INTRODUCTION This invention relates to sleeved bracing useful in the construction of earthquake resistant structures. This invention particularly relates to sleeved bracings useful for the construction of earthquake resistant framed buildings.

In order to understand the importance of the present invention, it would be beneficial to briefly explain the nature of the forces that act on a building/structure during an earthquake. For easy understanding the following description is given with reference to a building, but it should be born in mind that the same is applicable to any structure. During an earthquake, the ground on which a building structure is built, is subjected to the following primary vibratory motions: l Up and down motion Lateral drift Inverted Pendulum movement in the vertical plane Plan rotation Fig. la of the drawing accompanying this specification illustrates the framework of a typical multistory building comprising beams and columns. During the up and down vibratory motion of the ground the whole building moves up with a vertical acceleration as shown in Fig.. lb and then, after reaching a peak, will move downward with a vertical acceleration as shown in Fig. lc. This motion repeats during the duration of the earthquake. Because of the mass of the building and the vertical acceleration, the building frame will be subjected to additional vertical loads, up or down, depending on the direction of motion as shown by arrows in Figs. lb and lc. The beams and columns can be designed easily to withstand these additional loads.

During the lateral drift of the ground, the whole building will move laterally with an acceleration to one side as shown in Fig. ld and, after reaching a peak value of drift, will move in the opposite direction as shown in Fig. le. Because of the mass of the building and the lateral acceleration, the building frame will be subjected to cyclical lateral loads Fl, F2 & F3 as shown by arrows in Fig. ld and Fig. le. These lateral loads are very dangerous for the stability of the building frame. The design is also very complex.

Many such types of designs have been developed over the years to take care of these additional lateral loads due to the lateral drift of the building.

During the inverted pendulum motion of the ground, the whole building frame will rotate in a vertical plane with an angular acceleration and, after reaching a peak value of rotation, the building will rotate in the reverse direction. Because of the mass of the building and the angular acceleration, the building frame will be subjected to additional cyclical lateral loads Fl, F2 & F3 as shown by arrows in Figs. lf & Fig. lg.

During plan rotation of the ground, the building will rotate in plan with an angular acceleration and, after reaching a peak rotation, will rotate in the reverse direction. Because of the mass of the building and the angular acceleration, lateral forces will act on the frame as shown by arrows in Figs. Ih & li.

Many design procedures are available to design the building framework that can withstand these earthquake-induced additional lateral loads. In this context it is mentioned that many codes of practice in USA recommend that the building framework should remain elastic, or nearly so, under moderate earthquakes of frequent occurrence but must be able to yield locally without serious consequences if it is to resist a major earthquake.

In the description given below the state of art designs are discussed.

PRIOR ART Many types of structural frame configurations and designs to resist earthquake-induced loads are available presently. Some of the designs which are in common practice are described below.

Moment Resistant Frame design as shown in Fig. 3a of the drawing accompanying this specification.

Fig. 2a shows a normal building frame comprising beams (1) and columns (2) and the beams are simply supported on at the columns on seating cleats (3). Columns are supported on base plates (4).

Diagonal members are avoided so that it is easy to provide openings in the frames anywhere for fixing doors, windows service ducts etc. In the absence of diagonal members, the frame will sway laterally excessively as shown in Fig. 2b when lateral forces Fl, F2 & F3 act on the frame.

Therefore the connection between the beams and columns are made rigid to control the lateral drift of the building frame.

Fig. 3a shows the rigid frame design wherein, the building frame comprises beams (5), columns (6), stiffeners (7) and base plates (8) as shown in Fig. 2a.

Fig. (3b) shows a typical enlarged detail of rigid connection between beam and column Fig. (3c) shows view A-A of Fig. 3b.

Fig. (3d) shows the earthquake induced lateral forces Fl, F2 & F3 and the deflected shape of the frame. The end of beam (5) is connected to the flange of column (6) by full strength weld (9). Stiffeners (7) are welded to the column to prevent the column flange from opening out.

This forms a very rigid connection. (8) represents the base plate of the column (6) This moment resisting frame will be able to resist the lateral forces Fl, F2 & F32. This type of frame exhibits lower stiffness and higher ductility, which are desirable features in earthquake resistant structural systems. Energy is dissipated at the plastic hinge adjacent to the beam to column junction. Frequently, this system suffers severe drift as well as premature failure at connections and is rendered non-functional even after moderate earthquakes. Further, this system is not viable for tall buildings.

Frame with concentric'Tension only'intersecting diagonals system.

Fig. 4a shows a frame with concentric"tension only"intersecting diagonals.

Fig. 4b shows an enlarged detail of connection.

Fig. 4c shows the deflected shape of the frame.

Fig. 4d shows another deflected shape of the frame. In which 10-represents the columns of the frame.

11-represents the beams of the frame.

12-represents the concentric diagonal bracings fitted in the direction'X'. These are generally made of rolled steel angle sections.

13-represents the concentric diagonal bracings fitted in the direction'Y'. The diagonals (12) & (13) cross each other and hence these are known as intersecting diagonals.

14-represents an end plate welded to beam.

15-represents a gusset plate for attaching bracing (12).

16-represnts a gusset plate for attaching bracing (13).

(It is mentioned here that in actual practice, the two gussets (15 & 16) will be of different sizes depending on the force in the bracing member attached. Hence different numberings are given though both represents gusset plates) 17-represents the base plate.

Fl, F2 & F3 are the earthquake induced lateral load s acting on the frame at different floor levels.

