Buckling restrained braced frame
A buckling restrained braced frame for use in light-framed constructions which includes a ductile load bearing core surrounded by a lightweight casing which resists buckling of the core under compressive loads.
1. Field of the Invention
The present invention relates to hysteretic damping elements for use in light-framed constructions, and in particular to a buckling restrained braced frame for use in light-framed constructions which includes a ductile load bearing core surrounded by a lightweight casing which resists buckling of the core under compressive loads.
2. Description of the Related Art
Shear stresses due to natural phenomena such as seismic activity and high winds can have devastating effects on the structural integrity of light-framed constructions. Lateral forces generated during such natural phenomena may cause the top portion of a wall to move laterally with respect to the bottom portion of the wall, which movement can result in structural failure of the wall and, in some instances, collapse of the building. One method of preventing damage due to shear stresses is through the use of buckling restrained braced frames. Originally developed and deployed in Japan, and now widely used in the United States, buckling restrained braced frames are installed in large steel and concrete frame buildings and skyscrapers to provide passive hysteretic damping of shear stresses generated during seismic activity and high winds.
Buckling restrained braced frames used in such large-scale constructions comprise a central steel core mounted diagonally within a structural steel frame. The steel core is capable of withstanding high tensile loads. In order to provide stiffness under compressive loads, the central steel core is conventionally encased over its length in a steel tube filled with concrete or mortar. The concrete-filled steel tube prevents buckling of the steel core under compressive forces. Buckling restrained braced frames absorb the energy generated in seismic and other shear stress events by repeatable yielding without failure in both tension and compression to thereby prevent damage to the primary structural frame.
For the braced frame to yield properly under tensile and compressive loads, it is important that the loads are borne by the steel core as opposed to the casing. A slip interface, or “unbending” layer, is therefore provided between the steel core and the surrounding concrete to decouple the steel core from the concrete and ensure that compression and tensile loads are carried only by the steel core. In conventional braced frames, the materials and geometry of the slip layer had to be carefully designed and constructed to allow relative movement between the steel core and the concrete due to shearing and Poisson's effect, while simultaneously inhibiting local buckling of the core as it yields in compression.
Owing to their construction, conventional buckling restrained braced frames are not well suited for use in light-framed constructions. While effectively used in large-scale, multistory buildings and skyscrapers, the expense and offsite manufacturing requirements of conventional braces, together with their weight, have prevented their use in light-framed constructions. Moreover, upon a seismic event, it is impossible to inspect the steel core of conventional braced frames without having to destroy the braced frame itself.
SUMMARY OF THE INVENTIONIt is therefore an advantage of the present invention to provide a buckling restrained braced frame specifically adapted for use in light-framed constructions.
It is a further advantage of the present invention to provide a buckling restrained braced frame providing stable and predictable hysteretic behavior.
It is another advantage of the present invention to provide a buckling restrained braced frame which is easy to manufacture and which may be easily retrofit into existing light-framed constructions.
It is a still further advantage of the present invention to provide a buckling restrained braced frame which allows the integrity of the core to be inspected without having to replace the entire braced frame.
It is another advantage of the present invention to provide an effective buckling restrained braced frame without the use of a slip layer between the core and the casing composed of a separate material layer.
These and other advantages are provided by the present invention which in embodiments relates to a buckling restrained braced frame for use in light-framed constructions. The buckling restrained braced frame includes a frame and a buckling restrained brace diagonally mounted to opposed corners of the frame. The buckling restrained brace includes a ductile load-bearing core surrounded by a lightweight casing which resists buckling of the core under compressive loads. In embodiments of the present invention, the core may be formed of a steel rod having a circular cross-sectional area, and the casing may be formed of wood or other lightweight material having an opening defined through its longitudinal center for receiving the core. The casing may be formed from two separate halves which are affixed together to prevent slip with respect to each other by various affixation methods. Alternatively, the casing may be a single, unitary construction. Both the casing and the core may have a variety of cross-sectional shapes, and the opening in the casing in which the core is positioned may or may not conform to the shape of the core in alternative embodiments.
