GB2039582A - Beam with pretensioned member - Google Patents

Abstract

A beam arrangement comprises a beam 1, and an elongate, flexible pretensioned member 3 carried by the beam 1. The member 3 is so arranged that when the beam 1 bends under load, increased tension is produced in at least a part of the member 3. Struts 2 may be provided, and the beam may be prestressed in bending. A member 4 and struts 5 may be provided to protect member 3. The beam may be of wood and member 3 may be e.g. steel strip. <IMAGE>

Description

SPECIFICATION Beam arrangements The present invention relates to beam arrangements.

According to one aspect of the invention, there is provided a beam arrangement comprising a beam and a elongate, flexible, pretensioned member carried by the beam and so arranged that bending of the beam under load will cause increased tension in at least a part of said pretensioned member.

Preferably the beam is prestressed in bending by said pretensioned member.

Preferably at least one strut is provided extending from the beam, and the pretensioned member extends from a point on the beam over that end of a said strut remote from the beam and thence to a further point on the beam.

Preferably said points on the beam are at respective ends thereof.

Preferably said pretensioned member extends over two said struts.

Preferably the or each strut is perpendicular to the beam.

Preferably, two said struts extend from the beam at or near respective ends thereof and are joined by an elongate member which is secured to the or each strut over which said pretensioned member passes.

Said elongate member may be inclined relative to the beam.

Preferably, the pretensioned member is formed as a band.

Where the pretensioned member is a band, preferably the pretensioned member extends along one surface of the beam in contact therewith at least when the beam is unloaded.

In a preferred embodiment, the pretensioned member extends along said one surface of the beam, over each end of the beam, and between the beam and each of said struts provided at or near respective ends of the beam.

The pretensioned member is preferably of steel, although other materials could be used such as nylon rope, or aluminium.

The pretensioned member is preferably of steel strip.

The beam is preferably of wood.

According to a further aspect of the invention, there is provided a method of manufacturing a beam arrangement in which a beam is provided with an elongate, flexible member which is pretensioned and is arranged so that when the beam is subjected to load, bending of the beam causes increased tension in at least a part of said flexible member.

Preferably the beam has at least one strut extending therefrom and the flexible member is a steel band secured in tension around the beam and the or each strut.

Preferably the steel band is applied by a banding machine.

According to a further aspect of the invention, there is provided a method of prestressing a wooden beam in which bending stress is applied to the beam by means of an elongate flexible member in tension.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 shows a schematic side view a first embodiment of beam arrangement; Figure 2 shows a cross-sectional view taken along the line A-A of Figure 1; Figure 3 shows a second embodiment of beam arrangement in schematic side view; Figure 4 shows a third embodiment of beam arrangement in schematic side view; Figure 5shows in schematic side view a fourth embodiment of beam arrangement; Figure 6shows a sectional view taken along the line B-B of Figure 5; and Figure 7shows a further embodiment of beam arrangement in schematic side view.

In the following description of the figures, corresponding elements have been provided with like reference numerals.

Figure 1 shows a beam 1 provided with two strut members 2 extending perpendicularly therefrom. A steel band 3 is carried by the beam 1 and passes over the struts 2. The steel band is pretensioned so that it is initially, i.e. before application of any load, in a taunt condition. A lower elogate member 4 is secured to the struts 2 and has respective further struts 5 extending from its ends to the main beam 1.

The steel band 3 extends along the upper surface of the beam 1, over the ends thereof, between the lower surface of the beam 1 and ends of the struts 5, and thence over the ends of struts 2 which are remote from the beam 1. If the steel band 3 is suitably tensioned, the beam 1 may be given a hogging bending moment and therefore a preset chamber. Since the lower member 4 is shorter than the main beam 1,the beam 1 protrudes slightly to either side beyond the struts 5. This allows the beam arrangement to be supported by the protruding ends of the beam 1. When load is applied to the upper surface of beam 1, it will be apparent thatthe beam will tend to sag and that this will tension the steel band 3. The various struts 2 and 5 are secured to the beam 1 and to the member 4 by means of nails.The steel band 3 may be secured to the beam 1 at the ends of the beam by means of nails 8 passing through the band where it contacts struts 5. In this case, it is unnecessary to provide the part of the steel member extending along the upper surface of the beam 1. Thus, it is not essential that the steel pretensioned member is a band. However, it is preferred to utilise a band since this simplifies assembly. Nails 9 may also be provided as illustrated, if required.

