JP6984510B2 - Joint structure of square steel pipe columns and H-shaped steel beams - Google Patents

Description

The present invention relates to a joint structure of columns and beams in a building structure, and particularly to a joint structure of square steel pipe columns and H-shaped steel beams.

In a beam-column joint of a building steel structure, a joint element called a diaphragm is generally used to supplement the strength of the joint, and the form is as shown in FIG. 13, with a square steel pipe column 41. An inner diaphragm type for joining the H-shaped steel beam 43 using the inner diaphragm 45, a through diaphragm type for joining with the through diaphragm 47 as shown in FIG. 14, and an outer diaphragm 49 as shown in FIG. It is roughly divided into the outer diaphragm type to be joined.

The inner diaphragm type is generally joined by electroslag welding, and a very large heat input of several hundred thousand to several million J / cm acts on the welded part. The toughness of the weld is very small, and brittle fracture is likely to occur at the weld. This tendency is particularly remarkable in high-strength steel materials. Further, in a cross section having no right-angled angle such as a cold roll-formed square steel pipe or a circular steel pipe, electroslag welding cannot be applied due to the shape, so that it is difficult to apply the inner diaphragm type in the first place.

In the through-diaphragm type, the square steel pipe column is cut once at the position where the through-diaphragm is arranged, and then the divided square steel pipe column is reassembled after welding the through-diaphragm.
In addition, since the through diaphragm is structurally designed as a part of the column, the through diaphragm and the welding material between the through diaphragm and the square steel pipe column must be made of a material having a strength class higher than that of the steel used for the square steel pipe column. For example, when ^{a high-strength steel material such as 780 N / mm class 2} steel material is used for the column, the construction load and material cost become high.

On the other hand, in the outer diaphragm type targeted by the present invention, the welding material with the square steel pipe column may be adjusted to the strength on the beam side, and there is no need to worry about the toughness deterioration of the welded portion due to the large heat input. It is easy to apply even when high-strength steel materials are used.
In addition, the outer diaphragm type has advantages such as a smaller welding amount and processing amount than the through diaphragm type, and excellent workability when filling concrete into a square steel pipe column.

On the other hand, when the outer diaphragm is welded to the square steel pipe column at the construction site, in addition to the welded joint between the outer diaphragm and the square steel pipe column, there is also the welded joint between the outer diaphragm and the beam flange, or the high-strength bolt joint. There will be more work processes at the construction site.

In addition, since the current outer diaphragm has a large protrusion width from the pillar surface, when the outer diaphragm is attached at the factory and transported to the construction site, the number of pillars loaded on the truck is limited by the outer diaphragm with a large protrusion width, which improves efficiency. There is a problem of being bad.

From this point of view, Patent Document 1 and Patent Document 2 propose an outer diaphragm having a small protrusion width.
Patent Document 1 proposes an outer diaphragm using a band-shaped steel plate having a certain width, and can be applied as it is even when a beam having a different cause is attached to the same steel pipe column while reducing the protruding width. It is devised as such.

Further, in Patent Document 2, an outer diaphragm is formed by four L-shaped steel pieces, and a thick material is used for the L-shaped steel pieces to keep the outer dimension of the outer diaphragm small. The L-shaped steel pieces are joined to each other by fillet welding on the front and back surfaces, partial penetration welding, or the like, but the joint surface is limited to the inside of the beam flange to be attached.

Japanese Unexamined Patent Publication No. 2006-002351 Japanese Unexamined Patent Publication No. 2016-108868

However, in the case of Patent Document 1, when the band width (height of the outer diaphragm) becomes larger, the bending rigidity of the outer diaphragm in the out-of-plane direction (beam material axial direction) becomes smaller, the local strain increases, and the yield strength also decreases. Since it is easy to carry out, there is a drawback that the yield strength of the outer diaphragm to the tensile force in the outward direction of the steel pipe column surface transmitted from the beam flange to the outer diaphragm is reduced.

Further, in the case of Patent Document 2, there is a problem that it is difficult to verify the soundness of the beam flange-outer diaphragm weld, which is a structurally important part, by ultrasonic flaw detection inspection.
That is, in Patent Document 2, since the joint surface between the L-shaped steel pieces is inside the beam flange, the unwelded portion is attached to the joint surface between the L-shaped steel pieces when performing an ultrasonic flaw detection inspection at the beam end welded portion. If this is the case, this unwelded portion will be detected as a defect, so it is difficult to verify the soundness of the beam flange-outer diaphragm weld by ultrasonic flaw detection inspection.