Fig. 4b shows a typical joint of the frame. The beam (10) has an end plate (14) welded to it. The end plate has bolt holes for connecting to the column (11). The flange of the column (11) has matching bolt holes for connecting to end plate (14). A gusset plate (15) is welded to beam (10) and end plate (14) as shown. Diagonal bracing (13) is secured to the gusset plate (15) by bolts. A gusset plate (16) is welded to beam (10) and end plate (14) as shown. Diagonal bracing (12) is secured to the gusset plate (16) by bolts. In this connection the centerlines of column, beam bracings diagonals meet at point'a'and hence the bracing is called as concentric. In this design the tension diagonals (12) & (13) are very slender and they can resist tension well but buckle under even little compressive force.

When earthquake induced lateral forces Fl, F2 & F3 act at each floor level in the direction of the arrows as shown in Fig. 4c, the frame will deflect laterally as shown and the diagonals (12) will be subjected tension and the diagonal (13) will buckle due slight compressive force developed. When the direction of loading reverses as shown in Fig. 4d, diagonal (13) will be in tension and diagonal (12) will buckle and will become ineffective as shown. Therefore the diagonals (12 & 13) are designed to resist tension only.

This system resists the earthquake induced lateral loads very effectively because of the presence of diagonals in the framework. The connection details are also quite simple. However if the tension in the diagonals (12&13) exceeds their yield strength during a severe earthquake, they go in to a plastic state and absorb shock energy well. But however they will undergo permanent elongation in their length. Under repeated cyclic loading both the diagonals (12&13) undergo larger permanent elongation and the structure degrades. Afterwards, even under minor earthquakes, the lateral drift of the frame will be beyond acceptable limits.

Frame with Concentric Tension/Compression bracing system.

Fig. Sa shows the frame with concentric Tension/Compression diagonal bracing'wherein : 18-represents the beams of the framework.

19-represents the columns of the framework.

20-represents the Tension/Compression diagonal bracing.

21-represents an end plate welded to the end of beam (18).

22-represents a gusset plate for securing the diagonal (20) 23-represents the base plate.

An enlarged detail of the connection between the tension/Compression diagonal (20) and the beam and column junction is shown in Fig. 5b. The centerlines of beam, column and bracing meet at point 'g'and hence the bracing is concentric.

The diagonal (20) will be subjected to compression when the direction of lateral loads Fl, F2 & F3 are as shown by arrow marks in Fig. 5c. When the direction of loading reverses, as shown in Fig. 5d, the same diagonals will now be in tension.

In this design, when the diagonal (20) is in tension, it will undergo plastic deformation when subjected to load beyond its yield strength and absorb shock energy. However when the same diagonal is subjected to compression, it will buckle at a far lesser load without absorbing any shock energy. In order to prevent premature buckling, it is necessary to increase the stiffness of the diagonal (20) by adopting a much larger structural section. This makes the diagonal bracing member very heavy and expensive. The lateral drift of the building is very much less in this design but providing a very stiff diagonal bracing increases the total stiffness of the frame which in turn generates larger lateral shears (loads) at foundation level which is not desirable. Also, compression diagonals, when subjected to a compressive force beyond yield strength, will buckle suddenly without absorbing much energy.

Eccentric Bracing System This design is an improvement on the designs described earlier and is being currently extensively adopted all over the world and is shown in Fig. 6 wherein: 24-represents the beams of the framework.

25-represents the columns of the frame.

26-represents the eccentric diagonal bracing member, generally are made of rolled steel angles.

This bracing member is between two beams.

27-represents an eccentric bracing member between beam and column. 28-illustrates an end plate welded to end beam (24). This end plate has holes for securing the beam to the column.

29-represents a gusset plate welded to the beam (24) and has bolt holes for securing the bracing member.

30-represents the base plates welded to column.

31-represents a gusset plate welded to the columns and has bolt holes for securing the bracing member.

It can be seen in Fig. 6 that the centerline of bracing member (26) and the centerline of the beam (24) meet at point'k'whereas the centerline of column (25) and the centerline of beam (24) meet at point'h'. Thus there is an eccentricity of'el' (i. e. the distance h-k. Hence this bracing system is called eccentric bracing system.

The eccentric bracing system is of lesser stiffness as compared to concentric bracing systems.

Under severe seismic load a hinge is formed at point'k'leading to dissipation of considerable energy. However, Eccentric Braced Frame suffers considerable drift even under moderate earthquake, due to severe plastic hinge deformation of the beam link at point'k'. Even though the repair of this system, after suffering a moderate earthquake, is expensive, yet this system is currently very popular in its use.

2.5 Nippon Steel's'Unbonded Bracing members'.

Nippon steel company of Japan, in a published report in the year 1988, has described a bracing member called"Unbonded Braces"for use in the design of earthquake resistant frames as bracing diagonals. Our patent search shows that this system has not been patented by Nippon Steel Corporation. However this system uses the basic principles already disclosed in our Indian Patent No. 155036 for which an application was filed on 30t April 1981. The basic principles of the Nippon system is also disclosed in our US Patent 5,175,972 dated 5th Jan. 1993. Our patent application for the US Patent was made on in the year 1982, i. e., prior to the above-referred report of Nippon Steel. Therefore, a brief description of our said Indian and US patents is described below before describing the Nippon'System.

Our Indian Patent No. 155036 for a"column" Fig. 7a shows the column disclosed of the said Indian Patent.

