AU2017279714A1 - A structual member and method of manufacture - Google Patents

Abstract

a 1 V3 I. .JUJ - I n~ u - . A structural member having an enclosed profile is formed from a sheet of material having a longitudinal axis between a front edge and a rear edge, and a transverse axis across the longitudinal axis between a first side edge and a second side edge. The structural member includes a first flange having a first side and a second side, a first web extending from the first side of the first flange towards the first side edge, a second web extending from the second side of the first flange towards the second side edge, and a second flange. The second flange includes a first return between the first web and the first side edge, the first return further including an outwardly facing hook between the first return and the first side edge. The second flange also includes a second return between the second web and the second side edge, the second return further including an inwardly facing hook between the second return and the second side edge. The outwardly facing hook and the inwardly facing hook project into each other to provide an overlapping section, and the second flange includes at least one crimp in the overlapping section, where a width of the crimp in the direction of the transverse axis of the sheet is less than that of the overlapping section. 112-1 112-6 154-1 300-1 154-2 112-2 112-4 158-4 158-1 - -152-12 ,,152-2 102 112-3 158-2 158-3 156-1 300-2 156-2

Description

A STRUCTUAL MEMBER AND METHOD OF MANUFACTURE STATEMENT OF CORRESPONDING APPLICATIONS

This application is based the provisional specification filed in relation to New Zealand Patent Application No. 727847, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a structural member, more particular a hollow structural section cold-rolled from a sheet of material.

BACKGROUND

Cold-formed structural members are known for use in construction, for example as purlins, rafters, girts, and the like. Cold-forming refers to the shaping of flat steel sheet without the use of applied heat. Light gauge steel in particular when cold-formed typically provides considerably more strength for its weight than hot-formed steel (i.e. processed above its recrystallisation temperature).

The roll-forming process is the method generally preferred for volume production of cold-formed structural members, and is often termed cold-rolling. In this process the steel sheet is passed through a series of rollers which incrementally 'roll' the shape from the flat sheet.

Known roll-formed light-gauge steel structural members are typically open shapes. C and Z shaped purlins are the most common, although other shapes and variations on these shapes may be found.

Such open shapes are easier to manufacture than enclosed shapes. However, enclosed shapes such as rectangles, squares, and other hollow shapes are more stable and less prone to buckling and distortion when subjected to load than open shapes. Further, open shapes such as in the case of C and Z shaped purlins are also notorious for providing bird roosting perches when used on the underside of roofing. This can be a serious nuisance due to the accumulation of bird excrement both on the beams where it is very corrosive, and beneath the beams where it can also pose serious problems. A method that has been employed is the use of continuous welding to join the edges of a single sheet of steel together after roll-forming, to form a hollow section. However, in order to provide protection against corrosion of the structural member it is desirable to galvanise the light gauge steel. Welding precludes galvanising of the sheet prior to cold-rolling, which is much more economical and convenient than finishing or galvanising the structural member after cold-forming.

Another method that has been employed is the use of one of a variety of lock-seam methods to join the edges of a single sheet of steel together after roll-forming, to form a hollow section. Lock-seaming folds the edges of the sheet over each other to produce a mechanical joint. However, these methods require the production of very tight radii to achieve the fold, and are difficult to apply to steel sheet other than very thin thicknesses, typically 1.0mm or less. Furthermore, it is particularly difficult to design a single roll-forming machine that will perform lock-seaming for a range of sheet thicknesses which may be required to form a structural member, for example between 1.0 mm to 3.0 mm - which would otherwise necessitate investment in additional capital equipment.

It is an object of the present invention to address at least one of the foregoing problems or at least to provide the public with a useful choice.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

SUMMARY

According to one aspect of the present disclosure there is provided a structural member having an enclosed profile formed from a sheet of material having a longitudinal axis between a front edge and a rear edge, and a transverse axis across the longitudinal axis between a first side edge and a second side edge, the structural member including: a first flange having a first side and a second side; a first web extending from the first side of the first flange towards the first side edge; a second web extending from the second side of the first flange towards the second side edge; and a second flange including: a first return between the first web and the first side edge, the first return further including an outwardly facing hook between the first return and the first side edge; a second return between the second web and the second side edge, the second return further including an inwardly facing hook between the second return and the second side edge, wherein the outwardly facing hook and the inwardly facing hook project into each other to provide an overlapping section, and wherein the second flange includes at least one crimp in the overlapping section, and the width of the crimp in the direction of the transverse axis of the sheet is less than that of the overlapping section.

