GB2162915A

GB2162915A - Reinforcing bar coupling

Info

Publication number: GB2162915A
Application number: GB08420285A
Authority: GB
Inventors: Christopher Robin Edwards; Michael Stephen Wright
Original assignee: Allied Steel and Wire Ltd
Current assignee: Allied Steel and Wire Ltd
Priority date: 1984-08-09
Filing date: 1984-08-09
Publication date: 1986-02-12
Also published as: EP0171965A2; HK62193A; ZA855837B; EP0171965B1; DE3578537D1; JPH06102233B2; AU578964B2; AU4573685A; GB2162915B; GB8420285D0; EP0171965A3; JPS6171142A

Abstract

In order to provide a screw-threaded connection at an end of a concrete reinforcing bar of the kind having a deformed outer surface with ribs 11, the screw thread is rolled directly on to the bar without first machining it to a round form. The resulting thread may be incomplete on some parts of the surface of the bar. Two such bars may be joined by screwing them in to a common sleeve. <IMAGE>

Description

1 GB 2 162 915A 1

SPECIFICATION

Reinforcing bar joints The invention relates to reinforcing bar joints 70 and -particularly to screw-threaded joints be tween two lengths of deformed reinforcing bar. - Two ca tegories of bar used for reinforce ment are known as round bae 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 in clined to a circumferential line and provide means whereby concrete -poured around the bar is locked to the bar against relative axial movernent. Throughout this specification and the appended claims the term -deformed rein forcing 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 char acteristics of any such joint are its ultimate tensile strength and-the longitudinal displace ment 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 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 in the form of a coarse screw niques applied to deformed bar produce effective screw threads and that these screw theads are satisfactory for reinforcing bar joints.

In accordance with the present invention there is provided a screwthreaded joint between two lengths of deformed reinforcing bar (as herein defined) comprising an internally screw-threaded sleeve open at both ends, and an externally screw-threaded end portion of each of the lengths of bar engaged in the sleeve, wherein the threads on the bars have been roiled directly onto the deformed surface thereof.

The effect of rolling a screw thread on to a deformed bar is to produce a 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 the sleeve so that a joint produced by a sleeve and two such bars has threads bedded firmly together and does not suffer from excessive elongation under load.

An embodiment 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 11 of Fig. 1; Figure 3 is a view corresponding to Fig. 2 but in the direction of arrow 111; thread. Although the intention is that such bar 110 Figure 4 is a view corresponding to Fig. 1 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.

The alternative known approach is to ma- chine the 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 tech- 130 in the direction of arrow IV; and Figure 5 is a diagrammatic representation of a complete joint produced by two bars in accordance with the invention.

Figs. 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 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 Fig. 4. The ribs taper gradually and run out into the general surface of the bar along two notional axial lines 12 and 13 running the length of the bar. Apart from the ribs 11, the bar has a core of slightly oval shape as best seen in Fig. 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 Fig. 2. The ribs and the non-circular cross section are 2 GB2162915A 2 formed in the bar in the last 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 70 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 undeformed 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 Fig. 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. There is a tendency for a few threads to be formed completely around 90 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.

Once these threads have been rolled on to the end portions of the deformed bar, two bars may be joined together in conjunction with a sleeve as shown in Fig. 5. Fig. 5 shows an internally screw-threaded 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 Figs. 1 to 4. In Fig. 5, the effective threaded areas are indicated by shading and those areas where there is only a shallow thread or no effective thread are left un- shaded. The two bars 17 and 18 have flat ends with chamfered edges 19 and 21. As a result, the two bars can be screwed in to the sleeve 16 until they abut tightly against each other, 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 bar 17-18 is loaded in tension, the additional 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 apears that the discontinuities in the thread in the bars does not result in excessive stress concentrations and presumably beca.6se 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 circular cores.

Claims

1. A screw-threaded joint between two lengths of deformed reinforcing bar (as herein defined) comprising an internally screwthreaded 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 deformed surface thereof.

2. A screw-threaded joint as claimed in claim 1 wherein before thread rolling, the bars have a non-circular core and transverse ribs projecting from the core. -

3. A screw-threaded joint as claimed in claim 1 or claim 2 wherein the screw thread is discontinuous in that portions of the screwthreaded end regions are without thread or have thread of insufficient depth to carry substantial load.

4. A screw-threaded joint as claimed in. any one of the preceding claims wherein the two bars have flat ends and chamfered edges and are screwed tightly against each other within the sleeve.

5. A screw-threaded joint between two lengths of deformed reinforcing bar substantially as herein described with reference to and as illustrated by the accompanying drawings.

6. A length of deformed reinforcing bar (as herein defined) having an externally screwthreaded end portion wherein the thread is rolled directly on the deformed surface thereof.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935. 1986, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.