How to design a base plate

Abstract

To provide a design method for a base plate that can design the base plate safely on a construction method changing a S construction column or a CFT structure column to a RC construction column.SOLUTION: A design method for a base plate disclosed herein is a design method for a base plate provided at a lower part of a second column included in a stress changeover part in a joint structure for joining a first column with reinforced-concrete construction having column main reinforcements extending in a vertical direction and the second column with steel construction or concrete filling steel pipe structure placed above the first column through the stress changeover part. On the base plate, classifying by a neutral axis position ratio as the ratio of a neutral axis position not acting on bearing stress degree to the base plate size, a yield strength curve is calculated by considering concrete bearing stress degree acting on a part projecting from a joint steel pipe at an undersurface of the base plate.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for designing a base plate. [Background technology]

[0002] Traditionally, there have been cases where the stress on the ground floors has been switched from steel-framed (sometimes called "S construction") columns or concrete-filled steel pipe (sometimes called "CFT construction") columns on the ground floors to reinforced concrete (sometimes called "RC construction") columns on the basement floors. In this case, to ensure sufficient stress transfer at the switching point, it is common for the columns on the first two or three basement floors to be made of steel-framed reinforced concrete (sometimes called "SRC construction"). With SRC construction, depending on the size of the building, the columns may reach the foundation partway through the relevant section, increasing the number of labor hours and temporary structures, and the basement floors may become a narrow space, which is one factor in increasing costs.

[0003] To resolve these issues and problems, we have already devised a construction method that switches the steel columns or CFT columns on the ground floor to reinforced concrete columns on just one basement floor (see Patent Documents 1 and 2 below). In this construction method, by wrapping a steel pipe around the stress switching section (hereinafter, this section will be referred to as the "junction steel pipe"), it is possible to switch the stress on the ground floor on just one basement floor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-181350 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-181545 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a need to develop a specific design method for a construction method that switches steel columns or CFT columns to reinforced concrete columns on just one basement floor.

[0006] Therefore, the present invention provides a base plate design method that allows for safe design of base plates in a construction method in which steel columns or CFT structure columns are replaced with reinforced concrete columns. [Means for solving the problem]

[0007] The design method of the base plate according to the present invention is a design method for a base plate provided at the bottom of a second column provided with a stress switching section in a connection structure in which a first column of reinforced concrete construction having main column reinforcements extending in the vertical direction is connected to a second column of steel frame construction or concrete-filled steel pipe construction arranged above the first column via a stress switching section, and the design method is carried out by dividing the cases into two groups based on a neutral axis position ratio, which is the ratio of the neutral axis position at which no bearing stress acts on the base plate to the dimension of the base plate. In the case of conditional expression (A), The lower surface of the base plate Contact Consideration of bearing stress of concrete acting on the protruding part from the steel pipe According to the following conditional expressions (1) and (2), M BS / B BS D BS 2 σ B and N BS / B BS D BS σ B and are the coordinate axes Calculate the strength curve, The base plate is designed to satisfy the load-bearing curve.

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[0008] The base plate design method configured in this manner allows the base plate to be designed safely. [Effects of the Invention]

[0009] According to the present invention, the base plate can be designed safely. [Brief explanation of the drawings]

[0010] [Figure 1] 1(a) is a longitudinal cross-sectional view showing a part of the skeleton of a structure equipped with a column joint structure according to an embodiment of the present invention, and FIG. 1(b) is a cross-sectional view taken along line XX of FIG. 1(a). [Figure 2] 10 is an example showing the arrangement of stress switching parts in a structure including a column joint structure according to an embodiment of the present invention. [Figure 3] 10 is another example showing the arrangement of stress switching parts in a structure including a column joint structure according to an embodiment of the present invention. [Figure 4] 1A and 1B are diagrams showing the distribution of bearing stress around the base plate, where (a) is a vertical cross-sectional view and (b) is a horizontal cross-sectional view. [Figure 5] FIG. 1 is a diagram showing an effective bearing area and an effective support area. [Figure 6] 1A and 1B are diagrams showing the distribution of bearing stress when a tensile axial force is applied, where (a) is a vertical cross-sectional view and (b) is a horizontal cross-sectional view. [Figure 7] This figure shows the distribution of bearing stress when compressive axial force is dominant, (a) is a vertical cross-sectional view, and (b) is a horizontal cross-sectional view. [Figure 8] FIG. 10 is a diagram showing a comparison between the MN yield strength curve calculation results and the experimental results. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of a column joint structure according to the present invention will be described with reference to the drawings.

[0012] 1(a) and 1(b), in a structure 100 equipped with the column joint structure of this embodiment, the column main reinforcement bars 11 of a reinforced concrete column (first column) 1 are provided such that the lower main reinforcement bars 11A penetrate the slab 43 and are joined to the upper main reinforcement bars 11B via mechanical joints 15. In addition, a CFT column (second column) 2 of a concrete-filled steel pipe structure is fixed to a steel beam 42.

[0013] The joint structure between the RC column 1 and the CFT column 2 comprises an upper part 16 of the RC column 1, a lower part 23 of the CFT column 2, and a stress switching part 5 that joins the RC column 1 and the CFT column 2 in the axial direction, between a slab 43 of the underground structure and a slab 44 of the aboveground structure, which are arranged at a distance from each other above and below. At a stress switching section 5 that is lower than the height of one story between the upper and lower slabs 43, 44, the structure of the underground skeleton and the structure of the aboveground skeleton are switched.

[0014] The RC column 1 is a known reinforced concrete column with a plurality of main column reinforcements 11 made of reinforcing bars embedded inside a concrete portion 10, and has a rectangular cross-sectional shape. The main column reinforcements 11 extend in the column axis direction. The RC column 1 may also have a circular cross-section.

[0015] The CFT column 2 has a square steel pipe 20, a concrete portion 21 filled inside the steel pipe 20, and a base plate 22 that is square in plan view and attached to the lower end of the steel pipe 20. The outer shape of the steel pipe 20 in cross section is smaller than the outer shape of the RC column 1 in cross section. The outer shape of the base plate 22 in plan view is larger than the outer shape of the steel pipe 20 in cross section and smaller than the outer shape of the RC column 1 in cross section. The steel pipe of the CFT column 2 may be formed in a cylindrical shape.

[0016] Furthermore, an opening (not shown; the same applies below) is formed in the center of the base plate 22. This opening allows the concrete portion 21 of the CFT column 2 and the filled concrete portion 31 in the connecting steel pipe 3 (described later) to be formed integrally without being separated.

[0017] The stress switching part 5 comprises the base plate 22 of the CFT column 2, the upper part (main reinforcement part) 14 of the column main reinforcement 11, the connecting steel pipe 3 extending in the axial direction and arranging the upper part 16 of the RC column 1 and the lower part 23 of the CFT column 2 inside, and a filled concrete part 31 filled inside the connecting steel pipe 3. The RC column 1, which is an underground structure, and the CFT column 2, which is an above-ground structure, are connected via the stress switching part 5.

[0018] In this embodiment, the connecting steel pipe 3 is a square tubular steel pipe extending along the axial direction, and is placed above the concrete portion 10 of the RC column 1. The outer shape of the connecting steel pipe 3 in a cross section is the same as the outer shape of the RC column 1 in a cross section, and the outer surface of the connecting steel pipe 3 is formed flush with the outer surface of the RC column 1. The connecting steel pipe 3 may also be formed in a cylindrical shape.

[0019] The inside of the connecting steel pipe 3 is filled with a filled concrete section 31 from the lower end to the upper end of the connecting steel pipe 3. The concrete section 10 and the lower part of the filled concrete section 31 may be integrally constructed from precast concrete, and the filled concrete section 31 may be formed by pouring it from above on-site.

[0020] The lower part (column base) 23 of the CFT column 2 is inserted into the inside of the connecting steel pipe 3 from the upper end opening of the connecting steel pipe 3, and the lower part 23 of the CFT column 2 is fixed in the filled concrete part 31 inside the connecting steel pipe 3. The column base 23 of the CFT column 2 extends to the middle position in the axial direction of the connecting steel pipe 3, and a sufficient embedment length into the filled concrete part 31 is ensured, for example, an embedment length in the filled concrete part 31 that is more than twice the width or height of the CFT column 2.

[0021] Inside the connecting steel pipe 3, the anchoring portions 14 of the multiple column main reinforcements 11 extending from the upper portion 16 of the RC column 1 are inserted from the lower end openings of the connecting steel pipe 3, and are anchored together with the column base 23 of the CFT column 2 in the filled concrete portion 31 inside the connecting steel pipe 3. An enlarged anchoring end 13 is provided at the upper end of each column main reinforcement 11.

[0022] The anchoring portions 14 of the multiple column main reinforcements 11 extend to a height below the upper end of the connecting steel pipe 3, and there is a predetermined covering thickness between the upper end surface of the column main reinforcements 11 and the upper end surface of the filled concrete portion 31.

