GB2138861A

GB2138861A - Purlin for a sloping roof membrane

Info

Publication number: GB2138861A
Application number: GB08409591A
Authority: GB
Inventors: Karl-Gerd Kramer
Original assignee: Theodor Wuppermann GmbH
Current assignee: Theodor Wuppermann GmbH
Priority date: 1983-04-29
Filing date: 1984-04-12
Publication date: 1984-10-31
Also published as: AT382187B; DE3315694A1; FR2545131A1; CH663439A5; ATA101384A; BE899529A; FR2545131B1; NL8400998A; GB2138861B

Abstract

A purlin section has a web (2) and flanges (3, 4) on the latter for a sloping roof membrane, the flanges (3, 4) of the purlin section (1) form with the web (2) an angle which deviates from 90 DEG . The size of the angle is determined by the fact that the main axis for the smaller moment of inertia of the cross section of the section coincides, at least approximately, to the direction of the load of the roof membrane which is to be sustained by the purlin. The web (2) is thereby provided at the two end regions with a portion (5, 6), which extends rebounding to the flange (3, 4), at an angle relative to the flange which is smaller than 90 DEG . A lip (7, 8), which is bent off inwards, is arranged at the end of the flanges (3, 4). The buckling resistance of the section is thereby substantially increased. <IMAGE>

Description

SPECIFICATION Purlin for a sloping roof membrane This invention relates to a purlin consisting of a section (for example, an I-, U-, C-, or Zsection) which has a web and flanges on the latter, for a sloping roof membrane, in the case of which purlin the flanges of the purlin section form with the web an angle which deviates from 90 and the size of this angle is determined in such a way that the main axis for the smaller moment of inertia of the cross section of the section coincides, at least approximately, to the direction of the roof.membrane load which is to be sustained by the purlin.

A purlin of this kind is know from German Patent Specification No. 1 759866. On account of the construction of this purlin, the cross sections of its section are, in the main, only subjected to bending stress in the direction of the main axis for the smaller principal moment of inertia, as the main axial direction for the smaller principal moment of inertia and moment of resistance does not deviate or only deviates slightly from the principal load direction. It is then not necessary to support the purlin crossways in respect of the direction of stress.

The object of the invention is to improve the design of the purlin with regard to its resistance to bending, buckling, and tilting and to obtain higher values for this resistance.

The invention, relating to the purlin described, is characterised by the fact that the web is provided at the two end regions with a portion, which extends rebounding to the flange, under an angle to the flange which is smaller than 90 and by the fact that a lip, which is bent off inwards and which is in a defined aspect ratio in respect of the flange side, is arranged at the end of the flanges.

By constructing the cross sectional shape of the purlin in this way the buckling surface of the web and of the flange may be reduced and the tolerable buckling stresses may thereby be increased. The resistance of the section to buckling is essentially increased on account of the increase in joint-bearing width of flange and web. The surface of the web is also reinforced in the region of the greatest shear stress with the result that the bearing strength of the section is at the same time increased in respect of bending. In addition, the lips improve the rigidity of the section, more particularly if they lie within the pressure range, and improve the buckling conditions within the framework of the whole cross section of the section.The increased jointbearing width of the flanges of the section allows there to be a degree of compressive stress due to bending, which is increased in respect of the usual amount, and also critical buckling stress on the section. With strict observance of these pre-conditions, after reaching the tensile yield point as a function of the thickness of the material, there remains a residual moment which allows calculations to be made according to the load-factor method of design or according to a partial load-factor method of design.

The angle between flange and rebounding portion of the web is advantageously in the region of approximately 60 in order to give the restraining effect for the flange surface a controlled and effective order of magnitude.

The angular position of the lip in respect of the reinforced surface portion of the flange is also of importance. It is most effective if, in this case, the lips are square to the surface of the flange, because then a move of deflection in respect of other angular positions is not initiated until very much later.

According to a further feature of the invention, the flange width is to be kept substantially in the region of twenty to forty times the material thickness of the section bar, that is, when calculated with deduction of the radii of curvature of the flange edges. With a ratio of this kind, the flange width as a support subjected to bending without reduction can be exploited just as fully as the buckling stress.

As a result, the buckles advantageously come to lie substantially in the region of the spaces where the section is fixed to the roof membrane. The ratio of the length of lip to the flange width, which is reduced by the radii, advantageously lies in the range of 1:1.8. The range can extend to approximately 2.5.

The invention is described, by way of example, with reference to, the drawing, which shows a sectional and diagrammatic view of a purlin according to the invention.

