US3831924A

US3831924A - Torisonal energy absorber

Info

Publication number: US3831924A
Application number: US00318279A
Authority: US
Inventors: R Skinner
Original assignee: R Skinner
Current assignee: Development Finance Corp of New Zealand
Priority date: 1971-12-23
Filing date: 1972-12-26
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27
Also published as: JPS4872923A; JPS5725739B2

Abstract

A device is disclosed which absorbs from a structure such as a building energy imparted by an earthquake or a high wind and so inhibits damage to the building. Two members of the structure which move in opposite directions during vibration are coupled by levers to a bar of annealed low carbon steel in such a way that the bar is strained in torsion during vibration. A bar having a square cross-section with a side five inches long strained up to 4 percent in torsion has a capacity of approximately 70,000 N for several hundred cycles. The failure of such an absorber at the end of its useful life is not catastrophic.

Description

United States Patent [191 Skinner [451 Aug. 27, 1974 TORISONAL ENERGY ABSORBER [22] Filed: Dec. 26, 1972 {21] Appl. No.: 318,279

[30] Foreign Application Priority Data Dec. 23, 1971 New Zealand 165913 [52] US. Cl. 267/154, 267/57 [51] Int. Cl F16i l/16 [58] Field of Search 267/154 V, 57 V [56] References Cited UNITED STATES PATENTS 2,591,281 4/1952 Masschoot 267/154 3,406,523 10/1968 Baker et al. 267/154 3,490,756 l/l970 Spier 267/154 Primary Examiner.lames B. Marbert Attorney, Agent, or Firm-Holman & Stern [57] ABSTRACT A device is disclosed which absorbs from a structure such as a building energy imparted by an earthquake or a high wind and so inhibits damage to the building. Two members of the structure which move in opposite directions during vibration are coupled by levers to a bar of annealed low carbon steel in such a way that the bar is strained in torsion during vibration. A bar having a square cross-section with a side five inches long strained up to 4 percent in torsion has a capacity of approximately 70,000 N for several hundred cycles. The failure of such an absorber at the end of its useful life is not catastrophic.

8 Claims, 4 Drawing Figures .PATENTEDAUBZIIBH SHEEI 1 (f 4 1 TORISONAL ENERGY ABSORBER BACKGROUND OF THE INVENTION 1. Field of the Invention.

This invention relates to a torsional energy absorber. In particular, it relates to torsional energy absorbers connected into a structure such as a building which is exposed to earthquakes.

2. Description of the Prior Art.

Energy absorbers, otherwise known as shock absorbers or dampers, are well known. There are many designs which will markedly reduce the energy, whether impact or cyclic, transmitted by forces of up to tens of newtons. There are, however, applications in which forces several orders greater than this may cause damage. As one example only, we may consider the effect of a major earthquake on a structure such as a building or a bridge. Forces now involved are tens of thousands of newtons, and it is well known that they may cause heavy damage to the structures.

It is standard practice so to design structures that the energy transmitted to the building by such a natural phenomenon as an earthquake (high winds can also cause damage) is absorbed within the building by additional material which costs a considerable amount of money and is redundant from statical considerations.

To reduce this extra expense, attempts have been made to prevent the transmission to the building of earthquake forces by mounting the building on rubber or by incorporating in it special panels, commonly of reinforced concrete, which are designed to be energy absorbers. It is not known how effective the rubber damper is. It is known that the special concrete panels are expensive, and that they will be destroyed in one or a few severe earthquakes and will have to be replaced at appreciable expense.

Thus it is clear that the prevention of damage in structures by absorbing the energy imparted by earthquakes by processes that do not involve damage to the structure has attracted expert attention. Although this has been the subject of much research and development in this country and abroad there remains room in this technology for the development of more satisfactory devices.

