AU627578B2 - Sheet metal structural member - Google Patents

Description

S627578 COMMONWEALTH OF AUSTRALIA Form PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Class Int. Class pplication Number: Lodged: r Complete Specification-Lodged: Accepted: Published: Priority: Related Art: t tt Name of Applicant: Address of Applicant: t a TO BE COMPLETED BY APPLICANT JOHN LYSAGHT (AUSTRALIA) LIMITED 55 Sussex Street, Sydney, New South Wales, ,lustralia Actual Inventor: ALEXANDER MARICIC Address for Service: SMITH SHELSTON BEADLE Suite 8, 207 Great North Road FIVE DOCK NSW 2046 Complete Specification for the invention entitled: "SHEET METAL STRUCTURAL MEMBER" S014911 1 /OS/5 0 The following statement is a full description of this invention, including the best method of performing it known to me:- Complete after provisional PJ 4447 dated 29 May 1989 -I This invention relates to metal beams of the kind made from a single piece of sheet material by a cold roll-forming or pressing process, and which comprise a web and two flanges. Typical of the beams in question are Z sectioned steel purlins, as used for the support of roof cladding.

Being necessarily made from relatively thin sheet metal, the webs and flanges of the subject beams have high slenderness ratios by comparison with those of conventional fabricated or hot-rolled beams, such as standard I beams and other rolled steel joists. Indeed, failure of cold roll-formed or pressed metal beams is usually attributable to local buckling of a slender web or flange, rather than to failure of the metal to withstand the stress imposed on it, as in the case of conventional hot-rolled or fabricated beams.

The position is exacerbated in that hot rolled beams may be, and, if intended to bear heavy loads, usually are, symmetrical about their centre plane, with O each flange then projecting in both directions from the web. This enables twice the metal to be in the flange (for a given thickness and length of projection from the web) than is possible in beams which are cold-rolled or pressed from a single sheet, where each flange 2 necessarily projects in one direction only.

The design criteria for all web and flange beams favour:a web of great depth, so as to space apart the tension and conpression zones (flanges) to increase the moment of the forces in those zones resisting bending, and flanges of great width, so as to increase the metal in each flange and reduce the intensity of the stresses therein.

3 However, when the beam is roll-formed or pressed from light gauge material, the extent to which the depth of the web and the width of at least the compression flange (those dimensions being the relevant "lengths" of their slenderness ratios) is limited by the correspondingly increasing tendency for them to buckle locally.

In view of the foregoing it has become usual (in respect of the subject beams) to restrict the depth of the web and provide a so-called lip flange extending longitudinally of the free edge of each flange and projecting towards the neutral axis of the beam.

Indeed, Australian Standard 1538 of 1988 relating to such beams stipulates minimum design criteria for the second moment of area for such lip flanges.

Such lip flanges certainly stiffen the main flange.

They also leave substantial planar flange areas as the outermost surfaces of the beam, that is to say the Ssurfaces farthest from the neutral axis. This is 42(D important as it facilitates the affixture of cladding panels or the like to the beam, and enables site worke-s tiltto walk on exposed beams as construction proceeds.

However, lip flanges have only a limited effect on the crucial width of the main flanges. In this regard, the proportion of the flange metal forming the lip flange cannot be increased markedly because of the need for the flange metal to be as widely spaced as possible from the neutral axis.

The nett result is that, in prior known beams of the kind in question, all of the flange metal cannot be fully effective, that is able to be stressed to the design limit for the metal concerned, because of the likelihood of a buckling failure before that condition 4 is reached.

The present invention aims to provide a superior alternative to the conventional lip flange for increasing the resistance of the beam's flanges to buckling. The invention does so by providing stiffening ribs projecting towards the neutral axis at or near the free edges of the flanges and, in preferred embodiments, by also flattening or depressing the longitudinally extending corners between the flanges and web.

Therefore, the invention consists in a beam of the kind made by cold roll-forming or pressing a metal strip and comprising a web and two flanges, characterised in that a stiffening rib projecting towards the neutral axis extends along each flange at or near the free edge thereof.

By virtue of the ribs having two flanks they may account for a greater proportion of the flange metal than a lip flange may, thereby effecting a greater reduction of the "length" which determines the slenderness ratio of the flange, without unduly approaching the neutral axis.

The flattening or depression of the corner zones in preferred embodiments of the invention appears to have a stiffening effect on both the flanges and the web and, x provided the depression is not excessive, utilises less metal in the corner zone than is used by a conventional, sharp 900o corner. It is thought that, whereas the flange rib is the prime factor, the corner flattening or dep iston further contributes to the improvement in loe Jarrying capacity found in beams according to the invention.

