JP7811604B2 - Connection between concrete-filled steel pipe column and steel beam - Google Patents

Description

The present invention relates to column-beam joints in concrete-filled steel tubular structures (hereinafter referred to as CFT structures), particularly to the structure of joints between concrete-filled steel tubular columns and steel beams.

In CFT structures, when through diaphragms or internal diaphragms are installed in the steel column pipes, voids are likely to form on the underside of the through diaphragms or internal diaphragms when concrete is filled in. To prevent this, it is common to install air vent holes in the diaphragms to prevent voids from forming.

In addition, installing three or more diaphragms at one beam-column joint is not desirable in terms of concrete filling. Furthermore, if multiple diaphragms are installed, the minimum spacing between the diaphragms will be greater than the dimension required for welding. Previously, these issues were addressed by taper- ing the beam ends to unify the beam depth at the beam-column joint, but this had the disadvantage of increasing manufacturing costs.

On the other hand, there is a construction method that allows for the thickness of the steel pipe columns to be reduced by inserting rebar cages into the columns of a CFT structure. With this method, the larger the cross-sectional dimensions (width and diameter) of the inserted rebar cage, the greater the effect of reducing the thickness of the steel pipe columns. Therefore, with the through diaphragm type and internal diaphragm type, the effect of reducing the steel pipe thickness cannot be expected compared to the external diaphragm type. However, with the external diaphragm type, the diaphragm tends to be larger, which can lead to poor manufacturing costs and poor design, so the through diaphragm type is often adopted.

If the steel column pipes in a CFT structure are made thinner, local buckling may occur in the steel column pipes during a major earthquake, raising concerns about a reduction in column strength. One solution to this problem is to insert a reinforcing bar cage into the CFT structure to suppress local buckling of the steel column pipes. In this case, too, the larger the cross-sectional dimensions of the reinforcing bar cage, the greater the effect of suppressing local buckling of the steel column pipes, but there is a problem in that the size of the reinforcing bar cage is limited depending on the type of diaphragm.

In response to this, for example, Patent Documents 1 and 2 propose a structure in which a diaphragm is not installed at the column-beam joint of a CFT structure.

Patent Document 1 discloses a joint structure between a steel concrete-filled column and a beam, which consists of a thick-walled steel pipe with internal ribs welded between an upper steel pipe column and a lower steel pipe column, concrete filled inside the internally ribbed steel pipe, and a beam butt-jointed to the outer surface of the internally ribbed steel pipe.

Patent Document 2 discloses a joint structure between a concrete-filled steel pipe column and a beam, which consists of a connecting cylinder interposed between upper and lower steel pipe columns, an outer surface of the connecting cylinder that is flush with the outer surface of the steel pipe columns, an inner surface of the connecting cylinder positioned inward from the inner surface of the steel pipe columns, and concrete filled into the steel pipe columns and the connecting cylinder.

Japanese Patent Application Publication No. 02-080729 Japanese Patent Application Publication No. 04-106240

The invention described in Patent Document 1 uses steel pipes with internal ribs at the column-beam joint to ensure adhesion between the steel pipe column and concrete at the column-beam joint, but has the problem of relatively low adhesion strength in areas other than the column-beam joint.

The invention described in Patent Document 2 eliminates the need for a diaphragm by increasing the thickness of only the connecting tube installed at the column-beam joint. However, even in this case, the bond strength in areas other than the column-beam joint is relatively low, and there is a risk that the force will not be transmitted sufficiently from the column steel pipe to the concrete compared to when a diaphragm is installed.

The present invention aims to provide a joint between a concrete-filled steel pipe column and a steel beam that compensates for the load-bearing capacity of a column-beam joint in a CFT structure without the need for a diaphragm, thereby improving the strength and durability of the entire structure.