SUMMARY OF THE INVENTION Accordingly the purpose of this invention is to reduce the cost and improve the safety of structures such as building in regions liable to earthquakes by providing a design for a compact and inexpensive device which will absorb large amounts of energy when the structure is vibrated. The invention resides in new torsional energy absorbers and methods of construction thereof. In particular an absorber in which the absorbing element is a block of low carbon, annealed steel, rectangular in cross-section and strained up to 4 percent in torsion will absorb 60-65 percent of the energy imparted to it over a sufficient number of cycles to cope with all the earthquakes statistically to be expected during the economic life of the structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a stress-strain diagram for a sample of steel,

FIG. 2 shows a property of heavily stressed sections which is used in the invention,

FIG. 3 is a layout of one embodiment of the invention.

FIG. 4 shows a second embodiment.

DETAILED DESCRIPTION FIG. 1 shows the property of steel which enables it to be used as the absorbing member of the present invention. In the experiment of which this Figure is the result, a block of hot rolled mild steel, half inch square in cross-section, was strained in torsion over a one inch length by a lever. Loads cyclically applied at the end of the lever are shown as ordinates. Before straining, the steel was at point A. Up to point B the strain is elastic. From point B to point C there is a flat portion showing yield. From point C to point D there is a linear hardening portion. On reversal the torque twist curve does not follow the line D-C-B-A but curve E and on successive reversals almost the same closed loop is followed. There is very little hardening as is shown by the closeness of lines F and G.

If the material were behaving :in a perfectly elastic fashion there would be no curve E but a straight line, a prolongation in both directions of A-B, would be followed. In that case all the energy imported in straining the sample would be returned. If, at the other extreme, all the energy imparted in straining the sample were converted to heat a rectangle composed of vertical and horizontal lines including the points H and I as corners would be followed. The area within curve E is a measure of the energy absorbed and converted to heat. From a number of tests the area within the loop is estimated to be in the range 60-65 percent of the whole energy supplied as strain. From this it follows that hot rolled mild steel is a relatively efficient absorber of energy when strained in torque. When for example a torque of around 1,950 lb.in. is developed in a one inch length of half inch square bar at an angle of rotation of 23 the energy density per cycle is 50 X 10 N/M the beginning of the decay in peak torque is at 40 cycles with a peak torque around 1,950 lb.in. (213 NM). The cumulative energy density dissipation is 2.3 X 10 N/M Peak load begins to decay at around 2.8 X 10 N/M accumulated dissipation for this material.

The results of this test series indicate that the hot rolled mild steel has highly favourable behaviour from the point of view of an energy absorbing device in a structural system, if used as strain ranges around 4 percent. At this range cyclic hardening is small and achieved within the first few cycles and a range of 200 to 300 cycles of uniform peak load follows with a substantial reserve of strength left. On the basis of these results the design of a suitable device incorporating torsion as the primary mechanism was explored.

Other tests were made on:

a. Cold rolled mild steel as delivered, severely worked b. Cold rolled mild steel annealed c. Hot rolled mild from half inch, 1 inch and 2% inch sizes (1. Stainless steel From these tests it was found that best results were obtained from carbon steel having less than 0.3 percent of carbon in a hot-rolled or annealed condition. A fibrous structure is to be preferred.

FIG. 2 illustrates a property of sections of which use was made in this invention. FIG. 2A shows a circular bar in cross-section when stressed in torsion into the range in which part of the cross-section is in the plastic range and part is in the elastic range. The line 10 shows the boundary between the two. The part 11 in the elastic range does not extend to the surface of the bar.

FIG. 2B is a corresponding diagram for a square bar. It will be seen that the corners of the bar are still in an elastic mode. This condition encourages the distribution of plastic strain over the length of the bar (it is normally to be preferred that the distribution should be uniform) and discourages that plastic deformation which might be caused by bending moments.

It has been found that with such a material stressed into the plastic range, the energy absorbed per cycle decreases only very slowly for several hundred cycles and that the material can be expected not to fail for this number of cycles. Since the number of cycles during which an earthquake imposes approximately its maximum stresses is normally between and 30, this means that such a device can be expected to continue to operate satisfactorily over any normally predicted life of the structure into which it is incorporated.

It has been found that an absorbing member made of a good sample-of the material previously specified and so loaded that bending moments did not induce failure, could be expected to have a life lying between 500 cycles and 50 cycles as the maximum cyclic plastic strain was varied from 1% to 5 percent.