Fig. 7b_ represents the view along section B-B of Fig. 7a 32-represents a sleeve of tubular cross section.

33-represents a core rod housed inside the sleeve (32) with a pre-determined annular gap between the core rod (33) and the inside surface of the sleeve (32). The core rod is projecting beyond the sleeve by a pre-determined amount.

34-represents a base plate secured to the sleeve (32) W-is the axial load acting on the core (32) only.

The column shown in Fig. 7a supports the load W in the following manner: The load W is resisted by the core rod (33) only and not by the sleeve (32). The core rod (32), being very slender, will try to buckle laterally when subjected to compressive load. Then the side of the core rod (33) will come in to contact with the inside surface of the sleeve (32). Then the sleeve (32), by the virtue of its flexural stiffness, will prevent any further lateral buckling of the core rod (33) and thus the core rod (32) alone supports the entire load and the sleeve (32) acts only as a buckling restraining member. It is possible to load the core rod (33) beyond its yield strength and make it absorb energy by giving suitable flexural stiffness to sleeve (32).

It is also disclosed in the said Indian Patent that"The gap between the core and the sleeve can be filled with rubber washers. This disclosure has a bearing on the said Nippon's bracing member and will be discussed later.

Our US Patent 5,175,972 dated 5th Jan. 1993.

Fig. 8a shows the Scaffolding Prop of the said US Patent.

Fig. 8b shows the view along section Bl-B1 of Fig. 8a.

35-represents plurality of core rods placed inside the sleeve (37) with a small pre-determined annular gap as shown in Fig. 8b. One long core rod can also be used instead of plurality of core rods.

36-represents a short length core rod with threads at top end. This core rod (36) projects slightly beyond sleeve (37). There is a pre-determined annular gap between the core rod (35) and the surface of the sleeve (37). This core rod (17) is freely resting on core rod (35).

37-represents a sleeve of tubular cross section.

38-represents a socket that can be screwed on to top core rod (36). This socket (38) should not come in to contact with the top edge of sleeve (37) when the said prop is carrying the load of slab (40) 39-represents a base plate that is rigidly secured to the sleeve (37). The bottom most core rod (35) freely rests on the base plate.

40-represents a roof slab of a building that is supported by the scaffolding prop.

The scaffolding prop of Fig. 8a supports the load of the roof slab in the following manner : The weight of the slab roof slab (40) is transferred to the ground through socket (38), core rod (36), core rods (35) and base plate (39) in sequence. The core rods (35), being discontinuous and very slender, will try to buckle laterally when subjected to compressive load due to the weight of the roof slab (40). Then the sides of the core rods will touch the inside surface of the sleeve (37). And the sleeve (37), by the virtue of its flexural stiffness, will prevent the further lateral buckling of the core rods (35) and the core rods thus will resist the entire load. The sleeve (37) does not carry any part of the applied load and acts only as a buckling restraining member. It is possible to load the core rod beyond its yield strength and make it to absorb shock energy by giving suitable flexural stiffness to sleeve (37).

Description of Nippon Steel's Unbonded Braces.

Nippon's'Unbonded Bracing member'that can be fitted to a building framework as a diagonal bracing according the disclosure published is shown in Fig. 9a.

Fig. 9b shows the view along section C-C of Fig. 9a Fig. 9c shows the view along section D-D of Fig. 9a Fig. 9d shows the view along section E-E of Fig. 9a Fig. 9e shows the view along section F-F of Fig. 9a Fig. 9f shows the view along section G-G of Fig. 9a Referring to Fig. 9a (41)-represents a thin core of rectangular cross-section. This core has holes provided at both ends for securing the same to the building frame as a bracing diagonal.

(42)-represents an unbonding flexible coating given to the core (41) (refer FIG. 9c). This coating. ensures that the axial load in the core is not transferred to the sleeve (43).

(43)-represents a square hollow steel sleeve.

(44)-represents a grout filling the gap between the coating (42) and sleeve (43).

(45)-represents a pair gusset plates with holes welded perpendicularly to the flat core (42) at both the ends so as to form a plus cross section.

S-represents a hollow pocket left at both ends of the grout.

(46)-represents flexible polystyrene filling the pocket (s).

Ll is the predetermined gap between the end of grout (44) and one end of the plus section.

L2 is the length of unbonding coating (42) on the core (41).

Fig. 9b shows the core (41), unbonding coating (42) and cross gussets (45).

Fig. 9c shows a cross section passing through core (41), unbonding coating (42), grout (44) and Sleeve (43).

Fig.9d shows a cross section passing through the core (41), flexible polystyrene filling (46), grout (44) and sleeve (43).

Fig. 9e shows a cross section passing through core (41), smaller width Gussets (45) of plus section, flexible polystyrene filling (46), grout (44) and sleeve (43).

Fig. 9f shows a cross section passing through core (41) and gusset plates (45).

Behavior of Nippon's Unbounded Braces.

In Nippon's construction, because of the unbonding coating (42) on the core (41), no part of the axial load acting at the ends of core (41) is transferred either to the grout (44) or to sleeve (43). The axial load is resisted by the core (41) only. The sleeve (44), by the virtue of its flexural stiffness prevents the core from lateral buckling.

The Nippon's construction is fundamentally similar to the construction disclosed in our above said Indian and US patents. All the three said constructions have a core and a sleeve to restrain the core from buckling. The performance of the core inside the sleeve is identical in Nippon's construction and the constructions disclosed in the said Indian and US Patents.

