IMPROVEMENTS RELATING TO ROOF TRUSSES

Abstract: A roof truss strengthening device (7) comprising first and second clamping members (8) each having at least two roughened timber-engaging areas (11) on confronting clamp faces, each said areas on one face being opposite a respec­ tive area on the other face to form opposed pairs and threaded means (9) linking said clamping members between each two adjacent opposed pairs of confronting roughened areas, whereby by drawing the clamping members towards one another on opposite sides of the timber lengths (1, 2, 3, 4) meeting at a junction point of a prefabricated roof truss (5), using said threaded means passing between timber lengths adjacent to the junction point, the device strengthens the bond between timber lengths meeting at said junction point.

Description: This invention relates to timber roof trusses and in particular to prefabricated roof trusses factory made by securing lengths of timber together using galvanised steel fixing plates. For some years, a widely accepted method of forming a pitched roof structure of a building has been to use one or more prefabricated timber roof trusses, in which the timber lengths forming the rafters, struts and ties of each truss are secured together by fixing plates pressed in to the timber from opposite sides thereof to sandwich adjacent ends of the timber lengths and thereby secure them together in a structurally sound bond at each junction point. Computer techniques have been employed to ensure adequate strength is obtained for the roof structure in question with a minimum amount of timber. For economic reasons the safety factor in the structural design of such prefabricated roof trusses is considerably lower than it has traditionally been with on-site erected trusses. US-A-3 172 125 discloses apparatus for fabricating timber roof trusses by means the use of such fixing plates pressed into the timber from opposite sides. The structural strength of such prefabricated roof trusses is heavily dependent on the integrity of the fixing plates. Each of these is commonly made from a single plate of galvanised steel stamped to form a plurality of fixing prongs projecting from one side of the plate, a plurality of prongs being available for penetrating each timber length meeting at each junction point. However, the protection against rusting afforded by the galvanised coating is impaired by the stamping operation and there is therefore a risk that the small safety factor built into these prefabricated roof trusses will be lost due to corrosion of the fixing plates making total roof collapse a real possibility. This invention relates to a method of strengthening a prefabricated roof truss and to a strengthening device for use in the method. In its method aspect, the invention consists in augmenting the bond created, at a junction point where two or more lengths of timber meet in a prefabricated roof truss, by a pair of spiked fixing plates pressed into the timber lengths from opposite sides, said method consisting of sandwiching the timber lengths in the vicinity of the junction point and adjacent to the said pair of fixing plates between a pair of clamping members at least one of which members includes a clamp face providing roughened areas designed to fixedly engage with the timber lengths and providing between said timber lengths means to draw the clamp faces towards one another. Conveniently the means to draw the clamp faces towards one another is a threaded stud and a nut screwed thereon, the stud extending from one clamping member between two adjacent timber lengths meeting at the junction point and passing through a hole in the other clamping member. The stud could be fixed to said one clamping member but is conveniently the stem of a bolt which passes through clearance holes in both clamping members. Suitably the roughened timber-engaging areas of the clamp face of each clamping member are each provided with an array of points or projections which securely bond each clamping member to the timber lengths as the clamp faces are moved towards one another to complete the bond-augmenting operation. Suitably each clamping member comprises a first part adjacent to the timber lengths which provides the roughened area and a second part which is engaged by the means to draw the clamp faces towards one another. Desirably the first part is a metal plate (e.g. of stainless steel) whose edge region defines teeth bent normal to the plane of a central region of the plate, said central region being contacted by said second part. In terms of a device suitable for carrying out the aforesaid method, the invention can be expressed as a roof truss strengthening device comprising first and second clamping members each having at least two roughened timber-engaging areas on confronting clamp faces, each said areas on one face being opposite a respective area on the other face to form opposed pairs and threaded means linking said clamping members between each two adjacent opposed pairs of confronting roughened areas, whereby by drawing the clamping members towards one another on opposite sides of the timber lengths meeting at a junction point of a prefabricated roof truss, using said threaded means passing between timber lengths adjacent to the junction point, the device strengthens the bond between timber lengths meeting at said junction point. In a preferred arrangement each clamping member is of generally U-, V- or Y-shape in plan and exhibits three or four spaced-apart roughened areas. The clamping members can be made of metal or hard plastics material. The clamping members can be machined or cast to the desired shape and are preferably designed so