GB2086971A

GB2086971A - Structural Beam

Info

Publication number: GB2086971A
Application number: GB8132572A
Authority: GB
Original assignee: Kwikform Ltd
Current assignee: Kwikform Ltd
Priority date: 1980-11-05
Filing date: 1981-10-29
Publication date: 1982-05-19
Also published as: GB2086971B

Abstract

A structural beam having a Moment of Inertia not less than 200 cm<4> along the longitudinal axis of the beam, and not less than 20 cm<4> in a transverse direction of the beam comprises a web (11) which, in use, extends substantially vertically, a pair of limbs (13a, 13b) at one end (12) of the web (11) which limbs (13a, 13b) are inclined to the plane of the web (11) obliquely so that they extend away from the plane on opposite sides thereof, and outwardly extending flanges (19a, 19b) at the free end of each limb (13a, 13b). The flanges (19a, 19b) may each extend substantially perpendicular to the web (11) and be interconnected by a non- structural part (21) to which a shuttering panel may be connected. <IMAGE>

Description

SPECIFICATION Structural Beam This invention relates to a structural beam and more particularly, but not exclusively, to a structural beam for use in supporting shuttering in formwork during casting of concrete structures.

By "structural beam" we mean a beam having a Moment of Inertia not less than 200 cm4 along the longitudinal axis of the beam, and not less than 20 cm4 in a transverse direction of the beam.

A structural beam of this type will be referred to hereinafter as "of the type described".

The object of the present invention is to provide a new or improved structural beam.

According to the invention we provide a structural beam of the type described comprising a web which, in use, extends substantially vertically, a pair of limbs at the one end of the web which are inclined to the plane of the web at an oblique angle so as to extend away from the plane on opposite sides thereof, and an outwardly extending flange at the free end of each limb.

The flanges may each extend substantially perpendicular to the web.

Preferably, a structural beam in accordance with the invention also has, at the other end of the web, a further pair of limbs similar to those at the one end of the web, said further limbs being inclined to the plane of the web at an oblique angle so as to extend away from the plane on opposite sides thereof.

The further limbs may also have flanges at the free ends thereof which extends outwardly substantially perpendicular to the web.

Thus both pairs of limbs each define a triangular void therebetween.

Conventional beams, for example an I-shaped beam, need to be of thicker material than a beam in accordance with the present invention to achieve the same calculated beam strength. This is because it is necessary to increase the thickness of the component parts of conventional beams to ensure that the beam does not buckle in use. Thus, conventional beams are considerably heavier than a beam in accordance with the present invention.

Buckling is a type of failure in which one or more of the component elements of the beam deform into a series of wave-like folds.

Considering, for example, an I-shaped beam, wherein the horizontal flanges are each tf mm thick and the flanges each extend a distance bf mm from the central web, i.e. the length of the flanges is bf mm.

The flange buckling co-efficient Af can be shown to be directly proportional to bf and indirectly proportional to tf. See for example the British Standard Code of Practice -- CP1 18: 1969 (March 1 973 edition) - "Structural use of Aluminium" published by the British Standards Institute.

Thus to achieve a low buckling coefficient Af, the thickness tf must be large and the length bf of the flanges must be small.

Similarly, the buckling coefficient Aw of the web of the beam is directly proportional to the length bw of the web and indirectly proportional to the thickness tw of the web. Thus, to achieve a small buckling coefficient Aw, bw must be small and tw large.

In a beam in accordance with the present invention, both the flange lengths bf and the web length bw are reduced compared to a conventional I-beam due to its Y-shaped configuration. As the web is not continuous over the entire height of the beam, the effective length bw of the web over which the beam could buckle, is reduced. Further, the flanges do not extend inwardly to a web and their effective lengths bf are also reduced.

Still further, the triangular void defined between the limbs of the beam serves to increase the torsional stiffness of the beam.

Preferably, the beam is symmetrical, the limbs of the or each pair being inclined to the plane of the web at the same angle and being of the same length.

The flanges may be provided at their outermost ends with lips normal to the flange and extending toward the opposite end of the web. These lips will further improve the flange buckling coefficients Aw of the webs.