In addition, considering the case where L-shaped steel pieces are joined by complete penetration welding, ultrasonic flaw detection inspection of this joint surface should be performed in advance before beam flange welding to confirm that there are no defects. For example, it becomes difficult to identify the presence / absence and position of defects in the beam flange-outer diaphragm weld by ultrasonic flaw detection inspection. In this case, there is a problem that the ultrasonic flaw detection inspection process is increased and the construction man-hours are increased.

The present invention has been made to solve such a problem, without reducing the yield strength of the outer diaphragm against the tensile force in the outward direction of the square steel pipe column surface, and surpassing the soundness verification of the beam flange-outer diaphragm weld. It is an object of the present invention to provide a joint structure of a square steel pipe column and an H-shaped steel beam that does not interfere with the sound detection inspection.

(1) The joining structure of the square steel pipe column and the H-shaped steel beam according to the present invention is to join the square steel pipe column and the H-shaped steel beam via the outer diaphragm.
The outer diaphragm is formed by abutting the ends of two L-shaped steel pieces having the same shape to form a rectangular ring, and the inner peripheral surface of the rectangular ring is welded to the skin plate of the square steel pipe column.
Two joint surfaces of the L-shaped steel pieces are located at the corners of the square steel pipe column, and the angle formed by the extension surface of each of the orthogonal skin plates of the square steel pipe column and the joint surface is 25. It is characterized in that the degree is set to be 65 degrees or more, and the joint surface is joined by fillet welding, and the length of the welded portion is 10% or more of the protruding dimension of the outer diaphragm. Is to be.

(2) Further, in the above-mentioned (1), the skin plate of the square steel pipe column and the outer diaphragm are welded and joined by partial penetration welding, and the L-shaped steel pieces are opened on the front and back surfaces thereof. It is characterized by being welded and joined by partial penetration welding with a tip depth of less than 8 mm.

(3) Further, in those described in the above (1) or (2), the exit dimensions of the outer diaphragm l _d, outer diaphragm width of the attachment rather the pillar and _{D c, l d ≦ D c} / 2 Relationship It is characterized by satisfying.

(4) In any of the above (1) to (3) _{, the protrusion dimension of the outer diaphragm is l d} , the plate thickness is t _d , the yield strength is σ _d , and the beam flange to which the outer diaphragm is attached. The width is B _f , the plate thickness is t _f , the yield strength is σ _f , the widening width by the horizontal flange is B _s , the width of the column to which the outer diaphragm is attached is D _c , and the plate thickness is t _c . It is characterized by satisfying the relation of the formula.

In the welded structure of the square steel pipe column and the H-shaped steel beam of the present invention, the outer diaphragm is formed into a rectangular ring by abutting the ends of two L-shaped steel pieces having the same shape, and the inner peripheral surface of the rectangular ring is formed. Is welded to the skin plate of the square steel pipe column, two joint surfaces of the L-shaped steel pieces are located at the corners of the square steel pipe column, and each skin plate orthogonal to the square steel pipe column is located. The angle formed by the extension surface and the joint surface is set to be 25 degrees or more and 65 degrees or less, and the joint surface is joined by fillet welding, and the length of the welded portion is the outer diaphragm. By increasing the protrusion dimension by 10% or more, the resistance of the outer diaphragm to the tensile force in the outward direction of the steel pipe column surface is not reduced, and the soundness verification of the beam flange-outer diaphragm welding is performed by ultrasonic flaw detection inspection. There is no problem in doing it in.