When the design is satisfactory, it will be found that the first indication of incipient failure due to pure torsion is the growth of fine longitudinal cracks. At failure due to pure torsion the absorbing member has become a bundle of fibres. It is easy to prevent failure in shear, but not nearly so easy to prevent failure from bending moments. These cause a failure approximately in a cross-section of the beam. There are many methods of reducing the effect of a bending moment. In a good design symmetry discourages bending moment on the absorbing member and encourages torsion. It has been found that the length of the arm by which the torque is applied should be so related to the axial lengths of the absorbing member that the maximum bending stress is not more than percent of the maximum torsional stress.

Limiting torque is defined by:

Limiting torque A; A I yield, where I yield equals the yield stress in shear, and A is the length of a side of the square section.

FIG. 3 shows a layout for an embodiment that has been used. The force which is the source of energy to be absorbed is applied at the end of arm l. The device is anchored to foundations at 2 and 3. The symmetrical arrangement which may be inverted if that is preferred is significant in reducing the chance of premature failure due to a bending moment. Other means of reducing this bending moment can readily be envisaged. Arm 1 and the arms attached to supports 2 and 3 are all firmly attached by welding or otherwise to an absorbing member 4. Any material which has a stress-strain diagram of the form shown in FIG. 1 that is to say, in which there is considerable energy absorption in the plastic range can be used for the absorbing member 4.

In practice it has been found that considerable experience and skill are needed in using a design identical with that of FIG. 3. It has already been stated that the preferred material is low carbon steel in a soft condition. Unless very great care is taken the welds shown as attaching absorbing member 4 will cause hardening and stress raising. It is much to be preferred that any welding should be away from a region of plastic deformation due to stress and preferably away from any highly stressed region.

In a modification found satisfactory V notches were machined in the levers attached to absorbing member 4 and the levers were bolted to the bar by high tensile bolts. In this approach the forces are transmitted to the bar through surface control pressures. In order to keep the contact pressure below the yield point of the material fairly thick levers are required or, alternatively, the cross-section of the bar may be shaped down to reduce the moment to be transmitted. Both of these approaches were used and both worked.

However, an alternative approach which was found to lead to a simpler form of device was to use a rectangular bar as the torsional element. In this case the bar produced, for the same surface area per unit length, a much smaller moment than the square bar and thus the gripping pressures are less critical. The final device is shown in FIG. 4. It will be noted that there are two middle levers 21. The space between them and the spaces 23 beyond the outer levers are unstressed and welds made at these places exact no penalty.

The moment is transmitted from the levers into the bar by contact between the surfaces of slots machined in the levers, the welds being used only to maintain the levers in place on the bar. This device using a 2 inch by /2 inch bar developed a force of two tons (20,000 N) with a displacement of l centimetre, developed an energy absorption per unit volume of 1 L6 X 10 N/M per cycle and maintained this for 200 cycles with a subsequent very gradual decay. At 500 cycles the peak load had reduced to percent of the maximum. Accompanying this decay was the predicted pattern of longitudinal cracks.

Absorption of energy involves the production of heat. A metal structure of significant size, such as the absorber here described, is capable of losing heat by radiation, convection and conduction. The energy which is produced by absorption but is not dissipated as heat will cause a temperature rise, particularly of the absorbing member. Whether this heats significantly when it is cyclically stressed to the plastic regime, will depend principally on the frequency of the cyclic reversal of stress. In most of the tests to which reference has been made previously this rate was very low and the absorbing member did not heat significantly.

Another series of tests was carried out at high cyclic rates. From this it was found that the favourable properties of the steel degenerated if the temperature rose much above 450C.

It is considered desirable then that the design of an absorber in relation to the expected manner of stressing, and in relation to any steps that may be taken to encourage the dissipation of heat, should be such that the absorbing member does not rise in temperature above 450C.

