AU2001265685A1

AU2001265685A1 - A truss and a method of fabricating same

Info

Publication number: AU2001265685A1
Application number: AU2001265685A
Authority: AU
Inventors: Murray Ellen
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-06-15
Filing date: 2001-06-15
Publication date: 2002-03-14
Anticipated expiration: 2021-06-15

Description

A Truss and a Method of Fabricating Same

FIELD OF THE INVENTION

The first invention relates to a truss and a method of fabricating same. The invention has been developed primarily for use in roof structures of large industrial, commercial and sporting complexes. BACKGROUND OF THE INVENTION

A roofing structure is disclosed in Australian Patent No. 505,679 that is comprised of a pair of trusses connected together by purlins and covered with a sheeting material. The trusses each comprise a bottom member and a top member connected together by struts. The trusses are each fabricated by connecting the top and bottom members at their ends and connecting the struts therebetween. Initially, the top members are straight and the bottom members are upwardly curved. The bottom members are hollow and each have a cable placed therethrough that can be tensioned relative to their respective bottom member.

After the trusses have been assembled and connected together by the purlins, the cables are tensioned to compress, shorten and straighten the bottom members. This lengthens and downwardly curves the top members and places them in tension. As the top members are in tension they can resist downward compressive loads on the roof structure. Tensioning the cables in the bottom members deflects the trusses into their final shape. If the tensioning is performed before the trusses are connected together by the purlins, the trusses will buckle.

It would be desirable to connect diagonal members between the struts to increase the strength of the trusses. However, this can not be done with post tensioned trusses as large compression forces would be developed in some of the diagonal members and buckle them.

The bottom members and diagonal members can also be increased in size to resist buckling. However, this increases the material cost of the trusses.

It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages. SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the present invention provides a method of fabricating a truss, the truss including: a bottom member with at least one cable therein adapted for tensioning relative thereto; a top member; a first plurality of struts; and a second plurality of diagonal members, the method comprising the following steps:

(a) connecting the ends of the top and bottom members;

(b) connecting the struts between the top and bottom members in a spaced apart relationship, the distance between the bottom end of the struts along the bottom member defining a pre-compressed distance;

(c) connecting a pair of diagonal members, in between each adjacent pair of struts, between the top and bottom members; and, after steps (a) to (c),

(d) increasing the tension in the cable(s) to reduce the length of the bottom member; and

(e) adjusting the lengths of the diagonal members and/or maintaining the pre- compressed distance between the bottom end of adjacent struts at their connections to the bottom member to respectively reduce and/or avoid any compressing or tensioning of the diagonal members. In one form, the method further includes connecting each one of each of the pairs of diagonal members to extend from the top end of a strut to the bottom end of an adjacent strut. In an embodiment of this form, the length of diagonal members is adjusted by rotation of a threaded joining member relative to a pair of member portions whose adjacent distal ends are threaded for engagement with the joining member. In another embodiment of this form, the length of the diagonal member is adjusted by inserting a distal end of one member portion into an adjacent distal end of the other member portion in an overlapping relationship and fixing the member portions relative to each other by grouting adjacent the overlapping. In another form, the method further includes connecting each one of each of the pairs of diagonal members to extend from the bottom end of a strut and join together intermediate the top ends of the two adjacent struts. In an embodiment of this form, the pre-compressed distance is maintained by connecting the ends of the diagonal member pairs to a sleeve adapted to slide relative to the bottom member and fixing the sleeve relative to the top or bottom member, preferably by grouting or welding. In a second aspect, the present invention provides a truss including: a bottom member with at least one cable therein adapted for tensioning relative thereto to shorten the bottom member; a top member, the top and bottom members being connected at their ends; a first plurality of struts connected between the top and bottom members in a spaced apart relationship, the distance between the bottom end of the struts along the bottom member defining a pre-compressed distance; and a second plurality of diagonal members, a pair of the diagonal members being connected between the top and bottom members between each adjacent pair of struts, wherein the diagonal members are length adjustable and/or the pre-compressed distance between the ends of the struts at their com ections to the bottom member is maintainable to respectively reduce and/or avoid any compressing or tensioning of the diagonal members caused by tensioning and shortening of the cable(s) relative to the bottom member.