that each pair making up a device can be secured in place on a roof truss without intruding upon the timber lengths to any extent beyond that caused by pressing the roughened areas against the timber lengths. US-A-2365501 discloses a twin plate bracket unit for connecting timbers together at their ends, the ends of the timbers being sandwiched between the twin plates as the latter are clamped together by bolts which do not pass through the timbers. Spikes are provided on the plates to bite into the surfaces of the timbers and assist in holding the timbers firmly between the plates. The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figures 1, 3, 5 and 6 are side views of different junction points of a prefabricated roof truss reinforced by the method of this invention, Figures 2 and 4 are sectional views on the lines II-II and IV-IV of Figures 1 and 3, respectively, Figure 7 is a schematic view of a prefabricated roof truss showing junction points such as those indicated in Figures 1, 3, 5 and 6, Figures 8 to 12 are side views of modified strengthening devices, Figure 13 is a view in the direction of the arrow XIII in Figure 9, and Figure 14 is a section on the line XIV-XIV in Figure 12. Figure 7 shows a prefabricated roof truss 5 formed of rafters 1, 2, struts 3, 4, 14 and 15 and a tie 13 joined together by conventional Gangnail plates (not shown in this Figure) at each junction point. Figures 1 and 2 show the junction point between four timber lengths defining the rafters 1 and 2 and the struts 3 and 4 of the roof truss 5. One of a pair of Gangnail plates 6 used during factory prefabrication of the truss 5 to secure the timber lengths together at the illustrated junction point is reinforced by a device 7 consisting of a pair of generally U-shaped clamping members 8 and three bolts 9. Each clamping member 8 has a clamp face 10 exhibiting spaced-apart areas 11 which are roughened with an array of projections 12, the bolts 9 passing through clearance holes located between the timber lengths. For convenience, projections 12 have been shown in only one area 11 in Figure 1 but all four areas of each member 8 are provided with such projections. When a pair of members 8 are clamped in place (an operation that can be effected simply as a DIY-activity without any special tools or disturbing the roof structure) around the junction point, the projections 12 of each area 11 are pressed into the respective timber lengths from both sides thereof to fixedly locate the device 7 in place. Figures 3 and 4 show a more simple device 7A for securing a base tie 13 to a rafter 1 and the same reference numerals have been used, where appropriate, as were used in Figures 1 and 2. Each areas 11 has an array of projections 12 but these are not shown in Figure 3. Figure 5 shows a generally Y- or V-shaped device 7B for reinforcing a Gangnail-plate-formed junction between a rafter 1 and a strut 14. Once again each area 11 is provided with projections that bite into the timber from both sides due to the pressure generated by the bolts 9 passing in the gaps between the timber lengths. Figure 6 shows a generally inverted U-shaped device 7C for reinforcing a junction between two struts 14, 4 and the base tie 13 of a roof truss 5. Projections (not shown) in the areas 11 spike the timber lengths by virtue of the clamping action of the intra-timber bolts 9. The clamping members 8 shown in Figures 1 to 6 could be machined from stainless steel or die cast in a suitably strong aluminium alloy. Figures 8 to 12 show modified devices which include two parts in each clamping member 8, the first part being provided with teeth for biting into the timber lengths and the second part being a pure squeeze device which presses the teeth on the respective first part into the timber length(s). Figure 8 shows two-part members of a device 27 which is similar to the device 7 of Figure 1, Figure 9 a two-part device 27A similar to the device 7A of Figure 3, Figure 10 a two-part device 27B similar to the device 7B of Figure 5, Figure 11 a two-part device 27C similar to the device 7C of Figure 6. Figure 12 is a two-part device 27D for strengthening a Gangnail plate 6 bridging a join between collinear timber lengths 13a and 13b, and Figures 13 and 14 are sections through the devices 27A and 27D, respectively. The two-part construction is shown clearly in Figures 13 and 14, a toothed gripping plate 20 (e.g. of stainless steel) being provided with teeth 21 formed from bent-down edge regions 22 of the plate 20. The force to drive the teeth 21 into the timber length(s) is generated by pressure members 28 acted on by a bolt 29 (preferably of stainless steel) passing through all four parts 20, 28 as close as possible to the lengths of timber whose junction point is being strengthened. The pressure members 28 can be of mild steel or stainless steel. In order to show the teeth 21 more clearly, some regions of the timber length 13 have not been shown in Figure 13. A full U-shaped construction with four bolts 29 as shown in Figure 10 is preferred, but it is possible to use L-shaped units with just two bolts 29 to clamp the plates 20 into the timber lengths. L-shaped units would terminate where shown dotted at 30 in Figure 10. The device of Figure 11 could be clamped in place either with three bolts 29 or five bolts 29, the two extra bolts being shown dotted at 31 in Figure 11. The plates 20 can be extended over the space to one side of a timber length and each can be bent down to create a compression member to absorb the force created by the bolt 29 and ensure the teeth 21 are driven into the timber lengths as the bolts are tightened. Figure 14 shows such a compression member at 32.