According to another aspect of the present invention there is provided a method of manufacturing a structural member substantially as described herein from a sheet of material having a longitudinal axis between a front edge and a rear edge, and a transverse axis across the longitudinal axis between a first side edge and a second side edge, wherein the method includes the steps of: roll-forming the sheet of material to form the first return and the second return; roll-forming the sheet of material to form the first flange, the first web, and the second web; directing the first return towards the second return and forcing the first return away from the first flange and/or forcing the second return towards the first flange such that the first return and second return pass each other to overlap; releasing the first return and/or second return, such that the outwardly facing hook and the inwardly facing hook project into each other to provide the overlapping section; and crimping the overlapping section, wherein the width of the crimp in the direction of the transverse axis of the sheet is less than that of the overlapping section.

In an exemplary embodiment, the structural member may be formed from a single sheet of material. However, it is also envisaged that in exemplary embodiments the structural member may be formed from two sections, such that the first flange is also configured in the manner described with respect to the second flange (i.e. having overlapping hooks crimped in the manner described).

According to one aspect of the present disclosure there is provided a method of joining two edges of a sheet of material, wherein the method includes: forming a first hook on one edge, and a second hook on the second edge, positioning the first and second hooks such that they project into each other to provide an overlapping section, and crimping the overlapping section, wherein the width of the crimp in the direction of the transverse axis of the sheet is less than that of the overlapping section.

In an exemplary embodiment, the structural member is an elongate beam. The length of the beam may be that of the sheet of material from which it is formed, or cut to length.

Reference to the structural member having an enclosed profile should be understood to mean that the cross-sectional shape of the member is hollow, with the interior surfaces of the flanges and webs enclosing an interior space of the member - i.e. a hollow structural section. In addition to providing a structurally efficient shape, such a profile is envisaged as avoiding creating ledges for birds to perch on -particularly when used in the construction of roofs. It should be appreciated that reference to an enclosed profile is not intended to exclude the production of apertures in the structural member (for example, for the insertion of fasteners to secure fittings to the structural member).

Where the structural member is used in roof structures, it is envisaged that a flange may be a horizontal element of the structural member, predominantly resisting most of the bending moment experienced by the structural member in use. Similarly, in such an orientation a web of the structural member may be a vertical element of the structural member, predominantly resisting most of the shear forces experienced by the structural member in use. It should be appreciated that this is not intended to be limiting, as use cases are envisaged in which the structural member may be oriented such that the flanges are vertical elements - for example where used as a girt in a wall structure to resist wind loads.

Reference to a return should be understood to mean portion of the sheet of material bent out of alignment with the adjacent section, typically inwardly. In an exemplary embodiment, the returns may be substantially 90° returns. However, it should be appreciated that this is not intended to be limiting; for example, the angle of the returns may be such that they are biased to clamp against the adjacent portion of the structural member.

In an exemplary embodiment, the first web and/or the second web may be formed such that they are biased towards an angle of greater than 90° relative to the first flange. It is envisaged that this outwardly acting bias may assist in maintaining the shape of the structural member.

In an exemplary embodiment, the sheet of material may be made of steel. In an exemplary embodiment, the sheet of material is light gauge steel, more particularly steel having a thickness between about 0.3 mm and about 3.0 mm. In an exemplary embodiment, the light gauge steel may have a thickness between about 0.6 mm and about 2.5 mm. In an exemplary embodiment, the light gauge steel may have a thickness between about 0.75 mm and about 2.25 mm. The form of the structural member is such that tight folds are not required, which allows the same roll-forming equipment to be used for a range of thicknesses of sheet. It should be appreciated that this is not intended to be limiting, and that the sheet of material may be made of another metal or ductile material suitable for cold roll-forming.

In an exemplary embodiment, the sheet of material may be treated for corrosion protection prior to forming the structural member. For example, the sheet of material may be galvanised. The form of the structural beam allows for an enclosed profile to be produced without the use of welding or other heat based joining techniques, which could otherwise compromise corrosion protection applied prior to formation of the member.

Reference to crimping should be understood to mean the act of localised deformation of two or more layers of sheet material to produce a friction joint between them. It should be appreciated that reference to a "crimp" may therefore be understood to mean the localised deformation resulting from this action. In an exemplary embodiment, the crimping may be performed by applying the deforming force with a tool on one side of the overlapping section, with the other side of the overlapping section bearing against an anvil.

In an exemplary embodiment, crimping may be performed on opposing sides of the overlapping section - i.e. the flange having the overlapping section may include a first crimp on a first side of the overlapping section, and a second crimp on a second side of the overlapping section.

As a point of reference for explanation, each hook may include a lead-in portion, a return bend, and return portion ending at the side edge of the sheet of material. In an exemplary embodiment, at least one of the hooks may include a recurved section in the lead-in portion. This recurved section may allow the outward facing surfaces of the respective lead-in portions to be substantially level.

In an exemplary embodiment, the return bend of the hooks may be such that the return portion is open beyond parallel relative to the lead-in portion - i.e. more open that a "U" shape. It is envisaged that this may be particularly applicable to embodiments in which the structural member is manufactured from a single sheet, allowing for the overlap to be achieved by a lateral motion with a relatively small degree of deflection of the returns as they pass each other. However, it should be appreciated that this is not intended to be limiting to all embodiments.

Further, it should be appreciated that in exemplary embodiments in which the structural member is formed from two sections the sections may be positioned relative to each other through a sliding motion along the longitudinal axis. In such a configuration, the hooks may be generally "U" shaped or closed beyond parallel.

For ease of understanding, the joint created by the crimped hooks will be described as if the crimping force is applied from the outside of the structural member - i.e. is applied inwardly. However, it should be appreciated that this is not intended to be limiting to all embodiments, and in exemplary embodiments the crimping force may be applied from inside the structural member.

In an exemplary embodiment, the width of the crimp along the transverse axis of the sheet may be about, or less than, the distance between the respective return bends of the hooks through the overlap. In doing so, formation of the joint does not rely on flattening of the return bends - which for thicker sheets of material would likely require a bend radius below a recommended minimum bend radius.

Determination of the minimum bend radius may consider factors such as sheet material (for example, a higher tensile steel may require a higher bend radius), sheet thickness, the bending angle (generally a tighter radius may be used for a more open angle - e.g. a tighter radius may be used for a 90° bend than a 180° bend), and tolerance for tensile cracking (dependent on the finish and intended application). Minimum bend radius is typically expressed as a multiple of the thickness (T), e.g. 3T. A traditional lock seam must use a 0.5T radius to achieve flatness.

In an exemplary embodiment, the minimum bend radius may be greater than 0.5T. In an exemplary embodiment, the minimum bend radius may be about or greater than 1.0T. In an exemplary embodiment, the minimum bend radius may be about or greater than 2.0T.

It is envisaged that the joint described herein may enable a single former to be used to manufacture structural members from different thicknesses of sheet. For example, a former configured to produce a minimum bend radius of approximately 1.0T from 2.00 mm sheet may also be used to process 1.00 mm sheet (with a resulting bend radius of approximately 2.0T) - with the design of the joint being such that this difference in bend radius does not negate the effectiveness of the joint.

In an exemplary embodiment, the crimp in the overlapping section may be substantially centred in the overlapping section. In an exemplary embodiment including a first crimp on a first side of the overlapping section, and a second crimp on a second side of the overlapping section, a centre point of the first crimp may be offset from a centre point of the second crimp along the overlapping section.

In an exemplary embodiment, the crimp may include an indentation in the hook to which the deforming force is applied. In an exemplary embodiment, the indentation may be provided in the lead-in portion of the hook.

In an exemplary embodiment, the depth of the indentation may be sufficient to bear the end of the return portion of one of the hooks against the return portion of the other hook, without flattening the returns entirely - i.e. the returns do not bear against each other in their entirety through the overlap. A further understanding of the functional and advantageous aspects of the present disclosure can be realised by reference to the following detailed description and drawings.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present disclosure will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which: FIG. 1A is a cross-sectional view of an exemplary structural member according to one aspect of the present disclosure; FIG. IB is a cross-sectional view of an exemplary structural member according to one aspect of the present disclosure; FIG. 2A is a cross-sectional view of an exemplary structural member in a first stage of manufacture according to one aspect of the present disclosure; FIG. 2B is a cross-sectional view of the exemplary structural member in a second stage of manufacture; FIG. 2C is a cross-sectional view of the exemplary structural member in a third stage of manufacture; FIG. 2D is a cross-sectional view of the exemplary structural member in a fourth stage of manufacture according to one aspect of the present disclosure; FIG. 2E is a cross-sectional view of the exemplary structural member in a fifth stage of manufacture; FIG. 3A is a cross-sectional view of an exemplary joint for an exemplary structural member in a first stage of formation according to one aspect of the present disclosure; FIG. 3B is a cross-sectional view of the exemplary joint in a second stage of formation; FIG. 3C is a cross-sectional view of the exemplary joint in a third stage of formation, and FIG. 3D is a cross-sectional view of the exemplary joint in a fourth stage of formation.

DETAILED DESCRIPTION FIG. 1A illustrates an exemplary structural member in the form of an elongate beam 100. The beam 100 is formed from a single sheet of material, for example light gauge galvanised steel having a thickness of between about 0.3 mm to 3.0 mm.

The beam 100 includes a first flange 102. A first web 104 extends from a first side of the first flange 102. A first return 106 is formed at an end of the first web 104 distal from the first flange 102, substantially parallel with the first flange 102. A second web 108 extends from a second side of the first flange 102, substantially parallel with the first web 104. A second return 110 is formed at an end of the second web 108 distal from the first flange 102, substantially parallel with the first flange 102.

In the exemplary embodiment illustrated, swages 112-1 to 112-6 are provided in the first flange 102, first web 104, and the second web 108 for additional strength.

The first return 106 and second 110 are joined to provide a second flange by a joint 300, as shown in greater detail in FIG. 3C and described below.

In an exemplary embodiment, the beam 100 may be installed with the first flange 102 against the material to which the beam 100 is to be secured. For example, where the beam 100 is used to support roofing, the beam may be installed with the first flange 102 facing upwardly, and the roofing material fastened thereto. In doing so, a fastener (such as a screw) is only required to pass through one layer of the sheet of material from which the beam 100 is formed. FIG. IB illustrates an exemplary structural member in the form of an elongate beam 150. The beam 150 is formed from two sheets of material, for example light gauge galvanised steel having a thickness of between about 0.3 mm to 3.0 mm.

The beam 150 includes a first section formed of a web 152-1 having a first return 154-1 at one end, and a second return 156-1 at the distal end, substantially parallel with the first return 154-1. The beam 150 further includes a second section formed of a web 152-2 having a first return 154-2 at one end, and a second return 156-2 at the distal end, substantially parallel with the first return 154-2.

The first and second sections of the beam 150 are joined by joints 300-1 and 300-2 to create the box section illustrated.

In the exemplary embodiment illustrated, swages 158-1 to 158-4 are provided in the webs 152-1 and 152-2 for additional strength.

It should be appreciated that while beam 100 and beam 150 are illustrated as having the joints 300 in the ends of the respective beams, it is also envisaged that in exemplary embodiments the joints may be provided in the sides of those beams (for example, in one of the first web 104 or the second web 104 of beam 100, or in webs 152-1 and 152-2 of beam 150). FIG. 2A-2E illustrate the manufacture of the beam 100 from a single sheet 200 of light gauge steel using cold-rolling techniques. The method is illustrated after a first "outward facing" hook 202 has been formed in a first edge of the sheet 200, and a second "inward facing" hook 204 has been formed in a second edge of the sheet 200. In a first step, rollers act to bend the sheet 200 about points 202-1 and 202-2 (see FIG. 2A) to produce the first and second returns 106 and 108 (see FIG. 2B).

