JP7687352B2 - Buckling Restrained Brace - Google Patents

Description

The present invention relates to a buckling restraint brace consisting of a steel core material and wooden restraint materials arranged around the core material.

In buildings, braces are sometimes installed as components to resist shear forces during earthquakes. Braces are installed diagonally within the plane of the column-beam frame, and are primarily used to resist shear deformation of a story caused by earthquake loads.
During an earthquake, axial tensile and compressive loads are repeatedly applied, so the braces are required to have sufficient strength and energy absorption capacity against both loads.

Generally, when an axial compressive load is applied, there is a risk of buckling occurring in a component, and buckling reduces the component's strength and deformation capacity.
Since braces can also experience a decrease in strength and energy absorption capacity due to buckling, there are buckling-restrained braces that consist of a core material that resists earthquake loads and a restraining material that is arranged to cover the core material to suppress buckling deformation of the core material.
10 and 11 show such a buckling restraint brace 31 disposed within the plane of a frame formed by columns 33 and beams 35.

The restraining material is required to have sufficient strength to resist bending deformation due to buckling of the core material, and the restraining material or its joints are required to have sufficient strength to resist localized compressive forces due to buckling deformation of the core material.
FIG. 12 is a cross-sectional view of a buckling restraint brace 31 consisting of a core material 37 and a restraint material 39. Conventional cross-sectional shapes of the core material 37 include a plate (see FIG. 12(a)), circle (see FIG. 12(b)), cross (see FIG. 12(c)), H-shape (see FIG. 12(d)), etc., and the restraint material 39 covering the core material 37 is often box-shaped (see FIGS. 12(a), (c), and (d)) or circle (see FIG. 12(b)).

Since the local compressive force due to the buckling deformation of the core material 37 is proportional to the compressive load acting on the core material and the amount of buckling deformation of the core material 37, the larger the gap between the restraining material 39 and the core material 37, the higher the strength required of the restraining material 39. For this reason, some constructions provide a filler 41 such as mortar in the gap between the restraining material 39 and the core material 37 (see FIG. 12(a)).
In addition, in order to prevent the axial force of the core material 37 from flowing to the restraint material 39, an unbond material 43 is applied or attached (see FIG. 12(a)).

The joints between both ends of the buckling restraint brace 31 and the frame can be of two types: bolt joints 45 shown in FIG. 10, which are joined by bolts; and pin joints 47 shown in FIG.
In either case, the core material 37 is made wider near the joint, high-strength steel is used, reinforcing ribs are provided, etc. to increase the strength of the core material and prevent plasticity, so that the joint does not break first. In addition, because the restraining material 39 is made of steel, localized breakage due to contact with the core material 37 is unlikely to occur.
Furthermore, since the restraint material 39 is made of a single steel member, there is no need to worry about breakage at the joint.

On the other hand, the use of wood has been promoted in recent years due to the issue of _CO2 emissions. Since wood burns up in the event of a fire, it is sometimes used for building columns and beams after being made fireproof by fireproofing. In addition, wood is sometimes used in some parts of steel structures, such as floor slabs, and the use of wood as a restraining material for braces is currently being considered.

Patent Document 1 proposes a buckling restraint brace that consists of a steel plate-shaped core material and a pair of wooden restraint materials arranged on both sides of the wide surface of the core material, with the two being bolted together at both axial ends.
Patent Document 1 also proposes a buckling restraint brace in which a pair of wooden restraint members are arranged to sandwich both sides of the web of a core material with an H-shaped cross section.

Patent Document 2 proposes a buckling restraint brace consisting of a steel plate-shaped core material, a pair of wooden restraint members arranged on both sides of the wide side of the core material, and a pair of wooden side plates arranged on both sides of the narrow side of the core material and joined to the restraint plates.

JP 2019-214881 A JP 2020-51186 A

The buckling restraint braces disclosed in Patent Documents 1 and 2 each have wooden restraint members joined with bolts.
When the core material buckles, the restraining material is pressed against the core material, and a tensile force acts on the bolted joint. In that case, the bolted joint (wood or bolt around the bolted joint) may break, and the desired buckling restraint effect may not be obtained. In particular, because wood has a low elastic modulus, the wood at the bolted joint may deform locally, which may increase the buckling deformation of the core material and the local compressive force.

Therefore, when a restraining material is bolted, the bolted joint must have a certain level of performance, but References 1 and 2 make no mention of the necessary performance, and there is a risk that the bolted joint may break when the core material buckles.

The present invention was made to solve these problems, and aims to provide a buckling-restrained brace that will not break at the bolt joints even when the core material buckles.

