Method for evaluating the strength of joint structures

Abstract

This invention provides a method for evaluating the strength of a joint structure that can bring the design strength closer to the actual strength. [Solution] The method for evaluating the strength of a joint structure is a method for evaluating the strength of a joint structure 1 in which an upper rectangular steel pipe 11 and a lower rectangular steel pipe 12 are joined via a through diaphragm 13, wherein the diameter of the upper rectangular steel pipe 11 is smaller than the diameter of the lower rectangular steel pipe 12, and the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 have flat plate portions 11f, 12f and corner portions 11c, 12c which have higher strength than the flat plate portions 11f, 12f, and the method includes calculating the bending strength of the joint structure 1 considering the strength of the corner portions 11c, 12c of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the load-bearing capacity of a joint structure in which an upper rectangular steel pipe and a lower rectangular steel pipe are joined via a through diaphragm. [Background technology]

[0002] In steel-framed buildings, when the diameter of columns changes between floors, a variable-width joint system with a thickened diaphragm is used as one of the joint types. Reducing the column diameter saves material, which has the advantage of reducing costs and GHG (Green House Gas: greenhouse gases such as CO2). As shown in Non-Patent Document 1 below, the application condition for this variable-width joint system is that the diaphragm does not collapse until the column collapses. The collapse of the diaphragm is the minimum load-bearing capacity of the joint, which can be determined by yield line theory assuming an arbitrary collapse mechanism (hereinafter, j M u ) and the collapse of the column is due to the full plastic moment of the upper column (hereinafter, c M p ) is determined by, j M u ≧ c M p Satisfying this condition is a requirement for applying the differential width joint type. Non-patent documents 2 and 3 below disclose the load-bearing capacity evaluation that forms the basic concept of Non-patent document 1. Patent document 1 below discloses a method for analyzing and modeling the out-of-plane bending strength of a diaphragm that reflects the curvature (shape) of the corners of a square steel pipe. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-162148 [Non-patent literature]

[0004] [Non-Patent Document 1] Architectural Institute of Japan: Steel structure joint design guidelines 4th edition (2021) [Non-Patent Document 2] Tsuyoshi Tanaka, Masao Terashima, and Mototsugu Tabuchi: Evaluation of the load-bearing capacity of through diaphragms in irregular-width rectangular steel pipe column-beam joints, Annual Proceedings of Steel Structures, Vol. 16, (2008), pp. 95-102. [Non-Patent Document 3] Seiji Miyoshi, Tsuyoshi Tanaka, Mototsugu Tabuchi, Masao Terashima: Out-of-plane bending strength of through diaphragm in irregular-width rectangular steel pipe column-beam joint, Proceedings of the Architectural Institute of Japan Annual Meeting, (2011), pp. 1087-1088. [Overview of the project] [Problems that the invention aims to solve]

[0005] Cold-formed square steel pipe columns, the main target of the variable-width joint system, experience increased strength due to cold working during the bending process at the corners. The strength evaluations disclosed in Non-Patent Documents 2 and 3 are performed under the assumption that the effects of the corners of the steel pipe column (curvature and strength increase) mentioned above are not considered, in order to simplify the design. While there are designs that reflect the curvature (shape) of the corners, such as the configuration disclosed in Patent Document 1, the strength evaluation does not take into account that the strength of the corners is higher than that of the flat sections. Thus, conventional methods do not take into account that the strength of the corners is higher than that of the flat sections when evaluating strength, raising concerns that the actual strength cannot be accurately evaluated.

[0006] This invention was made to solve the above-mentioned problems, and one of its objectives is to provide a method for evaluating the strength of a joint structure that can bring the design strength closer to the actual strength. [Means for solving the problem]

[0007] The method for evaluating the strength of a joint structure according to the present invention, in one embodiment, is a method for evaluating the strength of a joint structure in which an upper rectangular steel pipe and a lower rectangular steel pipe are joined via a through diaphragm, wherein the diameter of the upper rectangular steel pipe is smaller than the diameter of the lower rectangular steel pipe, and the upper rectangular steel pipe and the lower rectangular steel pipe have a flat portion and a corner portion with higher strength than the flat portion, and the method includes calculating the bending strength of the joint structure considering the strength of the corner portions of the upper rectangular steel pipe and the lower rectangular steel pipe. [Effects of the Invention]

[0008] According to one embodiment of the method for evaluating the strength of a joint structure of the present invention, the bending strength of the joint structure is calculated by considering the strength of the corners of the upper and lower square steel pipes, thereby bringing the design strength closer to the actual strength. [Brief explanation of the drawing]

[0009] [Figure 1] This is a plan view showing a joint structure whose load-bearing capacity can be evaluated by the load-bearing capacity evaluation method for joint structures according to an embodiment of the present invention. [Figure 2] This is a cross-sectional view of the joint structure along line II-II in Figure 1. [Figure 3] This is a schematic side view showing a column joint structure with varying widths, including the joint structure shown in Figure 1. [Figure 4] This is a plan view showing the collapse mechanism assumed in the load-bearing capacity evaluation method for the joint structure according to this embodiment. [Figure 5] Figure 4 is an explanatory diagram showing the radius of curvature of the corners of the upper and lower rectangular steel pipes in the collapse mechanism. [Figure 6] This is an explanatory diagram showing the state in which the yield line is formed in the lower rectangular steel pipe in the collapse mechanism shown in Figure 4. [Figure 7] Figure 4 is an explanatory diagram showing the state in which the yield wire is formed in the diaphragm during the collapse mechanism. [Figure 8] This is an explanatory diagram showing the distribution of internal force work to the flat and angular sections in the axial yield strain region of the upper rectangular steel pipe shown in Figure 4. [Figure 9] This is a perspective view showing the general shape of the analysis model set up in the example. [Figure 10] This is a side view showing the general shape of the analysis model set up in the example. [Figure 11] This graph shows the nominal stress-nominal strain relationship set in the example. [Figure 12] This graph shows the relationship between moment M and member angle θ at the upper surface of the through diaphragm, obtained from the analysis of the embodiment. [Figure 13] This is an explanatory diagram showing the general yield method used to evaluate yield strength in the examples. [Modes for carrying out the invention]

[0010] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The present invention is not limited to each embodiment, and can be materialized by modifying the components without departing from the spirit of the invention. Furthermore, various inventions can be formed by appropriately combining the multiple components disclosed in each embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, components from different embodiments may be appropriately combined.