In one form, each one of each of the pairs of diagonal members extends from the top end of a strut to the bottom end of an adjacent strut. In an embodiment of this form, the diagonal members include a threaded joining member and a pair of member portions whose adjacent distal ends are threaded for engagement with the joining member. In another embodiment of this form, the distal end of one member portion is open for receiving the adjacent distal end of the other member portion therein in an overlapping relationship, whereby the member portions are fixable relative to each other by grouting adjacent the overlapping.

In another form, each one of each of the pairs of diagonal members extends from the bottom end of a strut to join together intermediate the top ends of the two adjacent struts. In an embodiment of this form, the ends of the diagonal member pairs are connected to a sleeve adapted to slide relative to the bottom member, whereby the sleeve is fixable relative to the top or bottom member, preferably by grouting or welding.

In a third aspect, the present invention provides a spacer adapted for positioning within the interior of a bottom member of a truss, the interior of the bottom member adapted to receive one or more cables whose intrusion into the truss member interior define a cable(s) reduced interior, the spacer being adapted to have a cross section that is slightly smaller than, and substantially complementary to, the cable(s) reduced interior.

In one preferred form, the cross section of the bottom member is substantially square and the cross section of the spacer is substantially cross shaped.

In a fourth aspect, the present invention provides a roof structure comprised of a plurality of the trusses according to the first aspect arranged in an intersecting relationship and using common struts at each part of intersection. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described, by way of examples only, with reference to the accompanying drawings in which:

Fig. la is a top view of a (one dimensional) roof structure utilising a truss according to an embodiment of an aspect the invention; Fig. lb is a side view of the roof structure shown in Fig. la;

Fig. 2a is a top view of a roof structure using a truss according to another embodiment of an aspect the invention;

Fig. 2b is a side view of the truss shown in Fig. 2a;

Fig. 3 is an enlarged partial side view of the truss shown in Figs. 2a and 2b; Fig. 4 is a partial enlarged side view of an embodiment of a joining member used in the truss shown in Fig. 2a and 2b;

Fig. 5 is a partial enlarged side view of a diagonal member used in the truss shown in Figs, la and lb;

Fig. 6 is a partial enlarged side view of an alternative form of diagonal member for use in the truss shown in Figs, la and lb;

Fig. 7 is a side view of a spacer according to another embodiment of another aspect the invention;

Fig. 8 is a cross sectional view of a bottom member with the spacer of Fig. 7 installed therein; Fig. 9 is an exaggerated side view of a prior art tensioned bottom member;

Fig. 10 is an exaggerated side view of a tensioned bottom member utilising the spacer shown in Fig. 7; and

Fig. 11 is an upper perspective view of a (two-dimensional) roof structure utilising the trusses shown in Figs, la and lb.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figs, la and lb show a (one-dimensional) roof structure 10a formed from a pair of trusses 12, according to an embodiment of the invention, which are joined together by purlins 14. The sheeting material that covers the purlins 14 has been omitted from the drawings for clarity purposes. The expression "one-dimensional" is intended to convey that the roof structure spans between a pair of opposed walls with trusses oriented in one direction only.

The trusses 12 include a top member 16, a bottom member 18, struts 20 and diagonal members 22. These components are preferably steel with the top member 16 being steel rod up to about 300mm in diameter, the bottom member 16 being steel tube up to about 300mm in diameter, the struts 20 being steel rod up to about 20mm in diameter and the diagonal members being steel rod or tube up to about 40mm in diameter. The latter will be described in more detail below. The components can also be produced from other materials, such as timber or carbon composites.

The top and bottom members 16 and 18 are connected at their ends 24 and held spaced apart by the struts 20. The bottom member 18 is shorter than the top member 16 which results in the upwardly arched shape of the truss 12. In this embodiment, the diagonal members 22 extend between the top end of one of the struts 20 to the bottom ends of each of the adjacent struts 20.