Claims: A method of augmenting the bond created, at a junction point where two or more lengths of timber (1-4) meet in a prefabricated roof truss (5), by a pair of spiked fixing plates (6) pressed into the timber lengths from opposite sides, characterized in that said method consists of sandwiching the timber lengths in the vicinity of the junction point and adjacent to the said pair of fixing plates (6) between a pair of clamping members (8; 20,28) at least one of which members includes a clamp face providing roughened areas (11, 21) designed to fixedly engage with the timber lengths and providing between said timber lengths means (9, 29) to draw the clamp faces towards one another. A method as claimed in claim 1, characterized in that the means to draw the clamp faces towards one another is a threaded stud and a nut (9, 29) screwed thereon, the stud extending from one clamping member (8, 28) between two adjacent timber lengths meeting at the junction point and passing through a hole in the other clamping member (8, 28). A method as claimed in claim 2, characterised in that the stud is fixed to said one clamping member. A method as claimed in claim 2, characterized in that the stud is the stem of a bolt (9, 29) which passes through clearance holes in both clamping members. A method as claimed in any preceding claim, characterised in that the roughened timber-engaging areas (11, 22) of the clamp face of each clamping member are each provided with an array of points or projections (12, 21) which securely bond each clamping member to the timber lengths as the clamp faces are moved towards one another to complete the bond-augmenting operation A method as claimed in any preceding claim, characterised in that each clamping member comprises a first part (20) adjacent to the timber lengths which provides the roughened area (21) and a second part (28) which is engaged by the means (29) to draw the clamp faces towards one another. A method as claimed in claim 6, characterised in that the first part is a metallic plate (20) whose edge region (22) defines teeth (21) bent normal to the plane of a central region of the plate (20), said central region being contacted by said second part (28). A roof trus strengthening device (7, 27) suitable for carrying out the method of claim 1, characterised by first and second clamping members (8: 20,28) each having at least two roughened timber-engaging areas (11, 21) on confronting clamp faces, each said areas on one face being opposite a respective area on the other face to form opposed pairs and threaded means (9, 29) linking said clamping members between each two adjacent opposed pairs of confronting roughened areas, whereby by drawing the clamping members (8: 20,28) towards one another on opposite sides of the timber lengths (1-4) meeting at a junction point of a prefabricated roof truss (5), using said threaded means (9, 29) passing between timber lengths adjacent to the junction point, the device strengthens the bond between timber lengths meeting at said junction point. A strengthening device as claimed in claim 8, characterised in that each clamping member (8) is of generally U-, V- or Y-shape in plan and exhibits three or four spaced-apart roughened areas (11). A strengthening device as claimed in claim 8 or 9, characterised in that each clamping member (27) comprises a first part (20) adapted to be located adjacent to the timber lengths which provides the roughened area (21) and a second part (28) which is engaged by the means (29) to draw the clamp faces towards one another. A strengthening device as claimed in claim 10, characterised in that the first part is a metallic plate (20) whose edge region (22) defines teeth (21) bent normal to the plane of a central region of the plate (20), said central region being contacted by said second part (28). A method as claimed in claim 7 or a strengthening device as claimed in claim 11, characterized in that the plate (20) is of stainless steel.

Inventors: QUARMBY, ARTHUR
Applicants: QUARMBY, ARTHUR
Application year: 1989
Main classification: E04C3
IPC codes: E04C3, E04B1