[0011] First, with reference to Figures 1 to 3, an overview of the joint structure 1 whose strength is evaluated by the strength evaluation method for joint structure 1 according to an embodiment of the present invention will be described. Figure 1 is a plan view showing the joint structure 1 whose strength can be evaluated by the strength evaluation method for joint structure 1 according to an embodiment of the present invention, Figure 2 is a cross-sectional view of the joint structure 1 along line II-II in Figure 1, and Figure 3 is a schematic side view showing the irregular width column joint structure 2 including the joint structure 1 in Figure 1.

[0012] As shown in Figures 1 and 2, the joint structure 1 is formed by joining an upper rectangular steel pipe 11 and a lower rectangular steel pipe 12 via a through diaphragm 13. The entire lower end of the upper rectangular steel pipe 11 may be joined to the upper surface of the through diaphragm 13, and the entire upper end of the lower rectangular steel pipe 12 may be joined to the lower surface of the through diaphragm 13. The joining may be performed by welding.

[0013] As shown in Fig. 1, the upper square steel pipe 11 and the lower square steel pipe 12 have corner portions 11c, 12c and flat portions 11f, 12f. The corner portions 11c, 12c are arranged at the four corners of the upper square steel pipe 11 and the lower square steel pipe 12 when viewed in a plane (Fig. 1) or a cross-section (Fig. 2), and may be in a region of 45 degrees from the center of the corner arc to both wings. The flat portions 11f, 12f may be portions that extend linearly between the corner portions 11c, 12c when the upper square steel pipe 11 and the lower square steel pipe 12 are viewed in a plane or a cross-section.

[0014] In Figs. 1 and 2, the upper square steel pipe 11 and the lower square steel pipe 12 are coaxially arranged. However, the upper square steel pipe 11 and the lower square steel pipe 12 may be eccentric with respect to each other. For example, one flat portion 12f of the lower square steel pipe 12 may overlap one flat portion 11f of the upper square steel pipe 11, or one flat portion 12f of the lower square steel pipe 12 may overlap one corner portion 11c of the upper square steel pipe 11.

[0015] The diameter of the square steel pipe is generally defined by the distance between the outer surfaces (outer diameter), but the diameter of the square steel pipe in the present invention may be defined at the center of the plate thickness. The diameter D of the upper square steel pipe 11 cu is smaller than the diameter D of the lower square steel pipe 12. The diameter D of the upper square steel pipe 11 cl may be the width between the centers of the plate thicknesses of a pair of flat portions 11f facing each other. Similarly, the diameter D of the lower square steel pipe 12 cu may be the width between the centers of the plate thicknesses of a pair of flat portions 12f facing each other. As shown in the embodiment of Fig. 1, the cross-sectional shapes of the upper square steel pipe 11 and the lower square steel pipe 12 may be square. Alternatively, the cross-sectional shapes of the upper square steel pipe 11 and the lower square steel pipe 12 may be rectangular with short sides and long sides. When at least one of the upper square steel pipe 11 and the lower square steel pipe 12 is rectangular, at least one of the long side and the short side of the upper square steel pipe 11 may be smaller than the diameter of the lower square steel pipe 12.

[0016] ​​The upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 are typically formed rectangular steel pipes. Formed rectangular steel pipes are made by bending steel plates to form a rectangle, and then joining the ends of the steel plates that are butted together. The bending process may be performed cold.

[0017] The corners 11c and 12c have higher strength than the flat sections 11f and 12f (the corner 11c of the upper rectangular steel pipe 11 has higher strength than the flat section 11f of the upper rectangular steel pipe 11, and the corner 12c of the lower rectangular steel pipe 12 has higher strength than the flat section 12f of the lower rectangular steel pipe 12). When the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 are formed rectangular steel pipes, the strength of the corners 11c and 12c is higher than the strength of the flat sections 11f and 12f due to work hardening.

[0018] The joint structure 1 in Figure 1 may be included in the variable-width column joint structure 2 in Figure 3. In the variable-width column joint structure 2, the column 21 on the upper floor and the column 22 on the lower floor are joined via a joint 23. The joint 23 has an upper diaphragm 23a, a rectangular joint panel 23b whose upper surface is joined to the lower surface of the upper diaphragm 23a, and a lower diaphragm 23c whose upper surface is joined to the lower surface of the joint panel 23b. The lower end of the column 21 on the upper floor is joined to the upper surface of the upper diaphragm 23a, and the upper end of the column 22 on the lower floor is joined to the lower surface of the lower diaphragm 23c. The diameter or width of the column 21 on the upper floor is smaller than the diameter or width of the joint panel 23b and the column 22 on the lower floor. The diameter or width of the joint panel 23b is about the same as the diameter or width of the column 22 on the lower floor. In Figure 1, the upper rectangular steel pipe 11 may be a column 21 on the upper floor, the lower rectangular steel pipe 12 may be a connecting panel 23b, and the through diaphragm 13 in Figure 1 may be an upper diaphragm 23a. The connecting portion 23 may also be composed of only one diaphragm. In this case, the upper rectangular steel pipe 11 in Figure 1 may be a column 21 on the upper floor, and the through diaphragm 13 in Figure 1 may be a diaphragm constituting the connecting portion 23.

[0019] A beam 24 may be joined to the joint 23. Figure 3 shows a configuration in which two beams 24 extending left and right in the figure are joined to the joint 23, but the number of beams 24 joined to the joint 23 is arbitrary.

[0020] The method for evaluating the strength of a joint structure 1 according to an embodiment of the present invention includes calculating the bending strength of the joint structure 1 by considering the strength of the corners 11c and 12c of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12. An arbitrary collapse mechanism is assumed, and the one with the smallest internal force work is defined as the true collapse mechanism. When a collapse mechanism is assumed, a yield line occurs at the contact position between the lower rectangular steel pipe 12 and the through diaphragm 13. The internal force work at this yield line is calculated using the smallest value of the total plastic moment per unit length between the lower rectangular steel pipe 12 and the through diaphragm 13. In a conventional evaluation method that does not consider the strength of the corner portion 12c of the lower rectangular steel pipe 12, if a yield line occurs in the lower rectangular steel pipe 12, taking the strength of the corner portion 12c into consideration may increase the internal force work in the lower rectangular steel pipe 12 or shift the yield line position to the through diaphragm 13, and thus the bending strength of the joint structure 1 will also increase. In addition, among the collapse mechanisms, there is also a mechanism in which the upper rectangular steel pipe 11 undergoes axial yielding, and in some cases, the true collapse mechanism occurs when axial yield strain occurs in the upper rectangular steel pipe 11. Since the axial yield strain of the upper rectangular steel pipe 11 is formed mainly around the corner portion 11c, if calculations are made considering that the strength of the corner portion 11c is higher than that of the flat portion 11f, the internal force work in the axial yield strain region will increase, and therefore the bending strength of the joint structure 1 will also increase. As in the load-bearing capacity evaluation method for the joint structure 1 of this embodiment, by calculating the bending strength of the joint structure 1 while considering the strength of the corners 11c and 12c of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12, the calculated load-bearing capacity can be brought closer to the actual load-bearing capacity.