The joint between a concrete-filled steel pipe column and a steel beam according to the present invention is characterized in that, at the joint between a concrete-filled steel pipe column and a steel beam, where concrete is filled inside the column steel pipe, a joint steel pipe that is thicker than the column steel pipes above and below is interposed in the section where the end of the steel beam is joined to the concrete-filled steel pipe column.

This invention is based on a CFT structure in which steel beams are joined to steel pipe columns and filled with concrete. At the joints between the steel pipe columns and steel beams in a CFT structure, by using thicker steel pipes at the joints where the ends of the steel beams are joined to the concrete-filled steel pipe columns, it is possible to eliminate the need for diaphragms, making it easier to fill with concrete. It also eliminates the need to provide haunches at the beam-column joints, and eliminates constraints such as adding diaphragms. Beams can be attached to any beam depth and at any position (height) within the range of the thick steel pipe joints.

Furthermore, in the joint between a concrete-filled steel pipe column and a steel beam of the present invention, the joint steel pipe may be thicker on the inside, and the outer surfaces of the joint steel pipe and the upper and lower column steel pipes may be flush.

Conventionally, force (column axial force) was transmitted from the column steel pipe to the column concrete via a diaphragm, but by eliminating the diaphragm, it is necessary to consider a method of transmitting force from the column steel pipe to the column concrete. When a column steel pipe and a thick-walled joint steel pipe are joined with their outer surfaces together, the joint steel pipe becomes thicker on the inside, creating an internal step at the joint between the column steel pipe and the joint steel pipe. It is thought that this internal step allows the vertical force from the steel beam to be transmitted to the concrete filled in the column steel pipe.

Furthermore, the joint between the concrete-filled steel pipe column and the steel beam according to the present invention may have a protrusion provided on the inner surface of a specified upper section of the steel column pipe connected to the lower side of the joint steel pipe. By providing a protrusion on the inner surface of the steel column pipe connected to the lower side of the joint steel pipe, force can be transmitted to the column concrete via the protrusion.

In addition, at the joint between a concrete-filled steel pipe column and a steel beam, a protrusion may also be provided on a predetermined lower section of the inner surface of the column steel pipe connected to the upper side of the joint steel pipe.

In addition, in the joint between a concrete-filled steel pipe column and a steel beam of the present invention, the joint steel pipe may be thicker on the outside, and the inner surfaces of the joint steel pipe and the upper and lower steel column pipes may be flush. In this case, there is no step on the inner surface of the joint between the thick-walled joint steel pipe and the steel column pipe, but the protrusions on the steel column pipe allow the vertical force from the beam to be transmitted to the concrete filled in the steel column pipe.

Furthermore, at the joint between a concrete-filled steel pipe column and a steel beam, the protrusions may be protrusions formed by weld beads or rebar welding, or protrusions on the inner surface of a checkered steel plate. The protrusions on the inner surface of the steel pipe column need not pose any problems in terms of strength or durability, and may be formed on the steel pipe column by, for example, welding beads or rebar. Furthermore, irregularities on the inner surface of a checkered steel plate can also be treated as protrusions.

Furthermore, at the joint between the concrete-filled steel pipe column and the steel beam of the present invention, a reinforcing bar cage consisting of main reinforcement and hoops may be inserted inside the concrete-filled steel pipe column. When installing a reinforcing bar cage in a conventional CFT structure, only a small reinforcing bar cage that matches the size of the concrete pour hole in the diaphragm can be used. However, since the present invention does not use a diaphragm, a larger reinforcing bar cage can be used, allowing the thickness of the steel pipe column to be thinner and suppressing local buckling.

Since the present invention is a joint between a concrete-filled steel pipe column and a steel beam as described above, it has the following effects.
(1) In conventional CFT structures using diaphragms, voids tend to form on the underside of the diaphragm, raising concerns about the ability to fill the concrete. However, in the present invention, by eliminating the diaphragm, the ability to fill the concrete is improved.