The said Indian Patent also discloses that"The sleeve can be isolated from the core by providing rubber washers with the result that performance is better under vibratory conditions."In Nippon's construction, there is the flexible unbonding coating (42) instead of the rubber washer of the said Indian patent.

The bracing member of Nippon's construction is used as a diagonal in earthquake resistant building framework to control the lateral drift and also to absorbs energy.

Fig. lOa shows a building frame fitted with this bracing member wherein: (46)-represents the columns of a building framework.

(47) represents the beams of the building framework.

(48)-represents the Nippon Unbonded Bracing member of Fig. 9a.

(49)-represents the gussets attached to the framework. The gusset (49) is provided with bolt holes for securing the bracing member to the framework.

Fig. 1 Ob shows the earthquake induced lateral loads Fl, F2 & F3 acting in the direction of the arrow. Under this loading the bracing member (48), as shown in detail in Fig. 9a will be in tension. The core (41) will resist this tension and it has the capacity to absorb energy when subjected to a tensile force beyond the core's yield strength. Thus substantial energy will be absorbed during severe earthquake. The lateral drift is also controlled.

Fig. lOc shows the reversed earthquake induced lateral loads Fl F2 &F3 acting in the direction of the arrow. Under this loading the bracing member (48), as shown in detail in Fig. 9a, will be in compression. Then the core (41) of the bracing member (48) will start to buckle but the grout (44) and sleeve (43) will prevent the core (41) from buckling. The core (41) can absorb significant energy, even under compressive force, when loaded beyond its yield strength during a severe earthquake.

The drawbacks of the Nippon construction are described below.

Draw backs in the Nippon Construction: The unbonding coating may get damaged during the course of time. If this happens, then friction will develop between the core (41) and grout (44). and as a consequence axial load on the core (41) will be transferred to the grout (44) and sleeve (43) which is not desirable.

Flexible polystyrene filling (46) which is used is not fully fire resistant.

The core (41) is a thin flat steel section. This thin section is not having a firm lateral support in the gap Ll (see Fig. lla). The flexible polystyrene filling (46) cannot be relied upon to give sufficient lateral support to the core (41). The system, as shown in Fig. lla, works well provided the axial force acting on the core is concentric, i. e. the center lines of the bracing member, beam and column should meet at a single point. If there is an eccentricity'e2' (Ref. Fig. lla) due to fabrication deviations, then the core (41) will no longer be carrying purely axial load but will be subjected to a bending moment Ml equal to the axial force F3 multiplied by the eccentricity'e2'. Because the core (41) has a very thin cross section and because of the presence of flexible filling (46), the core (41) may bend in the gap Ll as shown in Fig. llb. This bending of the core will cause premature failure of the bracing member. Furthermore, the bracing member is rigidly connected to the building frame with several bolts instead of a single pin joint. This type of multiple bolted connection causes secondary moments on the core (41). This secondary moment M also causes the core to bend as shown in Fig. llb. Also the grout (44) will be generally of considerable self weight and due to lateral acceleration of the building during a severe earthquake, this self weight of grout itself generates lateral forces and bending moments on the thin core (41). Furthermore, during a severe earthquake, the cladding materials like bricks, tiles etc., may loosen first and fall on the bracing member. This falling debris will also cause the core to bend in the gap Ll..

In the United States, The American Institute of Steel Construction (AISC) has published specifications over the years for the design of steel structures and the specifications are widely followed by design engineers. As on date, there are no AISC specifications for the type. of construction disclosed by the said Indian and US Patents and the Nippon construction. Now a committee of AISC has prepared a draft of the specification for this new system and is likely to be incorporated in the AISC Code of Practice as an appendix. The draft specification specially mentions that the bracing member should be capable of resisting any bending moment and lateral forces caused are eccentricity of connections and other factors.

Another drawback of the Nippon system is that if a very long bracing member is to be designed for a large structure, then the axial deformation of the core (41) will also be very large. Hence the gap LI (ref. Fig. 9a) also will have to be large. Here again this can cause problems due to local buckling of the core in the gap L1. The Nippon bracing tend to be very heavy because of the weight of grout.

In the present invention, the grout is optional. Furthermore in embodiment 3 of the present invention the grout is completely eliminated." In the Nippon bracing member a hollow square section is used for the sleeve (43). Actually circular hollow sections have a higher buckling load than square hollow sections for a given area of cross section, wall thickness and length.

Objects of the present invention.

During the earthquake in Kobe in Japan, the recent earthquakes in San Francisco in USA and the devastating earthquake in Turkey, many buildings were totally destroyed even though many of them had been designed according to any one of the systems described above. Each of the systems had some drawback as explained earlier which resulted in building failure. There is therefore an urgent need now to develop a safer bracing system.

The main object of the present invention is, therefore, to provide a bracing system that is devoid of the drawbacks of the state of art systems described earlier.

Another object of this invention is to provide an improved bracing system that can be retrofitted to the buildings already constructed in earthquake prone areas so as to make them safer.

Further object of the invention is to retrofit the new bracing system in the buildings that had earlier been damaged due to earthquakes and restore them for safe habitation.

Yet another objective of the present invention is to provide a sleeved bracing member that is useful for the construction of structures such as buildings which is cost effective.

Description of the present Invention The present invention has been developed based on our above said Indian and US Patents.

The principle embodiment of the invention is shown in Fig. 12a.

Fig. 12b shows the view along section H-H of Fig. 12a.

Fig. 12c shows the view along section I-I of Fig. 12a.