[0021] The method for evaluating the strength of the joint structure 1 according to this embodiment involves using the upper boundary theorem assuming a yield line, and when calculating the internal force work in the upper boundary theorem, using the smaller of the full plastic moment per unit length of the lower rectangular steel pipe 12 calculated using the yield stress (strength) of the corner 12c of the lower rectangular steel pipe 12 and the full plastic moment per unit length of the through diaphragm 13 to calculate the bending strength of the joint structure 1. By calculating the bending strength of the joint structure 1 in this way, the internal force work at the yield line that occurs at the contact position between the corner 12c of the lower rectangular steel pipe 12 and the through diaphragm 13 can be correctly evaluated.

[0022] The method for evaluating the strength of the joint structure 1 according to this embodiment includes using the upper boundary theorem, which assumes an axial yield region and a yield line, and calculating the bending strength of the joint structure 1 by considering the difference in strength between the flat portion 11f and the corner portion 11c of the upper rectangular steel pipe 11 when calculating the internal force work in the upper boundary theorem. By calculating the bending strength of the joint structure 1 in this way, the internal force work in the axial yield strain region can be correctly evaluated when an axial yield strain region occurs at the corner portion 11c of the upper rectangular steel pipe 11.

[0023] Next, the method for evaluating the load-bearing capacity of the joint structure 1 according to this embodiment will be explained in more detail with reference to Figures 4 to 7. Figure 4 is a plan view showing the collapse mechanism assumed in the load-bearing capacity evaluation method of the joint structure 1 according to this embodiment, Figure 5 is an explanatory diagram showing the radius of curvature of the corners 11c and 12c of the upper rectangular steel pipe 11 and lower rectangular steel pipe 12 in the collapse mechanism of Figure 4, Figure 6 is an explanatory diagram showing the state in which a yield line is formed in the lower rectangular steel pipe 12 in the collapse mechanism of Figure 4, and Figure 7 is an explanatory diagram showing the state in which a yield line is formed in the through diaphragm 13 in the collapse mechanism of Figure 4.

[0024] In the load-bearing capacity evaluation method for the joint structure 1 according to this embodiment, the collapse mechanism shown in Figure 4 is assumed. The following is a description of the collapse mechanism.

[0025] The upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 are formed by cold working.

[0026] The corner portion 12c of the lower rectangular steel pipe 12 includes the lower first corner portion 12c1, the lower second corner portion 12c2, the lower third corner portion 12c3, and the lower fourth corner portion 12c4.

[0027] The flat plate portion 12f of the lower rectangular steel pipe 12 includes a lower first flat plate portion 12f1 extending between the lower first corner portion 12c1 and the lower second corner portion 12c2, a lower second flat plate portion 12f2 extending between the lower second corner portion 12c2 and the lower third corner portion 12c3, a lower third flat plate portion 12f3 extending between the lower third corner portion 12c3 and the lower fourth corner portion 12c4, and a lower fourth flat plate portion 12f4 extending between the lower fourth corner portion 12c4 and the lower first corner portion 12c1.

[0028] The corner portion 11c of the upper rectangular steel pipe 11 includes the upper first corner portion 11c1 closest to the lower first corner portion 12c1, the upper second corner portion 11c2 closest to the lower second corner portion 12c2, the upper third corner portion 11c3 closest to the lower third corner portion 12c3, and the upper fourth corner portion 11c4 closest to the lower fourth corner portion 12c4.

[0029] The flat plate portion 11f of the upper rectangular steel pipe 11 includes an upper first flat plate portion 11f1 extending between the upper first corner portion 11c1 and the upper second corner portion 11c2, an upper second flat plate portion 11f2 extending between the upper second corner portion 11c2 and the upper third corner portion 11c3, an upper third flat plate portion 11f3 extending between the upper third corner portion 11c3 and the upper fourth corner portion 11c4, and an upper fourth flat plate portion 11f4 extending between the upper fourth corner portion 11c4 and the upper first corner portion 11c1.

[0030] Point A is set at the center of the thickness of the lower first corner 12c1, point B is set at the center of the thickness of the lower first flat section 12f1, point C is set at the center of the thickness of the lower second corner 12c2, point D is set at the center of the thickness of the lower third corner 12c3, point E is set at the center of the thickness of the lower third flat section 12f3, and point F is set at the center of the thickness of the lower fourth corner 12c4.

[0031] Point G is set at the center of the thickness of the upper second flat section 11f2 or the upper second corner section 11c2, point H is set at the center of the thickness of the upper second flat section 11f2 or the upper third corner section 11c3, point I is set at the center of the thickness of the upper fourth flat section 11f4 or the upper fourth corner section 11c4, and point J is set at the center of the thickness of the fourth flat section 11f4 or the upper first corner section 11c1.

[0032] Yield lines extend between each of the following points: A-B, B-C, C-D, D-E, E-F, F-A, G-B, G-C, G-H, H-D, H-E, I-E, I-F, I-J, J-A, and J-B.

[0033] Point K is set at the intersection of the thickness center of the upper first flat plate section 11f1 and the yield line JB, point L is set at the intersection of the thickness center of the upper first flat plate section 11f1 and the yield line GB, point M is set at the intersection of the thickness center of the upper third flat plate section 11f3 and the yield line HE, and point N is set at the intersection of the thickness center of the upper third flat plate section 11f3 and the yield line IE.

[0034] Point O is set at the intersection of the extension of the thickness centerline of the upper fourth flat plate section 11f4 and the extension of the thickness centerline of the upper first flat plate section 11f1. Point P is set at the intersection of the extension of the thickness centerline of the upper first flat plate section 11f1 and the extension of the thickness centerline of the upper second flat plate section 11f2. Point Q is set at the intersection of the extension of the thickness centerline of the upper second flat plate section 11f2 and the extension of the thickness centerline of the upper third flat plate section 11f3. Point R is set at the intersection of the extension of the thickness centerline of the upper third flat plate section 11f3 and the extension of the thickness centerline of the upper fourth flat plate section 11f4. These points O, P, Q, and R are sometimes referred to as the vertices of the corner section 11c.

[0035] Axial yield strain occurs in the upper rectangular steel pipe 11 in the region that overlaps with the triangles defined by points A-B-J, B-C-G, D-E-H, and E-F-I, respectively.

