BACKGROUND OF THE INVENTION In the past box section beams formed from metal and typically from steel have been used in a number of different applications. Heavy duty beams typically formed from hot or cold rolling from steel billots are used in many structural applications either as beams or columns and are commonly referred to by the name RHS (rectangular hollow section).

In many applications there is no need for the strength and weight of an RHS beam and many proposals have been made for the use of light weight metal box section beams formed by bending (typically rolling or folding) from relatively thin gauge sheet steel. Such beams have been made in many different configurations, the most common of which are formed by pre-bending two C sections and joining those sections together in a fabricating operation. Constructions of this type are clearly shown and described in French Utility Model FR 2 568 668-A3, British patent 849,321 and International specification PCT/AU90/00577.

Although the overlapping C section box beams provide a workable product, they have a number of disadvantages including manufacturing cost due to the necessity to roll each C section separately followed by a joining operation which commonly requires manual handling. Furthermore, it is difficult to provide accurately aligned transverse apertures in the webs of box section beams formed in this way (for the passage of building services, etc.) due to the difficulty in accurately aligning preformed apertures in the manual assembly operation.

The use of material in an overlapping C section box beam is also often inefficient as the flange thickness formed by the overlapping C sections at the top and bottom of the beam must, by nature of the beam construction, be exactly two times the web thickness in the beam. The steel grade used in both C sections must also be the same. This distribution of material over the cross section of the beam is often not optimum in providing the most efficient strength to weight ratio.

SUMMARY OF THE INVENTION The present invention therefore provides an elongate structural member formed from sheet metal arranged in an enclosed box section having top and bottom flange members spaced apart by web members, the flange members being formed separately from the web members, the flange members having at least a flange portion extending across the width of the box section and web portions extending from the flange portion, lying against and being fastened to the web members.

Preferably the flange members are formed from thicker sheet metal than the web members.

Preferably the web members are provided in the form of side faces of an elongate box section also having top and bottom faces.

Preferably the elongate box section is formed from a single elongate strip of sheet metal.

Preferably the flange portions of the flange members lie against the top and bottom faces of the elongate box section.

Preferably the flange members are enclosed within the elongate box section.

Preferably the edges of the web portions of the flange members distal from the flange portions are provided with laterally extending edge portions.

Preferably the web portions of the flange members are fastened to the web members by integral stakes burst through the overlying sheet metal.

In a further aspect the invention provides a method of manufacturing an elongate structural member having an enclosed box section, said method comprising the steps of providing an elongate length of sheet metal having notional longitudinal parallel spaced flange areas, separated by notional longitudinal parallel spaced web areas, overlaying elongate sheet metal flange reinforming members over the notional flange areas, and bending the elongate length into a box section having opposing flanges formed by the flange area overlaid by the flange reinforcing members, separated and supported by opposing web members formed by the web areas.

Preferably the flange reinforcing members are wider than the notional flange areas, overlapping the notional web areas to either side of the notional flange areas and

forming sub-web portions overlaying the internal surface of the web members adjacent the flanges when bent into a box section.

Preferably the method includes the step of fastening the flange reinforcing members to the elongate length of sheet metal in the notional flange areas before bending into the box section.

Preferably the method includes the step of fastening the flange reinforcing members to the elongate length of sheet metal in the sub-web portions after bending into the box section.

Preferably the step of fastening the flange reinforcing members to the elongate length of sheet metal comprises punching stakes through the overlapping members.

BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms that may fall within its scope, one preferred form of the invention will now be described by way of example only with reference to the accompanying drawings in which: Fig. 1 is a cross section through an assembled elongate structural member according to the invention, Fig. 2 is a similar view to Fig. 1 showing the incorporation of aligned apertures in the web members, Fig. 3A is a cross sectional view showing the formation of the box section in a rolling operation with the sheet metal flange reinforcing members overlaid on and fastened to the base elongate length of sheet metal, Fig. 3B is a similar view to Fig. 3A showing the partial forming of the first two corners of the box section, Fig. 3C is a similar view to Fig. 3B showing the first two corners of the box section completely formed with the remaining two corners partially formed, and Fig. 4 is a similar view to Fig. 1 showing an alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred form of the invention the elongate structural member is formed by bending in a cold rolling process from sheet steel, although it will be

appreciated that the invention may also be applied to other materials and to other forms of bending e. g. by folding.

The finished box section beam as shown in Fig. 1 comprises top and bottom flange members 1 and 2 spaced apart by web members 3 and 4 forming a rectangular hollow box section shape as can be clearly seen in Fig. 1.

The profile of the beam may be provided with ribs or other indentations such as those shown at 5 and 6 to improve the structural performance of the beam (as is well known) or for any other desired end use purpose.