It is explanatory drawing of the joint structure of the square steel pipe column and the H-shaped steel beam which concerns on this embodiment. It is explanatory drawing of the L-shaped steel piece which constitutes the outer diaphragm of the joint structure of the square steel pipe column and the H-shaped steel beam shown in FIG. 1. It is explanatory drawing of the joint portion of the L-shaped steel piece in the joint structure of the square steel pipe column and the H-shaped steel beam which concerns on this embodiment. It is explanatory drawing of the analysis model in the process of determining the shape of the L-shaped steel piece in the joint structure of the square steel pipe column and the H-shaped steel beam which concerns on this embodiment (the 1). It is explanatory drawing of the analysis model in the process of determining the shape of the L-shaped steel piece in the joint structure of the square steel pipe column and the H-shaped steel beam which concerns on this embodiment (the 2). It is explanatory drawing of the explanatory drawing of the method of determining the outer diaphragm joint strength in the process of determining the shape of the L-shaped steel piece in the joint structure of the square steel pipe column and the H-shaped steel beam which concerns on this embodiment. It is a graph explaining that the regression type proof stress is correlated with the FEM proof stress. It is explanatory drawing of the analysis model of the invention example in an Example (the 1). It is explanatory drawing of the analysis model of the invention example in an Example (the 2). It is explanatory drawing of the analysis model of the comparative example in an Example (the 1). It is explanatory drawing of the analysis model of the comparative example in an Example (the 2). It is a graph explaining the effect in an Example. It is explanatory drawing of the mode in the case of joining a square steel pipe column and an H-shaped steel beam with a diaphragm (No. 1). It is explanatory drawing of the mode in the case of joining a square steel pipe column and an H-shaped steel beam with a diaphragm (No. 2). It is explanatory drawing of the mode in the case of joining a square steel pipe column and an H-shaped steel beam with a diaphragm (No. 3).

As shown in FIG. 1, the joint structure 1 of the square steel pipe column and the H-shaped steel beam according to the present embodiment joins the square steel pipe column 3 and the H-shaped steel beam 7 via the outer diaphragm 9. The outer diaphragm 9 is formed by abutting the ends of two L-shaped steel pieces 11 having the same shape to form a rectangular ring, and the inner peripheral surface of the rectangular ring is welded to the outer peripheral surface of the square steel pipe column 3. It is a thing.
Hereinafter, the details of the outer diaphragm 9 will be described.

The two L-shaped steel pieces 11 constituting the outer diaphragm 9 have the same shape. By making the L-shaped steel pieces 11 have the same shape, mistakes in taking parts are eliminated, and management at the time of manufacturing becomes easy.

The two joint surfaces 13 of the L-shaped steel pieces 11 are located symmetrically with respect to the corners of the square steel pipe column 3. The angle formed by the joint surface 13 with the extension surfaces of the adjacent skin plates 5a, b, c, and d of the square steel pipe column 3 is 25 degrees or more and 65 degrees or less. To specifically explain this point with respect to one of the two joint surfaces 13 shown in FIG. 1, the angle at which _{the joint surface 13 forms with the extension surface A of one skin plate 5a is θ A} , and the joint surfaces 13 are adjacent to each other. If the angle formed by the extension surface B of the other skin plate 5b is θ _B , it means _{that both θ A} and θ _B are 25 degrees or more and 65 degrees or less. In other words, the shape of the end surface 15 (see FIG. 2) of each L-shaped steel piece 11 is set so that the joint surface 13 has the above angle. When the L-shaped steel piece 11 is cut out from a thick steel plate, it may be cut out so that the end face 15 has the above angle at the time of cutting out.

The joint surface 13 is positioned at the corner of the square steel pipe column 3 so that the welded portion between the beam flange of the H-shaped steel beam 7 and the outer diaphragm 9 and the welded portion of the joint surface 13 do not overlap in terms of structure. The soundness of the beam flange-outer diaphragm weld, which is an important part, can be verified by ultrasonic flaw detection inspection.
It is also due to the fact that the stress load on the portion is small by locating the joint surface 13 at the corner portion. This point is also related to the welding specifications of the joint surface 13, and will be described later.

Further, the reason why the angle formed by the joint surface 13 with the extension surface of each of the skin plates 5a, b, c, and d is set as described above is as follows.
First, the angle of the joint surface 13 is specified in order to secure a long weld length on the joint surface 13 and prevent quality deterioration of the welded portion such as extreme hardening due to a short bead.
The angle formed by the extension surfaces of the skin plates 5a, b, c, and d of the joint surface 13 is 25 degrees or more and 65 degrees or less so that the joint surface 13 is orthogonal to one of the skin plates 5. This is because the joint surface 13 is 10% or more longer than the case where it is provided (in this case, the angle formed by the joint surface 13 with the other skin plate 5 is 0 degree). For example, when the angle formed with one skin plate 5 of the joint surface 13 is 25 degrees (in this case, the angle formed by the joint surface 13 with the other skin plate 5 is 65 degrees), as compared with the above-mentioned orthogonal case. The welding length is about 1.1 times longer.