[0036] Let x (mm) be the distance from point P to point G, the distance from point Q to point H, the distance from point R to point I, and the distance from point O to point J. The distance x is the radius of curvature R of the plate thickness center at the corner 11c of the upper rectangular steel pipe 11. cu It may be smaller than (mm), radius of curvature R cu It may be equal to (mm), and the radius of curvature R cu It may be larger than (mm).

[0037] Let y (mm) be the distance between points B and E and points G and H in the direction in which the lower first flat plate section 12f1, the lower third flat plate section 12f3, the upper first flat plate section 11f1, and the upper third flat plate section 11f3 extend.

[0038] In the method for evaluating the strength of the joint structure 1 according to this embodiment, the bending strength of the joint structure 1 is calculated using the following equations (1) to (14) in such a collapse mechanism.

[0039]

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[0040]

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[0041]

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[0042]

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[0043]

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[0044]

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[0045]

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[0046]

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[0047]

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[0048]

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[0049]

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[0050]

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[0051]

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[0052]

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[0053] Here, D cu is the diameter (mm) of the upper rectangular steel pipe 11, and t cu This is the plate thickness (mm) of the upper rectangular steel pipe 11, and R cu This is the radius of curvature (mm) of the center of the plate thickness at the corner 11c of the upper rectangular steel pipe 11. The diameter D of the upper rectangular steel pipe 11. cu This is the distance between the centers of the plate thickness.

[0054] D cl This is the diameter (mm) of the lower square steel pipe 12, and t cl This is the plate thickness (mm) of the lower square steel pipe 12, R cl This is the radius of curvature (mm) at the center of the plate thickness at the corner 12c of the lower rectangular steel pipe 12. The diameter D of the lower rectangular steel pipe 12. cl This is the distance between the centers of the plate thickness.

[0055] t d This is the thickness of the diaphragm (mm).

[0056] fu σ y The yield stress (N / mm²) of the flat plate portion 11f of the upper rectangular steel pipe 11 is 2 ) and fl σ y The yield stress (N / mm²) of the flat plate portion 12f of the lower rectangular steel pipe 12 is 2 ) and d σ yThe yield stress (N / mm²) of the through diaphragm 13 is 2 ) The yield stress of the flat plate sections 11f, 12f and the through diaphragm 13 may be calculated using the F value, which is the material standard. For example, if it is JIS G3136 SN490 steel or cold press-formed square steel pipe BCP325 made from the same steel (Japan Iron and Steel Federation Product Standard MDCR0003-2017, etc.), then its F value is 325 (N / mm²). 2 )

[0057] cu σ y The yield stress (N / mm²) of the corner 11c of the upper square steel pipe 11 is 2 ) and cl σ y The yield stress (N / mm²) of the corner 12c of the lower square steel pipe 12 is 2 The yield stress of the corners 11c and 12c is calculated by setting an appropriate corner strength increase rate, which is the ratio of the yield stress of the corners 11c and 12c to the yield stress of the flat sections 11f and 12f, and expressing it as the product of the yield stress of the flat sections 11f and 12f and the corner strength increase rate. The yield stress of the corners 11c and 12c in the actual upper square steel pipe 11 and lower square steel pipe 12 is estimated by tensile testing (for example, JIS Z2241:2011 No. 12B full-thickness tensile test specimen (an arc-shaped test specimen is taken from the corner) or No. 4 round bar tensile test specimen (a bar-shaped test specimen is taken with the axis at the position of t / 4 of the plate thickness of the corner)) or by the indentation method. Strength estimation using the indentation method follows the method described in "ISO / TR 29381 Metallic materials - Measurement of mechanical properties by an instrumented indentation test - indentation tensile properties".

[0058] δ is the axial yield displacement (mm), F δ is the axial yield displacement (mm) at the flat plate portion 11f of the upper rectangular steel pipe 11. C δ is the axial yield displacement (mm) at the corner 11c of the upper rectangular steel pipe 11, and N is the axial force (N).

[0059] Equation (1) is the bending strength of joint structure 1, calculated using yield line theory (equilibrium of internal and external work). j M u This is an expression that represents [something].

[0060] Equation (2) is the total plastic moment per unit length in the through diaphragm 13. D m P This is the formula that represents the total plastic moment per unit length in the through diaphragm 13 obtained by formula (2). D m P This is used in equation (1).

[0061] Equation (3) is the full plastic moment per unit length at the yield line on the flat plate portion 12f of the lower square steel pipe 12. Fx m P This is the equation that represents the total plastic moment per unit length in the through diaphragm 13 obtained in equation (2). D m P And, the full plastic moment per unit length in the flat plate portion 12f of the lower rectangular steel pipe 12 Fcl m p The smaller of the two is the full plastic moment per unit length at the yield line on the flat plate portion 12f of the lower rectangular steel pipe 12. Fx m P This indicates that it is used as follows. This is because a yield line is formed in the lower rectangular steel pipe 12 when the yield strength of the lower rectangular steel pipe 12 is lower than the yield strength of the through diaphragm 13 (see Figure 6), and a yield line is formed in the through diaphragm 13 when the yield strength of the through diaphragm 13 is lower than the yield strength of the lower rectangular steel pipe 12 (see Figure 7). The full plastic moment per unit length at the yield line on the flat plate portion 12f of the lower rectangular steel pipe 12 obtained by equation (3) Fx m P This is used in equation (1).

[0062] Equation (4) is the full plastic moment per unit length at the yield line on the corner 12c of the lower square steel pipe 12. Cx m PThis is the equation that represents the total plastic moment per unit length in the through diaphragm 13 obtained in equation (2). D m P And, the full plastic moment per unit length at the corner 12c of the lower square steel pipe 12 Ccl m p The smaller of the two is the full plastic moment per unit length at the yield line on the corner 12c of the lower square steel pipe 12. Cx m P This indicates that it is used as follows. This is because, as described above, a yield line is formed in the lower square steel pipe 12 when the yield strength of the lower square steel pipe 12 is lower than the yield strength of the through diaphragm 13 (see Figure 6), and a yield line is formed in the through diaphragm 13 when the yield strength of the through diaphragm 13 is lower than the yield strength of the lower square steel pipe 12 (see Figure 7). The full plastic moment per unit length at the yield line on the corner 12c of the lower square steel pipe 12 obtained by equation (4) Cx m P This is used in equation (1).

[0063] Equation (5) is the full plastic moment per unit length in the flat plate portion 12f of the lower square steel pipe 12. Fcl m p This is the formula that represents the full plastic moment per unit length in the flat plate portion 12f of the lower rectangular steel pipe 12 obtained by formula (5). Fcl m p This is used in equation (3).

