JP7616130B2 - Steel beam - Google Patents

Description

The present invention relates to steel beams used in structures such as buildings.

To prevent buildings and other structures from collapsing during an earthquake, they are generally designed so that the beams become plastic before the columns. The beams in such structures need to have the plastic deformation capacity to adequately absorb the energy input during an earthquake.

Figure 8 shows a schematic diagram of the bending moment-deformation angle relationship for a steel beam. As shown by the dotted line in Figure 8, when local buckling occurs in a steel beam, the strength drops sharply and the plastic deformation capacity cannot be sufficiently ensured. Therefore, in the design of steel beams, the width-thickness ratio of the flanges and webs is limited to prevent local buckling and ensure sufficient plastic deformation capacity.

In a steel beam designed by restricting the width-thickness ratio of the flanges and webs, the plastic deformation performance is ensured, but as shown by the dashed line in Fig. 8, even after the steel beam reaches the full plastic strength _Mp , the strength continues to increase due to strain hardening after material yielding. Therefore, in order to design a building so that the beams plasticize before the columns as described above, it is necessary to make the strength of the columns and column-beam joints exceed the maximum strength _Mmax of the steel beam. Therefore, when the width-thickness ratio of the flanges and webs of the steel beams is restricted, it becomes necessary to increase the size of the columns and column-beam joints, which impairs manufacturability and economic efficiency.

In addition, it is also possible to use steel with a small yield ratio in order to suppress the increase in strength due to strain hardening after the steel beam reaches its full plastic strength _Mp . However, this narrows the plasticized region formed at the axial end of the steel beam, reducing the total amount of seismic energy absorbed by the steel beam during an earthquake.

Here, Patent Document 1 discloses a steel beam having a slit portion 94 in a web 93 of the steel beam 9A, which extends from a position away from the end face of the steel beam 9A toward the center in the material axis direction of the steel beam 9A, as shown in FIG. 9. According to the steel beam 9A of Patent Document 1, the slit portion 94 reduces the strength of the web 93, suppressing fracture of the welded portion between the flange 91 at the end of the steel beam 9A and the column 2 or diaphragm 21, and also makes it easier for the steel beam 9A to plasticize before the column 2. In addition, the web 93 of the steel beam 9A is easier to plasticize before the flange 91, which increases the energy absorption capacity during an earthquake.

Patent Document 2 and Patent Document 3 disclose a column-beam joint structure in which reinforcing plates 92 are joined to both sides of flanges 91 at the ends of steel beams 9B and 9C, and notches 95 and 96 are further formed, as shown in Figures 10 and 11, respectively. According to the steel beams 9B and 9C in Patent Document 2 and Patent Document 3, by forming notches 95 and 96 at the sites where the reinforcing plates 92 are joined, it is possible to suppress fracture of the welded portion between the flanges 91 at the ends of the steel beams 9B and 9C and the column 2 without reducing the strength of the steel beams 9B and 9C.

JP 2019-78102 A JP 2001-207533 A JP 2004-353419 A

"2007 Edition: Commentary on Technical Standards for Building Structures," edited by the Editorial Committee for the Commentary on Technical Standards for Building Structures, National Official Gazette Sales Cooperative, August 2007

Here, the inventors focused on utilizing the buckling that occurs in steel beams to suppress the increase in strength due to strain hardening after the steel beams reach their full plastic strength while ensuring the plastic deformation capacity of the steel beams.

When a horizontal force such as an earthquake force acts on a structure, bending buckling or shear buckling occurs in the steel beam, as shown by buckling waveform B in Figures 12 and 13, respectively. In steel beam 9, after reaching full plastic strength, the strength continues to increase until buckling occurs, at which point the increase in strength stops. In other words, the increase in strength after the steel beam reaches full plastic strength can be suppressed by the occurrence of buckling.

However, if buckling is caused by relaxing the width-thickness ratio restriction, as described above, low-mode local buckling occurs, and as shown by the dotted line in Figure 8, the strength of the steel beam drops sharply, making it impossible to ensure sufficient plastic deformation performance for the steel beam. The inventors therefore came up with the idea of a method to ensure plastic deformation performance while suppressing the increase in strength of the steel beam after it reaches its full plastic strength by increasing the buckling mode that occurs in the steel beam, and after extensive investigations, they have developed the steel beam according to the present invention.

In contrast, the steel beam 9A of Patent Document 1 does not take into consideration suppressing local buckling of the steel beam by increasing the buckling mode of the steel beam. Also, in the steel beam 9A of Patent Document 1, the slit portion 94 extends in the material axis direction of the steel beam 9A, and is therefore subject to interference from secondary members such as small beams attached to the steel beam 9A and through holes for equipment piping provided in the steel beam 9A, and it may not be possible to ensure the required length of the slit portion 94.