(1) A buckling restrained brace according to the present invention comprises a steel core material and a wooden restraint material disposed around the core material,
The core material comprises a plasticized portion in a longitudinal center portion and an elastic portion in a longitudinal end portion having a brace axial strength higher than that of the plasticized portion,
The restraint material is formed by bolting two or more wooden materials arranged so as to sandwich the core material,
The bolt joint portion is composed of at least a bolt, a nut, and a washer, and the bolts are arranged in two or more rows along the entire axial length of the brace material at both side positions of the core material,
The bolt insertion hole and the counterbore are provided at the position of the bolt joint in the restraint member,
The bolt joint is characterized in that its rigidity in the bolt axial direction and its yield strength in the bolt axial direction satisfy the following formulas.

(2) In addition, in the above-mentioned (1), the bending strength of the restraining material satisfies the following formula.

(3) In the above-mentioned (1) or (2), the core material has a rectangular cross section and is plate-shaped, and reinforcing ribs are welded to both surfaces of the plate at the longitudinal end of the core material,
The restraining material is composed of at least two pieces of laminated wood, the width direction of the laminated wood being arranged perpendicular to the width direction of the core material, and a groove is provided in the portion of the restraining material corresponding to the plasticized portion of the core material, which is shaped to match the shape of the core material.

(4) In addition, in the above (3), the two members of the laminated material that constitutes the restraining material, in which the groove is provided, are joined in a manner that prevents the brace material from shifting in the axial direction.

(5) In addition, in the above (3) or (4), the restraining material is made of two pieces of laminated wood, and the groove is formed by arranging the step-shaped portions of the two pieces of laminated wood facing each other, each having a step-shaped portion.

In the present invention, the rigidity and strength of the bolt joint in the axial direction of the bolt material are adjusted to satisfy predetermined values while taking into account the deformation of the bolt joint, so that destruction of the bolt joint when the core material is compressed can be prevented and a high buckling restraint force can be achieved.

1A is an explanatory diagram of a buckling restraint brace according to an embodiment of the present invention, and FIG. 1A shows the inside of the buckling restraint brace as seen from the arrow A-A in FIG. 2, and FIG. 1B is a view taken from the arrow B-B in FIG. 1A. FIG. 1( a ) is an explanatory diagram of a buckling restraint brace according to an embodiment of the present invention, and is a cross-sectional view taken along the line CC in FIG. FIG. 2 is an explanatory diagram of a buckling restraint brace according to an embodiment, and is a cross-sectional view taken along the line DD in FIG. 4A and 4B are diagrams showing the buckling restraint brace shown in FIG. 1 in a pre-assembled state, with FIG. 4A showing a portion corresponding to FIG. 2 and FIG. 4B showing a portion corresponding to FIG. 13A and 13B are diagrams showing another example of a restraining member in a pre-assembled state. FIG. 11 is an explanatory diagram of another embodiment of the restraining material (part 1). FIG. 2 is an explanatory diagram of another embodiment of the restraint material (part 2). FIG. 4 is an explanatory diagram of another embodiment of the restraining material (part 3). FIG. 1 is a diagram showing a model of a buckling restraint brace according to an embodiment. This is an explanatory diagram of a typical buckling restraint brace installed within the frame plane (part 1). This is an explanatory diagram of a typical buckling restraint brace installed within the frame plane (part 2). FIG. 1 is an explanatory diagram of the core material and restraint material of a typical buckling restraint brace.

A buckling restraint brace 1 according to this embodiment will be described with reference to FIGS. 1 to 3. FIG.
FIG. 1(a) is an explanatory diagram illustrating the internal structure of the buckling restraint brace 1, and shows the inside as seen through from the direction of the arrows A-A in FIG.
FIG. 1B is a diagram showing a state seen from the direction of arrows BB in FIG.
Furthermore, Fig. 2 is a cross-sectional view taken along line CC of Fig. 1(a), and Fig. 3 is a cross-sectional view taken along line DD of Fig. 1(a).

As shown in FIGS. 1 to 3, the buckling restraint brace 1 according to this embodiment is made up of a steel core material 3 and wooden restraint materials 5 arranged around the core material 3.
Each component will be described in detail below.

<Core material>
The core material 3 consists of a steel plate-shaped body with a rectangular cross section, and has a plasticized portion 7 in the longitudinal center and an elastic portion 9 at the longitudinal end portion, which has a higher brace axial strength than the plasticized portion 7.
To prevent the elastic part 9 from becoming plastic, the elastic part 9 has a higher brace axial strength than the plasticized part 7. Therefore, the member strength of the elastic part 9 is determined by the size and strength of the plasticized part 7. In this embodiment, reinforcing ribs 10 are welded to both surfaces of the plate of the elastic part 9 to increase the brace axial strength.
The reinforcing rib 10 is joined by fillet welding or partial penetration welding so as to be perpendicular to the wide surface of the core material 3. Therefore, in the joining area of the reinforcing rib 10, the core material 3 and the reinforcing rib 10 form a cross section.