Such a structural beam may comprise a formwork member, the flanges of the beam in use supporting shuttering panels for casting concrete structures.

The shuttering supported by the beam may be attached to the flanges at the free ends of the pairs of limbs. In this case a pressed spring clip device may be used to attach the shuttering, to the beam. The shuttering may comprise plywood, or another composite sheeting material.

The clip may comprise one or more shuttering connecting parts having means to connect the clip to the shuttering, and a pair of elements extending transversely from said shuttering connecting parts, each having a retaining formation adapted to cooperate with the flanges of the beam to attach the clip to the beam.

However, preferably, a connecting part interconnects the flanges of the or each pair of limbs, or the limbs adjacent the flanges, and the shuttering is attached to said connecting part.

One or more fixing devices such as a screw, a power driven nail or a self-drilling screw for example, may be used for attaching the shuttering to the or one of the connecting webs of the beam, said fixing device or devices perforating the connecting part.

The strength of a beam in accordance with the present invention is however calculated independently of the horizontal connecting part or parts. Thus repeated perforation of the connecting part or parts does not detract from the strength of the beam as originally calculated.

In one embodiment, the connecting part or parts is or are integral with the flanges, providing a portion therebetween depressed inwardly of the triangular void.

This depressed portion can easily be recognised by a person attaching shuttering to the beam, as being the connecting part, and thus the region between the flanges which it is safe to perforate, to attach the shuttering to the beam.

The beam may be made conveniently from aluminium whereby a very light, strong and corrosion resistant beam is formed. Preferably, the beam is extruded although it may be manufactured by other methods if desired.

A beam in accordance with the present invention may have a bending moment in the longitudinal direction of the beam of not less than 2.0 kNm and a bending moment in a transverse direction of the beam of not less than 0.3 kNm.

The beam may have a radius of gyration of not less than 40 mm along the longitudinal axis of the beam, and a radius of gyration of not less than 10 mm in a transverse direction of the beam.

The invention will now be described with the aid of the accompanying drawings in which: Figure 1 is a cross-sectional view through a structural beam in accordance with the invention; Figure 2 is a plan view of a spring clip device for attaching shuttering to the beam of Figure 1; Figure 3 is a side view of the spring clip of Figure 2.

Referring first to Figure 1, there is shown an extruded aluminium structural beam 10 comprising a web 11 which, in use, extends substantially vertically. At the upper end 12, the web 11 is divided thereby providing an upper pair of limbs 1 3a, 1 3b, the limbs being inclined to the plane of the web at an oblique angle a thereto so as to extend upwardly and outwardly of the web 11 on opposite sides thereof and so define a triangular void 14 therebetween.

The upper ends of the limbs 1 3a, 1 3b each have integral therewith a substantially horizontal flange 1 spa, 1 sub respectively which flanges 1 spa, 1 Sb each extend outwardly of the associated limb 13a, or 13b.

At the lower end 16, the web 1 1 is also divided and has a lower pair of limbs which, as they are similar to the upper pair of limbs, are designated by the same reference numerals 1 3a, 1 3b. Each lower limb 1 3a, 1 3b also has a flange 1 5a, 1 Sb at the lower end thereof.

The angle a subtended between the limbs 1 3a, 1 3b is, in the beam shown, 600 although, in another beam, the angle may be different.

Flanges 1 spa, 1 sub each have at their outermost end a lip 1 9a, 1 9b, which lips are substantially parallel to the web 11.

The inner ends of each of the flanges 1 5a, 1 5b of each pair are interconnected by a connecting part 21 which in the beam 10 described, is of less thickness t' than the flanges 1 spa, 1 Sb thereby providing a depressed portion 23.

In another beam, the thickness t' of the connecting part 21 need not be reduced, but a depression produced such as shown in dotted lines at 20, on the inside face 21' of the connecting part 21 as shown.

The beam 10 is particularly suitable for supporting shuttering in formwork while concrete is being cast, although the beam 10 obviously has many other applications. Where the beam is used for supporting shuttering in formwork, the shuttering may be supported on the upper flanges 1 Sa, 1 Sb and attached to the beam 10 by one or more fixing devices such as a screw, power driven nail or self-drilling screw which is or are attached to the upper connecting part 21.

