GB2194809A - Energy absorption joint - Google Patents

Abstract

An energy absorption joint, eg for tunnel support arches 6, comprising a tapered plug 2 and a hollow section socket 1 having yieldable walls, the dimensions and shapes of the components being such that the tapered sides of the plug abut the socket walls on entry into said socket and deform but not fracture the walls on telescoping, whereby the joint manifests a predictable load v deformation characteristic. The extent of the telescoping action thus indicates the load forces to which the joint has been subjected. <IMAGE>

Description

SPECIFICATION Energy absorption joint This invention relates to a method and apparatus for providing a archlike or other structural support system with a telescoping controllable energy absorption yielding joint.

In the underground environment of a deep mine tunnel the floor and the roof in particular, due to the earth's geology, are continuously moving together. Usually this geological movement results in the tunnel cross sectional area decreasing most generally in height. It is virtually impossible to prevent this movement, but its effect on the workable life of a tunnel can be reduced by use of appropriate tunnel supports.

In recent years, the life span of mine tunnel supports has assumed greater importance, brought about by the economical transportation of ore over long distances by conveyors and the improved carriage of men and materials. These factors have allowed mineral ore faces to be worked more profitably at ever increasing distances from the mine shafts. In the mineral mining industry due to the prohibitive cost and in some cases unsuitability of the conventional long term tunnelling support systems, which often take the form of circular shell segments joined at regular intervals, short term support to provide a restraint to geological movement is usually achieved by the use of heavy rigid arches usually formed from rolled steel H-Section. This practice is especially common in the Coal Mining Industry.It is necessary when installing these and other conventional forms of archlike supports to carefully block off the arch from the rock face at frequent intervals in an attempt to achieve a more even load distribution and to avoid excessive bending stresses.

To minimise buckling, some form of tie bar is often employed between individual arches.

Among the disadvantages found with this form of tunnel support are their initial high material and installation costs and when the geological movement has caused the arch design load to be reached, hence the end of their useful life span, their replacement cost in terms of labour cost, new support equipment and the disruption to normal mine operations.

It is well known in the Mining industry that there are recognised advantages if the tunnel support system allows some initial controlled relaxation of the ground so that in part natural arching may occur. However, expert opinion strongly advises that the upmost caution be exercised when seeking these advantages.

Arches of the yielding type designed for colliery support are becoming widely used in the Mining industry in recognition of their ease of erection, reduced tendency to distort and that they can, in favourable circumstances, go some way towards allowing natural arching to take place. These arches usually incorporate a conventional form of friction type sliding joint and can inpart overcome some of the disadvantages found in using the conventional rigid H-Section design. However, normal environmental conditions found in underground workings acting on these joints, can cause varying load resistance factors which result in inconsistent roof support, which can, if not rectified, eventually lead to tunnel collapse.A typical problem experienced with known types of yielding joints is that although the yielding effect in say dry conditions can be within an acceptable working range, the same joint in wet conditions will very often display quite different characteristics to the detriment of the overall system.

The invention now disclosed, seeks to overcome the previously stated disadvantages and provides a solution to other tunnel support problems found in the Mining and other associated industries especially where the tunnelling and/or working conditions are hostile.

In particular, the present invention provides an energy absorption joint comprising a tapered plug and a hollow section socket having yieldable walls, the dimensions and shapes of the components being such that the tapered sides of the plug abut the socket walls on entry into said socket and deform but not fracture the walls on telescoping therein, whereby the joint manifests a predictable load v deformation characteristic.

The extent of the telescoping action thus indicates the load forces to which the joint has been subjected.

Conveniently, the plug may be circular in section, whilst the socket may be square in section.

The invention will be fully understood from the following detailed description aided by the accompanying drawings which shcw some embodiments by way of example.

Fig. 1 of the drawings shows the two components of the joint assembled in a relaxed state.

Fig.2 shows the joint in section and alternative modes of joining to conventional tunnel supports.

Fig.3 illustrates typically two energy absorption joints in the supports of a conventional tunnel arch.

Fig.4 shows in cross section, the action of a joint in the process of deformation.

Fig.5 illustrates the relationship of load to joint deformation.

Fig.6 demonstrates a section through a typical joint fully telescoped.

Fig.7 shows a typical configuration of several joints incorporated into a circular tunnel support.

The joint, typically illustrated in Fig,1, consists of a hollow section steel socket or base (1) and a circular-section steel plug (2), having a tapered portion (3) dimensioned such as to provide a controlled interference fit into the internal dimensions of the base. The base has four straight sides, which can be fitted with a shoe plate (not shown). Under load the plug causes the shell or walls of the base to deform plastically. The plug (2) can be in the form of a solid or hollcw section.

Where initial stability is required (Fig.2), the plug can be formed with a spigot portion (4) such as to provide a sliding fit into the base (1). When the joint is required to support tubular structures, the plug can have a second slideable fitting spigot (5) formed at its other end. This second spigot can be further fitted with an additional support collar (not shown). Where other forms of attachment are considered necessary, either or both of the plug ends can be fashioned accordingly.

A wide range of loads can be supported in the initial yielding phase, but once the full yield pattern has been established in the shell, the full restraint load will have been reached also and no further load increase can occur. From this point onwards, when the strata load rises to the maximum designed restraint load of the arch support, the joint telescopes and the load on the support can increase no further. Various restraint loads can be achieved by varying the shell size, the shell wall thickness and the interface between the shell and the plug.