Furthermore, in the steel beams 9B and 9C of Patent Documents 2 and 3, no consideration is given to suppressing local buckling of the steel beams by increasing the buckling mode of the steel beams, and there is a risk of low-order buckling modes as shown in Figures 12 and 13 occurring.

In view of the above problems, the present invention aims to provide a steel beam that can ensure sufficient plastic deformation performance while suppressing the increase in strength due to strain hardening after the steel beam reaches its full plastic strength by increasing the buckling mode that occurs in the steel beam.

To solve the above problems, the present invention has the following features:

[1] A steel beam having an upper flange, a lower flange, and a web connecting the upper flange and the lower flange, the web having a slit formed therein that extends in the height direction of the web.

[2] The width of the slit in the axial direction of the steel beam is equal to or less than the plate thickness of the web. [1] A steel beam.

[3] A steel beam according to [1] or [2], in which the slits are formed in multiple rows at equal intervals in the axial direction of the steel beam, and the total length of the slits in each row is 0.5 times or more the height of the web.

[4] A steel beam according to any one of [1] to [3], in which the corners of the openings at the ends of the slits are rounded.

According to the steel beam of the present invention, a slit extending in the height direction of the web is formed in the web of the steel beam, which makes it possible to increase the buckling mode that occurs in the steel beam and expand the range in which buckling occurs. As a result, it is possible to ensure sufficient plastic deformation performance for the steel beam while suppressing the increase in strength due to strain hardening after the steel beam reaches its full plastic strength.

In addition, in the steel beam of the present invention, the slits are formed to extend in the height direction of the web, so the slits can be placed without interference from secondary components such as small beams attached to the steel beam, or through holes for equipment piping provided in the steel beam.

1(a) and 1(b) are a side view and a side view, respectively, showing an example of a steel beam of the present invention, and FIG. 1(c) is a schematic diagram showing a buckling waveform that occurs in the steel beam of FIG. 1(a). 2(a) and 2(b) are respectively a plan view and a side view showing an example of a position where the steel beam of the present invention is used in a structure. FIG. 3(a) is a side view showing an end of a slit in an example of a steel beam of the present invention, and FIG. 3(b) is a partially enlarged view of FIG. 3(a). FIG. 4(a) is a side view showing an example of a steel beam of the present invention, and FIG. 4(b) is a graph showing a schematic diagram of the full plastic moment and the bending moment at ultimate strength of the steel beam of FIG. 4(a). FIG. 5 is a side view showing another example of a steel beam according to the present invention. FIG. 6 is a side view showing still another example of the steel beam of the present invention. FIG. 7 is a side view showing still another example of the steel beam of the present invention. FIG. 8 is a graph showing a schematic diagram of the bending moment-deformation angle relationship of the steel beam of the present invention and the conventional steel beam. FIG. 9 is a side view of the steel beam of Patent Document 1. FIG. 10 is a plan view of the steel beam of Patent Document 2. FIG. 11 is a plan view of the steel beam of Patent Document 3. FIG. 12 is a side view showing an example of bending buckling that occurs in a conventional steel beam. FIG. 13 is a side view showing an example of shear buckling that occurs in a conventional steel beam.

Below, an embodiment of the steel beam of the present invention will be described in detail with reference to the drawings.

Figures 1(a) and 1(b) show a side view and a cross-sectional view, respectively, of a steel beam according to the first embodiment of the present invention.

As shown in Fig. 1(a) and Fig. 1(b), the steel beam 1A of this embodiment is a steel beam made of H-shaped steel having an upper flange 11, a lower flange 12, and a web 13 connecting the upper flange 11 and the lower flange 12. As shown in Fig. 1(a), the upper flange 11 and the lower flange 12 at the ends in the axial direction of the steel beam 1A are welded to a diaphragm 21 provided on a steel column 2 made of a steel pipe. In addition, the web 13 at the end in the axial direction of the steel beam 1A is welded to the side of the steel column 2 or is joined by a high-strength bolt to a shear plate (not shown) provided on the side of the steel column 2.

As shown in FIG. 1(a), slits 14A and 14B extending in the height direction of the web 13 are formed in three rows in the axial direction of the steel beam 1A in the web 13 of the plasticized region at the axial end of the steel beam 1A. In each row, slits 14A are formed in the area adjacent to the upper flange 11 of the web 13, and slits 14B are formed in the area adjacent to the lower flange 12.

Figure 1(c) shows a schematic diagram of the buckling waveform B that occurs in the steel beam 1A of this embodiment when antisymmetric bending acts on the steel beam 1A. As shown in Figure 1(c), slits 14A, 14B extending in the height direction of the web 13 are formed in the web 13 of the steel beam 1A, so that the buckling mode that occurs in the steel beam 1A can be made higher order than when no slits are formed (see Figures 12 and 13). As a result, as shown in Figure 1(c), the range in which buckling occurs in the steel beam 1A is expanded, and sufficient plastic deformation performance can be ensured for the steel beam 1A while suppressing the increase in strength after the steel beam 1A reaches its full plastic strength.