The strength of the beam 10 is calculated independently of the connecting parts 21 and thus the part 21 may be repeatedly perforated without detracting from the calculated strength of the beam.

When the one connecting part has many perforations due to much use, the beam 10 may be inverted and the other connecting part used to attach shuttering panels.

However instead of perforating the part 21 to attach shuttering to the beam 10, a fastening device such as shown at 30 in Figures 2 and 3 may alternatively be used.

The device 30 comprises a spring clip having a body 31, a plurality of fastening plates 32 which each have pressed out studs 34 and screw apertures 35 whereby for example, a plywood or other composite shuttering sheet material panel can be connected to the clip 30. The plates 32 may be substantially perpendicular to the body 31 as shown or, they may be downwardly inclined relative to the body 31 thus providing a spring force to accommodate variations in thickness of the shuttering sheet.

The clip 30 further comprises a pair of downwardly directed elements 36, 37 which, with the body 31, define a recess 38. The elements 36, 37 each have an inwardly directed retaining formation 40.

As the clip 30 is made of a pressing in spring steel, when the clip 30 is moved downwardly onto the beam of Figure 1 in the direction of the arrow from the position shown, the elements 36, 37 will be forced apart as the undersides 40a of the retaining formations 40 engage the corners 42 of the flanges 15a, 1 sub of the beam.

As the body 31 engages the flanges 15a, 1 sub, the elements 36, 37 will spring inwards and the retaining formations 40 will abut the under edges 41 of the lips 19a, 19b and thus the clip 30 will be firmly attached to the beam 10.

A plurality of such clips 30 may be utilised along the length of each beam 10 of the formwork whereby the shuttering may be connected to the beam.

The web 11 of the beam 10 described above has a thickness tw of approximately 2.5 mm. and a length bw of 68 mm.

The overall height H of the beam 10 is 156 mm., and the overall width W of the beam 10 is 80 mm.

The flanges 15a, 1 sub have a thickness t, of 3.0 mm., the limbs 13a, 13b and the connecting parts 21 being 2.0 mm. wide in the middle.

Thus the total cross-sectional area of the beam is approximately 10.15 cm2. However, as the beam is made of aluminium, the weight of the beam is only 2.75 kg per metre.

The strength of such a structural beam is: Moment of Inertia along the longitudinal axis of the beam 388 cm4 Moment of inertia in a transverse direction of beam 38.1 cm4 Section Modulus along the longitudinal axis of the beam 42.4 cm3 Section Modulus in a transverse direction of the beam 9.5 cm3 Radius of gyration in the longitudinal axis of beam 61.8 mm Radius of gyration in a transverse direction of beam 1 9.4 mm Although these values relate to the beam manufactured in the particular embodiment described above, it will be appreciated that a beam in accordance with the invention may be of other dimensions and strengths for different applications, but a beam in accordance with the invention will be considerably lighter than a conventional beam of the same strength as the thickness of the web tw and the flanges tf are reduced.

However, a beam suitable for use as a structural beam has a minimum Moment of, Inertia of 200 cm4 in the longitudinal direction of the beam and a minimum Moment of Inertia 20 cm4 in a direction transverse of the beam.

As the length bw of the web 11 is less than the length of the web of, for example, a conventional I-shaped beam, the buckling coefficient Aw of the web is considerably reduced compared to that of an I-shaped beam.

Further, the lengths bf of the flanges 1 Sa, 1 Sb are less than the lengths of the flanges of the Ishaped beam as the flanges to do not extend to a central web, and the triangular void 14 defined between the limbs 13a, 1 3b supports the flanges of the beam in a horizontal plane. Thus the effective buckling length of the flanges and hence the buckling coefficient Af of the flanges, is considerably reduced. The upper and lower triangular voids 14 also have the effect of increasing the torsional stiffness of the beam.