[0023] The upper parts of the multiple main column reinforcements 11 are arranged between the connecting steel pipe 3 and the base 23 of the CFT column 2, and are arranged in a rectangular shape in plan view along the inner surface of the connecting steel pipe 3 so as to surround the base 23.

[0024] As shown in Figures 2 and 3, the stress switching units 5 can be installed only on basement floors surrounded on all sides by the ground and basement exterior walls W. As shown in Figure 2, multiple stress switching units 5 may all be installed on the same floor (although in Figure 2 they are installed on the first basement floor, they may also be installed on other basement floors), or as shown in Figure 3, multiple stress switching units 5 may be installed across multiple floors.

[0025] Next, the design of the stress switching portion will be described. The design of this part is based on allowable stress design, so stress calculations based on elastic calculations are required. Therefore, the distribution of bearing stress acting on the underside of the base plate embedded in the connecting steel pipe through concrete and on the top surface of the protruding part is assumed to be a triangular distribution assuming plan view retention.

[0026] 1. Bearing stress distribution around the base plate Figure 4 shows the stress state around the base plate. Here, the position where the bearing stress becomes zero (neutral axis position x n : distance from the tip of the base plate), and σ BS is the edge stress acting on the underside of the base plate, and σ v is the edge stress acting on the top surface of the protruding part. Furthermore, the concrete strength (σ BS ) (bearing strength coefficient), the relationship between the following equations (1) and (2) holds.

[0027]

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[0028]

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[0029] 2. Evaluation of cross-sectional force acting on the neutral axis position Axial force acting on the base plate (N BS : Compression (positive) and bending moment (M BS ) is the neutral axis position ratio x n Depending on the range of possible values ​​of ´, it can be evaluated as follows:

[0030] <case1>In the case of the following formula (A) (see Figure 4):

[0031]

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[0032] [Axial force balance] This is expressed as the following equation (3).

[0033]

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[0034] From equation (1) and the following equation (B) (see FIG. 4), equation (3) becomes the following equation (4).

[0035]

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[0036]

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[0037] where α BS or α V If is given, then α BS Erase x n ´ and solve the quadratic equation (5) to get x n ´ is required.

[0038]

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[0039] In addition, x n If ' is given, then α in Eq. (2) BS and α V From this relationship, the axial force when the bearing strength or allowable stress is reached can be determined.

[0040] Bending moment The following equations (6) and (7) are obtained.

[0041]

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[0042]

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[0043] [Effective bearing area (A BS ) and effective bearing area (A ce )] If the distribution of bearing stress is not uniform, it is assumed that an eccentric load acts. The assumed effective bearing area and effective bearing area are shown in Figure 5. Here, the area (x b ) is calculated from the condition that the bearing pressure (axial force) is equal, and the effective bearing area is also given at the same time. BS ) can be calculated using the following equations (8) and (9).

[0044]

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[0045]

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[0046] On the other hand, the effective bearing area (A ce ) is given by the following equation (10) based on the idea shown in the "Prestressed Concrete Design and Construction Standards and Commentary 1998," which defines the bearing area as twice the distance from the load center to the edge.

[0047]

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[0048] <case2>In the case of the following formula (C) (mainly when tensile axial force is applied, see Figure 6)

[0049]

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[0050] Here, we consider the case where a tensile axial force is applied and consider the resistance mechanism provided by the protruding part of the base plate. The distribution of bearing stress when a tensile axial force is applied is shown in Figure 6. Note that the bearing stress on the underside of the base plate is not taken into consideration as it is assumed to be small. Bearing stress and stress gradient (σ´) at the CFT column core V ,Δσ V ) is expressed by the following equation (11).

[0051]

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[0052] <case2-1>In the case of the following formula (D):

[0053]

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[0054] [Axial force balance] The following equations (12) and (13) are obtained.

[0055]

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[0056]

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[0057] α V If x is given, n ´ is rearranged to obtain the following equation (14).

[0058]

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[0059] x n When ' is given, the following equation (15) is given.

[0060]

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[0061] Bending moment The following equations (16) and (17) are obtained.

[0062]

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[0063]

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[0064] <case2-2>In the case of the following formula (E):

[0065]

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[0066] [Axial force balance] The following equations (18) and (19) are obtained.

[0067]

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[0068]

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[0069] α V If x is given, n ´ is rearranged to obtain the following equation (20).

[0070]

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[0071] x n When ' is given, the following equation (21) is given.

[0072]

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[0073] Bending moment The following equations (22) and (23) are obtained.