The bottom member 18 is hollow and includes at least one cable therein which is adapted for tensioning relative thereto, as is well known in the art. As the cable(s) is/are tensioned the bottom member 18 is compressed and shortened. The shortening of the bottom member 18 and thus the shortening of the distance between the bottom end of each strut 20 tends to increase the length of the diagonal members 22 that connect the bottom of a strut to the top of an adjacent strut. More importantly, shortening of the bottom member 18 and thus the shortening of the distance between the bottom end of each strut 20 tends to reduce the length of the diagonal members 22 that connect the top of a strut to the bottom of an adjacent strut, which would normally cause them to become compressed and may lead to buckling. However, in accordance with the present invention, this induced tension and compression can be reduced to the point of being completely released by respectively lengthening and shortening the diagonal members 22.

Fig. 5 shows a first embodiment of a diagonal member 22a that is length adjustable. The diagonal member 22a is comprised of two member portions 26 and 28, preferably produced from 40mm steel rod, that have adjacent, inwardly facing, externally threaded, distal ends 30 and 32 respectively. An internally threaded joining member 34 connects the member portions 26 and 28. In accordance with the present invention, the overall length of the diagonal member 22a is adjusted by rotation of the joining member 34 in order to initially assemble the truss 12 into its unstressed state. As the cable(s) in the bottom member 18 is/are tensioned, tension or compression will be induced in the diagonal member 22a. Compression of the diagonal members 22a can be reduced by rotating the joining member 34 to bring the ends 30 and 32 of the member portions 26 and 28 closer together, which shortens the overall length of the diagonal members 22a. Tension of the diagonal members 22a can be reduced by rotating the joining member 34 to drive the ends 30 and 32 of the member portions 26 and 28 farther apart, which lengthens the overall length of the diagonal members 22a. For best results, the cable(s) in the bottom member 18 is/are progressively tensioned in a number of stages and the length of the diagonal members 22a are adjusted to release the compression or tension induced therein in between each stage.

In this embodiment, the trusses 12 are preferably connected together by the purlins 14 after the cable(s) are tensioned.

An alternative form of diagonal member 22b suitable for use with the truss 12 is shown in Fig. 6. The member portion 26 is again preferably produced from 40mm steel rod and the member portion 28 is preferably produced from 40mm steel tube. In this embodiment, the member portion 26 has a male distal end 30 which is insertable within the enlarged female distal end 32 of the member portion 28. The internal diameter of the female portion 32 is of a larger size than the external diameter of the male portion 30 so as to allow a gap therebetween in overlapping region 36 for introduction of grout. In this embodiment, the truss 12 is initially assembled substantially as described previously and the diagonal members 22b are installed without any grout therein. As the cable(s) in the bottom member 18 is/are tensioned and the bottom member 18 is shortened, the member portions 26 and 28 of the diagonal members 22b move towards or way from one another as a result of the respective compressive or tensional load attempted to be induced thereon. When the tensioning of the bottom member 18 is complete the final length of the diagonal members 22b can be fixed by grouting the member portions 26 and 28 together. The zig-zag shape of the male and female ends 30 and 32 increases the strength of the grouted bond therebetween.

Figs. 2a and 2b show another (one-dimensional) roof structure 10b comprised of trusses 12, according to another embodiment of the invention, joined together by purlins 14. In this embodiment, the pairs of diagonal members 22 between each adjacent pair of struts 20 have their bottom end connected to the bottom end of each of the adjacent struts 22 and the tops ends connected together intermediate the top ends of each of the adjacent struts 20. Fig. 3 shows an embodiment of a sleeve 40a which is positioned around, and slidable relative to, the bottom member 18. The sleeve 40a includes a bracket 42 to which the bottom ends of the strut 20 and two diagonal members 22 are pivotally connected by fasteners 44. In this embodiment, when the bottom member 18 is compressed by tensioning of the cables, the tensional and compressive forces attempted to be induced respectively in each one of the adjacent pairs of diagonal members 22 push and pull on the sleeves 40a and cause them to slide relative to the bottom member 18. The sleeves 40a thereby maintain the spacing they had along the bottom member 18 before compression of same. The original (pre-bottom member compression) lengths of the diagonal members 22 are therefore also maintained.

When the bottom member 18 has been fully compressed, the ends 46 of the sleeves 40 are fixed to the bottom member 18 by welding at ends 46. In this embodiment, the trusses 12 are preferably connected together by the purlins

14 before the cable(s) are tensioned.

An alternative form of sleeve 40b is shown in Fig. 4. The sleeve 40b has an internal diameter larger than the external diameter of the bottom member 18. In this embodiment, after the bottom member has been tensioned into its final position, the sleeve 40b is fixed relative to the bottom member 18 by grouting in the zig-zag overlapping region 48 in a similar manner to that described with reference to the diagonal member 22b of Fig. 6.

The trusses described above are advantageous in that they allow the bottom member 18 to be compressed into its final position without inducing any tensional or compressive forces into the diagonal members 22. As a result, the diagonal members 22 have no initial compression or tension and are able to resist greater compressive or tensile loads respectively, as generated by external loads on the roof structure 10, before buckling or otherwise failing. This allows the roof structures using the trusses according to the present invention to withstand far greater loads than conventional roof trusses produced from similar materials. Put another way, a truss according the invention can be produced of greater length than a conventional truss using the same materials and be able to withstand a similar external load. As an example, if a conventional truss is able to produce a span of 30 metres, a truss according to the embodiment of the present invention produced from similar materials can be produced in a span of up to 80 metres. The trusses described above can be produced utilising a conventional bottom member and cable tensioning arrangement. However, the ability of the bottom member itself to better withstand compressive buckling can be improved by utilising the spacer 60, according to another embodiment of the invention, shown in Fig. 7. The spacer 60 has two end flanges 62 spaced apart by a joining member 64 welded thereto at each end. Fig. 8 shows the spacer 60 positioned within the interior of a square cross section bottom member 18. Four tensioning cables 66 are shown positioned within the interior of the bottom member 18, which reduces the cross sectional area of same. The cross section of the spacer 60 is star shaped and slightly smaller than the reduced interior cross section of the bottom member 18 available after the cables 62 have been installed therein.

The spacer 60 is initially produced with a cross section almost complimentary to, and slightly smaller than, the interior of the bottom member 18 and then cut outs 68 are formed therein for passage of the cables 66.

Fig. 9 shows an exaggerated cross section view of a conventional bottom member 18 with a single cable 66 passing therethrough. A curve is induced into the bottom member 18 by tensioning of the cable 66, which causes compression and buckling of the bottom member 18. As Fig. 9 shows, the cable 66 attempts to find a straight line path between the ends of the bottom member 18. This causes the ends of the bottom member 18 to deflect away from the centre point 18a of the bottom member 18 by an amount roughly equivalent to the interior dimension "a" of the bottom member until such buckling is resisted by the cable 62 becoming incident on the centre point 18a of the interior member 18.

The spacer 60 effectively reduces the interior dimension of the bottom member to "b", which is roughly equal to the length of the cut out 64 (see Fig. 8). As a result, the cable 62 resists buckling of the interior member 18 after a much smaller amount of deflection, as illustrated (with exaggeration) in Fig. 10.

For best results, multiple spacers 60 are inserted into the bottom member at approximately one half of the spacing of the struts 20.