Tubular extensions may be connected to each end of the core to maintain the casing in position around the core, and to allow affixation of the brace to the corners of the frame. The extensions fit partially within recessed sections formed in the ends of the casing, and may be screwed, welded bolted, glued and/or otherwise affixed onto the ends of the core.
With such a configuration, tensile and compressive loading of the buckling restrained brace resulting from shear stresses within a wall are effectively dampened by repeatable yielding of the buckling restrained brace in both tension and compression. The inherent ductile properties of the core allow the buckling restrained brace to yield under tensile loads, and the restraint of the casing against the core allows the buckling restrained brace to yield under compressive loads. Moreover, as there is low friction between the core and the casing, the tensile and compressive loads are borne by the core and are not transferred to the casing.
BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will now be described with reference to the drawings in which:
The present invention will now be described with reference to
Referring now to
Referring to
According to the present invention, the casing 112 may be formed for example of wood having an opening 114 (
In embodiments of the present invention, casing 112 may be formed from two separate halves 112a and 112b such as shown in
The halves 112a, 112b are affixed together to prevent slip with respect to each other. Various affixation methods may be applied to prevent slip between the halves 112a, 112b including an adhesive, epoxy, screws, nails, bolts, or a combination thereof. The affixation method may additionally include one or more mending plates at the interface between halves 112a, 112b. Such mending plates, available for example from Simpson Strong-Tie, Co., Inc., of Dublin, Calif., may include sharp prongs protruding from the front and back surfaces of the plate which can be driven into the respective opposed surfaces of the halves 112a, 112b when the halves are affixed to each other to prevent slip between the halves.
The halves may additionally or alternatively be affixed to each other by one or more ropes, straps or bands applied tightly around the halves 112a, 112b. A further method of preventing slip between the halves 112a, 112b that may be used in addition to or instead of the above-described methods is a wrap applied around the outer surface of the casing once the halves are joined together. The wrap may be applied in liquid form that dries hard, and may be for example a fibrous resin.
As shown for example in
As indicated above, casing 112 may alternatively be formed of other lightweight metals and materials in a single, unitary construction, such as for example aluminum, copper, brass, plastic or fiberglass. In such embodiments, the opening 114 may be formed during an extrusion, casting or other process for fabricating the unitary casing. A casing 112 formed of these lightweight metals and materials may alternatively be formed in separate halves, as explained above, having channels 116a, 116b which are then affixed together using at least some of the affixation methods described above.
In addition to the advantages of using lightweight materials in light-framed constructions, the material of the casing 112 and the fit of the core 110 within the opening 114 allow the buckling restrained brace to provide hysteretic damping of shear forces on the BRBF 100 without the use of a slip layer conventionally required between the casing and core. In particular, while the core undergoes axial loads (i.e., along the central axis of the core), the relatively low friction between the casing and core prevents the core from transferring any appreciable axial loads to the casing.
The buckling restrained brace 104 may further include tubular extensions 120 connected to each end of core 110. The extensions may be formed of the same material as the core 110 or another material having a like coefficient of thermal expansion so that the extensions remain securely fixed to the ends of the core 110 at different temperatures and conditions. In embodiments of the invention, each tubular extension 120 includes a threaded bore 122 (
The extensions 120 are provided to maintain the casing 112 in position around the core 110, and protrude out of the ends of the casing 112 as shown to allow affixation of the brace 104 to the frame 102. In one embodiment, a bolt 126 may fit through holes formed in the front and rear gusset plates 106 at opposed corners of frame 102, and a hole 128 may be formed through each tubular extension allowing the brace 104 to be bolted to frame 102. The bolts 126 through holes 128 allow some relative movement between the frame 102 and brace 104 without generating stress within the tubular extensions 120 or gusset plates 106 as a result of such relative movement.
