The PB System is characterised by a special form of glazing bead provided with substantially horizontal leg, which is retained by the bar at the inner and outer edges of the leg, and having a bend formed in the leg. As used in this specification the word"bend"means only that part of the leg is relatively inclined with respect to another part of the leg and that this shape may be produced otherwise than by an act of bending and in fact the shape normally would be produced by extrusion. The shape and orientation of the bend is such that if an attempt is made to prise the bead from its mounting, the applied force has the result of driving the opposed edges of the leg into urged engagement with the leg retaining means. Accordingly, the PB System provides a window unit in relation to which it is very difficult or impossible to remove the external glazing beads by means of the small hand tools conventionally used by petty criminals for such purposes.

Although the PB System provides improved security benefits compared with prior window systems higher thermal insulation standards for window units require an increase in the width of the insulating bridge between the inner and outer alloy sections of the glazing bar.

Although the need for improved insulation applies to all types of thermally broken glazing bars there is a particular problem implementing improvements with PB system type bars.

One way to improve thermal efficiency is to simply provide the bar with a wider insulating bridge fabricated using cast polyurethane or by means of wider extruded polyamide webs. However this results in a correspondingly wider window unit, which is undesirable. Another way to accommodate wider insulation elements is to reduce the width of the alloy sections by an amount sufficient to compensate for the increased length of the insulating bridge, so as to retain the original overall depth of the window unit. In the case of a PB system type glazing bar, however, reducing the width of the outer alloy section and glazing bead adversely affects the security of the system and is therefore not acceptable and the width of the rear section alone cannot be reduced sufficiently.

We have now established that a thermally broken alloy glazing bar can be produced with improved thermal insulation having the further advantage of having a significantly lower alloy mass per unit length in comparison with the known PB System glazing bar, and without compromising the security of the glazing unit. That is to say, it is now possible, in accordance with this invention, to reduce manufacturing costs while substantially retaining all of the security advantages of the PB System.

The term"alloy mass per unit length"as used herein refers to the mass of aluminium alloy per unit length of bar, in order to distinguish that characteristic, which is the main indicator of the cost of production, from the total mass per unit length, which would include the mass of thermal break material present in thermally broken glazing bars which are now specified for most building applications in preference to non-thermally broken bars.

From mechanical considerations of a light alloy glazing bead subjected to forces intended to prise it from an installed window unit we have concluded that the ability of the PB system to resist attack is mainly dependent, at any particular wall thickness, on the length of the said horizontal leg of the bead which extends from the outer towards the inner alloy section.

Generally speaking, in the case of a thermally broken window frame, the narrowing of the depth of the glazing bar relative to the leg length of the glazing bead means that the inner edge of the leg will extend into the region of the insulating bridge, which connects the inner and outer alloy sections of the glazing bar.

Accordingly, the innermost edge portion of the leg cannot be retained by a retaining formation provided on the outer alloy section, as in the known system, since the free end of the leg extends beyond the outer alloy setion and projects above the insulating web.

This invention consists in a glazing bar for a fixed or opening light having an alloy outer section and an alloy inner section connected together by a web of thermally insulating material to form a glazing rebate, a glazing bead being engageable within the glazing rebate, the bead having a leg extending from the outer alloy section towards the inner alloy section the opposed inner and outer edge portions of the leg being retained between retaining formations in the glazing rebate, the inner retaining formation being disposed on the thermally insulating web, the leg being formed with a bend and being of sufficient length such that a force applied to lever the bead from the glazing rebate on the outer side causes the leg to deflect in such a way as to cause the inner and outer edges of the leg to be urged further into engagement with their respective retaining formations and thereby to resist the forced removal of the bead from the rebate.

Preferably the inner retaining formation is formed integrally with an insulating web by extrusion and is adapted to resist deflection by the applied force transmitted to it by the inner edge portion of the leg.

Preferably the inner retaining formation is located intermediate the opposed end portions of the upper web to lie, in use, beneath the bottom edge of a sealed double glazing panel.

The interengagement of the the leg and the retaining means is desirably such that when a force is applied to the bead the inner edge of the leg subjects the web member to a compressive force.

To facilitate compression of the web member under the influence of such an applied force, preferably the inner edge portion of the leg is turned down at a small angle to enter the inner retaining formation in the form of a groove provided in the upper portion of the web, the groove being inclined upwardly to receive the downwardly inclined edge of the leg, the arrangement being such that a force applied to remove the bead would put the web member into compression to resist attack.

The insulating web is preferably in the form of a polyamide extrusion.

A new and useful glazing frame fabricated from such glazing bars to provide, compared with the known PB System, a relatively lightweight, high insulating more economic glazing frame of slim profile having relatively lower alloy mass per unit length of frame.

Important benefits arising from this invention are that secure window units can be produced at lower cost due to a lower alloy weight per unit length, which can be achieved without compromising the security of the system and makes possible the production a slimmer window unit than would otherwise be the case.

An embodiment of the invention will now be described in further detail by way of example only with reference to Figure 1 of the accompanying drawings.

Figure 1 is a cross section of a glazing bar and associated bead.

Figure 2 is cross section representing a PB System bar (and therefore not in accordance with this invention) but having, for the purposes of comparison only, a thermally insulating bridge formed by two 30 mm wide extruded polyamide webs which is required to meet current insulation requirements. It should be noted that the conventional PB system glazing bar has a 12 mm deep insulating bridge.

Referring to Figure 1 a thermally broken alloy glazing bar has an outer aluminium section 1, and an inner aluminium section 2 connected together by upper and lower extruded polyamide webs 3, and 4 respectively. The insulation polyamide webs of this embodiment are 30 mm in width and offer improved thermal insulation compared with standard insulation bridge width of 12 mm. The upper web 3 is formed with a slightly upwardly and outwardly facing groove 5, which receives the downwardly turning edge of the inwardly extending leg 6 of glazing bead 7.

A window unit constructed with the glazing bar shown in Figure 1 resists attack as well as the PB System bar (Figure 2), constructed generally in accordance with GB 2292170, but illustrated here with improved 30 mm thermal insulating webs, notwithstanding an overall reduction in depth to 66.5 mm from 77 mm, resulting in cost and weight savings. The improvement is achieved by keeping the length of the leg at least the same as in the corresponding conventional PB System bar while reducing the depth of the alloy sections.

The way in which a window unit in accordance with the present invention resists attack is described above and in GB2292170 and will not be described in further detail here.

It will be appreciated that the term"window units"as used herein includes fixed and opening lights generally, including lights formed in glazed doors and panels other than glazing panels may be substituted as required.

Important benefits arising from this invention are that secure window units can be produced at lower cost due to a lower alloy weight per unit length, which can be achieved without compromising the security of the system and makes possible the production a slimmer window unit than would otherwise be the case.