GB2248862A

GB2248862A - Structural member resistant to buckling

Info

Publication number: GB2248862A
Application number: GB9121600A
Authority: GB
Inventors: Katsuhiko Imai
Original assignee: Kawasaki Steel Metal Products and Engineering Inc
Current assignee: Kawasaki Steel Metal Products and Engineering Inc
Priority date: 1990-10-12
Filing date: 1991-10-11
Publication date: 1992-04-22
Also published as: GB9121600D0; JPH0742759B2; DE4133203C2; DE4133203A1; JPH04149345A

Abstract

A structural member (1) resistant to buckling comprises two elongate members (2, 3) disposed one within the other. The elongate members (2, 3) are movable relatively to one another in the longitudinal direction so that an axial compressive load (P) applied to one of the elongate members (2) is initially not resisted by the other elongate member (3). When the elongate member (2) which initially resists the said axial compressive load (P) begins to buckle, the two elongate members (2, 3) come into contact and together resist buckling. Thus, member (2) is deformed in the axial direction only when it begins to deform plastically. When a buckling resistant structural member (1) of the type described above, is used in the construction of a truss, the truss is prevented from collapsing suddenly, since the structural members (1) are able to deform to a large extent and in a stable manner before failure occurs. Such an arrangement is particularly suitable for the construction of earthquake resistant building; the structural members (1) are capable of absorbing a large quantity of energy and hence should prevent the building from collapsing or at least delay collapse so that occupants of the building may escape. <IMAGE>

Description

2243362 1 STRUCTURAL MEMBER RESISTANT TO BUCKLING This invention relates

to structural members, particularly although not exclusively for use in a truss. More particularly, the invention relates to a structural member with high resistance to elastic and plastic buckling and which is capable of large controlled axial plastic deformation under high compressive loads.

In constructing a large structure by using a large number of structural members comprising long steel pipes, a truss or brace structure is often adopted. For example, in designing a truss structure which can withstand the axial compressive force attributed to the dynamic external forces generated by an earthquake, it is desirable that the structural member can deform to a large degree without decreasing its strength before buckling. Because, generally, the buckling of a steel pipe suddenly decreases the strength thereof. Accordingly, the structural member should be designed on the assumption that a load lower than the buckling strength acts on the structural member, or alternatively, the steel pipe should be necessarily reinforced with stiffeners fitted inside the pipe. However, in both of the above cases, the structural member behaves elastically against the external force. A structural member which behaves elastically should be designed assuming a larger stress than that of a structural member which utilizes plastic deformation.

Further, in the brace structure analogous to the truss structure, if the compressive brace remains within its elastic region, large st resses occur in the brace when an earthquake occurs and very large forces act on the adjacent columns or beams in the structure. There are of ten cases where columns or beams which can withstand such force are too large and heavy to be used in practice. There may be a method of designing -2 compressive braces by means of estimating the strength after buckling, however, in the state of the art, it is not easy to give a desired earthquake resistance to the truss structure by appropriately evaluating the sudden decrease of strength after buckling.

In Japanese Patent Application No. Heisei 1-340441 filed on 28th December 1989, the present inventor has proposed a structural member which has at the right and left ends thereof weakened parts which are plastically deformed by external forces smaller than the buckling force of the steel pipe. When an axial compressive force more than the predetermined load acts on the structural member, the weakened parts at both ends can be plastically deformed intentionally.

In the compressive member made of a single steel pipe, when a buckling load exceeds a maximum buckling strength based on buckling theory, the strength of the steel pipe generally decreases suddenly as shown by a solid line in Fig. 13, and the axial deformation of the compressive member becomes extremely small. On the other hand, the above mentioned structural member provided with a weakened part is able to deform to a large extent in the axial direction, as shown by a dotted line. However, as plastic deformation does not take place along the entire length of the steel pipe, the available deformation of the structural member is limited. Further, the weakened part which is provided at the end of the steel pipe is complicated and expensive.