Figure 2 shows a sectional view taken along the line A-A of Figure 1. In this figure, the width of the band 3 may be seen in relation to the depth of the beam 1 and the member 4. The beam 1, the member 4, and all the struts 2 and 5 are preferably of wood.

Figure 3 shows a further embodiment of beam arrangement which is similar to that shown in Figures 1 and 2 but which differs in that the beam 1 is of the same length as the lower member 4. Also, the steel member is not formed as a band, buttermin ates at the ends of the beam 1 and is secured by nails 8. In this embodiment, the beam arrangement may be supported on the lower surface of the member 4.

Figure 4 shows a further embodiment which is again similarto that shown in Figure 1, but which differs in that the beam 1 is not parallel to the lower member 4. Also, the beam 1 extends beyond only one of the struts 5. This embodiment may be supported by the overhanging end of beam 1 at one end of the arrangement, and at a point on the lower surface of member 4 as illustrated in the figure.

The embodiment of Figure 5 is similar to that of Figure 3, the only significant difference being that the steel member 3 passes over only one strut 2.

Figure 6 is a sectional view taken along the line B-B of Figure 5.

Figure 7 shows an embodiment in which a larger number of components are used so that the beam arrangement is longer. It is illustrated in a multiply supported condition. The centre portion of the beam arrangement including steel member 3 and struts 2 and 5 is similar to the embodiment of Figure 5.

However, the beam 1 and the lower member 4 extend laterally from the central portion of the arrangement by a substantial distance and are linked together at each end via a respective further strut 6.

Between the upper end of each strut 6 and the lower end of the adjacent strut 5 a further steel reinforcing strip 7, of similar material to member 3, is provided under tension.

Although in each of the embodiments described the member or band 3 and the members 7 are of steel strip, this is not essential. The members may be made of steel cable or wire, or may be made of aluminium or nylon.

Where a steel band 3 is applied, this may be secured in place by means of a banding machine such as is used for securing steel sheets for transport. The ends of the steel strip may be secured together to form a band by means of a crimping device.

It will be appreciated that in all the described embodiments a force is applied to a pre-fabricated timber beam arrangement by means of a high tensile steel strip. This enables the beam arrange mentto be contructed of relatively small section timber but still to have the load bearing properties of a conventional timber beam.

The timber used in the construction of the arrangement can be either hard wood or soft wood, can be solid or laminated and may be treated with a rot or insect preservative.

As will be apparent, the size and type of timber to be used and the depth of the arrangement as a whole are determined by the load to be carried and the amount of deflection which is allowable over the required span.

Although it is preferred to hold the members of the arrangement together by means of wire ringed nails, ply gussets, or glue or pre-punched multi-nail plates may also be used.

The high tensile steel can be a continuous length, or it can be assembled in separate pieces with its width, gauge and position within the arrangement being determined by the load and deflection allowable in the arrangement in operation. The steel can be either "bright" or weather protected, e.g. galvanised or printed.

Although all the struts joining the upper beam to the lower beam member are shown as extending vertically, this is not essential, and they may be inclined to the vertical. For example, in the embodiment of Figure 1 each of the strut 2 may be inclined so that its axis will bisect the angie included between the two portions of steel strip passing over its end.

Pretensioning of the steel strip or band may either be applied during construction of the beam arrangement or on completion of fabrication, or may be applied in situ.

The advantages of the prestressed timber beam arrangement over a solid or otherwise manufactured arrangement is that smaller timber sections can be used and the maximum deflection occurring in use can be controlled by the application of a calculated tensile force by means of the high tensile steel strip or band.

A particular application of the beam arrangement is to carry water tanks in the roof space of a building such as domestic house. The space available under the ridge of a house is often fairly restricted with modern low-pitched roofs and anything which allows more room for the water tank is advantageous. By use of the described beam arrangements, the support for a water tank occupies less vertical height. Furthermore, by use of the arrangement of Figure 1, the beam arrangement may sit in a low position between the ceiling joists. As a result of the use of such a support, it is possible to use, in many instances, one single large water tank whereas with prior art arrangements two small water tanks would have been required. This leads to a considerable saving in cost and installation time.

As already mentioned, the maximum deflection of the beam 8 occurring in use can be controlled by application of appropriate tension of the member 3.

However, if too much tension is applied, although the deflection can be made small, the ultimate strength of the beam arrangement is reduced. In practice, therefore, a compromise must be made between maximum deflection and ultimate strength.

When using a steel band, provision of nails 8 is not essential, although they reduce the maximum deflection whilst not substantially affecting the ultimate strength. This is thought to be because the nails prevent slipping of the band overthe ends of beam 1 and so help to retain tension in the lower parts of the band, whilst allowing the upper part to go slack.

When using a band, it has proved advantageous to round off the ends of beam 1 so that the band does not bite too much into the wood and so cause reducing band tension.

When manufacturing the arrangement of Figure 1, the band 3 may be applied around beam 1 and struts 2 by a banding machine and pretensioned to a predetermined extent, the ends of the strip being joined by a crimp on a point between struts 2. Then, when member 4 carrying struts 5 is secured to struts 2 by nails 9, ends of struts 5 bear on the band 3 and increase the tension by a predetermined amount, before nails 8 are driven into place. Thus, the initial tension applied by the banding machine is lower, in this method, than the finally required tension.

When manufacturing the arrangement of Figure 5, one may begin with the member 4 carrying struts 5 and secure the strip 3 to the free ends of struts 5.

Then beam 1 carrying one (or more) strut 2 is nailed into position by nails 8, the lower end of the or each strut 2 bearing on the strip 3 and thus tensioning it by an amount dependent on the initial length of the strip 3. A banding machine is thus unnecessary.

Claims (25)

1. A beam arrangement comprising a beam and an elongate, flexible, pretensioned member carried by the beam and so arranged that bending of the beam under load will cause increased tension in at least a part of said pretensioned member.

2. An arrangement according to claim 1 wherein the beam is prestressed in bending by said pretensioned member.

3. An arrangement according to claim 1 or 2 wherein at least one strut is provided extending from the beam and the pretensioned member extends from a point on the beam over that end of a said strut remote from the beam and thence to a further point on the beam.

4. An arrangement according to claim 3 wherein said points on the beam are at respective ends thereof.

5. An arrangement according to claim 3 or 4 wherein said pretensioned member extends over two said struts.

6. An arrangement according to any one of claims 3 to 5 wherein the or each strut is prependicu lartothe beam.

7. An arrangement according to any one of claims 3 to 6 wherein two said struts extend from the beam at or near respective ends thereof and are joined by an elongate member which is secured to the or each strut over which said pretensioned member passes.

8. An arrangement according to claim 7 wherein said elongate member is inclined relative to the beam.

9. An arrangement according to any one of the preceding claims wherein the pretensioned member is formed as a band.

10. An arrangement according to any one of the preceding claims wherein the pretensioned member consists of a plurality of sections joined together.

11. An arrangement according to any one of the preceding claims wherein the pretensioned member extends along one surface of the beam in contact therewith at least when the beam is unloaded.

12. An arrangement according to claim 11 when dependent on claims 7 and 9 wherein the pretensioned member extends along said one surface of the beam, over each end of the beam, and between the beam and each of said struts provided at or near respective ends of the beam.

13. An arrangement according to any one of the preceding claims wherein said pretensioned member is of steel.

14. An arrangement according to claim 13 wherein the pretensioned member is of steel strip.

15. An arrangement according to any one of the preceding claims wherein the beam is of wood.

16. A beam arrangement substantially as hereinbefore described with reference to Figures 1 and 2, Figure 3, Figure 4, Figures 5 and 6, or Figure 7 of the accompanying drawing.

17. A method of manufacturing a beam arrangement in which a beam if provided with an elongate, flexible member which is pretensioned and is arranged so that when the beam is subjected to load bending of the beam causes increased tension in at least a part of said flexible member.

18. A method according to claim 17 wherein the beam has at least one strut extending therefrom and the flexible member is a steel band secured in tension around the beam and the or each strut.

19. A method according to claim 18 wherein the steel band is applied by a banding machine.

20. A method of manufacturing a beam arrangement according to claim 17 and substantially as hereinbefore described.

21. A method of prestressing a wooden beam in which bending stress is applied to the beam by means of an elongate flexible member in tension.

22. A method of prestressing a wooden beam according to claim 21 and substantially as hereinbefore described.

23. A beam arrangement formed by the method of any one of claims 17 to 20.

24. A beam prestressed by the method of claim 21 or 22.

25. A structure including a beam arrangement or a beam according to any one of claims 1 to 16,23 and 24.