2(a) and 2(b) show an example of a position where the steel beam 1A of this embodiment is used in a building in plan and side views, respectively. As shown in FIGS. 2(a) and 2(b), the steel beam 1A of this embodiment is used as a main beam attached to a column 2 in a building. The slits 14A and 14B of the steel beam 1A of this embodiment are formed to extend in the height direction of the web 13, so that these slits 14A and 14B can be arranged without interference from secondary members such as the sub-beam 3 attached to the steel beam 1A or through holes for equipment piping provided in the steel beam 1A, as shown in FIGS. 2(a) and 2(b).

As shown in FIG. 1(a), in the steel beam 1A of this embodiment, the slits 14A and 14B are formed so that the width 14w of the slits 14A and 14B in the axial direction of the steel beam 1A is equal to or less than the plate thickness 13t of the web 13. This is preferable because it suppresses the deterioration of the cross-sectional performance of the steel beam 1A caused by forming the slits 14A and 14B in the web 13 of the steel beam 1A.

As shown in FIG. 1(a), the three rows of slits 14A, 14B are arranged at equal intervals 14i in the axial direction of the steel beam 1A. If the intervals between slits 14A, 14B in the axial direction of the steel beam 1A are not uniform, buckling is unlikely to occur in areas with narrow intervals, and buckling is likely to progress in a concentrated manner in areas with wide intervals, making it difficult to fully achieve the effect of increasing the buckling mode that occurs in the steel beam 1A. Therefore, arranging slits 14A, 14B at equal intervals in the axial direction of the steel beam 1A as described above is preferable because it allows buckling to progress evenly within the area where slits 14A, 14B are provided, and it is possible to fully achieve the effect of increasing the buckling mode that occurs in the steel beam 1A.

The slits 14A and 14B are formed so that the total length of the slits 14A and 14B arranged in each row, i.e., the sum of the height 14ha of the slits 14A and the height 14hb of the slits 14B, is 0.5 times or more the height 13h of the web 13. This is preferable because it more reliably increases the buckling mode that occurs in the steel beam 1A, and more reliably suppresses the increase in strength after the steel beam 1A reaches its full plastic strength while ensuring sufficient plastic deformation performance.

Furthermore, as shown in Figs. 3(a) and 3(b), a radius 14r is provided at the corners of the opening at the lower end of slit 14A. Similarly, a radius (not shown) is provided at the corners of the opening at the upper end of slit 14B. This is preferable because it can alleviate stress concentration in web 13 around the ends of slits 14A and 14B. It is preferable to set the radius of radius 14r to 5 mm or more, as this effect can be reliably obtained. In addition, it is preferable to form the lower end of slit 14A and the upper end of slit 14B in a semicircular shape and form the other parts of slits 14A and 14B in a linear shape, as this makes it easier to process the openings of slits 14A and 14B.

Figures 4(a) and 4(b) show schematic diagrams of the total plastic moment at each position in the material axis direction of the steel beam 1A of this embodiment, and the bending moment distribution at the ultimate strength of the steel beam 1A. As shown in Figure 4(b), the total plastic moment of the steel beam 1A is small at the positions of the steel beam 1A where the slits 14A and 14B are provided. Therefore, if the positions of the slits 14A and 14B are too close to the ends of the material axis direction of the steel beam 1A, or if the heights 14ha and 14b of the slits 14A and 14B are too large, the ultimate strength of the steel beam 1A may be reduced due to the influence of the reduction in the total plastic moment of the steel beam 1A at the positions where the slits 14A and 14B are provided. Therefore, it is preferable to adjust the distance of slits 14A and 14B from the axial end of steel beam 1A and the heights 14ha and 14hb of slits 14A and 14B so that the ultimate strength of steel beam 1A does not decrease even if the total plastic moment of steel beam 1A becomes small at the position where slits 14A and 14B are provided.

In addition, it is preferable that the width-thickness ratio of the upper flange 11, the lower flange 12, and the web 13 of the steel beam 1A is equivalent to the width-thickness ratio rank FA disclosed in Non-Patent Document 1. In this way, it is possible to prevent the occurrence of local buckling in the steel beam and a sudden decrease in strength, and it is possible to reliably ensure the plastic deformation performance of the steel beam 1A.

Figures 5 to 7 show side views of steel beams 1B to 1D according to the second to fourth embodiments of the present invention, respectively.

