Column-beam joint

Abstract

To expand conditions for undermatching welding for a column-beam joint where a column is composed of square steel pipes to prevent a seam welding portion from preceding yielding to a base metal by limiting the relation between a surface where the seam welding portion is formed in the column and a direction in which the beam is attached.SOLUTION: In a column-beam joint, a column is composed of square steel pipes having a rounded rectangular cross section including four edges and four corners, and a first beam and a second beam are attached to only two surfaces constituted by opposite edges of the square steel pipe. A seam welding portion extending in a longitudinal direction of the square steel pipe is formed on one or both of the edge to which the first beam is attached and the edge to which the second beam is attached. Tensile strength σu,WM[N / mm2] of the seam welding portion and the tensile strength σu,BM[N / mm2] of the base material of the edge satisfy the conditions with respect to a thickness t[mm] and an outer diameter D[mm] of the square steel pipe.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] The present invention relates to a column-beam joint.

Background Art

[0002] When welding steel members, in order not to cause premature failure of the welded part with respect to the base material, generally, overmatching is performed in which the tensile strength of the weld metal is made greater than the tensile strength of the base material. For example, for a base material of tensile strength grade 490 N / mm 2 a weld metal material and welding conditions exceeding 490 N / mm 2 in tensile strength are selected. However, in the welding of high-strength weld metal materials selected by overmatching, preheating is required to prevent weld cracking and ensure the strength and toughness of the weld metal, and the upper limits of the heat input and interpass temperature are strict, resulting in an increase in the labor involved in welding construction, an increase in cost, and a decrease in production efficiency.

[0003] Problems due to overmatching of the weld metal as described above can also occur in the seam weld of cold-formed square steel pipes used for columns in building steel structures. In recent years, the steel types of square steel pipes have diversified, and square steel pipes of multiple steel types are manufactured on the same production line. In such a case, if the weld metal material and welding conditions are set so as to achieve overmatching for each steel type, it is necessary to replace the weld metal material and change the welding conditions of the seam weld each time the steel type of the manufactured square steel pipe changes, which is a factor in reducing productivity.

[0004] On the other hand, for example, Patent Document 1 discloses a technique that enables undermatching welding, that is, welding in which the tensile strength of the weld metal is smaller than the tensile strength of the base material, for the welded part between a square steel pipe column and a continuous diaphragm.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] In the case of the seam welded portion of the cold-formed square steel pipe, since it receives a shearing force in the direction along the welding line direction, in the case of under-matching welding, the welded metal material of the seam welded portion may shear yield and plastically deform prior to the base material. When such plastic deformation occurs, the assumption of plane retention of the cross-section of the square steel pipe is no longer valid, and large deformation occurs with a bending moment smaller than the bending strength assumed based on the plane retention assumption at the time of design, resulting in excessive interlayer deformation. Since the under-matching welding in Patent Document 1 is applied to the welded portion that receives a tensile force in the direction orthogonal to the welding line direction, the technique of the under-matching welding described in Patent Document 1 cannot be directly applied to the seam welded portion of the square steel pipe as described above.

[0007] Therefore, the present invention aims to provide a more practical column-beam joint in which a column is formed of a square steel pipe with an under-matching weld for the seam welded portion by limiting the relationship between the surface on which the seam welded portion is formed in the column and the attachment direction of the beam for the column-beam joint formed of a square steel pipe, thereby expanding the conditions for under-matching welding so that the seam welded portion does not yield prior to the base material.