Fig. 12d shows the view along section J-J of Fig. 12a.

Fig. 12e shows the view along section K-K of Fig. 12a.

Fig. 12f shows the shape of gusset (53) to a larger scale.

Fig. 12g shows the shape of gusset (54) to a larger scale.

50-represents a core rod of solid round cross section and can be made of metal or any matrix materials like graphite composites. The core rod (50) can have rectangular or square cross section or hollow tubular or box section also.

51-represents a sleeve, which is a hollow round cross section. The sleeve (51) can be also be of hollow square cross section. The sleeve (51) can be made of metal or any matrix materials. The core rod (50) is placed inside the sleeve (51) concentrically leaving a pre-determined annular gap.

52-represents a grout material that fills up the annular gap between the core rod (50) and sleeve (51) over a distance of L3..

As per the present invention it is essential to leave a very small annular gap between the core rod (50) and the grout (52) to ensure that any axial load applied to the core rod is not transferred to the grout (52). The grout material when used can be selected from plain concrete, cement mortar, solidifying liquid grout.

53-represents a gusset plate of a predetermined length L4 and with a slot at one end. The shape of the gusset (53) is shown in Fig. 12f. The gusset (53) is attached to both ends of the core rod (50) by inserting the core rod (50) in to the slot provided in gusset (53) and welding along the length the slot. The gusset (53) has holes for securing the bracing member to the beams and. columns of the building frame.

54-represents a gusset plate of shape shown in Fig. 12g. A pair of these gussets (54) is welded to Gusset (53) over a pre-determined length L5 and welded to the core rod (50) over length L6 to form a plus section as shown in Figs. 12d & 12e respectively.

According to the present invention there is provided a sleeved bracing member useful as compression/tension diagonal bracing in structures, particularly buildings, to make them resistant to earthquakes, which comprises a core rod (50) which is loosely placed inside a sleeve (51), the annular space between the core rod (50) and the sleeve (51) optionally filled with grout materials (52) leaving a small predetermined gap around core rod (50), gusset plates (53 & 54) provided with holes being fixed at both ends of the core rod (50) for securing the bracing member to the beam- column junction of the frame of the structure to be made resistant to earthquakes, a pair of gusset plates (54) being fixed at right angles to gusset plate (53) at both ends of the core rod (50) so as to form a plus section,-the width of the gusset plates (34 &35) being such that they are capable of sliding inside the sleeve along edges al, bl, cl and dl even when it is subjected to a compression load and further that the pair of gusset plates (54) being fixed to gusset plate (53) perpendicularly to form a"plus"section, a predetermined gap L7 being left between the end of grout material (52) if present and the end of plus section (53 & a pair of 54), the plus section protruding beyond the sleeve (51) by a pre-determined length L5, part of the plus section being inside the sleeve (51) over a predetermined distance of L6 at both ends of the sleeve (51), an optional stiffening flange (55) being welded to the end of the sleeve (51) The core rod (50) may be made of mild steel, high strength steel or any other metal and may have a solid square cross section, solid rectangular or solid round cross section. The core rod may have a hollow cross section like a hollow square tube. The core rod may be rolled sections like angles, channels etc. of circular or square cross section.

The hollow tube may be made of mild steel, high strength steel or any other metal. The grout material when present is selected from plain concrete, cement mortar, solidifying liquid grout.

The cross sectional area of the core rod is such that under the action of tension compression loads, the core rod goes in to a plastic state. i. e., stressed beyond its yield strength.

The gap L7 between the end of plus section and end of the grout material, if present, is such that it is sufficient to allow for the core to shorten longitudinally under the action of compressive load.

It should be noted that when the compressive force acts, not only the plus section undergoes shortening in length but also bulges laterally due to"Poisson"effect. It is essential as per this invention that the plus section formed by the core rod (50) and the gussets (53 & 54) slides freely inside the sleeve along edges al, a2, a3 & a4 (refer Fig. 12c) even after lateral bulging. The gap between the plus section and the sleeve (50) should be just enough to meet this requirement and not more. A larger gap would make the plus section behave differently as will be explained in further chapters.

The core rod (50), grout material (52) and the sleeve (51) can be seen in Fig. 12b. The annular gap between the core rod (50) and grout (52} can also be seen in Fig. 12b. The core rod (50) and the sleeve (51) can be seen in Fig. 12c. The plus section welded to core rod (50) the sleeve (51) can be seen in Fig. j. The protruding portion of the plus section at the ends of the sleeve (51) can be seen in Fig. e.

Attachment of the bracing member of the present invention to the building frame work.

A building framework fitted with the bracing member as per the present invention is shown in Fig.13a.

Fig. 13b shows the earthquake generated lateral loads Fl, F2 and F3 acting on the building framework in the direction of the arrows.

Fig. 13C shows earthquake-generated loads Fl, F2 & F3 acting in the direction of the arrows.

56-represents the beams of the building framework.

57-represents the columns of the building framework.

58-represents the sleeved bracing of Fig. 12a 58a-represents the gussets attached to the building framework for securing the sleeved bracing.