The extensions 120 fit within recessed sections 130a, 130b (
The shape of the core 110 may vary from circular in alternative embodiments. For example,
In the embodiment shown in
It is further understood that the shape of the outer surface of casing 112 and/or the shape of opening 114 may vary in alternative embodiments. For example, the casing may have a circular cross-section as shown in
In a further alternative embodiment, each of the halves themselves may be formed in substantially triangular sections that fit together as shown in
Up to this point, casing 112 has been described as being either unitary or formed of two separate halves. However, in a further alternative embodiment, the casing 112 may be formed of more than two halves. For example, as shown in
It is understood that the various embodiments described above which are shown in, or are described variations of,
A further alternative embodiment of the present invention is shown in
As best seen in the cross-sectional view of
A spring 162 is provided around the core 110 within the gap between the conduits 150 and 152. When assembled, spring 162 is compressed between the conduit ends 156 and 158. In addition to maintaining the conduit ends 154 and 158 in contact with the respective tubular extensions 120, the spring 162 also provides some resistance to buckling of the core.
In order to provide greater resistance to core buckling at the gap between the conduits, a slightly larger diameter conduit 164 may be placed over the spring 162 and has ends which overlap by a few inches the ends 156 and 160 of the conduits 150 and 152. In embodiments, the wire diameter of the coil spring 162 is provided to be the same as the wall thicknesses of conduits 150 and 152. Thus, when assembled, the spacing between the core 110 and spring 162 may be the same as the spacing between the core 110 and conduits 150, 152. At the same time, the conduit 164 may be slid into position over both the conduits 150, 152 and the spring 162. The conduit 164 may be approximately 4 to 6 inches long, but it is understood that it may be shorter or longer than that in alternative embodiments. The conduit 164 may be held in place as by welding, screws, bolting, glue and/or other affixation methods.
Referring to
In the embodiments of the present invention described to this point, the BRBF 100 includes a frame 102 and a single buckling restrained brace 104 diagonally mounted in frame 102.
The BRBF 100 is scalable to different sizes, and may be provided with different aspect ratios. In one embodiment, the frame 102 may be 4 feet wide by 8 feet high. In such an embodiment, by way of example only and not limiting on the invention, the buckling restrained brace 104 shown in
The BRBF 100 can be mounted within a wall of a light-framed construction to add structural rigidity and resistance to shear. It may be mounted to an underlying support surface which may be a concrete building foundation, a floor diaphragm on the building foundation or a floor diaphragm on a top plate of a lower floor. It may be fastened to the underlying support surface by means of anchors provided through holes formed in the lower bar of frame 102. Other anchoring mechanisms may be used in alternative embodiments, such as for example by strap anchors, mudsill anchors, retrofit bolts, foundation plate holdowns, straps, ties, nails, screws, framing anchors, ties, plates, straps or a combination thereof. The BRBF 100 may similarly include holes in the top bar of the frame 102 for affixation to a top plate of a wall as by bolts or other anchoring mechanisms described above.
Upon both tensile and compressive loading of the buckling restrained brace 104 by shear forces exerted on a wall, for example during seismic activity or high winds, the brace 104 may repeatably yield in both tension and compression to provide hysteretic damping of the shear forces. The inherent ductile properties of the core 110 allows the buckling restrained brace to yield under tensile loads and the restraint of the casing 112 against the core 110 allows the buckling restrained brace to yield under compressive loads, thus damping the shear stresses that would otherwise bear on the frame 102. Moreover, as previously indicated, the nature of the materials used for the core and casing allow the tensile and compressive loads to be borne by the core and not the casing.
A further advantage of the BRBF 100 according to the present invention is that they may be constructed on-site during construction. Additionally, after a seismic event, the casing 112 may be easily removed and the core inspected. Thus, if the integrity of the core remains intact, a new casing may be assembled around the core and no replacement of the BRBF 100 is necessary.
Although the invention has been described in detail herein, it should be understood that the invention is not limited to the embodiments herein disclosed. Various changes, substitutions and modifications may be made thereto by those skilled in the art without departing from the spirit or scope of the invention as described and defined by the appended claims.