MEANS FOR SOLVING THE PROBLEMS

[0008] [1] A column-beam joint in which a column is formed of a square steel pipe having a rounded rectangular cross-section including four side portions and four corner portions, and first and second beams are attached only to two surfaces formed by the opposing side portions of the square steel pipe, wherein a seam welded portion extending in the longitudinal direction of the square steel pipe is formed only on either one or both of the side portions to which the first beam is attached or the side portions to which the second beam is attached, and the tensile strength σ u,WM [N / mm 2 of the seam welded portion and the tensile strength σ of the base material of the side portionu,BM [N / mm 2 satisfies the formulas (i) and (ii). The column-beam joint TIFF2025105538000002.tif17170[2] A column-beam joint in which a column is composed of a square steel pipe having a rounded rectangular cross-section including four side portions and four corner portions, and a beam is attached only to one surface formed by the side portions of the square steel pipe. A seam weld extending in the longitudinal direction of the square steel pipe is formed only on either one or both of the side portion to which the beam is attached or the side portion opposite to the beam. With respect to the plate thickness t [mm] and the outer diameter D [mm] of the square steel pipe, the tensile strength σ u,WM [N / mm 2 and the tensile strength σ of the base material of the side portion u,BM [N / mm 2 satisfies the formulas (i) and (ii). The column-beam joint TIFF2025105538000003.tif17170[3] The beam is an H-shaped steel beam including a pair of flanges and a web. The square steel pipe is divided at positions corresponding to the pair of flanges, and a pair of diaphragms are inserted at respective positions. The pair of flanges are joined to the end faces of the pair of diaphragms. The seam weld is not formed, or a seam weld that does not satisfy formula (ii) is formed, in any one or two of the portion above the pair of diaphragms of the square steel pipe, the portion between the pair of diaphragms, and the portion below the pair of diaphragms. The column-beam joint according to [1] or [2].

Effect of the Invention

[0009] According to the above configuration, since the seam welded portion of the square steel pipe is located on the flange side of the column, the shear force acting on the seam welded portion can be suppressed. Further, the magnitude relationship between the strain of the base metal corner of the square steel pipe where strain concentration occurs in terms of shape and the strain of the seam weld where strain concentration occurs in terms of strength due to under-matching is reversed from the magnitude relationship in these aspects in the conventional over-matching weld, and the seam weld can be made under-matching within a range where the reversal does not occur. Due to these effects, it is possible to achieve an under-matching weld in which the seam welded portion does not yield prior to the base metal of the square steel pipe at the column-beam joint.

Brief Description of the Drawings

[0010]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 1E

Figure 1F

Figure 1G

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11A

Figure 11B

Figure 11C

Embodiments for Carrying out the Invention

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions are omitted.

[0012] Figures 1A to 1G are diagrams showing an example of a column-beam joint according to an embodiment of the present invention. In the illustrated example, the column-beam joint includes a square steel pipe 1 column constituting the column and an H-shaped steel beam 2. The H-shaped steel beam 2 includes flanges 21, 22 and a web 23. The square steel pipe 1 is divided at positions corresponding to the flanges 21, 22 of the H-shaped steel beam 2, and diaphragms 31, 32 are inserted into their respective positions, and the flanges 21, 22 of the H-shaped steel beam 2 are welded to the end faces of the diaphragms 31, 32. In this case, the square steel pipe 1 is divided into a portion 1A above the diaphragms 31, 32, a portion 1B between the diaphragms 31, 32, and a portion 1C below the diaphragms 31, 32. The portions 1A, 1B are welded to the diaphragm 31, and the portions 1B, 1C are welded to the diaphragm 32, respectively. The web 23 of the H-shaped steel beam 2 is bolted to a gusset plate 33 welded to the portion 1B of the square steel pipe 1.

[0013] Note that the configuration of the column-beam joint as described above is an example, and various modifications are possible according to known techniques. For example, without dividing the square steel pipe 1, a diaphragm may be attached to its outer peripheral surface by welding or the like. Also, the diaphragm may be attached inside the square steel pipe 1, and the flanges 21, 22 of the H-shaped steel beam 2 may be attached to the outer peripheral surface of the square steel pipe 1 by welding or the like. Further, the web 23 of the H-shaped steel beam 2 may be directly welded to the portion 1B of the square steel pipe 1. The beam is not limited to the H-shaped steel beam, and for example, the beam may be constituted by a steel section of another cross-sectional shape. In FIGS. 1A to 1C, the height (beam depth) of the H-shaped steel beam 2 may be different on both sides of the square steel pipe 1. In this case, an intermediate diaphragm may be provided between the diaphragms 31, 32.