The maximum Bending Moment FZ for the beam described above can be calculated as follows: Referring to the British Standard Code of Practice Cup 1 1969 (March 1973 edition) "Structural Use of Aluminium" published by the British Standards Institute Section 4.5.1, the formula for calculating the Buckling co-efficient is given as mb = t where m has the maximum value assigned to it as shown in Table II entitled "Local Buckling Coefficients", Aw for the above described beam in accordance with the invention can be shown to be 36.00.

The graph on Page 29, i.e. Figure 2 of CP1 18, gives the permissible Bending compressive stresses for commercially available alloys. For the alloy HE30TF the maximum permissible compressive stress, for fw=36, is given as 122 N/mm2.

Thus the maximum Bending Moment FZ is 122 N/mm2 x Section Modulus (in the longitudinal direction of the beam i.e. 42.4 cm3) Thus FZ=122 N/mm2x42.4 cm3 =5.172 KNm For a conventional I-beam, Aw can be calculated to be 59.9, and thus the maximum compressive stress from Figure 2 of CP 118, is 94 N/mm2.

Thus the maximum Bending Moment FZ for the conventional I-beam is FZ=94 N/mm2x42.4 cm3 =3.98 KNm Thus FZ for a web in a beam in accordance with the present invention is considerably improved compared to a conventional I-beam.

Further comparisons may also be made to show the greater strength of a beam in accordance with the present invention when compared to known beams.

Claims

1. A structural beam of the type described comprising a web which, in use, extends substantially vertically, a pair of limbs at the one end of the web which are inclined to the plane of the web at an oblique angle so as to extend away from the plane on opposite sides thereof, and an outwardly extending flange at the free end of each limb.

2. A beam according to Claim 1 wherein the flanges each extend substantially perpendicular to the web.

3. A beam according to Claim 1 having at the other end of said web, a further pair of limbs similar to those at the one end of the web, said further limbs being inclined to the plane of the web at an oblique angle so as to extend away from the plane on opposite sides thereof.

4. A beam according to Claim 3 wherein the further limbs also have flanges at the free ends thereof which extend outwardly substantially perpendicular to the web.

5. A beam according to any one of the preceding claims wherein the beam is symmetrical, the limbs of the or each pair being inclined to the plane of the web at the same angle and being of the same length.

6. A beam according to any one of the preceding claims wherein the flanges are provided at their outermost ends with lips normal to the flange and extending toward the opposite end of the web.

7. A beam according to any one of the preceding claims which comprises a formwork member, the flanges of the beam, in use, supporting shuttering panels for casting concrete structures.

8. A beam according to Claim 7 wherein the shuttering supported by the beam is attached to the flanges at the free ends of the or each pair of limbs.

9. A beam according to Claim 8 wherein a pressed spring clip device is used to attach the shuttering, to the beam.

10. A beam according to Claim 9 wherein the clip comprises one or more shuttering connecting parts having means to connect the clip to the shuttering, and a pair of elements extending transversely from said shuttering connecting parts, each having a retaining formation adapted to cooperate with the flanges of the beam to attach the clip to the beam.

11. A beam according to Claim 7 wherein a connecting part interconnects the flanges of the or each pair of limbs, or the limbs adjacent the flanges, and the shuttering is attached to said connecting part.

12. A beam according to Claim 11 wherein one or more fixing devices is used for attaching the shuttering to the or one of the connecting parts of the beam, said fixing device or devices perforating the connecting part or parts.

13. A beam according to Claim 9 or Claim 10 wherein the connecting part or parts is or are integral with the flanges, providing a portion therebetween depressed inwardly of the triangular void.

14. A beam according to any one of the preceding claims wherein a beam is made from aluminium.

1 5. A beam according to Claim 14 wherein the beam is extruded.

1 6. A beam according to any one of the preceding claims wherein the beam has a bending moment in the longitudinal direction of the beam of not less than 2.0 kNm and a bending moment in a transverse direction of the beam of not less than 0.3 kNm.

1 7. A beam according to any one of the preceding claims wherein the beam has a radius of gyration of not less than 40 mm along the longitudinal axis of the beam, and a radius of gyration of not less than 10 mm in a transverse direction of the beam.

1 8. A beam substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

19. Any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawings.