The use of hollow metal sections (eg square or rectangular box sections) in steel construction is attractive due to economy, aesthetics and structural performance.

Connecting structural components to such hollow metal sections is problematic due to the lack of access to the interior of the hollow section (the"blind side") for completing the fastening operation. This has led to the development of so-called"blind fastening devices"which only require access to the exposed face of the hollow section.

Blind fastening devices may be used to secure inter alia fixing brackets, pipe supports, electrical conduits or any other such building component to a hollow metal section.

One such blind fastener ("the Hollobolt") is described in WO-A-96/22479 (Glynwed International Plc). This particular device relies on a threaded cone located on the blind side of the hollow section which is drawn into axial engagement with an expanding shank by means of a threaded fastener. This causes the shank to radially expand outwardly away from the blind face and anchor the blind fastening device. The expanding shank is slotted such that it is composed of a number of axially extending individual legs, each of which is adapted to be radially expanded outwardly from the blind face of the hollow section.

A further blind fastening device is described in US-A- 5213460 (Sadri et al). This is a relatively sophisticated device with a limited applicability as a very tight tolerance to the clearance hole diameter is required and special equipment is necessary for installation. The device consists of a threaded shank and a non-rigid main sleeve which is collapsible to bring an expanding sleeve into engagement with the blind face of the hollow section. The expanding sleeve is not slotted and adopts an expanded bulbous shape as it passes over a taper on the non-rigid main sleeve. In order to collapse the non-rigid main sleeve and complete the fastening operation, a high axial force must be applied through the shank using a tool.

Notwithstanding the extensive efforts which have been made to develop blind fastening devices which perform as well as standard bolts in primary structural connections, there remains a need for blind fastening devices with improved clamping and resistance to the moment caused by connected components. The present invention seeks to fulfil this need by providing a device which represents a significant departure from conventional blind fastening devices. The device is based on a reverse mechanism by which a radially expanding component of the device expands inwardly towards the blind face of the hollow section. More particularly, the expanding component of the blind fastening device of the invention operates in a reverse manner to the expanding component of the device disclosed in for example WO-A- 96/22479.

Thus viewed from one aspect the present invention provides a blind fastening device for use in securing a building component to a hollow structural section comprising: a substantially cylindrical slotted sleeve having a plurality of axially extending slots separated by substantially arcuate legs capable of radial expansion inwardly towards the blind face of the hollow section; a rigid, non-collapsible shank having a tapered end on the blind side of the hollow section which serves to permit radial expansion inwardly of the slotted sleeve towards the blind face of the hollow section; a threadedly apertured shank head at the end of the rigid shank on the exposed side of the hollow section adapted to prevent rotation of the shank head whilst the head of the threaded fastener is rotated; a threaded fastener threadedly receivable in the slotted sleeve, the shank head and shank; a fastener head for tightening the threaded fastener; whereby progressive tightening of the fastener causes the slotted sleeve to axially engage the tapered end of the shank and thereafter to radially expand inwardly towards the blind face of the hollow section.

By providing a rigid, non-collapsible shank without slots, there will be an improvement in terms of shear and bearing characteristics over the dual purpose shanks of the prior art (ie shanks fulfilling the expansion and load bearing functions of the device). In general, any increase in shear capacity provides significant benefits to the structural engineer.

In a preferred embodiment of the invention, the legs of the slotted sleeve are progressively expanded until they engage directly the blind face of the hollow section. This results in effective clamping of the connected components directly in line with the axis of the threaded fastener.

When compare to the conventional Hollobolt, the greater area of contact between the blind face and the legs of the device of the invention is believed to improve resistance to moment connections.

Whilst any number of slots may be used on the sleeve, four or more are generally preferred. Particularly preferably the sleeve is provided with four slots. The four slots are advantageously located equidistantly around the sleeve perimeter defining four arcuate legs of equal width which are radially and diametrically aligned. This ensures substantially equal bearing contact about the axis of the threaded fastener onto the blind face of the hollow section.

Not wishing to be bound by any theoretical considerations, the rate and the extent of radial expansion and the extent of clamping of the device of the invention is believed to be influenced by the angle of taper on the shank.

This is thought to be due to the frictional forces exhibited across the surface of the taper. In addition, the inter- relationship between the taper and the sleeve may effect the behaviour of the blind fastening device such as the ability to resist induced loads caused by the structural component eg creep.