In view of the above, the present invention seeks:

to facilitate manufacturing of a low-priced structural member which can deform plastically in an axial direction only without a complicated weakened part; to easily suppress the lateral deformation caused when a long structural member buckles, so that a large axial deformation thereof occurs; to facilitate a large deformation only of selected structural member(s) from the truss structure; torprevent the steel structure from being suddenly collapsed during a large earthquake by allowing large axial deformation of selected structural members only, so that high earthquake resistance of the truss structure can be achieved by utilizing plastic deformation such as that of the Rigid Frame Structure; and to enable the structural member to withstand the axial compressive force by means of maintaining the plastic deformation capacity even if the axial force acting on the steel pipe becomes equal to the yield load thereof.

According to the present invention, there if provided a structural member comprising two elongate members disposed one within the other, the elongate members being movable relatively to one another in the longitudinal direction, whereby an axial compressive load applied to one of the elongate members is initially not resisted by the other elongate member.

The first elongate member may be inserted in the second elongate member, or, it may cover the second elongate member.

The overall length of the second elongate member is preferably shorter than the overall length of the first elongate member by an amount substantially equal to the predetermined axial deformation which is allowed when the first elongate member deforms plastically in the axial direction thereof. Both the second elongate member and the first elongate member may together resist the axial compressive force after the first elongate member has shortened by the predetermined axial deformation.

Even when the axial compressive force acting on the first elongate member causes it to begin to buckle and to deform in a direction orthogonal to the longitudinal axis of the first elongate member, the first elongate member is prevented from deforming laterally by the action of the second elongate member, to which the axial compressive force is not conveyed. Accordingly, the first elongate member is deformed only in the axial direction when it begins to deform plastically.

If structural members according to the present invention are used to construct a truss, the truss is prevented from collapsing suddenly, because the first elongate member is able to deform stably and to a large extent. The plastic deformation of the first elongate member develops into an.axial symmetrical local buckling as a result of the supporting function of the second elongate member. The said deformation is stable, so that the first elongate member is not bent by means of the buckling phenomenon. A similar stable deformation of the first elongate member will occur both in the case in which it is inserted in the second elongate member and in the case in which it encloses the second elongate member.

If the overall length of the second elongate member is shorter than the overall length of the first elongate member by the predetermined axial deformation allowable for the first elongate member to deform plastically in the axial direction thereof, both the first and second elongate members can oppose the axial compressive force which also acts on the second elongate member after the first elongate member shortens by the said predetermined axial deformation. As a result, the strength of the structural member as a whole is substantially increased, and a truss structure using the above structural member becomes more resistant to dynamic loading.

k According to this invention, as the deformation of the first elongate members is suppressed by the second elongate member, large axial deformation of the first elongate member can be achieved without a large deformation in a direction orthogonal to the axis of the first elongate member. Increasing the axial deformation further facilitates the development of axial symmetrical local buckling at the local plastic zone, so that the plastic deformation of the first elongate member becomes stable. Maintaining large deformation and desired strength of the structural member prevents a truss structure, constructed from such structural members, from collapsing immediately, even if a large external force acts thereon.

If the overall length of the second elongate member is shorter than the overall length of the first elongate member by an amount equal to the axial deformation generated under axial symmetrical local buckling of the steel pipe structural member in addition to the axial plastic deformation thereof, the deformation of the structural member can further increase.

In manufacturing the above structural member, the second elongate member needs only to be fitted over or inserted into the first elongate member so that the structural member according to the invention can be supplied at a moderate price.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Fig. 1 is a sectional view of a first elongate member in which is inserted a second elongate member; Fig. 2 is a cross sectional view of Fig. 1; 35 Fig. 3 is a sectional view of the structural member under elastic buckling; Fig. 4 is a cross sectional, view of a structural member having first and second elongate members of different cross sectional shape; Fig. 5 is a section through another embodiment of structural member in which the second elongate member covers the first elongate member; Fig. 6 is a sketch of a truss structure in which buckling resistant structural members are - partially adopted; Fig. 7 is a partial enlarged view of Fig. 6; Fig. 8 is a further embodiment of bending resistant structural member; Fig. 9 is a brief sketch of a diagram of forces acting on a structural member during buckling; Fig. 10 and Fig. 11 are partial sectional views of a structural member undergoing axial symmetrical local buckling; Fig. 12 is a sketch of a structural member undergoing a bending deformation; Fig. 13 is a graph showing the axial deformation plotted against the buckling load of a single pipe; Referring to the drawings, Fig. 1 shows a structural member 1 comprising a first elongate element comprising a steel pipe structural member 2 and a second elongate element comprising a bending resistant steel pipe 3. As shown in Figure 2, the steel pipe structural member 2 is inserted into the bending resistant steel pipe 3, in such a way that an axial compressive force P which acts on the steel pipe structural member 2 is not conveyed to the bending resistant steel pipe 3.

