GB2074209A

GB2074209A - Lattice girder for bridges etc.

Info

Publication number: GB2074209A
Application number: GB8012129A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-04-11
Filing date: 1980-04-11
Publication date: 1981-10-28
Also published as: GB2074209B

Abstract

A lattice girder is made of triangular units 10, each having its base 12 uppermost with the sides 22 forming struts extending down to a vertex 24. The adjacent vertices are interconnected by chords 30. Parallel girders may support falsework or bridge decking 50, the parallel girders being braced by horizontal frameworks 36 interconnecting the bases 12 and by vertical frameworks 46. Shorter chords 30 may be used to produce a curved girder. <IMAGE>

Description

SPECIFICATION Load bearing system This invention relates to a load-bearing system, and more particularly to a composite beam structure.

The invention provides a load-bearing system comprising a plurality of first members which are adapted at opposing end portions thereof for interconnection end to end, each first member having a connection portion intermediate its end portions and displaced from the line between the end portions; and one or more second members joining the intermediate connection portions of adjacent first members; whereby loading on the first members from opposite the intermediate connection portions causes tensioning of the second members thereby tending to prevent flexing of the load-bearing system.

In use, the structural system will usually be arranged as a beam with the second members lowermost, so as to withstand gravitational loading.

Preferably each first member comprises a bar extending between the end portions, there being further structural elements extending between each end portion and the intermediate connection portion, forming a triangle with the bar. There may also be 3 structural element extending between the intermediate connection portion and an intermediate point on the bar.

The intermediate connection portion may have two side plates between which is received one end of the second member. The second member can then be connected to the side plates by means of a pin passing through apertures in the side plates and in the end portion of the second member.

The first members can be located together end to end by means of a longitudinally projecting dowel on one end portion which is received in a corresponding hole in the adjacent end portion of the next first member.

A structural system may be made from two or more such beams side by side, and such a system may include horizontal bracing members interconnecting (preferably diagonally) the respective first members of the two beams. For some applications, vertical bracing members may also be provided, suitably arranged diagonally between upper and lower portions of the structural element which extends between the intermediate connection point and the intermediate position on the bar.

In order that the invention may be more clearly understood, structural systems embodying it will now be described by way of example with reference to the accompanying drawings, wherein: Figs. 1, 2 and 3 are side, plan and end views of a triangular main member, Fig. 4 shows a partially exploded assembly of several such main members with interconnecting chord members, Fig. 5 is a plan view of a part of a structural system using such members, and Fig. 6 is a cross-sectional end view of another such structural system.

Referring to the drawings. and firstly to Figs. 1 to 3, there is shown a main member 10 comprising a horizontal bar 12. At one end portion 14 of the bar 12 there is a longitudinally projecting dowel 16, while at the opposite end portion 18 there is a corresponding hole 20. A plurality of bars 12 will therefore be interconnectable end to end, with the dowels 1 6 fitting in the holes 20.

The main member 10 also has two diagonal structural bars 22 which extend from the end portions 1 6, 1 8 respectively, meeting at a connection portion 24 and forming a triangle with the bar 12. At the connection portion 24, which is intermediate the end portions 14, 18, and displaced beneath the axis of the bar 12 joining those two end portions, there are provided two horizontally spaced side plates 26, which are arranged parallel to the bar 12. There is also a structural element 28 extending between the connection portion 24 and the mid-point of the bar 12.

As seen in Fig. 4, a plurality of the main members 10 are connected together end to end to form a beam, with the dowel 1 6 on one end of the bar 12 of each main member 1O fitting Into the corresponding hole 20 of the next member. The dowels are held in the holes 20 by means of a pin (not shown). To complete the beam, chord members 30 are fitted between the intermediate connection portions 24 of adjacent main members 10. These chord members 30 are chamfered at each end in order to fit against the diagonal structural elements 22. The chord members 30 fit between the side plates 26 of the connecting points 24, and are held there by means of a heavy duty pin which passes through holes 32 in the side plates 26 and a hole 34 in the end portion of the chord 30.

In use, the beam seen in Fig. 4 is arranged with the bars 12 uppermost and the chord members 30 lowermost, and can then withstand downward loading, the ends 14, 1 8 of adjacent bars 12 bearing against each other, and the chord members 30 being placed in tension. The structure is thus held rigid.

The main member 10 and chord member 30 are constructed with welded rolled hollow sections of steel to B.S. 4360 Grade 50C, but other grades of steel, of different cross-sections, or different methods of fabrication such as extrusion, or welding flat plates, could be used.

Different materials could also be used.

The beam is supported on suitable supports at either end through bearinys which fit either a male or female joint at each end of the beam.