(2) There are no longer any restrictions such as providing haunches at the beam ends or adding diaphragms, and beams can be attached to any beam depth and at any position (height) within the range of the thick-walled steel pipe joints.

(3) In columns of CFT structures in which reinforcing bar cages are inserted, the cross-sectional dimensions of the reinforcing bar cages can be maximized by not installing a diaphragm, which allows the thickness of the column steel pipe to be thinner and also improves the effectiveness of suppressing local buckling.

(4) There is no diaphragm, improving design.

1 shows one embodiment of the joint between a concrete-filled steel pipe column and a steel beam according to the present invention, where (a) is an elevation view and (b) is a cross-sectional view taken along the line A-A of (a). Regarding the joint between a concrete-filled steel pipe column and a steel beam according to the present invention, (a) is an explanatory diagram showing the transmission of vertical force from the step when the outer surfaces of the joint steel pipe and the upper and lower column steel pipes are flush, (b) is the transmission of vertical force from the step and the inner protrusion when the outer surfaces of the joint steel pipe and the upper and lower column steel pipes are flush, and (c) is the transmission of vertical force from the inner protrusion. This shows the structure of a typical joint between a concrete-filled steel pipe column and a steel beam using a diaphragm, where (a) is a plan view and (b) is a cross-sectional view. 1A and 1B show a joint between a typical concrete-filled steel pipe column with a diaphragm and a reinforcing bar cage and a steel beam, where (a) is a cross-sectional view and (b) is a plan view. 1A and 1B show a case where a reinforcing bar cage is provided at the joint between a concrete-filled steel pipe column and a steel beam of the present invention, where FIG. 1A is a cross-sectional view and FIG. 1B is a plan view. The figures show the performance of reinforcing steel CFT structures, where (a) is a normal reinforcing steel CFT structure and (b) is a reinforcing steel CFT structure according to the present invention. This is a graph comparing the results of calculations for a CFT structure with a normal diaphragm, a CFT structure with a normal diaphragm and conventional reinforcing bars, and a CFT structure of the present invention with no diaphragm and reinforcing bars.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows the joint between a concrete-filled steel pipe column 1 and a steel beam 2 of the present invention, with (a) being an elevation view and (b) being an A-A cross-sectional view of (a). The present invention assumes a CFT structure in which concrete 1a is filled in a steel column pipe 1b. FIG. 1 shows a steel column pipe 1b with a square cross section joined to a steel beam 2 with a different web height. The steel pipe where the steel beam 2 is joined is a thick-walled joint steel pipe 3. By using a thick-walled joint steel pipe 3 at the joint between the concrete-filled steel pipe column 1 and the steel beam 2 and not providing a diaphragm, it becomes easier to fill the interior of the steel column pipe 1b. Furthermore, the joint steel pipe 3 shown in FIG. 1 is joined to the steel column pipe 1b so that their outer surfaces are flush with each other, making it thicker on the inside. As a result, an internal step 5 occurs at the joint between the steel column pipe 1b and the joint steel pipe 3.

In addition, inner protrusions 4 are provided on the upper part of the inner surface of column pipe 1b connected to the lower side of joint steel pipe 3, and on the lower part of the inner surface of column pipe 1b connected to the upper side of joint steel pipe 3. By providing protrusions on the inner surface of column pipe 1b near joint steel pipe 3, it is possible to transmit force to the column concrete via the protrusions. Note that inner protrusions 4 may be provided only on the upper part of the inner surface of column pipe 1b connected to the lower side of joint steel pipe 3, rather than on the lower part of the inner surface of column pipe 1b connected to the upper side of joint steel pipe 3.