The bending resistant steel pipe 3 is not integral with the steel pipe structural member 2, i.e., it is merely put in the steel pipe structural member 2. By means of a compressive force acting on the steel pipe structural member 2, it begins to buckle and to bend 1 X laterally as shown in Fig. 3. The bending resistant steel pipe 3 opposes the bending of the steel pipe structural member 2, so that it prevents the steel pipe structural member 2 from deforming in a direction 1 orthogonal to the axis 2m thereof. In order to achieve the above characteristics, the sectional shape and dimensions of the bending resistant steel pipe 3 are selected so that it can prevent the steel pipe structural member 2 from buckling under load.

Referring again to Fig. 1, a clearance a between the inner surface 2a of the steel pipe structural member 2 and the outer surface 3b of the bending resistant steel pipe 3 is selected to be extremely small. For example, a bending resistant steel pipe 3 having an outer diameter of 53.6mm is applied to a steel pipe structural member 2 having an outer diameter of 60.5mm and a thickness of 3.2mm. The difference between the inner diameter of the steel pipe structural member 2 and the outer diameter of the bending resistant steel pipe 3 is 0.5mm, so that each clearance a is approximately 0.25mm.

Though the difference between the inner diameter of the steel pipe structural member 2 and the outer diameter of the bending resistant steel pipe 3 is small, the bending resistant steel pipe 3 can be easily inserted into the steel pipe structural member 2. As the difference between the inner diameter of the steel pipe structural member 2 and the outer diameter of the bending resistant steel pipe 3 is small, the following advantage is obtained. The buckling load by means of the compressive force P acting on the steel pipe structural member 2 makes the pipe yield and deform laterally as shown in Fig. 3, however, the axial force is not conveyed to the bending resistant steel pipe 3.

Therefore, the bending resistant steel pipe 3, loaded with the bending force only, prevents the steel pipe structural member 2 from deforming in the lateral direction.

Using a solid-drawn steel pipe as the bending resistant steel pipe 3 facilitates machining the bending resistant steel pipe 3, so that the desired dimensions of the pipe can be easily produced. The steel pipe structural member 2 may also be, for example, rectangular as shown in Fig. 4. A clearance a is maintained between the circular bending resistant steel pipe 3 and the rectangular steel pipe structural member 2.

Conversely to the above, the bending resistant steel pipe 3 may be disposed so as to cover the steel pipe structural member 2, as shown in Fig. 5. A clearance a is kept between the outer surface 2b of the steel pipe structural member 2 and the inner surface 3a of the bending resistant steel pipe 3.

Structural members 1 as shown in Fig. 1 and Fig. 5 are used as the structural members 5, 5 indicated by a double solid line at three positions at each side of the truss structure 4 shown in Fig. 6. They need not be used as structural member 6 indicated by a double broken line and as the structural members 7, 8 indicated by a single solid line. The buckling resistant structural members are distributed in the truss in this way, because the stress is greatest in the structural members 5, 5 when an external force acts on the truss structure 4 due to an earthquake or the like. It is noted that structural members 1 may also be used in other parts of the truss 4.

It is not necessary for the above bending resistant steel pipe 3 to be inserted for the entire length of the steel pipe structural member 2. As shown in Fig. 7, the bending resistant steel pipe 3 may be inserted for only part of the length of a horizontal long structural member 5. In order to hold a bending t X i resistant steel pipe 3 at a desired position, the bending resistant steel pipe 3 is welded to the steel pipe structural member 2 at a point 9. Joining the pipe 3 to the member 2 on1-y at one point prevents the axial compressive force P, acting on the steel pipe structural member 2, from being transmitted in the axial direction to the bending resistant steel pipe 3.

