A first stiffener 20A and a second stiffener 20B are provided in this order on a steel beam 1 from the beam end toward the beam center. Furthermore, the horizontal distance between the first stiffener 20A and the second stiffener 20B is set to be larger than the horizontal distance between the steel column 2 and the first stiffener. In other words, the distance between the steel column 2 and the first stiffener 20A and the distance between the first stiffener 20A and the second stiffener 20B are arranged unevenly. As a result, when an external load exceeding the yield strength acts on the steel beam 1, local buckling occurs not in the web 10 or flanges 11, 12 between the steel column 2 and the first stiffener 20A, but in the web 10 or flanges 11, 12 between the first stiffener 20A and the second stiffener 20B. In other words, compared to conventional methods, the position of local buckling is shifted toward the center of the beam, reducing the impact of local buckling on the steel column 2 and the beam-to-column welded joint between the steel beam 1 and the steel column 2. Therefore, local buckling of the steel beam 1 can be suppressed while improving the deformation performance of the steel beam 1 compared to conventional methods.

[0031] The present invention is not limited to the above-described embodiment, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention. [Explanation of symbols]

[0032] 1...Steel beam 2...Steel column 10...Web 11...Lower flange 12...Upper flange 20...Stiffener (vertical stiffener) 20A...1st stiffener (1st vertical stiffener) 20B...Second stiffener (second vertical stiffener) 20C...Third stiffener (third vertical stiffener)

Claims

1. A steel beam with an H-shaped or I-shaped cross section that is connected to a steel column, At least two vertical stiffeners, which are plate members extending in a direction perpendicular to the axial direction of the steel beam, are provided on the side surfaces of the web of the steel beam along the axial direction of the steel beam; The vertical stiffeners are, in order from the beam end side toward the beam center side of the steel beam, a first vertical stiffener and a second vertical stiffener, A steel beam characterized in that the horizontal distance between the first vertical stiffener and the second vertical stiffener is greater than the horizontal distance between the steel column and the first vertical stiffener.

2. A steel beam as described in claim 1, characterized in that the nth vertical stiffener, counting from the beam end side, of the total m vertical stiffeners is located at a horizontal distance L(n) from the beam end of the steel beam, satisfying the following equations (1) to (8). [Equation 1] [Equation 2] [Equation 3] [Equation 4] [Equation 5] [Equation 6] [Equation 7] [Equation 8] eMcr is the elastic buckling strength, the subscript f is the flange under compressive stress, wB is the web under pure bending stress, wS is the web under pure shear stress, w is the web under combined bending and shear stress, Mcr is the maximum bending strength due to local buckling, Mcr' is the end bending moment, d is the inner dimension of the web, tw is the web thickness, b is the half width of the flange, tf is the flange thickness, Z is the section modulus, Mp is the full plastic strength, E is the elastic modulus, ν is Poisson's ratio, L0 is the shear span, and L(0) = 0, L(m + 1) = L0

3. A steel beam as described in claim 1 or 2, characterized in that when an external load exceeding the yield strength acts on the steel beam, buckling occurs in the portion of the steel beam between the first vertical stiffener and the second vertical stiffener.

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