Figure 2(a) shows the transmission of vertical force from a step at the joint between a concrete-filled steel pipe column 1 and a steel beam 2 of the present invention when the outer surfaces of the joint steel pipe 3 and the upper and lower steel column pipes 1b are flush. The steel column pipe 1b and the thick-walled joint steel pipe 3 are joined with their outer surfaces facing each other, and the steel beam 2 is joined to the joint steel pipe 3. Because the steel column pipe 1b and the joint steel pipe 3 are joined with their outer surfaces facing each other, the joint steel pipe 3 is thicker on the inside, creating an inner step 5 at the joint between the steel column pipe 1b and the joint steel pipe 3. It is believed that this inner step 5 allows the vertical force from the steel beam 2 to be transmitted to the inside of the steel column pipe 1b.

Figure 2(b) also shows the transmission of vertical force from the step and inner protrusion 4 when the outer surfaces of the joint steel pipe 3 and the upper and lower steel column pipes 1b are flush. The inner step 5 transmits the vertical force from the steel beam 2 to the inside of the steel column pipe 1b, and the inner protrusion 4 on the inner surface of the steel column pipe 1b also transmits the vertical force from the steel beam 2 to the inside of the steel column pipe 1b.

Figure 2(c) is an explanatory diagram showing the transmission of vertical force from the inner protrusion when the inner surfaces of the joint steel pipe 3 and the upper and lower steel column pipes 1b are flush. The joint steel pipe 3 is thicker on the outside, and the inner surfaces of the joint steel pipe 3 and steel column pipe 1b are flush. As in (a) and (b), there is no inner step 5 at the joint between the joint steel pipe 3 and steel column pipe 1b, and the inner protrusion 4 on the inner surface of steel column pipe 1b transmits the vertical force from the steel beam 2 to the inside of steel column pipe 1b. Furthermore, the inner protrusion 4 on the inner surface of steel column pipe 1b does not need to pose any problems in terms of strength or durability, and can be formed on steel column pipe 1b by, for example, welding a weld bead or rebar.

Figure 3 shows the structure of a typical joint between a concrete-filled steel pipe column 1 and a steel beam 2 using a diaphragm 7, while Figure 4 shows a typical joint between a concrete-filled steel pipe column 1 and a steel beam 2 equipped with a diaphragm 7 and a reinforcing bar cage 6. The column steel pipe 1b is equipped with a diaphragm 7, which has a concrete pouring hole 7a and an air vent hole 7b. As shown in Figure 4, the reinforcing bar cage 6 is fitted to the size of the concrete pouring hole 7a provided in the center of the diaphragm 7. In conventional CFT structures, the cross-sectional size of the reinforcing bar cage 6 is determined to match the size of the concrete pouring hole 7a in the diaphragm 7, as shown in Figure 4, which results in a small diameter for the reinforcing bar cage 6.

Figure 5 shows the case where a reinforcing bar cage 6 is installed at the joint between a concrete-filled steel pipe column 1 of the present invention and a steel beam 2. In the CFT structure of the present invention, a diaphragm is not required by using a thick-walled joint steel pipe 3 for the column steel pipe 1b, which is the joint with the steel beam 2. This allows the cross section of the reinforcing bar cage 6 to be rectangular, following the inner surface of the joint steel pipe 3, and the cross-sectional dimensions of the reinforcing bar cage 6 can be increased. In addition, because the column steel pipe 1b has an inner protrusion 4, force can be transmitted to the column concrete via the inner protrusion 4. The inner protrusion 4 may be installed only on the upper part of the inner surface of the column steel pipe 1b, which is connected to the lower side of the joint steel pipe 3.

Figure 6 shows the performance of a reinforcing steel CFT structure, with (a) being a regular reinforcing steel CFT structure and (b) being a reinforcing steel CFT structure according to the present invention. Compared to a regular reinforcing steel CFT structure, the reinforcing steel CFT structure according to the present invention allows for larger cross-sectional dimensions (width and diameter) of the reinforcing steel cage, and also allows for a larger amount of main reinforcement in the reinforcing steel cage, thereby increasing the burden of the reinforcing steel. As a result, the reinforcing steel CFT structure according to the present invention can achieve higher performance than a regular reinforcing steel CFT structure. Conversely, the increased burden of the reinforcing steel can also reduce the burden of the steel pipes and concrete columns, resulting in cost savings due to the reduced steel pipes and concrete.