[0064] Equation (6) is the full plastic moment per unit length at the corner 12c of the lower square steel pipe 12. Ccl m p This is the formula that represents the yield stress of the corner 12c of the lower square steel pipe 12. The calculation of formula (6) involves the yield stress of the corner 12c of the lower square steel pipe 12. cl σ y The following is used: The full plastic moment per unit length at the corner 12c of the lower square steel pipe 12 obtained by equation (6) Ccl m p This is used in equation (4).

[0065] Equation (7) shows the yield axial force per unit length in the flat plate portion 11f of the upper rectangular steel pipe 11. Fcun y is an expression. The yield axial force per unit length in the flat plate portion 11f of the upper square steel pipe 11 obtained in Equation (7) Fcu n y is used in Equation (1).

[0066] Equation (8) is an expression representing the yield axial force per unit length at the corner 11c of the upper square steel pipe 11 Ccu n y is an expression. The yield axial force per unit length at the corner 11c of the upper square steel pipe 11 obtained in Equation (8) Ccu n y is used in Equation (1). In the collapse mechanism where axial yield strain occurs, the position where axial yield strain occurs is at the steel pipe corner. Therefore, for the calculation of this internal force work, the yield axial force per unit length at the corner 11c of the upper square steel pipe 11 Ccu n y as well as the yield axial force per unit length in the flat plate portion 11f of the upper square steel pipe 11 Fcu n y need to be referred to together. This is reflected in the fourth and fifth terms of Equation (1). That is, Equation (8) and the fourth and fifth terms of Equation (1) correspond to calculating the flexural strength of the joint structure considering the strength difference between the flat plate portion 11f and the corner 11c of the upper square steel pipe 11 when calculating the internal force work in the upper bound theorem assuming the axial yield region and the yield line.

[0067] Equation (9) is an expression representing the amount of misalignment l between the upper square steel pipe 11 and the lower square steel pipe 12 in the middle column form (the form in which the axial centers of the upper square steel pipe 11 and the lower square steel pipe 12 coincide). The amount of misalignment l obtained in Equation (9) is used in Equation (1).

[0068] Equation (10) is an expression representing the ratio of the internal force work in the region of the corner 11c of the upper square steel pipe 11 ( Ccu N y the region to be referred to) among the internal force work in the axial yield strain region. The ratio obtained in Equation (10) is used in Equation (1).

[0069] Equation (11) is for the internal force work in the region of the flat plate portion 11f of the upper square steel pipe 11 ( Fcu Ny It is an expression representing the ratio of internal force work in the region to be referred to). The ratio obtained by Equation (11) is used in Equation (1).

[0070] Equation (12) is an expression representing the side length L1 of the region assuming the strength increase at the corner in the axial yield strain region. Considering the axial yield strain region where the triangle defined by points A - B - J overlaps with the upper square steel pipe 11, the side length L1 is the side length of the region assuming the strength increase of the upper first corner 11c1. Equation (12) represents using the smaller of the distance x from point O to point J, which is one of the side lengths of the axial yield strain region, and the radius of curvature R of the center of the plate thickness of the upper first corner 11c1 cu as the distance L1. This means that if the side length of the axial yield strain region is less than or equal to the radius of curvature R cu then the distance L1 is equal to the side length of the axial yield strain region, but if it exceeds the radius of curvature R cu the corner region within the axial yield region does not increase and the distance L1 is fixed by the radius of curvature R cu This is because of this. In the axial yield strain region related to the triangle defined by points B - C - G, the distance L1 is the distance from point P to point G. In the axial yield strain region related to the triangle defined by points D - E - H, the distance L1 is the distance from point Q to point H. In the axial yield strain region related to the triangle defined by points E - F - I, the distance L1 is the distance from point R to point I. The distance L1 obtained by Equation (12) is used in Equation (10).

[0071] Equation (13) is an expression representing another side length L2 of the region assuming the strength at the corner in the axial yield strain region. Considering the axial yield strain region where the triangle defined by points A - B - J overlaps with the upper square steel pipe 11, the side length L2 is another side length of the region assuming the strength increase of the upper first corner 11c1. Equation (13) represents using the smaller of the distance x (D cu - y) / (l + x) from point O to point K, which is one of the side lengths of the axial yield strain region, and the radius of curvature R of the center of the plate thickness of the upper first corner 11c1 cu as the distance L2. This means that if the side length of the axial yield strain region is less than or equal to the radius of curvature R cu then the distance L2 is equal to the side length of the axial yield strain region, but if the radius of curvature R cuIf it exceeds this value, the corner region within the axial yield region does not increase, and the distance L2 is equal to the radius of curvature R. cu This is because it is fixed at that point. Furthermore, the side length x(D) of the axial yield strain region can be calculated using similar triangles. cu The equation -y) / (l+x) is obtained. Also, in the axial yield strain region related to the triangle defined by points E, F, and I, the distance L2 is the distance from point R to point N. The distance L2 obtained in equation (13) is used in equation (10).

[0072] Equation (14) represents another side length L3 of the region within the yield strain area where the strength of the corner is assumed. Considering the axial yield strain region where the triangle defined by points B-C-G overlaps with the upper rectangular steel pipe 11, side length L3 is another side length of the region where the strength increase of the upper second corner 11c2 is assumed. Equation (14) is given by xy / (l+x), which is one of the side lengths of the axial yield strain region, from point P to point L, and R, which is the radius of curvature of the plate thickness center of the upper second corner 11c2. cu This indicates that the smaller of the two values ​​is used as the distance L3. This means that the side length of the axial yield strain region is equal to the radius of curvature R. cu If the following conditions are met, the distance L3 will be equal to the side length of the axial yield strain region, but the radius of curvature R cu If it exceeds this value, the corner region within the axial yield region does not increase, and the distance L3 is equal to the radius of curvature R. cu This is because it is fixed at [the specified point]. Furthermore, the side length xy / (l+x) of the axial yield strain region can be obtained from calculations using similar triangles. In addition, in the axial yield strain region related to the triangle defined by points D-E-H, the distance L3 is the distance from point Q to point M. The distance L3 obtained in equation (14) is used in equation (10).