Behavior of the bracing member of the present invention Referring to Figs. 12a, 13b & 13c: When the lateral loads Fl, F2 & F3 act in the direction as shown in Fig. 13c, the bracing member (58) will be in compression. Then the core rod (50) (refer Fig. 12a) of the bracing member (58) will be subjected to a compressive force. It will start to buckle because of its long length and small cross section but the grout (52) and the sleeve (53) will prevent the core rod (50) from buckling any further and then onwards the core rod (50) alone starts resisting the compressive force. Therefore the bracing member will be safe. The sleeve (51) by itself is not secured to any part of building framework and therefore will not be subjected to any direct axial force. The core (50) is also capable of going in to a plastic state if the axial force exceeds its yield strength (during severe earthquake), thus absorbing considerable shock energy. When the axial compressive load acts on the core, the core rod undergoes axial shortening in length. Therefore the gap L7 between the plus section and the end of grout (52) will keep on diminishing. The designer should ensure that, even during severe earthquake, a small amount of gap is still left so that the plus section does not come in to contact with the grout. If the plus section comes in to contact with the grout, then part of the axial force will be transferred to the grout which in turn will transfer it to the sleeve by friction.

This will cause premature failure of the bracing member, as the sleeve (51) is not designed for to resist directly any large axial force.

When the axial force reverses its direction due to cyclic loading as shown in Fig. 13c, then the bracing member (58 of Fig. 13b) will subjected to a tensile force. The core rod (52) of the bracing member will now be subjected to tension and then stretch in its length and the gap L7 will keep on increasing with the increased tension. According to the present invention, even under severe earthquake, part of the plus section, even after elongation of the core (50) should be within the sleeve (51) so that the sleeve acts as a guide for the plus section to slide concentrically in the sleeve. In tension also the core can go in to a plastic state and absorb considerable shock energy.

Thus the core rod can go in to plastic state and absorb energy both in tension and in compression.

The most important feature of the present invention is that the bracing member is capable of resisting the induced secondary moments and lateral shear forces caused by the normal fabrication deviations in geometry. Under ideal conditions, the centerlines of bracing member, column and beam should meet at a point P as shown in Fig. 14a. But this may not be so in actual practice because of many reasons such as: The building frame comprising beams and columns might have undergone dimensional distortions due to welding.

The steel sections procured for fabrication may not be absolutely straight due to rolling tolerances.

Even if straightening is done, it has its own limits.

Generally, it is very difficult to fabricate a steel structure with absolute dimensional accuracy. Code of practice, in all countries, permits certain allowable dimensional deviations in rolling of steel sections and in fabrication.

The above deviations in geometry will cause shears and bending moments in the bracing member of this invention. In Fig 14b, F4 represents the axial compressive force load acting on the bracing member with an eccentricity of'e3'with reference to the centerline of the bracing member (58).

'M3'represents the bending moment acting on the bracing member. This bending moment is equal to the product F4 x e. M4 represents the secondary moment acting on the bracing member because of the rigidity of end connections of the bracing member. Q represents the lateral force acting on the bracing member. In the present invention, these bending moments and lateral force'Q'will be resisted by the sleeve as reactions'R'. This is because the plus section that is inside the sleeve for a predetermined length of L7 (refer Fig. 14b) will transfer the bending and lateral forces to the sleeve by locking action i. e., the plus section cannot bend independent of sleeve (51). However the plus section is free to slide longitudinally inside the sleeve (51) and therefore the axial force is not transferred to the sleeve (51). By this construction, the core resists the axial compressive/tensile force only and the sleeve (51) not only prevents the core from buckling but also resists lateral loads, secondary moments and the moment caused be the eccentricity of axial force. Therefore the bracing member of present invention will not bend locally as in Fig. lib of Nippon construction.

Design of the bracing member of the present invention.

While determining the maximum force in the bracing member, not only the earthquake induced loads on the building frame should be considered but also the other loads such as dead load, live load, wind load and other specified loads acting on the building and the various load combinations should be considered. The cross sectional area of the core (50) should be so determined that the axial compressive stress during a normal earthquake is with in elastic limits with the necessary factor of safety as stipulated in the code of practice. However it should be capable of going in to a plastic state during severe earthquake so that substantial energy is absorbed. The stability of the core over unsupported length L7 (Ref. Fig. 12a) is to be checked.

The sectional dimensions of the sleeve (51) are to be so determined that even during severe earthquake, the stresses are within elastic limits with the necessary factor of safety as stipulated in the code. The sleeve should have sufficient flexural stiffness to prevent the core from buckling. even during severe earthquake and also to with stand the lateral forces and bending moments transferred to the sleeve caused by fabrication deviations'The Euler Buckling Load'of the Sleeve should be not be less than the maximum force in the core multiplied by the required factor of safety. While designing the sleeve, the effect of friction between the Core (50) and grout (51) should also be considered. The effect of friction can be greatly reduced by covering the sleeve with an anti-friction coating.

The grout material which may be used should have enough compressive strength so that when the core tries to buckle, the grout should not get dented. The grout should be homogenous and free of defects like honeycomb.

The cross sectional area of the plus section is so determined that the stresses developed are within elastic limits with the required factor of safety even during severe earthquake.

The gap L7 is so determined that when the core goes in to plastic stage and shortens in its entire length, the plus section does not come in to contact with the end of grout (52). This will ensure that the axial load is carried by the core only.

The design should also check that even after the core rod (50) stretches in its length due to tension, part of the plus section is still within the sleeve (51) so that the locking action between the plus section. This will also ensure that the sleeve acts as a guide to the plus section when tension changes to compression during to cyclic earthquake loading.

A dynamic analysis of the entire frame with the sleeved braces has to be carried out using a computer to find out the frequency of the structure, response of the structure to the vibratory earthquake generated forces and find out the lateral drift. By choosing proper sections for the beams, columns, core rods and sleeves, an extremely safe building can be designed.