Figure 7 shows a bar graph comparing the estimated steel amounts for a CFT structure with a conventional diaphragm (Figure 3), a CFT structure with a conventional diaphragm and conventional reinforcing bars (Figure 4), and a CFT structure of the present invention with no diaphragm and reinforcing bars (Figure 5).

For steel column pipes with cross-sectional dimensions of approximately 800mm x 800mm, the comparison was made by calculating the amount of steel = steel pipe weight + rebar weight. If the amount of steel in a CFT structure with a normal diaphragm and no rebar, as in Figure 3, is set to 100, then the amount of steel in a CFT structure with a diaphragm and rebar, as in Figure 4, is 101. Furthermore, the amount of steel in the CFT structure of the present invention, which has no diaphragm and rebar, is 97. It can be seen that the amount of steel used in this invention is roughly the same as that used in a normal CFT structure and a conventional CFT structure with rebar.

Furthermore, if the steel weight breakdown is such that the steel weight of a CFT structure with a normal diaphragm and no rebar is 100, the steel pipe weight for a CFT structure with a diaphragm and rebar inserted is 92.3 and the rebar weight is 8.3, while the steel pipe weight for the CFT structure of the present invention with no diaphragm and rebar inserted is 86.2 and the rebar weight is 11.1.

Since rebar is cheaper in construction cost per unit weight (= material cost + construction cost) than steel pipe, for the same amount of steel, the higher the proportion of rebar weight, the cheaper the construction cost. The CFT structure of the present invention uses the same amount of steel as a regular CFT structure or a conventional rebar-reinforced CFT structure, but allows for a higher proportion of rebar weight, which is expected to result in significant cost reductions.

1...Concrete-filled steel pipe column 1a...Filled concrete 1b...Column steel pipe 2...Steel beam 3...Joint steel pipe 4...Inner surface protrusion 5...Inner surface step (step between joint steel pipe and column steel pipe)
6...Reinforcing bar cage 6a...Main reinforcement 6b...Hoop reinforcement 7...Diaphragm 7a...Concrete pouring hole 7b...Air vent hole

Claims (6)

A joint between a concrete-filled steel pipe column and a steel beam, in which concrete is filled inside the column steel pipe, characterized in that a thicker-walled joint steel pipe than the upper and lower column steel pipes is interposed in the section where the end of the steel beam is joined to the concrete-filled steel pipe column, and a protrusion is provided in an upper specified section of the inner surface of the column steel pipe connected to the lower side of the thick-walled joint steel pipe, and no protrusion is provided on the inner surface of the thick-walled joint steel pipe. A joint between a concrete-filled steel pipe column and a steel beam according to claim 1, characterized in that the joint steel pipe is thicker on the inside, and the outer surfaces of the joint steel pipe and the upper and lower column steel pipes are flush with each other. The joint between a concrete-filled steel pipe column and a steel beam according to claim 1, characterized in that a protrusion is also provided in a predetermined lower section of the inner surface of the column steel pipe connected to the upper side of the joint steel pipe. A joint between a concrete-filled steel pipe column and a steel beam according to claim 1 or 3, characterized in that the joint steel pipe is thicker on the outside, and the inner surfaces of the joint steel pipe and the upper and lower column steel pipes are flush with each other. A joint between a concrete-filled steel pipe column and a steel beam according to claim 1 or 3, characterized in that the protrusions are weld beads, protrusions formed by rebar welding, or inner protrusions of a checkered steel plate. A joint between a concrete-filled steel pipe column and a steel beam according to any one of claims 1 to 3, characterized in that a reinforcing bar cage consisting of main reinforcement and hoops is inserted inside the concrete-filled steel pipe column.