By defining the angle of the joint surface 13, if the protruding dimension of the outer diaphragm 9 is constant in the circumferential direction, the welding length of the joint surface 13 can be 10% or more of the protruding dimension of the outer diaphragm 9, and the outside Even when the protruding dimension of the diaphragm 9 is small, fillet welding can be performed soundly.
If the shape of the outer diaphragm 9 is such that the protruding dimension is shortened at the corners, the welding length cannot be sufficiently secured only by setting the above angle. Therefore, in the present invention, the outer diaphragm is assumed to be in such a case. We are trying to secure a welding length of 10% or more of the protrusion dimension of 9.

Normally, from the viewpoint of dimensional accuracy, the L-shaped steel pieces 11 are not joined to each other in advance, but each L-shaped steel piece 11 is welded to the skin plate 5 one by one, and then the L-shaped steel pieces 11 are joined to each other on the joint surface 13. Will be welded together. At this time, the inner surface side of the L-shaped steel piece 11 is welded and joined to the skin plate 5, and of course, there is a welding surplus on the joined surface 13. Therefore, looking at the joint surface 13, the skin plate side end portion is welded and joined in advance by a certain length, and the welding length of the fillet weld for joining the joint surfaces 13 to each other is shortened by that amount. From such a construction point of view, it is important to have a shape that can secure a long welding length of the joint surface 13, and in particular, it is more important to realize an outer diaphragm 9 having a small protrusion dimension. It becomes a viewpoint.

In the present embodiment, the skin plate 5 of the square steel pipe column 3 and the outer diaphragm 9 are welded and joined by partial penetration welding, and the joint surface 13 between the outer diaphragms 9 is the front and back surfaces thereof as shown in FIG. It is welded and joined by partial penetration welding with a groove depth of 8 mm or less. Note that FIG. 3 shows a case where the angle formed by the joint surface 13 with the extension surfaces of the skin plates 5a, b, c, and d is 45 degrees.
The reasons for such welding specifications are as follows.

In general, the horizontal force transmitted from the beam flange to the joint (column-beam joint) during an earthquake tends to flow to the one with higher rigidity in the direction of action of the horizontal force. According to this theory, in the structure of the present invention, most of the force flows in the vicinity of the skin plate 5, and almost no horizontal force is borne in the vicinity of the outer end corners of the joint surfaces 13 of the L-shaped steel pieces 11.
Further, since the outer diaphragm 9 and the skin plate 5 are welded and joined by partial penetration welding, it can be considered that the outer diaphragm 9 is welded and joined at least on the column surface.
From the above, it can be said that the welding joint between the L-shaped steel pieces 11 does not have to rely on complete penetration welding.

On the other hand, when the outer diaphragms 9 are not joined at all, thermal deformation due to welding work around the beam flange-outer diaphragm welding, expansion / contraction of the steel material due to temperature changes during the day and night, and wind load It is considered that the outer diaphragm 9 is greatly deformed by the slight vibration such as, and the appearance is spoiled, and the coating and the surface material are peeled off, and fine cracks are generated due to local stress concentration.
Therefore, in the present invention, at the joint portion between the L-shaped steel pieces 11, the front and back surfaces are welded and joined by partial penetration welding with a groove having a depth of 8 mm or less.
As mentioned above, this welded joint is not expected to bear or transmit a large horizontal force in the event of an earthquake, but is intended to maintain its shape. The groove depth is set to 8 mm or less so that it can be covered by construction.

<External dimensions of outer diaphragm>
As described above, if the outer dimension of the outer diaphragm 9 is large, there is a problem that the efficiency of transportation and handling is poor when the outer diaphragm 9 is attached and transported to the construction site at the factory. The protruding dimension of the _{diaphragm 9 is l d} , and the width of the pillar to which the outer diaphragm 9 is attached is D _c , so that the relationship of _{l d} ≤ D _{c / 2 is satisfied.}
The main reason for setting this is that the transportation efficiency and handling at the construction site are taken into consideration, but when l _d > D _c / 2, the outer diaphragm 9 is set in the out-of-plane direction ( Since local deformation (in the direction of the column material axis) also increases, structural performance such as yield strength is maintained, and thermal deformation during welding of the outer diaphragm 9 and skin plate 5 columns is suppressed to ensure dimensional accuracy during construction. This is also taken into consideration.