Claims
1. A buckling restrained brace for use in a buckling restrained braced frame, comprising:
- a core; and
- a casing surrounding at least a portion of the core for resisting buckling of the core under compressive loads on the core, the casing being formed of wood.
2. A buckling restrained brace as recited in claim 1, wherein the casing is formed from two separate halves that are affixed together.
3. A buckling restrained brace as recited in claim 2, wherein the separate halves are affixed to each other by at least one of an adhesive, an epoxy, screws, nails, bolts, and a mending plate provided at an intersection between the halves.
4. A buckling restrained brace as recited in claim 2, wherein the separate halves are affixed to each other by at least one of a rope, a strap, and a band applied around the halves.
5. A buckling restrained brace as recited in claim 2, wherein the separate halves are affixed to each other by a wrap applied around the outer surface of the casing once the halves are joined together.
6. A buckling restrained brace as recited in claim 2, wherein the separate halves are affixed to each other by means for preventing slip between the halves.
7. A buckling restrained brace as recited in claim 2, wherein each half of the casing is formed with a channel for receiving the core when the halves are affixed together, the core lying in contact with at least one surface defining at least one of the channels when the halves are affixed together.
8. A buckling restrained brace as recited in claim 1, wherein the core is ductile.
9. A buckling restrained brace as recited in claim 1, wherein the core has a circular diameter through a plane perpendicular to a central axis of the core.
10. A buckling restrained brace as recited in claim 1, wherein the core has a diameter through a plane perpendicular to a central axis of the core in the shape of one of a square, a rectangle, an oval, an ellipse, or cross-shaped.
11. A buckling restrained brace as recited in claim 1, wherein the casing has an outer surface which is in the shape of one of a square, a rectangle, a circle, an oval, and an ellipse.
12. A buckling restrained brace as recited in claim 1, wherein the casing has an outer surface including rounded edges.
13. A buckling restrained brace as recited in claim 1, wherein the casing includes an opening through its center, the opening being substantially square.
14. A buckling restrained brace as recited in claim 1, wherein the casing includes an opening through its center, the opening substantially conforming in shape to the core.
15. A buckling restrained brace as recited in claim 1, wherein the casing is formed of four separate sections which may be affixed to each other to define an opening capable of at least partially enclosing the core.
16. A buckling restrained brace as recited in claim 1, further comprising a pair of tubular extensions affixed to first and second ends of the core, the pair of tubular extensions affixing the core to a frame of the buckling restrained braced frame.
17. A buckling restrained brace as recited in claim 16, wherein a tubular extension of the pair of tubular extensions has a threaded bore for mating with threads on the first or second end of the core.
18. A buckling restrained brace as recited in claim 16, wherein a tubular extension of the pair of tubular extensions is welded to the first or second end of the core.
19. A buckling restrained brace for use in a buckling restrained braced frame, comprising:
- a core; and
- a casing surrounding at least a portion of the core for resisting buckling of the core under compressive loads on the core, the casing being formed of a material from a group of materials including: wood, metals including aluminum, copper, brass, bronze and alloys thereof, polymers including rigid plastic, and fiberglass.
20. A buckling restrained brace as recited in claim 19, wherein the casing is formed from two separate halves that are affixed together.
21. A buckling restrained brace as recited in claim 19, wherein the casing is formed from a unitary member with an opening capable of at least partially enclosing the core.
22. A buckling restrained brace as recited in claim 20, wherein the separate halves are affixed to each other by means for preventing slip between the halves.
23. A buckling restrained brace as recited in claim 19, wherein the core has a diameter through a plane perpendicular to a central axis of the core in the shape of one of a circle, a square, a rectangle, an oval, an ellipse, or cross-shaped.
24. A buckling restrained brace as recited in claim 19, wherein the casing has an outer surface which is in the shape of one of a square, a rectangle, a circle, an oval, and an ellipse.
25. A buckling restrained brace as recited in claim 19, wherein the casing has an outer surface including rounded edges.
26. A buckling restrained brace as recited in claim 19, wherein the casing includes an opening through its center, the opening substantially conforming in shape to the core.