[0073] Equations (1) to (14) include the following assumptions. The central column type is such that the axes of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 coincide. This study focuses on the collapse mechanism in which a yield line and an axial yield strain region of the upper rectangular steel pipe 11 occur, as shown in Figures 4, 6, and 7. The bending strength of joint structure 1 was calculated from the balance of internal and external force work in the yield line and axial yield strain region, among the assumed collapse mechanisms. The minimum bending strength is calculated by changing the positions of the points set on the flat plate sections 11f and 12f of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12, respectively, within the apex of the yield line (i.e., changing x and y). The stress state of the cross-section at the yield line is assumed to be fully plastic. As shown in Figure 5, the radius of curvature R of the corners 11c and 12c of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12. cu ,R cl Without considering this, assuming that the flat sections 11f and 12f of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 are perpendicular to each other, and assuming that the strength of the corners 11c and 12c is increased relative to the flat sections 11f and 12f, the range of corners 11c and 12c is the distance R from the vertices (points O, P, Q, and R) of the corners 11c and 12c. cu or distance R cl The range is limited to this. The increase in strength of the corner sections 11c and 12c relative to the flat sections 11f and 12f will affect the internal force work due to axial yielding of the upper square steel pipe 11 and the internal force work at the yield line generated in the lower square steel pipe 12 or through diaphragm 13.

[0074] The internal force work in the axial yield strain region of the upper rectangular steel pipe 11 is distributed to the flat section 11f and the corner section 11c in the following manner. Figure 8 shows the axial yield displacement δ of the corner section 11c and the flat section 11f near the corner section 11c of the upper rectangular steel pipe 11. From the area ratio of the similar figures in Figure 8, it is calculated as follows: L 1-3 The notation refers to the distances L1 to L3 in equations (12) to (14). Internal force work on the flat section : Internal force work on the corner section = (L 1-3 -R cu ) 2 :L 1-3 2 -(L 1-3 -R cu ) 2 Note that point V in Figure 8 represents points O to R in Figure 4, and point W in Figure 8 represents points G to N in Figure 4.

[0075] In Figure 4, the yield line on the lower rectangular steel pipe 12 or the through diaphragm 13 is formed on the lower rectangular steel pipe 12 when the yield strength of the lower rectangular steel pipe 12 is lower than the yield strength of the through diaphragm 13 (see Figure 6), and on the through diaphragm 13 when the yield strength of the through diaphragm 13 is lower than the yield strength of the lower rectangular steel pipe 12 (see Figure 7). When the yield line is formed on the through diaphragm 13, it is not affected by the increase in strength of the corner portion 12c relative to the flat portion 12f of the lower rectangular steel pipe 12. However, when the yield line is formed on the lower rectangular steel pipe 12, it is necessary to consider the effect of the increase in strength of the corner portion 12c relative to the flat portion 12f of the lower rectangular steel pipe 12. Of this yield line, the ratio of the side length for which the effect of the corner portion 12c should be considered is 2R cl / D cl Therefore, the ratio of the side length without considering the effect of corner 12c is (D cl -2R cl ) / D cl That is the case.

[0076] When performing the load-bearing capacity evaluation calculation, the radius of curvature R of the corners 11c and 12c of the upper rectangular steel pipe 11 and the lower rectangular steel pipe 12 is used. cu ,R cl This may be taken into consideration.

[0077] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention. [Examples]

[0078] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0079] The inventors conducted a simulation test of the joint structure 1 using a general-purpose finite element analysis (FEA) program. An analysis model, roughly shown in Figures 9 and 10, was created, and the relationship between the moment and member angle at the joint 23 was determined. The boundary conditions of the analysis model were pin roller support at the upper end and a fixed end at the lower end. A displacement incremental analysis was performed, applying a unidirectional forced displacement (200 mm) in the horizontal direction while maintaining an axial compressive force at the upper end support point with an axial force ratio of 0.3. The nominal stress-nominal strain relationship of the material was assumed to be of the perfect elastoplastic type as shown in Figure 11, and this was converted to a true stress-true strain relationship for use in the analysis. The nominal yield strength of the flat plate sections 11f and 12f was 325 (N / mm²). 2 ) and the elastic modulus is 205,000 (N / mm²). 2 ) and, for the sake of simplifying the analysis, the material properties of the weld metal and backing plate were set to be the same as those of the flat plate sections 11f and 12f. Yield strength of the corner 11c of the upper square steel pipe 11 Cu σ y and the yield strength of the corner portion 12c of the lower square steel pipe 12 Cl σ y Regarding the yield strength of each flat plate section 11f, 12f, Fu σ y , Fl σ y It is expressed as the product of the angle strength increase rate α. Cu σ y =α· Fu σ y , Cl σ y =α· Fl σ y The analytical variable was defined as α. Table 1 shows the various conditions of the analytical model.

[0080] [Table 1]

[0081] [Table 2]

[0082] Figure 12 shows the relationship between moment M and member angle θ obtained from the analysis. Here, moment M represents the moment at the top surface of the through diaphragm 13, obtained by multiplying the horizontal reaction force (horizontal load) at the loading point by the distance to the top surface of the through diaphragm 13 (2000 mm), and member angle θ represents the member angle obtained by dividing the horizontal displacement (0 to 200 mm) at the loading point by the distance to the top surface of the through diaphragm 13 (2000 mm).

[0083] Table 2 shows the analysis results and calculated values. In the analysis results column of Table 2, the maximum strength is the maximum value of moment M, and the yield strength is calculated using the general yield method (in the M-θ relationship in Figure 12, the secant stiffness at 1 / 5 of the maximum analysis M is used as the initial stiffness S, as shown in Figure 13). ini The moment M when the initial stiffness becomes 2 / 3 is... y This was determined by a method that uses yield strength. In the calculated values ​​column of Table 2, the conventional formula is the bending strength of joint structure 1 determined by equation (15) described later, which does not consider corner strength, the proposed formula is the bending strength of joint structure 1 determined by equation (1) described later, which considers corner strength, and the full plastic strength of the upper square steel pipe 11 is the full plastic moment of the cross section considering corner strength.

[0084] In the analysis conducted this time, the maximum load-bearing capacity was found to be lower than the full plastic load-bearing capacity of the upper rectangular steel pipe 11, indicating that the collapse of the diaphragm at the joint 23 is the determining factor for the load-bearing capacity. Therefore, we confirmed that this analysis result is appropriate for verifying the validity of the proposed formula for estimating the load-bearing capacity due to the collapse of the diaphragm.

[0085] The yield strength was used as the analytical result for comparison with the strength evaluation result (the calculated bending strength of joint structure 1). Table 3 shows the evaluation accuracy and the standard deviation of the evaluation accuracy when compared with the analytical yield strength for the bending strength of joint structure 1 evaluated using the conventional formula and the bending strength of joint structure 1 evaluated using the proposed formula of the present invention.