[0014] Figures 2A and 2B are diagrams showing examples of square steel pipes used for the column-beam joints shown in FIGS. 1A to 1G. In the illustrated examples, the square steel pipe 1 has a rounded rectangular cross-section including four side portions 11 and four corner portions 13, and a seam weld portion 15 extending in the longitudinal direction of the square steel pipe 1 (the X direction in the figure) is formed on at least one side portion 11. More specifically, the seam weld portion 15 is formed only on one side portion 11 as in the example of FIG. 2A, or the seam weld portion 15 is formed on two side portions 11 facing each other as in the example of FIG. 2B. Here, The side portion 11 is a straight portion without curvature in the cross-section (Y-Z cross-section in the figure) of the square steel pipe 1, and the corner portion 13 is a corner portion with curvature in the cross-section of the square steel pipe 1.

[0015] The square steel pipe 1 as described above is manufactured, for example, by joining the ends of cold-formed members with the seam weld portion 15. More specifically, the square steel pipe 1 in the example of FIG. 2A is manufactured by joining the ends of a member formed in a C-shaped cross-section with the seam weld portion 15. The square steel pipe 1 in the example of FIG. 2B is manufactured by joining the ends of two members formed in a U-shaped cross-section with the seam weld portion 15.

[0016] As already described, when the seam weld portion 15 in the square steel pipe 1 is subjected to under-matching welding, it is necessary to prevent the weld metal material of the seam weld portion 15 from shearing and yielding prior to the base materials of the side portion 11 and the corner portion 13. For this purpose, in the embodiment of the present invention, the seam weld portion 15 to be under-matching welded is formed only on the side portion of the square steel pipe 1 orthogonal to the material axis direction of the beam when the column-beam joint includes only one beam or two beams arranged opposite to each other with a column therebetween, as in the examples shown in FIGS. 1A to 1G. Thereby, as will be described later, the influence of under-matching welding when a load is applied to the beam is reduced, and under-matching welding becomes possible at a lower matching ratio (the ratio of the tensile strength σ u,BM [N / mm 2 of the base material of the side portion to the tensile strength σ u,WM [N / mm 2 of the weld metal or the seam weld portion).

[0017] More specifically, when the H-shaped steel beam 2 is attached only to two surfaces formed by the opposing side portions 11 of the square steel pipe 1 as shown in FIGS. 1A to 1C, if each H-shaped steel beam 2 is taken as the first beam and the second beam, the seam weld portion 15 that is an under-matching weld is formed only on either one or both of the side portions 11 to which the first beam is attached or the side portions 11 to which the second beam is attached. Further, when the H-shaped steel beam 2 is attached only to one surface formed by the side portion of the square steel pipe 1 as shown in FIGS. 1D to 1G, the seam weld portion 15 that is an under-matching weld is formed only on either one or both of the side portions 11 to which the H-shaped steel beam 2 is attached or the side portions 11 on the side opposite to the H-shaped steel beam 2.

[0018] In addition, in the column-beam joint according to the embodiment of the present invention, as shown in FIG. 3 which is a reference example, a seam weld portion 915 that is an under-matching weld is not formed on the side portion of the square steel pipe 91 that is parallel to the material axis direction of the H-shaped steel beam 92. When there is a column-beam joint as in the example of FIG. 3, the seam weld portion 915 becomes, for example, an over-matching weld or an under-matching weld under conditions different from those of the embodiment of the present invention. Such a column-beam joint and the column-beam joint according to the embodiment of the present invention may be mixed in a building as long as each joint is appropriately designed, and it is not necessarily the case that the embodiment of the present invention must be applied to all column-beam joints in the building.

[0019] When the square steel pipe 1 is divided by the diaphragms 31 and 32 as shown in FIGS. 1A to 1G above, the seam weld portion 15 that becomes an under-matching weld does not necessarily have to be applied to all of the portions 1A, 1B, and 1C of the divided square steel pipe 1. That is, among the portion 1A above the diaphragms 31 and 32, the portion 1B between the diaphragms 31 and 32, and the portion 1C below the diaphragms 31 and 32, a seam weld portion that becomes an under-matching weld is not formed in one or two of them, or a seam weld portion that is not an under-matching weld is formed. The seam weld portion that is not an under-matching weld means that the tensile strength of the base material is less than the tensile strength of the weld metal material and σ u,BM ≧σ u,WM is a seam weld portion that does not satisfy.