As shown in Figures 5 to 7, the shape and arrangement of the slits in the steel beams 1B to 1D of the second to fourth embodiments are changed compared to the steel beam 1A of the first embodiment.

Specifically, as shown in FIG. 5, in the steel beam 1B of the second embodiment, slits 14C extending in the height direction of the web 13 are formed in three rows at equal intervals in the material axis direction of the steel beam 1A. The slits 14C in each row are formed in the center of the height direction of the web 13, i.e., at a position away from the upper flange 11 and the lower flange 12. The slits 14C are formed so that the total extension of the slits 14C arranged in each row, i.e., the height 14hc of the slits 14C, is 0.5 times or more the height 13h of the web 13. This is preferable because it is not necessary to process the openings of the slits 14C in a position close to the upper flange 11 and the lower flange 12, making it easier to process the slits 14C.

6, in the steel beam 1C of the third embodiment, slits 14A, 14B, and 14D extending in the height direction of the web 13 are formed in three rows at equal intervals in the material axis direction of the steel beam 1A. In the two outer rows of the three rows of slits 14A, 14B, and 14D arranged in the material axis direction of the steel beam 1A, slits 14A are formed in the area adjacent to the upper flange 11 of the web 13, and slits 14B are formed in the area adjacent to the lower flange 12. In addition, in the center row of the three rows of slits 14A, 14B, and 14D arranged in the material axis direction of the steel beam 1A, slits 14D are formed in the center of the height direction of the web 13, that is, at a position away from the upper flange 11 and the lower flange 12. That is, in the steel beam 1C of the third embodiment, the slits 14A, 14B, and 14D are arranged in a staggered pattern. The slits 14A, 14B, and 14D are formed so that the total length of the slits 14A, 14B, and 14D arranged in each row, i.e., the sum of the height 14ha of the slits 14A and the height 14hb of the slits 14B, and the height 14hd of the slits 14D, are each 0.5 times or more the height 13h of the web 13. In this way, by arranging the slits 14A, 14B, and 14D in a staggered manner, the areas around the slits 14A, 14B, and 14D, i.e., the areas where buckling occurs, can be dispersed. As a result, the buckling waveforms generated in each area are prevented from combining to form a single large buckling mode, and the effect of increasing the order of the buckling mode generated in the steel beam 1A can be fully achieved, which is preferable.

As shown in FIG. 7, in the steel beam 1D of the fourth embodiment, the slits 1E extending in the height direction of the web 13 are formed in three rows at equal intervals in the material axial direction of the steel beam 1A. In each row, the slits 14E are formed in series at four intermittent locations in the center of the height direction of the web 13, i.e., at positions away from the upper flange 11 and the lower flange 12. The slits 14E are formed so that the total extension of the slits 14E arranged in each row, i.e., the sum of the heights 14he of the slits 14E arranged in each row, is 0.5 times or more the height 13h of the web 13.

In the steel beams 1B to 1D of the second to fourth embodiments, similar to the steel beam 1A of the first embodiment, the slits 14A to 14E are formed so that the width 14w of the slits 14A to 14E in the material axial direction of the steel beam 1A is equal to or less than the plate thickness 13t of the web 13.

As in the second to fourth embodiments, even if the slits 14A to 14E are positioned in the center of the height of the web 13, and even if the shapes and positions of the slits in adjacent rows in the axial direction of the steel beam are not the same, the same effect can be obtained as with the steel beam 1A in the first embodiment.

In the above embodiments, examples have been described in which the steel beams are formed from H-shaped steel, but they may also be welded assembly materials such as built-in H steel. In addition, the present invention can be similarly applied to steel beams that have an upper flange, a lower flange, and a web that connects the upper flange and the lower flange, even if the cross-sectional shape is not H-shaped, such as steel beams with a groove-shaped cross section.

Reference Signs List 1A to 1D, 9, 9A to 9C Steel beam 2 Steel column 3 Sub-beam 11 Upper flange 12 Lower flange 13 Web 13h Web height 13t Web thickness 14A to 14E Slit 14w Slit width 14i Slit spacing 14ha to 14he Slit height 14r Radius 21 Diaphragm 91 Flange 92 Reinforcement plate 93 Web 94 Slit portion 95, 96 Notch portion B Buckling waveform

Claims (3)

A steel beam having an upper flange, a lower flange, and a web connecting the upper flange and the lower flange,
The web is formed with a slit extending in a height direction of the web ,
The slits are formed in a plurality of rows at equal intervals in the material axial direction of the steel beam,
A steel beam in which the total length of the slits in each row is equal to or greater than 0.5 times the height of the web . The steel beam according to claim 1, wherein the width of the slit in the axial direction of the steel beam is equal to or less than the plate thickness of the web. The steel beam according to claim 1 or 2 , wherein the corners of the openings at the ends of the slits are rounded.

Images