Fig. 8 shows a steel pipe structural member 2 furnished with edge covers 11 having a screw type connection 10. This steel pipe structural member 2 is connected with other steel pipe structural members 2 by means of bolts (not shown). In such a steel pipe structural member 2, prior to welding the edge covers 11, 11 to the ends of the steel pipe structural member 2, the bending resistant steel pipe 3 should be inserted into the steel pipe structural member 2.

When a truss structure 4 into which the double steel pipe type structural members 1 are incorporated at the positions drawn by double solid lines in Fig. 6 is subjected to an earthquake, an axial compressive force P acts on the double steel pipe type structural members 1, 1. As shown in Fig. 3, each steel pipe structural member 2 begins to buckle and deforms in a direction orthogonal to the axis 2m thereof. As the bending resistant steel pipe 3 is free to move in the longitudinal direction, relative to the steel pipe structural member 2, it carries no axial load. However, since the bending resistant steel pipe 3 is a close fit in the steel pipe structural member 2, it exerts a lateral restraint on the member 2 and so prevents it from deforming laterally.

If the structural member consists of a single pipe only, the single pipe yields transitionally at the beginning of buckling by means of a compressive force acting on the structural member. Concentrating the lateral and axial deformations on said single pipe i makes it deform immediately. As above mentioned, separating the lateral and axial deformations, i.e., imposing axial deformation only of the steel pipe structural member 2 and imposing lateral deformation of the bending resistant steel pipe 3 respectively, prevents a double steel pipe type structural member 1 from deforming laterally and enables it to deform greatly in the axial direction.

In order to construct such a double steel pipe type structural member 1, a bending resistant steel pipe 3 may simply be inserted into a steel pipe structural member 2, thereby fabricating a double steel pipe type structural member 1 easily. Inserting bending resistant steel pipe 3 into necessary parts only of the steel pipe-structural member 2 reduces the amount of steel pipe required.

So that the steel pipe structural member 2 does not buckle even if its yields, an estimation of the cross sectional area of the bending resistant steel pipe 3 is made as follows. Assuming that the outer diameter of the steel pipe structural member 2 is 60.5mm, its thickness is 3.2mm and its sectional area A2 is 5.76 rM2, its buckling stress a. is 3. 0 t/CM2, its moment of inertia 12 is 23.7CM4, its section modulus Z2 is 7.84CM3 and its length 12 is 2,03Omm. Further, the long steel pipe structural member 2 having a slenderness ratio of 100 is supposed to buckle elastically.

The yield axial force PY of the steel pipe structural member 2 becomes; ay x A2 = 5.76 x 3.0 = 17.3 t The buckling strength P,, of the steel pipe structural member 2 becomes; f. x A 2 X -a = 0.954 x 5.76 x 1.9 = 10.4 t where f. is a buckling unit stress, and C is a safety factor of buckling.

Further, assuming that a lateral deformation of the steel pipe structural member 2 of 5mm is allowable when it yields, the lateral force P. based on the 5 balance of forces shown in Fig. 9 becomes; 2 x PY x tan e = 2 x 17.3 x 0.5/101.5 = 0.17 t When a lateral force of about 0.17 ton acts on the steel pipe structural member, in other words, when 0.17 ton reacts on the steel pipe structural member from the bending resistant steel pipe 3 owing to the deformation of 5mm of the steel pipe structural member 2, the double steel pipe type structural member 1 continuously deforms without buckling.

Referring to Fig. 3 and Fig. 9, moment of inertia 13 required for the bending resistant steel pipe 3 is calculated as follows. The relation between the deformation 8 and the movement of inertia 13 'S 6 = PL X l' / 48 E X 13 To substitute the related numerical values, 20 0.5 - 0.17 x 203 3 / 48 x 2,100 x 13P thus, 13 becomes 28.2CM4. From this estimation it is found that the moment of inertia 13 of the bending resistant steel pipe 3 is about 20% increased compared with the 23.7cm' value calculated for the steel pipe structural member 2. Furthermore, assuming the yield stress ay = 3.0 t/CM2, the section modulus Z3 required for the bending resistant steel pipe 3 in the elastic region is calculated from the relation of ay = WZ3 3.0 = 0.17 x 30 203/(Z3 x 4) results in Z= 2.87 Cn13. It is found that the section modulus Z3 of the bending resistant steel pipe 3 may be smaller than the section modulus,Z2 of the steel pipe structural member 2. It is obvious, from the above studies, that the bending resistant steel pipe 3 sufficiently functions as a bending resistant steel pipe for the double steel 1 pipe type structural member 1, even if it is smaller in size than the steel pipe structural member 2.

