EP0171965B1

EP0171965B1 - Method of providing connections for reinforcing bars and joint and connections for such bars

Info

Publication number: EP0171965B1
Application number: EP19850305382
Authority: EP
Inventors: Christopher Robin Edwards; Michael Stephen Wright
Original assignee: Allied Steel and Wire Ltd
Current assignee: ASW Ltd
Priority date: 1984-08-09
Filing date: 1985-07-29
Publication date: 1990-07-04
Anticipated expiration: 2005-07-29
Also published as: JPH06102233B2; GB2162915A; EP0171965A2; GB2162915B; HK62193A; AU578964B2; EP0171965A3; JPS6171142A; GB8420285D0; ZA855837B; DE3578537D1; AU4573685A

Description

This invention relates to a method of forming a connection for the end portion of a rolled steel reinforcing bar, to a screw-threaded joint between such bars and to other screw-threaded terminations for such bars.
Two categories of bar used for reinforcement are known as round bar and deformed bar. Deformed bar is bar which has surface deformations which cause a deviation from a circular cross section and cause irregularities such as transverse ribs which might be inclined to a circumferential line and provide means whereby concrete poured around the bar is locked to the bar against relative axial movement. Throughout this specification and the appended claims the term "deformed reinforcing bar" or "deformed bar" is intended to refer to such bar as described above.
It is often desired to join two lengths of reinforcing bar and one way of doing this is to apply screw threads to end portions of two bars and to screw them into an internally screw-threaded sleeve. Two fundamental characteristics of any such joint are its ultimate tensile strength and the longitudinal displacement between the bars as they are loaded. To acquire the required strength, it is desirable to remove a minimum amount of material from the bar in the vicinity of the screw threads. In order to avoid excessive extension under load, there is a requirement for accurately produced screw threads which mate with each other over large surfaces. This is because if there is only local contact between the screw threads at high spots, high loads and thus substantial deformation occur at these high spots until other parts of the threads come into contact and share the load.
One known form of reinforcing bar joint incorporates deformations along the whole length of the bar which are made in such a manner that they can act as a coarse screw thread. For example see GB-A-1 546 824. Although the intention is that such bar could be screwed into sleeves to produce threaded joints, the inevitable irregularities in the screw thread which is formed by hot-rolling the bar result in a joint which is subject to substantial deflection as it comes under load unless additional means such as lock nuts, torque loaded to induce bedding of the threads are used. These means are both expensive and bulky.
It is known from FR-A-2 488 820 to roll a screw-thread on an end portion of a deformed bar having a circular core and ribs projecting from the core. However, while threadrolling dies are not normally completely "filled" material during the threadforming process, according to this known method when threads are being rolled directly onto deformed bar a slightly fuller thread is formed at the point of coincidence between a hollow in the forming die and a deformation on the bar being threaded. Although this is not said to result in any variation in the strength of the thread so formed, the slight difference in height of individual threads is said to be detectable. Such a method is effective where the bar has a circular core, but cannot be used where the bar has a non- circular core.
The alternative known approach is to machine deformations off the surface of the bar to provide a round bar end portion and then to roll or otherwise produce a screw thread on the bar. This is a particularly time-consuming operation and it is unsatisfactory in so far as the machining inevitably removes some of the material of the bar and tends to reduce its strength.
As a basis for the present invention we have now discovered that thread rolling techniques applied to previously deformed bar having a non- circular core and ribs projecting from the core produce effective screw threads and that these screw threads are satisfactory for reinforcing bar joints and similar applications.
In accordance with one aspect of the present invention there is provided a method of providing a connection for the end portion of a rolled steel deformed bar (as herein defined) including the step of rolling a screw-thread in a generally circumferential direction on an end portion of a deformed bar having a non-circular core and ribs projecting from the core, the rolling being carried out in such a manner that the screw-thread is discontinuous insofar as regions of the screw- threaded end portion are without thread or have thread of insufficient depth to carry substantial load.
The effect of rolling a screw thread on to a deformed bar is to produce a full depth or substantially full depth screw thread with crests and troughs in some regions while a less deep and incompletely formed screw thread is formed over other parts of the surface of the bar. A thread rolling operation normally produces an accurate screw thread and the screw thread in accordance with the invention is accurate except in that portions thereof are missing or only partly formed. It has been discovered that thread rolling a typical deformed bar produces sufficient screw thread to provide effective contact with the corresponding screw threads of another member so that the threads bed firmly together and do not suffer from excessive elongation under load.
According to a second aspect of the invention there is provided a screw-threaded joint between two lengths of deformed rolled steel reinforcing bar (as herein defined) comprising an internally screw-threaded sleeve open at both ends and an externally threaded end portion of each of the lengths of bar engaged in the sleeve, wherein the threads on the bar have been rolled directly onto the previously deformed surface of a bar having a non-circular core and transverse ribs projecting from the core, the threads being rolled in such a manner that the screw thread is discontinuous insofar as regions of the screw-threaded end portion are without thread or have thread of insufficient depth to carry substantial load.
According to a third aspect of the invention there is provided a screw-threaded termination for a rolled steel reinforcing bar (as herein defined) comprising an externally threaded end portion of the bar, a plate carried by the bar and a nut screw threaded to the end portion of the bar to hold the plate in position, wherein the screw thread on the bar has been formed by rolling directly onto the previously deformed surface of a bar having a non-circular core and transverse ribs projecting from the core, the threads being rolled in such a manner that the screw thread is discontinuous insofar as regions of the screw-threaded end portion are without thread or have thread of insufficient depth to carry substantial load.
Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a side elevation, partly in section, of a portion of deformed reinforcing bar after rolling a length of thread on one end thereof;
Figure 2 is an end elevation in the direction of arrow II of figure 1;
Figure 3 is a view corresponding to figure 2 but in the direction of arrow III;
Figure 4 is a view corresponding to figure 1 in the direction of arrow IV;
Figure 5 shows a typical thread form in greater detail;
Figure 6 is a pictorial representation of an end of a single bar used as an anchor and also showing an anchorage plate and nut; and
Figure 7 is a diagrammatic representation of a complete joint produced by two bars in accordance with the invention.
Figures 1 2, 3 and 4 show a length of high-yield hot-rolled steel reinforcing bar of a kind used widely and known by the United Kingdom Registered Trade Mark UNISTEEL. The bar is provided with a series of inclined transverse ribs 11 arranged generally in a herringbone pattern, as shown in figure 4. The ribs each taper gradually and run out into the general surface of the bar along two notional axial lines 12 and 13 (see figure 2) running the length of the bar. Apart from the ribs 11 the bar has a core 10 of slightly oval shape as best seen in figure 2, with a major diameter D and a minor diameter d. The cross sectional shape taken in conjunction with the ribs 11 is circular as also shown in figure 2. The ribs and the noncircular cross section are formed in the bar in the last stand of a hot-rolling mill.