Fig.3 shows a typical arch profile (6) with its vertical legs supported on two energy absorption joints (1 & 2) and the whole assembly is in support of a typical tunnel cross section (7). The two groups of arrows (8 & 9) indicate the commonly expected direction of the most significant loading on the arch profile.

In the preferred arrangement, following installation, the frusto-conical plug (2)-Fig. 1-is located in the rectangular shell base (1) with the tapered portion (3) of the plug (2) engaging the inner faces of the base (1). As the tunnel height begins to decrease by reason of the geological strata movement, the resultant loads placed upon the joints (1 & 2), causes the plugs (2) to be gradually driven into their bases (1) until the load reaches the maximum restraint force of the arch assembly. At this point, the plug has entered into the base to a depth wherein the parallel sides of the plug have begun to deform the inner walls of the base.Additional movement of the strata causing loads grnater than the arch restraint force are absorbed by an increase in the telescoping action with no increase in the restraint force acting on the arch assembly (1,2 & 6) Fig.4 shows a cross sectional area of a typical joint in the process of deformation. The side faces of the shell (1) act in a beamlike manner when a transverse load P is exerted onto its faces by the circular plug (2) being forced into the rectangular section by the applied load N.

Since this force N will be equally distributed between the four faces of the shell, the transverse force P acting on each face will be Nan6/4 where 6 is the taper angle (3) on the leading edge of the plug.

Let i = intereference between shell and plug = d-(b-2t) dt = total deformation of shell wall = i/2 I = second moment of area of shell side acting as a beam with depth t and width c = ct3/12 E = elastic modulus c = (d-(b-2t))/2tane S = plastic modulus of shell side = ct2/4 Q = length of shell side acting as a beam For a fixed beam, the central deformation will be, 192EI and the Plastic moment, Mp = YsS = 8 where Py = transverse load to cause yield and Ys = shell yield strength and for a simply supported beam P::'3 6 = 48EI and Mp = Pyl ............................(1) 4 However, the sides of the shell act as a partially fixed beam midway between the above two extremes where, # = 5Pl and Mp = 3Pyl = YsS 384EI 16 (2) Hence Py = 16ysS and = 64YsS (3) 3:' 3ltan# where Ny = applied load at yield. Because of the size of the interference between the shell Fig.4 (1) and the plug (2), the shell will yield before the imposed deformation has been achieved.

Below yield, the deformation will be proportional to the elastic modulus E, however, above yield the slope of the load/deformation line will be greatly reduced and will be proportional to Er and because a plastic hinge will have formed, the beam will approximate to a simply supported beam rather than a partially fixed one.

Fig.5 shows the relationship between load and deformation where: Pt = transverse load required for deformation Py = transverse load at yield 6t = total deformation = i/2 = (d-b+2t)/2 6y = deformation at yield Then at yield load Py, from ....(2) #y = 5Pyl = 5Nyt2tane ............. (4) 384EI 4x384EI and from Py to Pt, from ....(1) 6p = Ppl = Npt3tane 48ErI 4x48ErI (5) where #p = plastic deformation = 6t-6y Pp = plastic transverse load = Pt-Py Np = applied load in plastic range Nt-Ny Nt = total applied load = restraint load Hence, #t = 6y+6p = i/2 = (d-.(b-2t))/2 Substituting for by and Jp from ...(4) and ...(5) d-b+2t = 5Nyi3tan + Npt3tan 2 4x384EI 4x48ErI Substituting for Np = Nt-Ny Nt = 96Er1(d-b+2t) + Ny [1 - 5Er l tan# 8E and from ... (3) Ny = 64YsS 3:'tano Substituting for I and S Restraint load Nt = 4Er(d-b+2t)2t3 + NY [1 - 5Er l tan# [1 SE and Ny = 8Y3sd (d-b+2t) t2 3::'tan2 Fig.6 illustrates a joint typically in a fully telescoped state.

A further embodiment wherein the invention is incorporated into a conventional circular tunnel support is shown in Fig.7 wherein a load acting at any point on the support structure causes the plurality of joints (1 & 2) to act in a manner herein before described.

Although the invention has been described with reference to the particular embodiments illustrated, it is to be understood that various modifications may readily be introduced without departing from the scope of their invention. For example, the plug need not be of circular section and the base or shell need not be rectangular. Material other than steel may be used for these joint components. Further, the joint may readily be used in fields other than mining, indeed anywhere where controlled 'collapse' is required e.g. in motorway crash barriers, buffers, anti run-under bumpers for lorries etc.

Claims (7)

1. An energy absorption joint comprising a tapered plug and a hollow section socket having yieldable walls the dimensions and shapes of the components being such that the tapered sides of the plug abut the socket walls on entry into said socket and deform but not fracture the walls on telescoping therein, whereby the joint manifests a predictable load v. deformation characteristic.

2. A joint according to claim 1, wherein the cross-section of the socket hollow is uniform throughout its length.

3. A joint according to claim 1 or claim 2 wherein the tapered plug has a shank of uniform cross-section.

4. A joint adcording to any one of claims 1 to 3, wherein the tapered plug is hollow in crosssection.

5. A joint according to any one of claims 1 to 4, wherein the plug is circular in cross-section and the socket is square in cross-section.

6. A joint according to any one of claims 1 to 5, adopted for use in arch supports in underground roadways.

7. An energy absorption joint substantially as herein described with reference to the accompanying drawings.