[0020] The following describes the results of verifying the conditions for under-matching welding in a seam weld formed only on the side of a rectangular steel pipe orthogonal to the beam's material axis direction using FEA. In FEA, the shear strength and deformation of the rectangular steel pipe under bending shear, as well as the strains of the base metal at the sides and corners and the seam weld, were calculated. The elements of the analysis model were hexahedral high-order elements, and element division was performed such that the element sizes and aspect ratios of the sides, corners, and seam weld of the base metal were approximately equal, considering the element size dependence at the strain and stress concentration parts. Figure 4 shows the boundary conditions in the analysis. In the analysis, reverse symmetric bending shear with a forced displacement of the interlayer deformation angle of 1 / 20 was reproduced in a model of 1 / 2 of the story height considering symmetry conditions with a constant compressive axial force P applied. Figure 5 shows the direction of the external force acting on the cross-section of the rectangular steel pipe in the analysis. The loading angle θ indicating the direction of the external force was set to 90 degrees when the loading direction was parallel to the side including the seam weld, 0 degrees when the loading direction was orthogonal to the side including the seam weld, and 45 degrees or 135 degrees for loading when the loading direction coincided with the diagonal direction of the cross-section. Note that, as described above, in the embodiment of the present invention, the seam weld that becomes under-matching welding is formed only on the side of the rectangular steel pipe orthogonal to the beam's material axis direction, so the loading angle θ = 0°. In each case of the analysis, a 1 / 2 model of the cross-section was used considering the symmetry of the rectangular steel pipe cross-section. Common to each case, the excess height on each of the outer surface side and the inner surface side of the seam weld was set to 5.05 mm on the outer surface side and 3.2 mm on the inner surface side. In this case, the thickness of the seam weld including the excess height (36 + 5.05 + 3.2 = 44.25 mm) is approximately 1.23 times the plate thickness (36 mm) of the rectangular steel pipe. In the case of general welding methods such as GMAW (gas metal arc welding) and SAW (submerged arc welding), the thickness of the seam weld including the excess height e is approximately 1.15 to 1.30 times the plate thickness t of the rectangular steel pipe (e + t ≒ 1.15t to 1.30t), so it was set to e + t ≒ 1.23t, which is approximately the median value in this range. The cross-sectional dimensions of the rectangular steel pipe, story height L, axial force ratio, loading angle θ, and tensile strength σ of the base metal on the side in each case u,BM [N / mm 2 of the weld metal or the seam weld u,WM [N / mm2 Ratio σ of u,WM / σ u,BM (When it is 1 or less, it becomes under-matching welding) is shown in Table 1 below. Regarding the cross-sectional dimensions of the square steel pipe, for example, "□600×36" means that the outer diameter D is 600 mm and the plate thickness t is 36 mm. In the case of "□600×36", the diameter-thickness ratio (D / t) of the square steel pipe is 16.6, and in the case of "□800×36", the diameter-thickness ratio (D / t) of the square steel pipe is 22.2. The bending radius R (see Fig. 5) on the outside of the corner is 3.5 times the plate thickness (126 mm) in any case. The floor height L (see Fig. 4) is 3 m or 1.5 m. The axial force ratio is the ratio of the axial force applied to the square steel pipe to the yield strength of the cross-section. In No.1~25, since the axial force applied to the square steel pipe is not considered, the axial force ratio is set to 0. In No.26~No.41, an axial force ratio of 0.3 or 0.5 is set.

[0021]

Table 1

[0022] Fig. 6 shows the strain history of the seam welded part when the matching ratio is 0.80 for each loading direction. Generally, since the design of plasticizing the column is avoided, when deforming within the range corresponding to generally the elastic limit to the full plastic strength as assumed in the design (when the square steel pipe reaches the full plastic strength M p and the calculated displacement δ p nearby (δ / δ p =1)), the magnitude relationship of the strains is compared. Specifically, three cases (No.2, No.10, and No.15 in Table 1) with a matching ratio of 0.80 at loading angles θ = 0°, 45°, and 90° were compared. As a result, it was found that when the loading angles θ = 0° and 45°, the ratio of the equivalent plastic strain of the seam welded part is smaller than that when θ = 90°, and the influence of under-matching welding can be reduced. In this regard, the design of the embodiment of the present invention with θ = 0° by forming the seam welded part that becomes under-matching welding only on the side part of the square steel pipe orthogonal to the beam material axis direction is more advantageous than the case when θ = 90°.