The upper flange 102 is then formed by bending the sheet about points 208-1 and 208-2 (see FIG. 2B), with the first web 104 and second web 108 rolled out (see FIG. 2C).

The first and second webs 104 and 108 are rolled closer together. When they begin to meet each other, as seen in FIG. 2D the second return 110 is depressed towards the upper flange 102 so that the first return 106 can be lifted over it, while at the webs 104 and 108 continue to be pushed closer together.

Referring to FIG. 2E, the inward facing hook 204 of the first return 106 overlaps with the outward facing hook 202 of the second return 110. The joint 300 may then be formed, as will be described with reference to FIGs. 3A to 3C.

Referring to FIG. 3A, the outward facing hook 202 includes a lead-in portion 302-1, a return bend 304-1, and a return portion 306-1 ending at the side edge of the sheet of material 200. The return portion 306-1 of the outward facing hook 202 includes a recurved section 308. The inward facing hook 204 includes a lead-in portion 302-2, a return bend 304-2, and a return portion 306-2 ending at the other side edge of the sheet of material 200.

It may be seen that in this exemplary embodiment the return portions 306-1 and 306-1 are open beyond parallel relative to the lead-in portions 302-1 and 302-2. This assists with positioning the hooks 202 and 204 such that they overlap each other in the manner illustrated.

As shown in FIG. 3B, an anvil 310 (for example, a roller wheel) is provided on the inward facing surface of the lead-in portion 302-1 of the outward facing hook 202, and a crimping tool 312 (for example, a bead on a roller wheel) is brought to bear on the outward facing surface of the lead-in portion 302-2 of the inward facing hook 204 in a position centred on the overlap of the hooks 202 and 204. The hooks 202 and 204 are deformed such that the lead-in portion 302-2 of the inward facing hook 204 bears against the return portion 306-1 of the outward facing hook 202, and the return portion 306-2 of the inward facing hook 204 bears against the lead-in portion 304-1 of the outward facing hook 202, and the lead-in portions 302-1 and 302-2 and return portions 304-1 and 304-2 are brought into parallel.

Referring to FIG. 3C, the crimping tool 312 (not shown in FIG. 3C, but see FIG. 3B) continues to deform the inward facing hook 204 such that an indentation 314 is formed in the lead-in portion 302-2. The indentation 314 deflects the return portion 306-1 of the outward facing hook 202 such that its distal end bears against the return portion 306-2 of the inward facing hook 204, while the end of the return portion 306-1 of the outward facing hook 202 adjacent the return bend 304-1 remains elevated above the return portion 306-2 of the inward facing hook 204 with a gap 316 therebetween. The numerous points of contact between the hooks 202 and 204 may reduce the potential for relative movement, particularly in circumstances in which the beam is subjected to twisting forces.

Referring to FIG. 3D, in an exemplary embodiment a second indentation 318 may be formed in the lead-in portion 302-1 of the outward facing hook 202. The second indentation 318 deforms the return portion 306-2 of the inward facing hook 204, and bears it against the return portion 306-1 of the outward facing hook 202. It is envisaged that this may further increase the tightness and integrity of the joint 300 in comparison with that illustrated in FIG. 3C, without increasing the minimum bend radii used. Flowever, it should be appreciated that the joint 300 in the stage illustrated in FIG. 3C may still have application for use in the exemplary beams described herein.

In an exemplary embodiment, the second indentation 318 may be formed subsequently to formation of the indentation 314. In an exemplary embodiment, the second indentation 318 may be formed simultaneously with formation of the indentation 314 - for example with a second crimping tool mirroring crimping tool 312.

In an exemplary embodiment, a centre point 320 of the indentation 314 may be offset from a second centre point 322 of the second indentation 318 along the length of the joint 300. For example, in the exemplary embodiment illustrated, the centre point 320 may be closer to the free end of the return portion 306-1 of the outward facing hook 202 than the second centre point 322 is. It is envisaged that this may assist with reducing the spring back of the return portions 306-1 and 306-2.