In this embodiment, both ends of the core material 3 are bolted to the frame with high-strength bolts, so the ends of the elastic parts 9 have bolt holes 11 at the joints 13. However, in the case of pin joints, clevises are provided at the ends of the elastic parts 9.

<Restraint material>
The restraint material 5 is formed by bolting two wooden members arranged to sandwich the core material 3.
The restraining material 5 sandwiches the core material 3 from both sides, and therefore a groove 15 having a rectangular cross section that matches the shape of the core material 3 is provided in the restraining material 5 at a portion corresponding to the plasticized portion 7 of the core material 3 .
As shown in FIG. 4, the grooves 15 may be provided in both wooden members constituting the restraining member 5, or may be provided in the wooden member on one side.
The groove shape is preferably adjusted so that the gap between the groove and the core material 3 is as small as possible.
The restraint member 5 is provided with a bolt hole 16 and a countersink 17 for inserting a bolt 18 therethrough (see FIGS. 4 and 5).

As a method for eliminating the need for groove machining, a pair of restraining members 5 having a length equivalent to the groove length may be provided in the portion corresponding to the groove wall, i.e., the portion facing the narrow surface of the core material 3, as shown in Figure 5.
Moreover, the restraining material 5 needs to be provided with a groove 15 that is larger than the shape of the reinforcing rib 10 so as not to interfere with the reinforcing rib 10. If the restraining material 5 may interfere with the welded portion between the reinforcing rib 10 and the core material 3, the restraining material 5 may be cut out to match the weld shape.

The wood used for the restraint material 5 may be a wood material such as laminated lumber, LVL, or CLT. In addition, taking into consideration the initial shape irregularity of the core material 3, it is preferable to make the size of the groove 15 slightly larger than the cross-sectional shape of the core material 3.

<Bolt joint>
The bolt joint 14 comprises at least a bolt 18 inserted through the bolt hole 16, a nut 19, and a washer 21, but a washer, a disc spring, and an anti-loosening mechanism may also be provided as necessary.
At least one row of bolts 18 is provided on each side of the core material 3, for a total of two or more rows, so as not to interfere with the core material 3. Buckling of the core material 3 can occur along its entire length, so the bolts are provided along the entire length of the bolt joint 14 in the axial direction of the brace material.
In the example shown in FIG. 1, the bolts 18 are provided in two rows, but the present invention is not limited to this, and the bolts 18 may be provided in three or more rows.

The restraining material 5 has a pilot hole for passing the bolt 18 through, and a countersink 17 in the fastening portion where the nut 19 and washer 21 are placed. Generally, the diameter of the bolt pilot hole 16 is about 1 to 2 mm larger than the bolt diameter. The number of bolts, bolt size, washer size, wood species and strength, etc. must be adjusted so that the bolt joint 14 meets the required rigidity and strength. Also, the wood species, strength, and size of the restraining material 5 must be adjusted so that the bending strength of the restraining material 5 meets the required strength taking into account the elongation of the bolt joint 14.

In this embodiment, since the restraint members 5 arranged to sandwich the core material 3 are made of wood, it is conceivable that the wood will sink into the bolt joint 14 when the core material 3 buckles. Therefore, when taking such sinking into consideration, it is necessary that the rigidity in the bolt axial direction and the yield strength in the bolt axial direction of the bolt joint 14 satisfy the following formula.

The derivation of the above formula will now be described.
When using a single restraining member 5 with a flat cross section as in the existing structure, the gap s between the core material 3 and the restraining member 5 can be considered to be constant.
However, since the restraint material 5 in this embodiment is formed by bolting a pair of wooden members together, the restraint material 5 is pressed when the core material 3 is compressed, and the gap s between the core material 3 and the restraint material 5 becomes larger by the amount of the elongation of the bolt and the embedding deformation of the wooden member.
Therefore, the existing design method, which assumes a constant gap s, may underestimate the gap s and result in a risky design.