[0086] [Table 3]

[0087] In Table 3, the "Conventional Strength Evaluation Accuracy" is a value calculated by (Conventional Strength Evaluation Formula) ÷ (Yield Strength) × 100 (%), and the "Average Accuracy" in the Conventional Evaluation column is the average of the "Conventional Strength Evaluation Accuracy" values ​​for No. 1 and 2. Similarly, the "Proposed Strength Evaluation Accuracy" is a value calculated by (Proposed Strength Evaluation Formula) ÷ (Yield Strength) × 100 (%), and the "Average Accuracy" in the Proposed Evaluation column is the average of the "Proposed Strength Evaluation Accuracy" values ​​for No. 1 and 2. As can be seen from the results in Table 3, it was confirmed that using the proposed evaluation formula brings the average accuracy closer to 100% and reduces the standard deviation. This signifies an improvement in prediction accuracy. Based on the above, it has been confirmed that when evaluating a joint type with different widths for an upper square steel pipe 11 and a lower square steel pipe 12 that are formed by cold working and in which the strength of the corners 11c and 12c is higher than the strength of the flat sections 11f and 12f, the design formula proposed in this study can be used to bring the design strength closer to the actual strength.

[0088] The calculation method for the evaluation formula proposed here is shown in equations (1) to (14). Equation (1) shows an example of a calculation method for joint strength that takes into account the corner strength increase rate.

[0089]

number

[0090] Equation (1) D m p This was calculated based on the following equation (2). In equation (2), d σ y is the yield stress of the through diaphragm 13, and td is the plate thickness of the through diaphragm 13.

[0091]

number

[0092] Equation (1) Fs m p This was calculated based on the following equation (3). In equation (3),D m p This is the total plastic moment per unit length in the through diaphragm 13, Fcl m p This is the full plastic moment per unit length at the flat plate portion 12f of the lower rectangular steel pipe 12.

[0093]

number

[0094] Equation (1) Cs m p This was calculated based on the following equation (4). In equation (4), D m p This is the total plastic moment per unit length in the through diaphragm 13, Ccl m p This is the full plastic moment per unit length at the corner 12c of the lower rectangular steel pipe 12.

[0095]

number

[0096] Equation (3) Fcl m p This was calculated based on the following equation (5). In equation (5), fl σ y is the yield stress of the flat plate portion 12f of the lower rectangular steel pipe 12, and t cl This is the plate thickness of the lower rectangular steel pipe 12.

[0097]

number

[0098] Equation (4) Ccl m p This was calculated based on the following equation (6). In equation (6), cl σ y is the yield stress of the corner 12c of the lower rectangular steel pipe 12, and t cl This is the plate thickness of the lower rectangular steel pipe 12.

[0099]

number

[0100] Equation (1) Fcu n y This was calculated based on the following equation (7). In equation (7), fu σ y is the yield stress of the flat plate portion 11f of the upper rectangular steel pipe 11, and t cu This is the plate thickness of the upper rectangular steel pipe 11.

[0101]

number

[0102] Equation (1) Ccu n y This was calculated based on the following equation (8). In equation (8), cu σ y is the yield stress of the flat plate portion 11f of the upper rectangular steel pipe 11, and t cu This is the plate thickness of the upper rectangular steel pipe 11.

[0103]

number

[0104] The value of l in equation (1) was calculated based on equation (9) below. In equation (9), D cl D is the diameter of the lower square steel pipe 12, cu This is the diameter of the upper rectangular steel pipe 11.

[0105]

number

[0106] ∫ in equation (1) c δ / ∫δ represents the ratio of the range in which the increase in strength at the corners is assumed to be relative to the volume of the axial yield strain region, and was calculated based on the following equation (10). In equation (9), Rcu This is the radius of curvature at the center of the plate thickness of the corner 11c of the upper rectangular steel pipe 11.

[0107]

number

[0108] ∫ in equation (1) F δ / ∫δ represents the ratio of the flat plate portion to the axial yield strain region volume, and since it is the ratio excluding the region where the increase in strength at the corners is not assumed, it was calculated based on the following equation (11).

[0109]

number

[0110] L1 is a portion of the side length within the range where an increase in corner strength is assumed relative to the axial yield strain region, and is the length from point O to point J, point P to point G, point Q to point H, point R to point I, or the radius of curvature R of the corner 11c of the upper square steel pipe 11. cu This was calculated based on equation (12).

[0111]

number

[0112] L2 is a portion of the side length in the range where an increase in strength at the corner is assumed with respect to the axial yield strain region, and is the length from point O to point K, point R to point N, or the radius of curvature R of the corner 11c of the upper rectangular steel pipe 11. cu This was calculated based on equation (13).

[0113]

number

[0114] L3 is a portion of the side length within the range where an increase in corner strength is assumed relative to the axial yield strain region, and is the length between points P and L, Q and M, or the radius of curvature R of the corner 11c of the upper square steel pipe 11. cuThis was calculated based on equation (14).

[0115]

number

[0116] Here, as a point of comparison, we present an example of joint strength evaluation using a conventional design method. Conventionally, calculations were performed based on equation (15) under the condition that the corners had the same strength as the flat sections in all regions.

[0117]

number

[0118] The first and sixth terms of equation (1) represent the internal force work at the yield line generated in the through diaphragm 13 and the external force work due to the axial force, respectively, and both take the same form in equation (15).

[0119] The second and third terms of equation (1), and the second term of equation (15), represent the internal force work of the yield line formed at the bonding position between the lower rectangular steel pipe 12 and the through diaphragm 13. In equation (1), the reference formula for the full plastic moment per unit length is divided into the flat plate portion 12f and the corner portion 12c, depending on the side length within the range where it is assumed that the corner portion 12c has the same strength as the flat plate portion 12f (terms 2 and 3). On the other hand, in equation (15), the calculation is performed by referring to the full plastic moment per unit length of the flat plate portion 12f without such division.