Fig. 11 shows a (two-dimensional) roof structure 10c. The expression "two- dimensional" is intended to convey that the roof structure spans between two pairs of opposed walls with trusses oriented in two directions. The structure 10c is comprised of the trusses 12 shown in Figs, la and lb arranged in an intersecting relationship forming a regular array or grid and with a common strut 20c at each point of intersection. In a further embodiment (not shown) the trusses shown in Figs. 2a and 2b are used. Although the invention has been described with reference to specific examples it would be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

Claims:

1. A method of fabricating a trass, the trass including: a bottom member with at least one cable therein adapted for tensioning relative thereto; a top member; a first plurality of struts; and a second plurality of diagonal members, the method comprising the following steps:

(a) connecting the ends of the top and bottom members

(d) increasing the tension in the cable(s) to reduce the length of the bottom member; and (e) adjusting the lengths of the diagonal members and/or maintaining the pre- compressed distance between the bottom end of adjacent struts at their connections to the bottom member to respectively reduce and/or avoid any compressing or tensioning of the diagonal members.

2. The method claimed in claim 1, further including connecting each one of each of the pairs of diagonal members to extend from the top end of a strut to the bottom end of an adjacent strut.

3. The method claimed in claim 2, wherein the length of diagonal members is adjusted by rotation of a threaded joining member relative to a pair of member portions whose adjacent distal ends are threaded for engagement with the joining member.

4. The method claimed in claim 2, wherein the length of the diagonal member is adjusted by inserting a distal end of one member portion into an adjacent distal end of the other member portion in an overlapping relationship and fixing the member portions relative to each other by grouting adjacent the overlapping.

5. The method claimed in claim 1, further including connecting each one of each of the pairs of diagonal members to extend from the bottom end of a strut and join together intermediate the top ends of the two adjacent struts.

6. The method claimed in claim 5, wherein the pre-compressed distance is maintained by connecting the ends of the diagonal member pairs to a sleeve adapted to slide relative to the bottom member and fixing the sleeve relative to the top or bottom member.

7. The method as claimed in claim 6, wherein the fixing is by grouting or welding

8. A truss including: a bottom member with at least one cable therein adapted for tensioning relative thereto a shorten bottom member; a top member; the top and bottom members being connected at their ends; a first plurality of struts connected between the top and bottom members in a spaced apart relationship, the distance between the bottom end of the struts along the bottom member defining a pre-compressed distance; and a second plurality of diagonal members, a pair of the diagonal members being connected between the top and bottom members between each adjacent pair of struts; wherein the diagonal members are length adjustable and/or the pre-compressed distance between the ends of the struts at their connections to the bottom member is maintainable to respectively reduce and/or avoid any compressing or tensioning of the diagonal members caused by tensioning and shortening of the cable(s) relative to the bottom member.

9. The truss claimed in claim 8, wherein each one of each of the pairs of diagonal members extends from the top end of a strut to the bottom end of an adjacent strut.

10. The trass claimed in claim 9, wherein the diagonal members include a threaded joining member and a pair of member portions whose adjacent distal ends are threaded for engagement with the joining member. -ll¬

l l. The truss claimed in claim 9, wherein the distal end of one member portion is open for receiving the adjacent distal end of the other member portion is open for receiving adjacent distal end of the other member portion therein in an overlapping relationship, whereby the member portions are fixable relative to each other by grouting adjacent the overlapping.

12. The truss claimed in claim 8, wherein each one of each of the pairs of diagonal members extends from the bottom end of a strut to join together intermediate the top ends of the two adjacent struts.

13. The truss claimed in claim 12, wherein the ends of the diagonal member pairs are connected to a sleeve adapted to slide relative to the bottom member, whereby the sleeve is fixable relative to the top or bottom member.

14. The truss as claimed in claim 12, wherein the fixing is by grouting or welding.

15. A spacer adapted for positioning within the interior of a bottom member of a truss, the interior of the bottom member adapted to receive one or more cables whose intrusion into the truss member interior define a cable(s) reduced interior, the spacer being adapted to have a cross section that is slightly smaller than, and substantially complementary to the cable(s) reduced interior.

16. The spacer claimed in claim 15, wherein the cross section of the bottom member is substantially square and the cross section of the spacer is substantially cross shaped.

17. A roof structure comprised of a plurality of the trusses claimed in any one of claims 8 to 14 arranged in an intersecting relationship and using common strats at each point of intersection.