In practical use, two or more such beams as seen in Fig. 4 willie used side by side. Fig. 5 shows a top view of part of such a system. To render the system more horizontally stable, there are provided transverse spacing members 36, and diagonal bracing members 38, extending between adjacent beams. The spacing members 36 and bracing members 38 are fixed through holes in their end portions over pins 40 which project from the top surface of the bars 12. The spacing members 36 also act as primary support for decking 50 placed over the system and act together with members 38 to prevent the system swaying in a horizontal plane. The design of the decking will depend on the use to which the system is to be put. The pins 40 are grouped in pairs, which assists in placing spacing and bracing members between one beam and adjacent beams on both sides.

In certain applications, including bridging, vertical bracing may also be required. For this purpose, lugs 42 project laterally from the vertical structural elements 28, and have holes 44 therein.

As seen in Fig. 6, a welded vertical rectangular framework 46 is secured between corresponding lugs 42 of adjacent beams, which framework comprises horizontal and vertical angle sections 48 braced by diagonal angle sections 52 welded between their midpoints. By means of the vertical and horizontal bracing seen in Figs. 5 and 6, the system is rendered stable.

The system described is modular, and has the advantages of prefabrication, including design flexibility, portability and speed and ease of erection. In addition, comparison with an alternative known system such as Bailey bridging shows that significant savings in materials and manufacturing costs can be achieved. The safe load-structural weight ratio for a beam of this system spanning thirty metres, is 1.44, while that of a similar Bailey bridge is 1.12. For eighteen metres these ratios become 4.99 and 4.21 respectively.

Another advantage of the system described is that it can be precambered, that is given an upward curve, by making the bottom chord member 30 slightly shorter than the top bar 12.

This is particularly useful where the system described is being used in the formation of structures of reinforced and prestressed concrete, where the alternative is to build in such a camber with timber packing. Using the same technique, but if necessary using a different method of locating the ends 14, 1 8 of the bars 1 2 than the dowels 16 and holes 20, arches can be formed very simply. Also, when used as false work to concrete, the structure is very simple to remove.

One bearing at the end of the beam is lowered slightly using a sand-jack or similar device, and this allows the bottom chord members 30 and then the main members 10 to be removed one at a time, starting from one end.

The main member seen in Fig. 1 is made approximately 1.5 metres deep (i.e. the distance from the connection portion 24 to the mid-point of the bar 12). However, alternatives can be used with a smaller depth but the same length. These would be used on shorter spans or where smaller loads are anticipated.

Claims

1. A load bearing system comprising a plurality of first members which are adapted at opposing end portions thereof for interconnection end to end, each first member having a connection portion intermediate its end portions and displaced from the line between the end portions; and one or more second members joining the intermediate connection portions of adjacent first members; whereby loading on the first members from the side opposite the intermediate connection portions causes tensioning of the second members thereby tending to prevent flexing of the load-bearing system.

2. A load bearing system according to claim 1 wherein each first member comprises a bar extending between the end portions, there being further structural elements extending between each end portion and the intermediate connection portion, forming a triangle with the bar.

3. A load bearing system according to claim 2 wherein each first member has a structural element extending between the intermediate connection portion and an intermediate point on the bar.

4. A load bearing system according to any one of the preceding claims wherein at least some of said intermediate connected portions each comprise two side plates between which one end of a second member is receivable.

5. A load bearing system according to claim 4 wherein an end of a second member received between two side plates is connectable thereto by means of a pin passing through apertures in the side plates and in the end portion of the second member.

6. A load bearing system according to any one of the preceding claims wherein the first members are mutually connectable end to end by means of a longitudinally projecting dowel on one end portion which is received in a corresponding hole in the adjacent end portion of the next first member.

7. A load bearing system according to any one of the preceding claims wherein the relative lengths of the first and second members are such that the system is precambered.

8. A load bearing system according to any one of the preceding claims comprising at least two beams extending side by side, each said beam being itself a load bearing system according to any one of the preceding claims.

9. A load bearing system according to claim 8 wherein adjacent'beams are connected by generally horizontal bracing and/or spacing members extending between respective first members of the beams.

10. A load bearing system according to claim 9 wherein said spacing members provide primary support for decking placed over the system.

11. A load bearing system according to any one of claims 8 to 10 further including vertical bracing means for enhancing stability to vertical displacement.

12. A load bearing system substantially as described herein with reference to and as illustrated in Figs. 1 to 4 or in Figs. 1 to 5 and/or 6 of the accompanying drawings.

1 3. A method of making a load bearing system according to any one of the preceding claims comprising prefabricating the first and second members and any bracing or spacing members, and assembling these on site.

14. A method of making a concrete structure comprising providing falsework consisting of a load bearing system according to any one of claims 1 to 12, casting concrete thereon, and subsequently dismantling the falsework.

1 5. A concrete structure made by the method of claim 14.