<Setting the thickness of the outer diaphragm>
If the protrusion dimension is shortened, the outer diaphragm 9 tends to yield when the outer diaphragm 9 receives a horizontal force from the beam during an earthquake. Therefore, when determining the protrusion dimension, it is necessary to increase the plate thickness of the outer diaphragm 9.
Therefore, in the present embodiment, the beam yields before the joint in the event of an earthquake, and the outer diaphragm joint has a proof stress P _d ≥ the beam flange proof stress P _f so that the building can absorb sufficient energy. The relationship between the plate thickness of the diaphragm 9 and the protrusion size is determined.
Specifically, the protrusion dimension of the outer diaphragm 9 is l _d , the plate thickness is t _d , the yield strength is σ _d , the width of the beam flange to which the outer diaphragm 9 is attached is B _f , the plate thickness is t _f , and the yield strength is. The following relationship is satisfied when σ _f , the widening width by the horizontal flange is B _s , the width of the column to which the outer diaphragm 9 is attached is D _c , and the plate thickness is t _c.
The widening width by the horizontal haunch is the beam flange width (width at the tip of the haunch) at the boundary between the outer diaphragm and the beam flange. If there is no haunch, Bs = Bf.

Here, the yield strength P _regression of the outer diaphragm junction is obtained by performing FEM analysis and regressing the analysis result by the least squares method.
Hereinafter, the method of FEM analysis and the calculation of the regression equation will be specifically described.

-Explanation of FEM analysis In order to obtain the yield strength of the outer diaphragm joint, FEM analysis was performed for each shape of the column, beam, and outer diaphragm shown in Table 1 below.

The analysis model 17 is composed of a column 19, an outer diaphragm 21, and a beam flange 23 at the beam-column joint shown in FIGS. 4 and 5. The beam web near the outer diaphragm joint is omitted in this model because it has a small effect on stress transmission.
As for the boundary conditions, the central surface of the beam flange is constrained in the z direction, the upper and lower ends of the column are constrained in the x direction, and the load P _f in the x direction is applied to the end of the beam flange to reproduce the stress state at the time of an earthquake. The Young's modulus of the steel material was set to 205,000 N / mm ² .

_{Here, the difference δ P} -δ _Q of the displacements of the two points P and Q on the inner surface of the column 19 shown in FIG. 5 is defined as the amount of deformation of the joint, and the beam flange load P _f (kN) and the deformation of the joint are defined. Figure 6 shows a graph of the δ _P- δ _{Q (mm) relationship.} In FIG. 6, the vertical axis is the beam flange load P _f (kN) and the horizontal axis is the joint deformation δ _P -δ _Q (mm).
In this example, the yield strength of the joint is defined as the time when the outer diaphragm 21 yields and the tangential rigidity of the graph drops to 1/3 of the initial rigidity (marked with a star in the graph).

Next, how to determine the regression equation will be described.
Regression equation of the strength of the outer diaphragm joint P _regression is set as the following equation using unknowns α, β, γ, δ, ε, ζ.

Taking the natural logarithm log on both sides, it becomes as follows.

未知数α、β、γ、δ、ε、ζについて、以下の６つの式が成り立つ。

Assuming that the proof stress of the No. _i test piece shown in Table 1 is P i, when the sum of squares of _{P regression} and log P _i shown by the following equation is minimized,

The following six equations hold for the unknowns α, β, γ, δ, ε, and ζ.

By solving the above equations simultaneously, the unknown variables α, β, γ, δ, ε, and ζ can be determined as follows.
α = 4.72, β = -0.24, γ = 1.22, δ = -1.21, ε = 0.74, ζ = 0.24
That is, the yield strength formula P _regression of the outer diaphragm joint is as follows.

In the design, the yield strength of the outer diaphragm joint expressed by the above equation is equal to _{or greater than the beam flange yield strength B f t f} σ _f , so the relationship is as follows.

これを展開して、外ダイアフラムの板厚t_dについて整理すると下式となる。

If this is expanded and the plate thickness t _d of the outer diaphragm is arranged, the following formula is obtained.