[0023] According to the graph in Fig. 6, the strain is the smallest when the loading angle θ = 45°. However, θ = 45° corresponds to the case of diagonal input, that is, when bending is simultaneously received from two-directional beams, and depending on the input direction of the horizontal force, specifically, for example, the direction of earthquake shaking, θ can be either 0° or 90°, so it is difficult to utilize in design.

[0024] Hereinafter, the under-matching rate achievable when the loading angle θ = 0° in the embodiment of the present invention will be further examined.

[0025] Fig. 7 is a graph showing the relationship between the bending moment and the inter-story deformation at the material end of the square steel pipe calculated by analysis. The bending moment on the vertical axis is the full plastic moment M p and the inter-story deformation on the horizontal axis is the elastic deformation (bending + shear) δ p at the time of the full plastic moment M p each normalized. For the cases where the matching rate is 1.00, 0.80, 0.75 at the loading angle θ = 0° (No. 1 to No. 3 in Table 1), in the relationship between the bending moment and the inter-story deformation, the results of each case almost overlapped, and the influence of the matching rate was not seen.

[0026] Fig. 8 is a graph showing the equivalent plastic strain of the seam welded part calculated by analysis. The vertical axis of the graph is the equivalent plastic strain of the seam welded part, and the horizontal axis is the inter-story deformation normalized in the same manner as in Fig. 7. For each case at the loading angle θ = 0° (No. 1 to No. 6 in Table 1), regarding the equivalent plastic strain of the seam welded part, the smaller the matching rate, that is, the greater the degree of under-matching welding, the greater the tendency. Fig. 9 calculates the ratio of the maximum equivalent plastic strain of the seam welded part to the maximum equivalent plastic strain of the base material part (hereinafter also referred to as the maximum equivalent plastic strain ratio) for each case at the loading angle θ = 0° (No. 1 to No. 8 in Table 1), and the maximum equivalent plastic strain ratio at the elastic deformation (bending + shear) δ p at the time of the full plastic moment M p of the square steel pipe is taken as the matching rate (σ u,WM / σ u,BMIt is a graph plotted against ( ). Note that the equivalent plastic strain of the base material part was maximized at the corner on the fixed end side of the model shown in Fig. 4 and on the side where tensile stress occurs due to the shape stress concentration at the corner and the strain concentration due to the Poisson's ratio effect. In this case, the Poisson's ratio effect is a phenomenon in which a strain that contracts in the circumferential direction (the direction orthogonal to the axial strain) of the square steel pipe occurs, and the square steel pipe is strained in the direction of diameter reduction, and in particular, the corners far from the cross-sectional center receive additional bending toward the cross-sectional center. When approximating the graph of Fig. 9 with an exponential function, the following equation (1) is obtained. Here, y represents the maximum equivalent plastic strain ratio on the vertical axis, and x represents the matching ratio on the horizontal axis. When y = 1 in Equation (1), x = 0.795. Therefore, in order to make the plastic strain of the seam welded part smaller than that of the base material part and not make the plasticization of the seam welded part precede that of the base material part, the tensile strength σ u,WM of the weld metal material should be 0.795 times or more of the tensile strength σ u,BM of the base material.

[0027]

Equation

[0028] Fig. 10 is a graph in which the results of No. 20 to No. 41 with the same loading angle θ = 0° are additionally plotted on the graph of Fig. 9 where the results of Case No. 1 to No. 8 are plotted as described above. Note that in Fig. 10, the area where the results of No. 20 to No. 41 are plotted is enlarged and shown. In the results of the illustrated Case No. 20 to No. 41, the maximum equivalent plastic strain ratio on the vertical axis with respect to the matching ratio (σ u,WM / σ u,BM ) on the horizontal axis does not exceed 1 in the range of σ u,WM / σ u,BM ≥ 0.795. From this result, it can be seen that the same conditions as those of Case No. 1 to No. 8 examined above are applicable also in Case No. 20 to No. 41.