Naturally some degree of taper is required otherwise sleeve expansion would not be induced. Equally an angle of taper approaching 90° would be likely to induce effective expansion but not effective clamping. Thus the optimum angle of taper reflects a balance of different factors and generally may be in the range 10-80°. In a preferred embodiment, the angle of taper on the shank is in the range 20 to 30°, particularly preferably about-30°.

The slot length is believed to be important in determining whether or not (or the extent to which) the expanding sleeve penetrates the clearance hole. The optimum slot (and therefore the leg) length provides a compromise between plastic buckling at higher leg lengths and brittle fracture at lower leg lengths. Preferably, the slot length is in the range 10 to 25mm, particularly preferably 15 to 20 mm.

The slot length may be used to ensure that the device has a broad clamping range to cover both narrow and heavy gauge steelwork in a single device. Thus the versatility of the device may be increased and the misuse of low range products by unskilled users is eliminated. The clamping range of the device of the invention may be up to 35-40 mm.

However the range may be as low as 10mm.

Preferably the device of the invention consists of a three part assembly. This preferred device is considerably less sophisticated and easier to use than existing devices and is therefore advantageous.

Thus viewed from a further aspect the present invention provides a three-component kit of parts for constructing a blind fastening device as hereinbefore defined consisting of: one or more substantially cylindrical slotted sleeves having a plurality of axially extending slots separated by substantially arcuate legs ; one or more rigid, non- collapsible shanks having a tapered end and a threadedly apertured shank head; and one or-more threaded fasteners receivable in the slotted sleeve, the shank head and shank.

The threaded fastener may be any suitable threaded component. However for ease and cost effectiveness, it is preferably a standard set screw which i-s the major component of a standard bolt.

The shank and sleeve are conveniently made of mild steel. The shank is preferably cylindrical. Not wishing to be bound by any theoretical considerations, it is believed that the ratio of shank wall thickness to shank diameter may be important in the overall effectiveness of this device.

The invention will now be described in a non-limitative sense with reference to the accompanying figures in which: Figure 1 illustrates an embodiment of the blind fastening device of the invention; and Figure 2 illustrates schematically the clamping action of a preferred device of the invention in a hollow section.

With reference to Figure 1, there is shown a blind fastening device of the invention designated generally by reference numeral 1. This comprises a three-part construction having a standard set screw 2 passing through a mild steel sleeve 3 and a mild steel shank 4 which permits expansion of the sleeve 3 by means of taper 18 at its end. The sleeve 3 comprises a plurality of slots 5 defining intermediate arcuate legs 6 which are generally equidistant around the perimeter of the sleeve. The shank 4 has a shank head 20 which prevents rotation of the shank whilst the screw head 7 of the set screw 2 is rotated.

With reference to Figure 2, as screw head 7 is progressively tightened on the exposed side of the hollow construction, the arcuate legs 6 of sleeve 3 radially expand inwardly towards the blind face 8 of the hollow construction 9 thereby anchoring the component 10 to the hollow section.

Test 1 A test was performed to induce the collapse of a slotted sleeve in the device of-the invention. The specimen was a device with a 10mm slot length and four equidistant slots. The angle of taper on the shank was 30°. The device was placed in a compression testing machine in order to determine the degree of sleeve expansion with respect-to the compressive force between the expanding sleeve and tapered shank. The compression testing machine consisted of a diaphragm load cell.

Prior to this test it was anticipated that the expansion vs compressive load relationship would be linear with the gradient being a function of the friction between the two parts. However it was surprisingly found that there was no visible expansion until a compressive load of 16.7kN was achieved. At this load it was evident that the frictional resistance between the components was reached.

Expansion of the sleeve at this point was-sudden and absolute. This suggests that approximately 0.33 of the nett force in the insert is used in expansion alone whilst 0.67 is reserved for clamping. This represents an efficient clamping mechanism.

Test 2 In a comparative test of the clamping of a device of the invention and a conventional M12 Lindapter Hollobolt, the device of the invention was found to clamp slightly more effectively. In the Hollobolt, a large surface area between the mild steel threaded cone and the expanding shank causes a loss in clamping force due to-friction and-there is a reduction in clamping due to plasticity being induced during expansion. The efficient reverse mechanism of expansion characteristic of the device of the invention results in the improved clamping forces.