[0074]

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[0075]

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[0076] <case3>In the case of the following formula (F) (when compressive axial force is predominant, see Figure 7)

[0077]

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[0078] Here, the bearing stress due to the upper surface of the protruding part of the base plate is ignored. Figure 7 shows the bearing stress distribution when the compressive axial force is dominant.

[0079] <case3-1>In the case of the following formula (G)

[0080]

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[0081] [Axial force balance] The following equations (24), (25), and (26) are obtained.

[0082]

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[0083]

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[0084]

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[0085] Bending moment The following equations (27) and (28) are obtained.

[0086]

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[0087]

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[0088] [Effective bearing area (A BS ) and effective bearing area (A ce )] The following equations (29) and (30) are obtained.

[0089]

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[0090]

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[0091] <case3-2>In the case of the following formula (H):

[0092]

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[0093] [Axial force balance] The following equations (31), (32), and (33) are obtained.

[0094]

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[0095]

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[0096]

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[0097] Bending moment The following equations (34) and (35) are obtained.

[0098]

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[0099]

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[0100] [Effective bearing area (A BS and effective bearing area (A ce )] The following equations (36) and (37) are obtained.

[0101]

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[0102]

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[0103] [Table 1]

[0104] The experimental values ​​of the axial force and bending moment acting on the base plate of each specimen were almost equal to or greater than the strength curve, which indicates that this strength curve provides a safe assessment.

[0105] The base plate design method configured in this manner allows the base plate to be designed safely.

[0106] The above describes one embodiment of the base plate design method according to the present invention, but the present invention is not limited to the above embodiment and can be modified as appropriate within the scope of the spirit thereof.

[0107] In the above embodiment, a CFT column has been described as an example of the second column, but the second column may also be an S column (a column of steel frame construction). [Explanation of symbols]

[0108] 1 RC pillar (first pillar) 2 CFT pillar (second pillar) 3 Jointed steel pipe 5 Stress switching section 10 Concrete section 11 Column main reinforcement 13 Fixed end 14 Fixing part 15 Mechanical couplings 20 Steel pipe 21 Concrete Section 22 Base Plate 23 Column base 31 Filled concrete section 42 Beam 43,44 Slabs

Claims

1. In a connection structure in which a first column of reinforced concrete construction having column main bars extending in the vertical direction is connected to a second column of steel frame construction or concrete-filled steel pipe construction arranged above the first column via a stress switching section, a design method for a base plate provided at the bottom of the second column equipped with the stress switching section is provided, In the case of conditional formula (A) which is divided into cases depending on a neutral axis position ratio, which is the ratio of the neutral axis position at which no bearing stress acts on the base plate to the dimension of the base plate, A base plate design method in which a strength curve with coordinate axes M BS / B BS D BS 2 σ B and N BS / B BS D BS σ B is calculated using the following conditional equations (1) and (2) taking into account the bearing stress of concrete acting on the protruding portion on the underside of the base plate that protrudes from the connecting steel pipe, and the base plate is designed to satisfy the strength curve. [Equation 1] [Equation 2]

2. In the case of conditional formula (B) which is classified according to the neutral axis position ratio, A method for designing a base plate as described in claim 1, in which the base plate is designed to satisfy the strength curve using the following conditional equations (3) and (4), which take into account the bearing stress of concrete acting on the protruding portion on the underside of the base plate that protrudes from the connecting steel pipe. [Equation 3] [Equation 4] [Equation 5]

3. In the case of conditional formula (C) which is classified according to the neutral axis position ratio, A method for designing a base plate as described in claim 1 or 2, in which the base plate is designed to satisfy the strength curve using the following conditional equations (5) and (6), which take into account the bearing stress of concrete acting on the protruding portion on the underside of the base plate that protrudes from the connecting steel pipe. [Equation 6] [Equation 7] [Equation 8]

4. In the case of conditional formula (D) which is classified according to the neutral axis position ratio, A method for designing a base plate according to any one of claims 1 to 3, in which the base plate is designed to satisfy the strength curve using the following conditional expressions (7) and (8), which take into account the bearing stress of concrete acting on the protruding portion on the underside of the base plate that protrudes from the connecting steel pipe. [Equation 9] [Equation 10] [0011]

5. In the case of conditional formula (E) which is classified according to the neutral axis position ratio, A method for designing a base plate according to any one of claims 1 to 4, in which the base plate is designed to satisfy the strength curve using the following conditional expressions (9) and (10), which take into account the bearing stress of concrete acting on the protruding portion on the underside of the base plate that protrudes from the connecting steel pipe. [0012] [0013] [0014]

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