FIG. 7 shows the relationship between the regression yield strength and the yield strength obtained by FEM (hereinafter referred to as FEM yield strength). In FIG. 7, the vertical axis shows the regression proof stress and the horizontal axis shows the FEM analysis proof stress. For each shape shown in Table 1, the regression proof stress and the FEM analysis proof stress (see the rightmost column of Table 1) are shown. It is a plot.
As shown in FIG. 7, the coefficient of determination R ² of the regression formula is 0.97, the correlation between the regression Strength and FEM strength is obtained.

An FEM analysis was performed to demonstrate the effect of the beam-column joint structure related to the present invention, which will be described below.
As shown in FIG. 8 which is a plan view and FIG. 9 which is a perspective view, the analysis model 25 of the invention example is a three-dimensional frame model in which an H-shaped steel beam 31 is joined to a square steel pipe column 27 via an outer diaphragm 29. .. Here, the dimensions of each member assumes the actual building, the pillars: □ -1500 × 50 (780N / mm 2 class steel), Beam: H-1200 × 400 × 19 × 28 (550N / mm 2 class steel ), Outer diaphragm: protrusion size 330, plate thickness 60 (550N / mm ^{class 2} steel), and columns filled with concrete. In the analysis, as shown in FIG. 9, the horizontal force during an earthquake was simulated by applying a load to the stigma.

The outer diaphragm 29 has a joining type simulating 45-degree direction welding at the corners. The welding depth is 6 mm, and the metal touch with a friction coefficient of 0 is used except for the welded part.
As shown in FIG. 10 which is a plan view and FIG. 11 which is a perspective view, the analysis model 33 of the comparative example is a three-dimensional frame in which the H-shaped steel beam 39 is joined to the square steel pipe column 37 in which the inner diaphragm 35 is built. Analysis was performed as a model. Table 2 shows a list of analysis cases.

FIG. 12 is a graph showing a comparison of the load-deformation angle relationship in both the invention example and the comparative example, in which the vertical axis indicates the load (kN) and the horizontal axis indicates the deformation angle (rad).
From FIG. 12, the skeleton rigidity and the proof stress of the invention example were higher than those of the comparative example, respectively, demonstrating that the present invention is effective.

1 Joint structure of square steel pipe column and H-shaped steel beam 3 Square steel pipe column 5 Skin plates 5a, b, c, d Each skin plate 7 H-shaped steel beam 9 Outer diaphragm 11 L-shaped steel piece 13 Joint surface 15 End face 17 Analysis model 19 Pillar 21 Outer diaphragm 23 Beam flange A, B Extension surface <Example>
25 Analytical model (invention example)
27 Square steel pipe column 29 Outer diaphragm 31 H-shaped steel beam 33 Analysis model (comparative example)
35 Inner diaphragm 37 Square steel pipe column 39 H-shaped steel beam <Conventional example>
41 Square steel pipe column 43 H-shaped steel beam 45 Inner diaphragm 47 Through diaphragm 49 Outer diaphragm

Claims (1)

It is a design method of the joint structure of the square steel pipe column and the H-shaped steel beam that joins the square steel pipe column and the H-shaped steel beam via the outer diaphragm.
The outer diaphragm is formed into a rectangular ring by abutting the ends of two L-shaped steel pieces having the same shape, and the inner peripheral surface of the rectangular ring is welded to the skin plate of the square steel pipe column.
Two joint surfaces of the L-shaped steel pieces are located at the corners of the square steel pipe column, and the angle formed by the extension surface of each of the orthogonal skin plates of the square steel pipe column and the joint surface is 25. The degree is set to be 65 degrees or more, and the joint surface is welded and joined by partial penetration welding with a groove depth of 8 mm or less on the front and back surfaces of the L-shaped steel pieces, and the length of the welded portion is set. Increase by 10% or more of the protruding dimension of the outer diaphragm.
The outer dimension of the outer diaphragm is l _d , the plate thickness is t _d , the yield strength is σ _d , the width of the beam flange to which the outer diaphragm is attached is B _f , the plate thickness is t _f , the yield strength is σ _f , and the horizontal haunch. The square steel pipe column and H are characterized in that the widening width is set to B _s , the width of the column to which the outer diaphragm is attached is D _c , and the plate thickness is t _c , so that the relationship of the following equation is satisfied. How to design a joint structure for shaped steel beams.

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