A reinforced concrete bridge is to be built of beams carried on 215 feet high A-frames with a base spread of 40 feet. A computer analysis of the proposed design has shown that energy absorbers with a capacity of 30,000 lbs (135,000 N) with a displacement of around 1 inch would be required on each leg to bring the earthquake induced oscillations to an acceptable level. Two energy absorbers according to the design of FIG. 4

when scaled up by 2.5 should provide the requirement of each of the legs of the bridge.

The designs shown in FIGS. 3 and 4 are symmetrical, that is to say, the absorber bar is supported at its two ends and is strained in torsion by a lever at or near its middle. This decreases but does not totally eliminate bending due to the force which not only operates the lever, but also bends the bar as a beam. It has already been said that the amount of this bending can be controlled by appropriate design of the lever, so that stresses in the bar due to torque are much greater than stresses due to bending. The two stresses are in quadrature, and their vector sum is very little greater than the stress due to torque alone. Because of this, a further embodiment of the invention is possible. In this, the bar is supported at only one end, and the lever is at the other end. This embodiment may be useful in some configurations of the structure to be protected.

What I claim is:

l. A cyclic energy absorber designed to be interposed between members of a structure which are caused by incoming energy to move relative to each other, said energy absorber comprising in combination:

a base adapted to be connected rigidly to a first part of the structure;

two supporting arms connected rigidly to said base;

a torsion bar, said two supporting arms being fixed rigidly to the ends of said torsion bar; and

a system of loading arms, one end of said loading arms being attached rigidly to the middle of said torsion bar, the other end thereof being linked to a second part of the structure so that relative toand-fro motion between said first and second parts of the structure cause said torsion bar to deform cyclically into the plastic range with a combination of torsional, bending and shearing deformations.

2. A cyclic energy absorber as defined in claim I, wherein the properties of said loading arms and said torsion bar, together with the direction of loading, are such that the cyclic plastic deformation of said torsion bar is predominantly torsional.

3. A cyclic energy absorber as defined in claim 1, wherein said torsion bar is constructed of steel having less than 0.3 percent of carbon and is in the annealed condition.

4. A cyclic energy absorber as defined in claim 1, wherein the ends of said supporting arms and of said loading arms are attached rigidly by welding to said torsion bar.

5. A cyclic energy absorber as defined in claim 1, wherein the ends of said supporting arms and said loading arms have slots which are adapted to fit over said torsion bar and wherein the ends of said supporting arms and said loading arms are attached rigidly by welding to those parts of said torsion bar which are not subject to plastic deformation.

6. A cyclic energy absorber as defined in claim 5, wherein said supporting arms and said loading arms are clamped to said torsion bar by bolts which close said slots.

7. A cyclic energy absorber as defined in claim 1, wherein said torsion bar is non-circular in it crosssection and is adapted to maintain parts of its crosssection within the elastic range when other parts of the cross-section have been stressed into the plastic range, whereby plastic strain is distributed more uniformly over the length of said torsion bar.

8. A cyclic energy absorber as defined in claim 7, wherein said torsion bar is rectangular in its cross section.

Claims

1. A cyclic energy absorber designed to be interposed between members of a structure which are caused by incoming energy to move relative to each other, said energy absorber comprising in combination: a base adapted to be connected rigidly to a first part of the structure; two supporting arms connected rigidly to said base; a torsion bar, said two supporting arms being fixed rigidly to the ends of said torsion bar; and a system of loading arms, one end of said loading arms being attached rigidly to the middle of said torsion bar, the other end thereof being linked to a second part of the structure so that relative to-and-fro motion between said first and second parts of the structure cause said torsion bar to deform cyclically into the plastic range with a combination of torsional, bending and shearing deformations.

2. A cyclic energy absorber as defined in claim 1, wherein the properties of said loading arms and said torsion bar, together with the direction of loading, are such that the cyclic plastic deformation of said torsion bar is predominantly torsional.

7. A cyclic energy absorber as defined in claim 1, wherein said torsion bar is non-circular in it cross-section and is adapted to maintain parts of its cross-section within the elastic range when other parts of the cross-section have been stressed into the plastic range, whereby plastic strain is distributed more uniformly over the length of said torsion bar.

8. A cyclic energy absorber as defined in claim 7, wherein said torsion bar is rectangular in its cross-section.