The conventional way of obtaining a threaded joint between two such sections of bar would be to machine end portions of both sections of the bar to the minor diameter d. This not only involves the time associated with the machining operation but also reduces the cross section of the bar and thus tends to reduce its strength at the joint. In contrast with this, in accordance with the present invention, the unmachined bar is subject to a thread rolling operation by thread rolling equipment incorporating three rotary dies set to roll a thread on a bar with a diameter intermediate between D and d.
This thread rolling operation results in a fully formed or substantially fully formed thread at two diametrically opposed regions associated with the lines 12 and 13 as shown in figure 1 and also produces a very shallow and only partially formed thread at regions such as 14 where the bar has a smaller transverse dimension. In some cases there may even be no thread at all in the regions 14. There is a tendency for a few threads to be formed completely around the whole or a major part of the circumference of the bar in the region of the crests of the ribs 11, as indicated at 15. Due to the circumferential spreading of the ribs during the rolling operation, the well formed thread extends slightly beyond the original position of the rib itself. A preferred threadform is illustrated in figure 5 and incorporates a relatively wide shallow root 20 and a relatively sharp narrow crest 21. This type of thread facilitates rolling and' leads to good fatigue strength.
Figure 6 shows one possible use of the bar with a thread rolled on its end and also provides a pictorial representation of the screw thread. The bar may be used as a rock anchor in which case it is secured in a hole in a rock surface by means such as grout in the usual way. A nut 22 serves to hold a plate 23 against a rock surface (not shown) so that the plate tends to hold rock near the surface in position, taking load on a part of the rock remote from the surface by virtue of the bar being anchored in its hole. The nut is tightened to a desired tension in order to hold the surface rock in position.
Two bars may be joined together in conjunction with a sleeve as shown in figure 7. Figure 7 shows an internally screwthreaded steel sleeve 16 within which two bars 17 and 18 have been engaged. The bars 17 and 18 each correspond to the screw- threaded bar shown in figures 1 to 5. In figure 7, the effective threaded areas are indicated by shading and those areas where there is only a shallow thread or no effective thread are left unshaded. The two bars 17 and 18 have flat ends, and a spacer disc 19 is positioned between them. The two bars can be screwed in to the sleeve 16 until they abut tightly against the spacer disc, thereby ensuring that even when there is no external load on the bars 17 and 18, the threads of the bars are firmly engaged against the threads of the sleeve in a direction corresponding to tensile loading. Although the threads are not continuous, there is sufficient area of thread to ensure positive bedding of each bar against the sleeve. When in use the now composite load applied through the screw threads results in very little extension at the joint. In practice, the extension at the joint may be no more than the elastic extension arising in a corresponding length of reinforcing bar remote from the joint.
The tensile strength of the bar at the joint remains substantially the same as that of the bar as a whole because there is no loss of material as a result of the thread rolling. It appears that the discontinuities in the thread in the bars does not result in excessive stress concentrations and presumably because of this the fatigue strength of the joint is particularly good. Although the invention has been described in relation to a bar having an oval core, it may also be applied to other forms of deformed bar including bars with non-circular cores.