[0029] More specifically, in Cases No. 20 to No. 23, No. 25, No. 34 to No. 41, the floor height L is 1.5 m, and in Cases No. 1 to No. 8, No. 24, No. 26 to No. 33, the floor height L is 3 m. Therefore, it can be said that the above conditions are applicable at least when the floor height L is 3 m or less. Also, in Cases No. 24 and 25, the cross-sectional dimension of the square steel pipe is "□800×36" (diameter-to-thickness ratio 22.2), and in Cases No. 1 to No. 8, No. 20 to No. 23, No. 26 to No. 41, the cross-sectional dimension of the square steel pipe is "□600×36" (diameter-to-thickness ratio 16.6). Therefore, it can be said that the above conditions are applicable at least when the diameter-to-thickness ratio of the cross-section of the square steel pipe is 16 or more. The upper limit of the diameter-to-thickness ratio of the cross-section of the square steel pipe is not particularly limited, but is, for example, 67 or less. In Cases No. 26 to No. 41, considering the axial force applied to the square steel pipe, the axial force ratio was set to 0.3 or 0.5. However, similar to Cases No. 1 to No. 8 where the axial force was not considered, σ u,WM / σ u,BM Since a result not exceeding 1 was obtained in the range of ≧0.795, it can be seen that when considering the axial force applied to the square steel pipe, more specifically, the above conditions are applicable at least when the axial force ratio is 0.5 or less.

[0030] Figures 11A, 11B, and 11C are contour diagrams of the equivalent plastic strain calculated by analysis. Among the cases with the loading angle θ = 0°, the equivalent plastic strains of Cases No. 1 (matching ratio 1.00), No. 2 (matching ratio 0.80), and No. 4 (matching ratio 0.70) are shown. When the matching ratios are 1.00 and 0.80, the maximum value of the equivalent plastic strain appears in the base material part, whereas when the matching ratio is 0.70, the maximum value of the equivalent plastic strain appears in the seam weld part. The above calculation results that the matching ratio should be 0.795 or more in order not to cause plasticization of the seam weld part to precede that of the base material part are supported.

Explanation of symbols

[0031] 1... Square steel pipe, 11... Side part, 13... Corner part, 15... Seam welded part, 1A, 1B, 1C... Parts, 2... H-shaped steel beam, 21, 22... Flanges, 23... Web, 31, 32... Diaphragm, 33... Gusset plate.

Claims

1. A column-beam joint in which a column is composed of a square steel pipe having a rounded rectangular cross-section including four side portions and four corner portions, and first and second beams are attached only to two surfaces formed by the opposing side portions of the square steel pipe, wherein a seam weld extending in the longitudinal direction of the square steel pipe is formed only on either one or both of the side portions to which the first beam is attached or the side portions to which the second beam is attached, For the plate thickness t [mm] and outer diameter D [mm] of the square steel pipe, the tensile strength σ of the seam welded part ｕ，ＷＭ [N / mm ２ ] and the tensile strength σ of the base material of the side part ｕ，ＢＭ [N / mm ２ ] satisfy the column-beam joint of formulas (i) and (ii).

2. A column-beam joint in which a column is composed of a square steel pipe having a rounded rectangular cross-section including four side portions and four corner portions, and a beam is attached only to one surface formed by the side portions of the square steel pipe, wherein a seam weld extending in the longitudinal direction of the square steel pipe is formed only on either one or both of the side portions to which the beam is attached or the side portions opposite to the beam, For the plate thickness t [mm] and outer diameter D [mm] of the square steel pipe, the tensile strength σ of the seam welded part ｕ，ＷＭ [N / mm ２ and the tensile strength σ of the base material of the side part ｕ，ＢＭ [N / mm ２ satisfy the column-beam joint of formulas (i) and (ii).

3. the beam is an H-shaped steel beam including a pair of flanges and a web, the square steel pipe is divided at positions corresponding to the pair of flanges and a pair of diaphragms are inserted into respective positions, and the pair of flanges are joined to end faces of the pair of diaphragms, the seam weld is not formed, or a seam weld not satisfying formula (ii) is formed, in any one or two of a portion above the pair of diaphragms of the square steel pipe, a portion between the pair of diaphragms, and a portion below the pair of diaphragms, the column-beam joint according to claim 1 or claim 2.

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