27. A buckling restrained brace as recited in claim 19, further comprising a pair of tubular extensions affixed to first and second ends of the core, the pair of tubular extensions affixing the core to a frame of the buckling restrained braced frame.
28. A buckling restrained brace for use in a buckling restrained braced frame, comprising:
- a core; and
- means for resisting buckling of the core under compressive loads on the core.
29. A buckling restrained braced frame for hysteretic damping in light-framed constructions, comprising:
- a frame; and
- a buckling restrained brace, including a core, and a casing surrounding at least a portion of the core for resisting buckling of the core under compressive loads on the core, the casing being formed of a material from a group of materials including: wood, metals including aluminum, copper, brass, bronze and alloys thereof, polymers including rigid plastic, and fiberglass.
30. A buckling restrained brace as recited in claim 29, wherein the casing is formed from two separate halves that are affixed together.
31. A buckling restrained brace as recited in claim 29, wherein the casing is formed from a unitary member with an opening capable of at least partially enclosing the core.
32. A buckling restrained brace as recited in claim 30, wherein the separate halves are affixed to each other by means for preventing slip between the halves.
33. A buckling restrained brace as recited in claim 29, wherein the casing has an outer surface which is in the shape of one of a square, a rectangle, a circle, an oval, and an ellipse.
34. A buckling restrained brace as recited in claim 29, wherein the casing has an outer surface including rounded edges.
35. A buckling restrained brace as recited in claim 29, wherein the casing includes an opening through its center, the opening substantially conforming in shape to the core.
36. A buckling restrained brace as recited in claim 29, further comprising a pair of tubular extensions affixed to first and second ends of the core, the pair of tubular extensions affixing the core to a frame of the buckling restrained braced frame.
37. A buckling restrained brace for use in a buckling restrained braced frame capable of hysteretic damping of shear forces, the buckling restrained brace comprising:
- a core; and
- a casing surrounding at least a portion of the core for resisting buckling of the core under compressive loads on the core, the casing being formed of a material from a group of materials including: wood, metals including aluminum, copper, brass, bronze and alloys thereof, polymers including rigid plastic, and fiberglass;
- wherein tensile and compressive loads exerted on the core are not transferred to the casing without employing a slip layer between the core and the casing.
38. A buckling restrained brace for use in a buckling restrained braced frame, comprising:
- a core having a length;
- a first conduit circumjacent about the core;
- a second conduit circumjacent about the core, the first and second conduits having a combined length less than the length of the core to define a gap between the first and second conduits; and
- a spring wrapped around the core within the gap between the first and second conduits, the spring biasing the first and second conduits away from each other.
39. A buckling restrained brace as recited in claim 38, further comprising:
- a third conduit circumjacent about the spring and at least end portions of the first and second conduits adjacent the gap.
40. A buckling restrained brace as recited in claim 39, further comprising:
- a casing surrounding the third conduit, and at least a portion of the first conduit, the second conduit and the core for resisting buckling of the core under compressive loads on the core, the casing being formed of wood.
41. A buckling restrained brace as recited in claim 38, the first and second conduits formed of steel.
42. A buckling restrained brace for use in a buckling restrained braced frame, comprising:
- a core having a length;
- a first conduit circumjacent about the core;
- a second conduit circumjacent about the core, the first and second conduits having a combined length less than the length of the core to define a gap between the first and second conduits;
- a spring wrapped around the core within the gap between the first and second conduits; and
- a third conduit circumjacent about the spring and at least end portions of the first and second conduits adjacent the gap.
43. A buckling restrained brace as recited in claim 42, further comprising:
- a casing surrounding the third conduit, and at least a portion of the first conduit, the second conduit and the core for resisting buckling of the core under compressive loads on the core, the casing being formed of wood.
Type: Application
Filed: May 18, 2004
Publication Date: Nov 24, 2005
Inventors: Steven Pryor (Dublin, CA), Patrick Flynn (Lafayette, CA), William Downs (Stockton, CA)
Application Number: 10/848,346