Advantages of the present invention 1. The lateral drift is well controlled because the bracing member is a concentric compression/ tension diagonal member in the building framework. Therefore the system can be adopted for very tall structures.

2. The sliding plus section at the ends helps in preventing premature failure of the laterally unsupported portion of the core.

3. The circular cross section of the core is unlikely to buckle in the gap L7 (ref. Fig. 12a) when compared to a thin flat section.

4. The bracing member is simple to fabricate.

5. There is no flexible material between the core rod and grout but only a small gap. Therefore the problem of any flexible material getting degraded and getting stuck does not arise.

6. The bracing member of this invention is capable of resisting lateral loads and secondary moments caused by the dimensional deviations in fabrication.

7. Since there is no flexible material, the bracing is less affected by fire.

8. A stable hysteric behavior of the core rod is achieved under cyclic tension/compression load on the brace. Thus there is good shock energy absorption both in compression and tension. The sleeve of the bracing member is independent of the building frame and therefore it does not increase the stiffness of the building frame. The core has a very small stiffness and does not increase the stiffness of the building frame much. These factors result in reduced earthquake generated lateral forces.

9. The repair of the bracing system is simple. After a severe earthquake, the core that would have gone in to a plastic state can be easily replaced. The gap between the core and sleeve helps in easy replacement of core.

10. The present invention satisfies the requirements defined in ASCI specifications.

Embodiments of the present invention The scope of the design of the present invention is illustrated by providing the various embodiments of the invention which are explained below.

Embodiment 1 as shown in Fig. 15a Fig. 15b is a view along section h-h of Fig. 15 a.

In Figs. 15 a & 15b (50) to (55) represent the parts explained earlier.

(56) represents a metal washer, which slides through the gap L7.

(57) represents a metal spring provided on either side of the metal washer (56).

(58) represents an end plate secured to the end of plus section to give support to the spring washer (57).

(59) represents an end plate at the end of grout (52) to give support the spring washer (57) A sleeved bracing member useful as compression/tension diagonal bracing in structures particularly buildings, to make them resistant to earthquakes, which comprises a core rod (50) which is loosely placed inside a sleeve (51), the annular space between the core rod (50) and the sleeve (51) filled with a grout materials (52) leaving a small predetermined gap around core rod (50), gusset plates (53 & 54) provided with holes being fixed at both ends of the core rod (50) for securing the bracing member to the beam-column junction of the frame of the structure to be made earthquake resistant,, a pair of gusset plates (54) being fixed at right angles to gusset plate (53) at both ends of the core rod (50) so as to form a plus section,-the width of the gusset plates (34 &35) being such that they should be capable of sliding inside the sleeve along edges al, bl, cl and dl even when it is subjected to a compression load and further that the pair of gusset plates (54) being fixed to gusset plate (53) perpendicularly to form a"plus"section, a predetermined gap being left between the end of (52) and the end of plus section (53 & a pair of 54), the plus section protruding beyond the sleeve (51) by a pre-determined length L4, part of the plus section being inside the sleeve (51) over a predetermined distance of L6 at both ends of the sleeve (51), an optional stiffening flange (55) being welded to the end of the sleeve (51) a metal washer (56) being provided, which slides through the gap L7., a metal spring (57) being provided on either side of the metal washer (56), end plates (58 & 59) being secured to the end of plus section and at the end of grout (52) respectively to give support to the spring washer (57).

If very high strength steel were used for the core rod (50), the result would be that the cross sectional dimensions of the core rod would be much less as compared to mild steel core rods. This smaller cross section may cause the thin core rod to buckle in the gap L7. The sliding washer (56) provided in this embodiment reduces the unsupported length of the core (50) in the gap L7 by half.

As the axial load on the core increases, the gap L7 keeps on reducing and at any given instance the moving washer would still be in the middle of the reduced gap because of the action of springs.

Thus very thin core rods can also be used in this embodiment.

This embodiment of the present invention also envisages the deployment of more than one sliding washer and more number of springs. For example two sliding washers and three springs can be used instead of one spring washer and two springs. An advantage of this is that larger axial deformation of the core in plastic state can be allowed, thus absorbing more shock energy. An experimental steel staging supporting a water tank was designed, fabricated and load tested where in the columns were designed like the bracing member of this invention and with two sliding washers plates and three spring washers. This embodiment of the invention validates the use of sliding washers and springs to prevent the local buckling of very thin cores in the gap L7.

Embodiment 2 is shown in Fig. 16a Fig. 16b shows a view of section I-I of Fig. 16a Fig. 16c shows a view of section H-H of Fig. 16a (50) to (55) represent the same parts as explained in the embodiment no 1 above (60) represents a thin metallic or non-metallic sleeve providedaround the core rod (50) with a pre- determined gap between the thin sleeve (60) and core rod (50).

Fig. 16b shows the section section L-L that passes through core (50), thin sleeve (60), grout (52) and sleeve (51).

Fig. 16c shows the section M-M that passes through core (50), Gusset (53), pair of gussets (54) and sleeve (51).