When the buckling shown in Fig. 9 is assumed, the local load per location is expressed as 4 dN _max s/l _n from the balance of moments within one buckling wavelength enclosed by the dotted line, and this local load occurs at L/l _n locations. Therefore, the total value P _B of the local load that presses the restraining material 5 due to the buckling of the core material 3 can be calculated as follows:

The above P _B indicates the total local load value when the gap dimension between the core material 3 and the restraining material 5 is the initial dimension s before buckling. If the total local load value due to this initial dimension s before buckling is P _B1 , the elongation δ _B1 of the bolt joint 14 of the restraining material 5 when P _B1 acts is expressed as follows.

As a result of the bolt joint 14 elongating by δ _B1 , the gap dimension between the core material 3 and the restraint material 5 increases from s to s + δ _B1 . The total local load value P _B2 at this time and the elongation δ _B2 of the bolt joint 14 of the restraint material 5 when P _B2 acts are expressed as follows.

Similarly, if the calculation is repeated for each elongation of the bolted joint 14 and the total local load value P _B is updated, it is shown as follows, and the formula below is the strength required for the bolted joint 14, that is, the right side of formula (2).

Here, x is any integer. As can be seen from the above formula, if K _B is not large enough, the total local load value P _B will diverge and become infinitely large. In other words, if K _B is not large enough, the bolt joint 14 will break. The condition for P _B not to diverge is as follows:

By rearranging the above equation for K _B , equation (1) is derived, which shows the rigidity required for the bolt joint 14 described above.
As described above, by satisfying the tensile rigidity and yield strength of the bolt joint 14 shown in equations (1) and (2), it is possible to prevent the bolt joint 14 from being destroyed by the load acting on the restraint material 5 due to buckling of the core material 3, and a sufficient buckling restraint effect can be expected.

The tensile rigidity and strength of the bolt joint 14 can be calculated as follows, assuming that the bolt 18 in the center and the wood at both ends (the parts where the washers are embedded) act as three series springs.

Next, the bending buckling constraint condition will be described.
According to a non-patent document (Guidelines for Vibration Control Design of Steel Structures, Architectural Institute of Japan, November 2014), buckling restraint conditions are required for buckling restrained braces to restrain bending deformation due to buckling of the core material 3.
The bending buckling constraint conditions shown in the non-patent document can be summarized as follows with respect to the bending strength M _yB of the constraint member 5:

However, the above formula is based on the premise that the restraining material 5 is a steel member that does not sink in like a wooden material. On the other hand, when a wooden material is used as the restraining material 5 as in the present invention, the gap dimension between the core material 3 and the restraining material 5 increases as described above, so there is a possibility that sufficient strength cannot be obtained with the above formula.
Therefore, it is necessary to take into consideration the elongation δ _B of the bolted joint 14, and the equation in this case is as follows:

As described above, according to this embodiment, the structure takes into consideration the possibility that the gap s will become larger, so even if the gap s becomes larger due to buckling, it is possible to obtain a buckling-restrained brace 1 that will not break at the bolt joint 14 even when the core material 3 buckles.

As shown in FIG. 6, the restraining material 5 may be a laminated material made up of a plurality of sawn boards 23 .
By using laminated timber, it is possible to prevent bending that is dependent on the direction of the wood fibers.

In addition, since two pieces of laminated wood are used for the restraint material 5, it is preferable to provide a mechanism for preventing the two pieces of laminated wood from shifting in the axial direction of the brace material.
As a mechanism for preventing slippage, as shown in Fig. 7, a wooden side panel 25 may be provided on the side of two pieces of laminated timber so as to span both pieces of laminated timber, and this may be doweled to the laminated timber with wooden dowels 27. Other mechanisms for preventing slippage include adhesively joining the laminated timber together, and using both dowels and adhesively joining the pieces.
Either joining method requires a shear strength sufficient to resist slippage between the two pieces of laminated timber.

By preventing the two laminated timber pieces from shifting in the axial direction of the brace material, the two pieces of laminated timber come together to resist bending during bending deformation, providing high rigidity and strength. This makes it possible to achieve a high buckling restraint effect with a smaller cross-sectional size.

In the example shown in FIG. 4, grooves are machined into the restraining material 5, but in order to facilitate machining, for example, as shown in FIG. 8, a step-shaped portion 29 may be formed on the opposing surfaces of a pair of restraining materials 5, and a groove 15 may be formed when the surfaces with the step-shaped portions 29 are joined together.
In this case, separate restraining members 5 are positioned at positions facing the two narrow surfaces of the plate-shaped core member 3 .

The step-shaped portion 29 in the restraining material 5 has a protruding shape of wood, but by making the step-shaped portion 29 of each restraining material 5 the same shape, the two restraining materials 5 have the same shape. This eliminates the need to manufacture them separately. Also, by making the protruding dimension of the step-shaped portion 29 the same as the narrow surface width of the core material 3, it is possible to adjust the gap between the core material 3 and the restraining material 5 to be minimized.