Be that as it may, it has been found in experiments leading to the invention that roll-formed steel beams with flange edge ribs and flattened or depressed corners in accordance with preferred embodiments of the invention, made from steel 1.4 mm thick, have the same maximum load carrying capacity as otherwise identical beams with conventional lip flanges made from steel 1.6 mm thick. In some instances this represents a material saving of 12%.

By way of example, two embodiments of the abovedescribed invention are described in more detail hereinafter with reference to the accompanying drawings.

Figure 1 is a perspective view of an end portion of 1 a beam according to the invention.

Figure 2 is an end elevationr of the beam of figure 1 drawn to a larger scale.

Figure 3 is a further enlarged detail of a part of a second embodiment of the invention corresponding to the part of the figure 2 embodiment within the enclosure :0 marked 3 in that figure.

Thr, beam illustrated by figures 1 and 2 is a so- Scalled Z sectioned roof purlin. It comprises a web 4 and flanges 5 and 6 respectively.

Flange 5 comprises a planar major part 7 and a stiffening rib 8 formed as almost a half of a full wave corrugation extending longitudinally of the flange. The rib 8 comprises two, inclined, planar flanks 9 and and an intermediate portion 11 of arcuate cross-section, but in other embodiments the portion 11 may be reduced, so that the rib has a more V like cross- section, and in x 1A others it may be increased, so that the :6 rib has a part circular or other wave like cross-section which is smoothly curved throughout. All such variations are embraced by the term "rib" as used herein.

The flange 6 is shaped similarly to flange 5 but its planar major part is a little wider, so that an inverted, identical, overlapping beam may nest with the illustrated beam, in the well-known manner of conventional Z sectioned purlins. To assist persons erecting a roof, the flange 6 preferably has an additional lip 12 to make it obvious at a glance which of the flanges is the "wider" flange.

In this embodiment, the longitudinal corners between the flanges 5 and 6 and the web 4 are flattened by virtue of narrow, planar, inclined junction zones 13 between the web and each flange. In the enclosure 3 of figure 2 a conventional unflattened corner is shown in broken outline for comparison purposes. This flattening has the effect of reducing the unstiffened width of the flange and the unstiffened depth of the web, and ii reducing the amount of metal in the junction zones, ii while leaving the distances between the flanges as a i whole and the neutral axis substantially unchanged.

The web 4 has two substantially rectangular, tapered embossments 14 pressed from it. Those embossments project in opposite directions and the crown of each i pierced by a fastener clearance hole 15. The embossments are positioned such that when two beams according to this embodiment are overlapped at their ends, with one beam inverted relative to the other, the embossments nest one within the other (one embossment of one beam enters one embossment of the other beam, and the other embossment of said one beam receives the other embossment of the other beam) with the clearance holes 7 in register. The two beams may then be bolted or riveted together and the embossments are then effective to maintain the beams in alignment and to transfer web stresses from one to the other. Thus the embossments 14 then materially assist the beams to function as if they were a continuous beam at the joint. For preference each beam has a second pair of embossments (not shown) spaced along the beam from the pair in the figure to enable four fasteners to be used at the joint.

Figure 3 shows an example of a beam (which may otherwise be the same as the above described embodiment of figures 1 and 2) wherein the corners between the flanges and the web are depressed. In this instance the stepped junction zone requires a little more me- than a corresponding flat zone, but the depressed cc, may be preferred because it is easier to roll-form by comparison with a merely flattened corner.

The above-described embodiments are essentially Zsectioned, that is to say they comprise a web with flanges projecting in opposite directions, however the invention is equally applicable to beams which are essentially channel sectioned with their flanges projecting in the same direction.

Claims (4)

1. A beam of the kind made by cold roll-forming or pressing a metal strip and comprising a web and two flanges, characterised in that a stiffening rib projecting towards the neutral axis extends along each flange at or near the free edge thereof, and further characterised in that the longitudinally extending corner between 2ach flange and the web is flattened or depressed.

2. A beam according to claim 1 wherein one flange is larger than the other, such that the beam may be inverted and then nested in overlapping relation with a second such beam.

3. A beam according to any one of the preceding claims wherein the web has at least two embossments formed in 5" it projecting from the web in opposite directions and g* o each pierced by a fastener clearance hole. 0

4. A Z sectioned steel purlin substantially as j^ o"o I described herein with reference to figures 1 and 2 or figure 3 of the accompanying drawings. I Dated this tenth day of June 1992 a JOHN LYSAGHT (AUSTRALIA) LIMITED Attorney: ROBERT G. SHELSTON Fellow Institute of Patent Attorneys of Australia of CARTER SMITH BEADLE