The beam is generally formed from a single elongate length of sheet metal 7 rolled into the box section shown by the process described below reinforced by separate sheet metal flange reinforcing members 8 and 9.

In the preferred form of the invention, the sheet metal flange reinforcing members extend around the corners 10 of the box section and partway up the web members 3 and 4, forming sub-web portions 11 overlaying the internal surfaces of the web members 3 and 4 adjacent the flanges 1 and 2.

The structural performance of the sub-web portions 11 can be further enhanced by providing inwardly turned lips 27 at the free edges of the sub-web portions. The lips 27 give enhanced strength by increasing the buckling resistance of the free edges of the sub-web portions 11 when in compression.

The flange reinforcing members 8 and 9 are typically fastened to the adjacent web members by multiple interlocks 12. The interlocks may take the form of any suitable fastener but are typically provided in the form of stakes burst through the overlapping metal areas in the positions shown during the rolling process.

Fig. 2 shows a similar view to Fig. 1 in which apertures 13 and 14 are punched into the webs 3 and 4 respectively to form aligned openings through the middle of the box section. Such openings are commonly needed when the beams are used in building construction where it is often necessary to pass building services through the centre of a beam. As the web members 3 and 4 are formed from the same elongate length of sheet metal 7 it is possible to form the apertures 13 and 14 during the rolling process and have those apertures accurately aligned, facilitating ease of use in placing building services once the beam is in position.

Although it is preferred to form the web members 3 and 4 from a single length of sheet metal 7, it is also possible to form the web members separately as shown in Fig. 4.

Although the box section beam according to the invention can be formed by any known process, it is typically formed by roll forming over a number of steps, some of which are progressively shown in Figures 3A, 3B and 3C.

In Fig. 3A the elongate length of sheet metal 7 is shown in substantially flat form (before being rolled into a box section) and can be described as having various notional areas which will ultimately be defined in the box section.

These areas are generally shown in Fig. 3A as notional flange areas 15 and notional web areas 16. It can be seen that the notional longitudinal parallel spaced flange areas 15 are separated by the notional longitudinal parallel spaced web areas 16.

The sheet metal flange reinforcing members 8 are overlaid over the notional flange areas, and in the preferred form of the invention the flange reinforcing members are wider than the notional flange areas, overlapping the notional web areas to either side of the notional flange areas. It is this overlap that ultimately forms the sub-web portions 11 shown in Fig. 1.

While the elongate length of sheet metal 7 is substantially flat, the flange reinforcing members 8 are secured in the desired location on the sheet metal 7 by punching in locating stakes 28. These stakes are used primarily for location during the roll forming process and have been omitted from Figs. 1 and 2 for clarity. The reinforcing ribs 5 may also be roll formed at this time.

As the rolling operation proceeds, the initial corners 17 and 18 are partially formed as shown in Fig. 3B and as the rolling continues, the corners 17 and 18 are formed to 90° as shown in Fig. 3C with corner 19 partially formed. Corner 20 is also partially formed as shown in Fig. 3C and at this point is bent to the stage where the sub-web 21 is substantially parallel to the opposing sub-web 22 on the same flange reinforcing member 8. As the corresponding free edges of the flange reinforcing member 8 and the free edge 23 of the elongate sheet metal member have been preformed to leave a gap 24 therebetween, the other free edge 25 of the elongate sheet

metal member may then be tucked into the gap 24 as the rolling process completes the corner 19 to a full 90° bend. Free edge 23 is then rolled down on top of free edge 25 forming the overlapping seam 26 shown in Figures 1 and 2. The fastening stakes 12 may then be burst through the overlapping seam portion to complete the box section member.

By rolling the box section member with separate flange reinforcing members 8 in this manner (rather than forming the flanges from overlapping portions of preformed C section members as in known in the prior art) it is possible to select the metal thickness and grade of the flange reinforcing members 8 to give ultimate performance of the final box section beam. In a typical beam construction, ultimate performance can be achieved where the flange thicknesses are more than twice the web thicknesses and this can be readily achieved by construction according to the present invention. The present invention also allows the efficient use of deeper sections with thinner material of higher steel yield stress. The structural performance of the beam shown is superior to multiple piece assemblies e. g. formed from two C sections by up to a 60% increase in bending strength capacity per unit weight.

It is a further advantage of the beam formed according to the present invention that the extended sub-web portions 11 of the flange reinforcing members together with the multiple interlocks 12 provide additional support for the highly stressed web/flange junction at the corners 10.

The sub-web portions 11 together with the multiple interlocks 12 also provide web support in shear and bearing by reducing the effective slenderness of the webs 3 and 4.