The steel pipe structural member 2 constituting the double steel pipe type structural member 1 obtains its maximum strength by means of the axial symmetrical local buckling 12 as shown in Fig. 10 or Fig. 11 generated after its sectional yield. And it deforms to a large extent for the duration from the beginning of yielding to presenting axial symmetrical local buckling 12. Then a shrinkage of about 1 of the overall length of the steel pipe structural member 2 (i.e., about 20mm in the steel pipe structural member 2 of the above mentioned size) is generated just before undergoing axial symmetrical local buckling. During this shrinkage, the bending resistant steel pipe 3 ensures axial symmetrical local buckling of the steel pipe structural member 2 and guides the deformation toward the outside or inside thereof. The deformation of the axial symmetrical local buckling is extremely stable, therefore, the bending deformation 13 as shown in Fig. 12 never occurs, and the strength of the steel pipe structural member 2 does not decrease suddenly. By means of such stable deformation, the truss structure is also severely distorted however, the sudden collapse thereof will be avoided or at least postponed. Members of the public within the truss structure 4 are given greater time to becomes aware of the distortion of the structure and to take refuge outside the structure.

As shown in Fig. 8, the overall length 13 of the bending resistant steel pipe 3 is chosen shorter than the overall length 12 of the steel pipe structural member 2 by the predetermined axial deformation 6 6/2 + 6/2) allowable for the steel pipe structural member 2 to deform plastically to an axial direction thereof. Both the steel pipe structural member 2 and the bending resistant steel pipe 3 can resist the axial A compressive force acting on the double steel pipe type structural member 1 together, after the steel pipe structural member 2 shrinks by said predetermined axial deformation 6.

When the steel pipe structural member 2 undergoes a section yield during the suppression of deformation thereof by means of the bending resistant steel pipe 3, it undergoes an axial symmetric local buckling 12 due to the lateral restraint and guiding action of the bending resistant steel pipe 3 as shown in Fig. 10 and Fig. 11, and attains its maximum strength. After the load on the steel pipe structural member 2 exceeds the maximum strength thereof, so that the member 2 deforms by the predetermined axial deformation 6, the subsequent compressive force acts on the bending resistant steel pipe 3 as well. The axial compressive force is resisted by both the steel pipe structural member 2 and t he bending resistant steel pipe 3. As a result, the strength of the double steel pipe type structural member 1 is substantially improved, and a high safety margin for the truss structure is achieved.

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Claims

1. A structural member comprising two elongate members disposed one within the other, the elongate members being movable relatively to one another in the longitudinal direction, whereby an axial compressive load applied to one of the elongate members is initially not resisted by the other elongate member.

2. A structural member as claimed in claim 1, in which one of the elongate members is a close fit within the other elongate member, so that when the elongate member which initially resists the said compressive load begins to buckle, the two elongate members come into contact and together resist the buckling aGtlen.

3. A structural member as claimed in claim I or claim 2, in which one of the elongate members is longer than the other.

4. A structural member as claimed in claim 3, in which the difference in length between the elongate members is equal to a predetermined axial deformation of the elongate member which initially resists the said axial compressive load.

5. A structural member as claimed in claim 4, in which after the elongate member which initially resists the said axial compressive load has shortened by the said predetermined axial deformation, both elongate members together resist the axial compressive force.

6. A structural member substantially as described herein, with reference to and as shown in Figures I to 3, 6, 7 and 9 to 12 or Figures 4, 6, 7 and 9 to 12 or figures 5 to 7 and 9 to 12 or figures 8 to 12 of the accompanying drawings.