The purlin 1, in the form of a Z-section, has a web 1 and flanges 3 and 4. In this case, the flanges 3 and 4 of the purlin section 1 form an acute angle, which deviates from 90', with the web 2. This angle varies depending on the pitch of the roof and the height of the purlin and is distinguished by the fact that the main axis for the smaller moment of inertia of the cross section of the section coincides, at least approximately, with the direction of the roof load which is to be sustained by the purlin. Generally, the roof pitch in industrial structures is below 15 . The range of the angle extends from approximately 17 to 0'. The medium range is of the order of 6 .

The web 2, which extends obliquely with respect to the flanges 3 and 4 of the section (in a sectional view) has in the end region a portion 5 and 6 which rebounds or bends back. The path of the portion which bends back is such that the angle between the flange 3 and the portion 5 or between the flange 4 and the portion 6 is smaller than 90 . An angle y of 60 is advantageous.

Furthermore, portions which are bent in wardly, i.e. lips 7 and 8, are arranged on the outer edges of the flanges 3 and 4, in which case the lips are advantageously square to or almost square to the surfaces of the flanges 3 and 4 in order to obtain optimum reinforcement of the flanges. Curved portions 9 and 10 are disposed between the portions 5 and 6 and the flanges 3 and 4, and also between the flanges 3 and 4 and the lips 7 and 8.

The material thickness of the section is preferably from approximately 1.5 to 4 mm.

The most commonly used thickness is of the order of 2 mm. The width B of the flanges advantageously corresponds to approximately twenty to forty times the material thickness of the section. This helps to optimise the utilizable bending and critical buckling stress of the section. The ratio of the height of the lip L to the flange width B is preferably from 1:2 to 1:2.5. Furthermore, it is advantageously to allow the portions 5 and 6 on the web 2, which portions extend obliquely, to begin substantially in the plane of the height of the lips 7 and 8 on the web 2. An approximation of the maximum bending and buckling resistance of the section is thereby achieved.

In practice, it has shown that a certain range of height of the purlins is sufficient so that it is possible to produce sections in a simplified cost-effective manner. On condition that the flange width remains unchanged for all heights of purlin and a constant incline of the web is chosen, a cross section which is favourable for all cases relating to techniques for building purposes, can be obtained.If the height range as used in practice lies in the range between 180 and 300 mm and the incline of the web is given as being 6 , the following roof pitches, which are optimized for practical operation, are, for example, applicable: H 180mm Incline of main axis equal to the optimum roof pitch 12" H 240mm Incline of main axis equal to the optimum roof pitch 6" H 300mm Incline of main axis equal to the optimum roof pitch 3" Tests have shown that the optimized cross sections of the sections, even in the event of deviation from the aforementioned most commonly used roof pitches on account of the turning bedding effect of the various roof membrane designs, show characteristics with regard to rigidity and stability, which make good the deviations, allowing there to be a broad range of application without substantial impairment of the bearing characteristics.

Claims

1. A purlin for a sloping roof membrane, comprising a section (for example an I-, U-, Cor Z-section) which has a web and flanges on the web, wherein the flanges of the section form with the web an angle which deviates from 90' and the size of this angle is such that the main axis for the smaller moment of inertia of the cross section of the section coincides, at least approximately, to the direction of the roof membrane load which is to be sustained by the purlin; wherein the web is provided at the two end regions with a portion which rebounds or bends back to the flange at an angle relative to the flange which is smaller than 90 ; and wherein an inwardly bent lip is arranged at the end of each flange.

2. A purlin according to claim 1, wherein the angle between the flange and the bent back portion of the web is 60 or approximately 60 .

3. A purlin according to claim 1 or 2, wherein the lips are square to the surface of the flanges.

4. A purlin according to any of claims 1 to 3, wherein the flange width is in the region of twenty to forty times the material thickness of the section.

5. A purlin according to any of claims 1 to 4, wherein the ratio of the height of the lips to the flange width is from 1:1.8 to 1:2.5.

6. A purlin according to any of claims 1 to 5, wherein the beginning of the portion which bends back on the oblique web is substantially in the plane of the top of the lips.

7. A purlin substantially as hereinbefore described with reference to, and as shown in, the- drawing.

8. A purlin for a sloping roof membrane, comprising a section (for example an I-, U-, Cor Z-section) which has a web and flanges on the web, wherein the flanges of the section form with the web an angle which deviates from 90 ; wherein the web is provided at the two end regions with a portion which rebounds or bends back to the flange at an angle relative to the flange which is smaller than 90 ; and wherein an inwardly bent lip is arranged at the end of each flange.