A sleeved bracing member useful as compression/tension diagonal bracing in structures particularly buildings, to make them resistant to earthquakes, which comprises a core rod (50) which is loosely placed inside a sleeve (51), the annular space between the core rod (50) and the sleeve (51) filled with a grout materials (52) leaving a small predetermined gap around core rod '50), gusset plates (53 & 54) provided with holes being fixed at both ends of the core rod (50) for securing the bracing member to the beam-column junction of the frame of the structure to be made earthquake resistant, a thin metallic or non-metallic sleeve (60) being provided around the core rod 50) with a pre-determined gap between the thin sleeve (60) and core rod (50), a pair of gusset slates (54) being fixed at right angles to gusset plate (53) at both ends of the core rod (50) so as to brm a plus section,-the width of the gusset plates (53 &54) being such that they should be capable if sliding inside the sleeve along edges al, a2, a3 & a4 even when it is subjected to a compression Dad and further that the pair of gusset plates (54) being fixed to gusset plate (53) perpendicularly to Drm a"plus"section, a predetermined gap L7 being left between the end of grout material (52) nd the end of plus section (53 & a pair of 54), the plus section protruding beyond the sleeve (51) y a pre-determined length L5, part of the plus section being inside the sleeve (51) over a redetermined distance of L6 at both ends of the sleeve (51), an optional stiffening flange (55) eing welded to the end of the sleeve (51) a metal washer (56) being provided, which slides through te gap L7, a metal spring (57) being provided on either side of the metal washer (56), an end plates (58 & 59) being secured to the end of plus section and at the end of grout (52) respectively to give support to the spring washer (57).

In this embodiment of the invention, the thin sleeve helps in achieving good quality fabrication of the bracing member. The gap between the core rod (50) and the thin sleeve (60) can be very well controlled. Also good compaction can be given to the grout (52) during the filling up of the gap between the inner thin sleeve (60) and the outer sleeve (51).

Embodiment 3 is shown in Fig. 17a Fig. 17b is a view along section N-N of Fig. 17a Fig. 17c is a view along section P-P of Fig. 17b (50), (51), (53) to (55) represent the parts as explained earlier.

(61) represents a thick walled inner sleeve.

(62) represents plurality of circular plate washers secured to the inner sleeve (61) at predetermined spacing L8. The outer edges of the circular washers (61) are free to slide longitudinally along the inner surface of the outer sleeve (51) so that, during the final assembly of the bracing member, the fitted sub assembly comprising parts (50), (53), (54), (61) and (62) can be slid into the outer sleeve (51).

Fig. 17b shows the section N-N passing through core (50), thick walled inner sleeve (61) and outer sleeve (51). In this section sliding washer (62) is seen in elevation.

Fig. 17c shows the section P-P passing through core (50), Gusset (53), pair of gussets (54) and sleeve (51).

. sleeved bracing useful as compression/tension diagonal bracing in structures particularly uildings, to make them resistant to earthquakes, which comprises a core rod (50) which is loosely ) laced inside a thick walled sleeve (61), leaving a small predetermined gap around the core rod 50), outer sleeve (51) enveloping the inner sleeve (61) with an annular gap all round, gusset plates 53 & 54) provided with holes being fixed at both ends of the core rod (50) for securing the tracing member to the beam-column junction of the frame of the structure to be made earthquake resistant, a gusset plate (53) and a pair of gussets (54) being fixed at right angles and pair of gussets (54) being fixed at right angles at both ends of the core rod (50) so as to form a plus section,-the width of the gusset plates (53 & 54) being such that they should be capable of sliding inside the outer sleeve (51) along edges al, a2, a3 & a4 even when it is subjected to compression load and further that the pair of gusset plates (54) being fixed to gusset plate (53) perpendicularly to form a"plus"section, a pre-determined gap 17 left between end of. the plus section (53 & a pair of 54) and the end of inner thick walled sleeve (61), the plus section protruding beyond the outer sleeve (51) at both ends, by a pre-determined length L5, part of the plus section being inside the outer sleeve (51) by a pre-determined length L6 at both ends of the outer sleeve (51), an optional stiffening flange (55) being welded to the end of the outer sleeve (51), plurality of circular plate washers (62) being secured to the inner sleeve (61) at pre-determined spacing L8, the outer edges of the circular washers (62) being free to slide longitudinally along the inner surface of the outer sleeve (51) so that the fitted subassembly comprising parts (50), (53), (54), (61) & 62 is capable of sliding in to the outer sleeve (51).

In this design the Inner sleeve prevents the core from buckling over the distance L8 and the outer sleeve (51) prevents the buckling of the whole bracing member. The washers act as lateral supports to the inner sleeve (61) so that the inner sleeve (61) does not buckle inside the outer sleeve (51).

It is to be noted that in this embodiment the use of grout is completely eliminated.

The design of the core (50) and outer sleeve (51) is same as described before. The'Euler Buckling Load'of the inner sleeve (51) over the length L8 should not be less than the'Euler Buckling load' of the outer sleeve (51) over the full length of the bracing member.

The additional advantages of this embodiment 3 are: Only steel is used and the use of the grout materials is eliminated. Therefore quality control is easy.

The weight of the bracing member will be very much less because of the absence of grout material.

This helps during transportation and erection.

In the case of grouting, it is difficult to check whether the grout material has deteriorated where as in this embodiment all steel parts can be checked by non-destructive testing like ultrasonic testing, radiography etc.

There is not much damage to the member if it falls accidentally during erection where as in grouted members the grout material may crack.

The total self-weight of the frame will be less if non-grouted bracings are used. With less self- weight, the earthquake-induced forces also will be less. This is cost effective.

Tests conducted using the various embodiments of the present invention have validated and confirmed the above-described behavior of sleeved bracing under tension/compression cyclic loading.