The entire disclosures of all applications, patents and publications cited above and below, if any, are herein incorporated by reference.

Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour in any country in the world.

Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth.

It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present invention.

The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof, as defined by the appended claims.

Claims (20)

1. A structural member having an enclosed profile formed from a sheet of material having a longitudinal axis between a front edge and a rear edge, and a transverse axis across the longitudinal axis between a first side edge and a second side edge, the structural member including: a first flange having a first side and a second side; a first web extending from the first side of the first flange towards the first side edge; a second web extending from the second side of the first flange towards the second side edge; and a second flange including: a first return between the first web and the first side edge, the first return further including an outwardly facing hook between the first return and the first side edge; a second return between the second web and the second side edge, the second return further including an inwardly facing hook between the second return and the second side edge, wherein the outwardly facing hook and the inwardly facing hook project into each other to provide an overlapping section, and wherein the second flange includes at least one crimp in the overlapping section, and a width of the crimp in the direction of the transverse axis of the sheet is less than that of the overlapping section.

2. The structural member of claim 1, wherein the structural member is formed from a single sheet of material.

3. The structural member of claim 2 or claim 3, wherein the structural member is an elongate beam.

4. The structural member of any one of claims 1 to 3, wherein at least one of the first web and the second web are biased towards an angle of greater than 90° relative to the first flange.

5. The structural member of any one of claims 1 to 4, wherein the sheet of material is light gauge steel,

6. The structural member of claim 5, wherein the light gauge steel has a thickness between about 0.3 mm and about 3.0 mm.

7. The structural member of any one of claims 1 to 6, wherein the crimp in the overlapping section is substantially centred in the overlapping section.

8. The structural member of any one of claims 1 to 7, wherein the second flange includes a first crimp on a first side of the overlapping section, and a second crimp on a second side of the overlapping section.

9. The structural member of claim 8, wherein a centre point of the first crimp may be offset from a centre point of the second crimp along the overlapping section.

10. The structural member of any one of claims 1 to 9, wherein each hook includes a lead-in portion, a return bend, and return portion ending at a side edge of the sheet of material.

11. The structural member of claim 10, wherein at least one of the hooks includes a recurved section in the lead-in portion.

12. The structural member of claim 10 or claim 11, wherein the return bend is configured such that the return portion is open beyond parallel relative to the lead-in portion.

13. The structural member of any one of claims 10 to 12, wherein the width of the crimp along the transverse axis of the sheet is about, or less than, the distance between the respective return bends of the hooks through the overlap.

14. The structural member of any one of claims 1 to 13, wherein the minimum bend radius is greater than 0.5T.

15. The structural member of any one of claims 1 to 14, wherein the minimum bend radius is greater than 1.0T.

16. The structural member of any one of claims 1 to 15, wherein the minimum bend radius is greater than 2.0T.

17. A method of manufacturing a structural member as claimed in any one of claims 1 to 16 from a sheet of material having a longitudinal axis between a front edge and a rear edge, and a transverse axis across the longitudinal axis between a first side edge and a second side edge, wherein the method includes the steps of: roll-forming the sheet of material to form the first return and the second return; roll-forming the sheet of material to form the first flange, the first web, and the second web; directing the first return towards the second return and forcing the first return away from the first flange and/or forcing the second return towards the first flange such that the first return and second return pass each other to overlap; releasing the first return and/or second return, such that the outwardly facing hook and the inwardly facing hook project into each other to provide the overlapping section; and crimping the overlapping section, wherein the width of the crimp in a direction of the transverse axis of the sheet is less than that of the overlapping section.

18. The method of claim 17, wherein the sheet of material is treated for corrosion protection prior to forming the structural member.

19. The method of claim 17 or claim 18, wherein crimping the overlapping section includes crimping on opposing sides of the overlapping section.

20. A method of joining two edges of a sheet of material, wherein the method includes: forming a first hook on one edge, and a second hook on the second edge, positioning the first and second hooks such that they project into each other to provide an overlapping section, and crimping the overlapping section, wherein the width of the crimp in the direction of the transverse axis of the sheet is less than that of the overlapping section.