Claims

1. A method of providing a connection for the end portion of a rolled steel deformed bar (as herein defined) characterised by the step of rolling a screw-thread (14, 15) in a generally circumferential direction on an end portion of the deformed bar having a non-circular core (10) and ribs (11) projecting from the core, the rolling being carried out in such a manner that the screw thread is discontinuous insofar as regions (14) of the screw-threaded end portion are without thread or have thread of insufficient depth to carry substantial load.

2. A length of deformed rolled steel reinforcing bar (as herein defined) characterised by a discontinuous externally screw-threaded end portion formed by a method according to claim 1.

3. A screw-threaded joint between two lengths (17 and 18) of deformed rolled steel reinforcing bar (as herein defined) comprising an internally screw-threaded sleeve (16) open at both ends and an externally threaded end portion of each of the lengths of bar engaged in the sleeve, characterised in that the threads on the bar have been rolled directly onto the previously deformed surface of a bar having a non-circular core (10) and transverse ribs (11) projecting from the core, the threads being rolled in such a manner that the screw-thread is discontinuous insofar as regions (14) of the screw-threaded end portion are without thread or have thread of insufficient depth to carry substantial load.

4. A screw-threaded termination for a rolled steel reinforcing bar (as herein defined) comprising an externally threaded end portion of the bar, a plate (23) carried by the bar and a nut (22) screw threaded to the end portion of the bar to hold the plate in position, characterised in that the screw thread on the bar has been formed by rolling directly onto the previously deformed surface of a bar having a non-circular core (10) and transverse ribs (11) projecting from the core, the threads being rolled in such a manner that the screw thread is discontinuous insofar as regions (14) of the screw-threaded end portion are without thread or have thread of insufficient depth to carry substantial load.