[0120] Terms 4 and 5 of equation (1), and term 3 of equation (15), represent the internal force work due to axial yield strain in the upper rectangular steel pipe 11. In equation (1), the reference formula for the yield axial force per unit length is divided into the flat section 11f and the corner section 11c according to the proportion of the range in which the strength increase of the corner section 11c included in the axial yield region is assumed (terms 4 and 5). On the other hand, in equation (15), the calculation is performed by referring to the yield axial force per unit length of the flat section 11f without such division. [Explanation of symbols]

[0121] 1: Joint structure 11: Upper square steel pipe 11c: Corner part 11c1: Upper first corner part 11c2: Upper second corner part 11c3: Upper third corner part 11c4: Upper fourth corner part 11f: Flat plate part 11f1: Upper first flat plate part 11f2: Upper second flat plate part 11f3: Upper third flat plate part 11f4: Upper fourth flat plate part 12: Lower square steel pipe 12c: Corner part 12c1: Lower first corner part 12c2: Lower second corner part 12c3: Lower third corner part 12c4: Lower fourth corner part 12f: Flat plate part 12f1: Lower first flat plate part 12f2: Lower second flat plate part 12f3: Lower third flat plate part 12f4: Lower fourth flat plate part 13: Through diaphragm

Claims

1. A method for evaluating the load-bearing capacity of a joint structure in which an upper rectangular steel pipe and a lower rectangular steel pipe are joined via a through diaphragm, The diameter of the upper rectangular steel pipe is smaller than the diameter of the lower rectangular steel pipe. The upper and lower rectangular steel pipes each have a flat section and a corner section with higher strength than the flat section. This includes calculating the bending strength of the joint structure, taking into account the strength of the corners of the upper and lower rectangular steel pipes. A method for evaluating the strength of a joint structure.

2. Using the upper bound theorem assuming a yield curve, The calculation of internal force work in the upper bound theorem includes using the smaller of the following two values ​​to calculate the bending strength of the joint structure: the full plastic moment per unit length of the lower rectangular steel pipe calculated using the yield stress of the corner of the lower rectangular steel pipe, and the full plastic moment per unit length of the through diaphragm. A method for evaluating the load-bearing capacity of a joint structure according to claim 1.

3. Using the upper bound theorem assuming an axial yield region and yield line, The calculation of internal force work in the upper bound theorem includes considering the difference in strength between the flat portion and the corner portion of the upper rectangular steel pipe when calculating the bending strength of the joint structure. A method for evaluating the load-bearing capacity of a joint structure according to claim 1 or claim 2.

4. The upper rectangular steel pipe and the lower rectangular steel pipe are formed by cold working. The corner portion of the lower rectangular steel pipe includes the lower first corner portion, the lower second corner portion, the lower third corner portion, and the lower fourth corner portion. The flat plate portion of the lower rectangular steel pipe includes a lower first flat plate portion extending between the lower first corner and the lower second corner, a lower second flat plate portion extending between the lower second corner and the lower third corner, a lower third flat plate portion extending between the lower third corner and the lower fourth corner, and a lower fourth flat plate portion extending between the lower fourth corner and the lower first corner. The corner portion of the upper rectangular steel pipe includes the upper first corner portion closest to the lower first corner portion, the upper second corner portion closest to the lower second corner portion, the upper third corner portion closest to the lower third corner portion, and the upper fourth corner portion closest to the lower fourth corner portion. The flat plate portion of the upper rectangular steel pipe includes an upper first flat plate portion extending between the upper first corner and the upper second corner, an upper second flat plate portion extending between the upper second corner and the upper third corner, an upper third flat plate portion extending between the upper third corner and the upper fourth corner, and an upper fourth flat plate portion extending between the upper fourth corner and the upper first corner. Point A is set at the center of the thickness of the lower first corner, point B is set at the center of the thickness of the lower first flat section, point C is set at the center of the thickness of the lower second corner, point D is set at the center of the thickness of the lower third corner, point E is set at the center of the thickness of the lower third flat section, and point F is set at the center of the thickness of the lower fourth corner. Point G is set at the thickness center of the upper second flat plate portion or the thickness center of the upper second corner portion, point H is set at the thickness center of the upper second flat plate portion or the thickness center of the upper third corner portion, point I is set at the thickness center of the upper fourth flat plate portion or the thickness center of the upper fourth corner portion, and point J is set at the thickness center of the fourth flat plate portion 11f4 or the thickness center of the upper first corner portion. Yield lines extend between each of the following points: A-B, B-C, C-D, D-E, E-F, F-A, G-B, G-C, G-H, H-D, H-E, I-E, I-F, I-J, J-A, and J-B. Point O is set at the intersection of the extension of the thickness centerline of the upper fourth flat plate portion and the extension of the thickness centerline of the upper first flat plate portion, point P is set at the intersection of the extension of the thickness centerline of the upper first flat plate portion and the extension of the thickness centerline of the upper second flat plate portion, point Q is set at the intersection of the extension of the thickness centerline of the upper second flat plate portion and the extension of the thickness centerline of the upper third flat plate portion, and point R is set at the intersection of the extension of the thickness centerline of the upper third flat plate portion and the extension of the thickness centerline of the upper fourth flat plate portion. In the region overlapping with the triangles defined by points A-B-J, B-C-G, D-E-H, and E-F-I, axial yield strain occurs in the upper rectangular steel pipe. Let x (mm) be the distance from point P to point G, the distance from point Q to point H, the distance from point R to point I, and the distance from point O to point J. Let y (mm) be the distance between points B and E and points G and H in the direction in which the lower first flat plate portion, the lower third flat plate portion, the upper first flat plate portion, and the upper third flat plate portion extend. Assuming a collapse mechanism, The bending strength of the joint structure is calculated using the following equations (1) to (14): A method for evaluating the load-bearing capacity of a joint structure according to claim 1 or 2. [Math 1] [Math 2] [Math 3] [Math 4] [Math 5] [Math 6] 【Number 7】 [Number 8] [Number 9] [Number 10] [Math 11] [Math 12] [Number 13] [Number 14] Here, D cu is the diameter (mm) of the upper rectangular steel pipe, and t cu This is the plate thickness (mm) of the upper rectangular steel pipe, R cu This is the radius of curvature (mm) at the center of the plate thickness at the corner of the upper rectangular steel pipe. D cl is the diameter (mm) of the lower square steel pipe, and t cl This is the plate thickness (mm) of the lower square steel pipe, R cl This is the radius of curvature (mm) at the center of the plate thickness at the corner of the lower square steel pipe. t d This is the thickness of the diaphragm (mm), fu σ y is the yield stress (N / mm 2 ) of the flat part of the upper square steel pipe, fl σ y is the yield stress (N / mm 2 ) of the flat part of the lower square steel pipe, d σ y is the yield stress (N / mm 2 ) of the through diaphragm, cu σ y This is the yield stress (N / mm²) at the corner of the upper square steel pipe. 2 ) and cl σ y The yield stress (N / mm²) of the corner of the lower square steel pipe is the yield stress of the corner of the lower square steel pipe. 2 ) and δ is the axial yield displacement (mm), F δ is the axial yield displacement (mm) in the flat section of the upper rectangular steel pipe. C δ is the axial yield displacement (mm) at the corner of the upper rectangular steel pipe, and N is the axial force (N).

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