According to a non-patent document (Architectural Institute of Japan, Guidelines for Vibration Control Design of Steel Structures, November 2014), the buckling restraint brace 31 requires bending buckling restraint conditions to restrain bending deformation caused by buckling of the core material 37, and these bending buckling restraint conditions are as follows:

The structure to be considered is shown in Figures 1 and 2, and the core material 3 is a plate-shaped cross-sectional member made of 490N/mm class ² steel with a width of 120mm, a thickness of 25mm, and a shaft length of 4000mm.
The buckling restraint members are a pair of wooden members with grooves 15 formed to match the shape of the core material 3 as shown in FIG. 9, each having a width of 300 mm, a height of 150 mm, and a total cross-sectional width of 300 mm, a height of 300 mm, and a length of 4600 mm.
The wood has a bending Young's modulus of 8000N/mm ² , a bending strength of 31.5N/mm ² and a compressive strength of 8.1N/mm ² .
The tangent modulus _Et of the core material 3 is set to 0.05 times the Young's modulus.

The results of the study on bending buckling restraint conditions are shown in Table 1.

If the existing design methods for buckling restraint braces were used as is, as shown in Table 1, this structure would satisfy the design conditions and would be considered to have sufficient bending performance as a buckling restraint material.
However, the above is based on the premise that the gap s is constant.
Therefore, in the case of using wooden material as in the present invention, the above structure was examined to see whether it satisfied the requirements of the present invention when the gap s was 1 mm (case a) and when it was 0.2 mm (case b).

When the specifications of the bolt joint 14 and the wooden member are adjusted so that K _B =594 (kN/mm), T _B =636 (kN), and n=40, the results are as shown in Table 2.

As shown in case a, when the gap s is 1 mm, the formula (2) is not satisfied. This shows that the conventional design method satisfies the requirements, but in reality it is a dangerous design.
On the other hand, in case b where the gap s is 0.2 mm, both formulas (1) and (2) are satisfied, which shows that a safe design is possible even under the conditions of the present invention.

REFERENCE SIGNS LIST 1 buckling restrained brace 3 core material 5 restraint material 7 plasticized portion 9 elastic portion 10 reinforcing rib 11 bolt hole 13 joint portion 14 bolt joint portion 15 groove 16 bolt pilot hole 17 countersink 18 bolt 19 nut 21 washer 23 sawn board 25 wooden side panel 27 wooden dowel 29 stepped portion 31 buckling restrained brace (conventional)
33 Column 35 Beam 37 Core material 39 Restraint material 41 Filler material 43 Unbonded material 45 Bolted joint 47 Pin joint

Claims (6)

A buckling restraint brace comprising a steel core and a wooden restraint member disposed around the core,
The core material comprises a plasticized portion in a longitudinal center portion and an elastic portion in a longitudinal end portion having a brace axial strength higher than that of the plasticized portion,
The restraint material is formed by bolting two or more wooden materials arranged so as to sandwich the core material,
The bolt joint portion is composed of at least a bolt, a nut, and a washer, and the bolts are arranged in two or more rows along the entire axial length of the brace material at both side positions of the core material,
The bolt insertion hole and the counterbore are provided at the position of the bolt joint in the restraint member,
A buckling restraint brace characterized in that the rigidity of the bolt joint in the bolt axial direction and the strength in the bolt axial direction satisfy the following formulas.

A buckling restraint brace as described in claim 1, characterized in that the bending strength of the restraint material satisfies the following formula.

The core material has a rectangular cross section and is plate-shaped, and reinforcing ribs are welded to both surfaces of the plate at the longitudinal ends of the core material.
The buckling restraint brace of claim 1 or 2, characterized in that the restraint material is composed of at least two pieces of laminated wood, the width direction of the laminated wood is arranged perpendicular to the width direction of the core material, and a groove is provided in the portion of the restraint material corresponding to the plasticized portion of the core material to match the shape of the core material. The buckling restraint brace described in claim 3 is characterized in that the two members of the laminated material that make up the restraint material that have the grooves are joined in a way that prevents the brace material from shifting in the axial direction. The buckling restraint brace described in claim 3, characterized in that the restraint material is made of two pieces of laminated wood, and the groove is formed by arranging the step-shaped portions of the two laminated wood pieces facing each other, each of which has a step-shaped portion. The buckling restraint brace described in claim 4, characterized in that the restraint material is made of two pieces of laminated wood, and the groove is formed by arranging the step-shaped portions of the two laminated wood pieces facing each other, each of which has a step-shaped portion.