GB2051906A - Strengthening Roof Beams - Google Patents

Abstract

A beam made of two sheets 18, 19 of material spaced apart by top 14 and bottom 15 battens is strengthened by fixing a steel strip 20 on the outside of one sheet side by side with the bottom batten. The steel strip is already perforated and is initially in a coil. It is secured by applying nails 24 or screws through the holes. The sheets 18, 19 may be plywood, which may be glued to the battens. Wood strips 16 may also be present between the battens. <IMAGE>

Description

SPECIFICATION Roofs and Roof Beams This invention relates to roofs and roof beams.

One common form of roof beam is of the kind which consists of two sheets of plywood spaced apart by top and bottom battens (or "chords") which in turn are spaced apart by other strips of wood, the plywood being glued to the battens and strips. These beams have been widely used and in general are satisfactory. However, instances have occurred in which beams have failed because of unusual loads and lack of adhesion over the entire glue surfaces.

Various possibilities have been considered for improving the load bearing capabilities of such beams both those already in buildings with restricted accessability and for new beams.

Nailing or screwing at all glue surfaces is the obvious approach but careful consideration shows that such close spacing of nails or screws would be necessary as might lead to splitting of wood.

Another possibility is to clamp pieces of wood on opposite sides of the beam but this is cumbersome, costly and difficult for accessability.

Again one could consider the use of various angle pieces but this also involves difficulties.

According to the present invention a beam of the kind referred to has applied to it on one side only a strip of mild steel 1.3 to 1.75mm thick which can be rolled into a coil, the steel strip having close spaced holes for nails or screws, the steel strip lying side by side with the lower batten and being nailed or screwed to it.

The steel strip may be approximately the same depth as the lower batten.

The steel strip coil can be easily transported and moved into position in an existing building and can be progressively unrolled as it is nailed or screwed to the beam.

Considerable tests have been necessary to establish the practicability of such a construction.

In these tests the steel strip was 1 6 gauge galvanised steel 7.290m long and 1 OOmm wide with 3.2mm diameter holes punched in two rows at 90mm centres offset by 45mm to be transported in coil from approximately 450mm diameter. The strip was nailed by 10 gauge x 50mm long twist nails. The roof tested was as shown in Figures 1, 2 and 3. The roof had pairs of beams 10, 11 and steel strips were attached on the outer surfaces of each pair. The beams were as shown in Figures 4 to 10. Figures 4 is an elevation, Figures 5 a plan, Figure 6 a detail of the end of the beam, and Figures 7 to 10 sections on planes W-W, X-X, Y-Y, Z-Z on Figure 4.The upper and lower battens 14, 1 5 are spaced apart by wood strips 1 6 and plywood sheets 18, 19 were glued to the battens and strips. The steel strips are shown at 20, 21. The beams are connected by cross-members and wood panels 22. The roof was about 25 feet 6 inches overall length and 8 feet four and a half inches wide. The roof was loaded with bricks (674 bricks average weight 4.67 Ibs each) giving a load of 14.74 lb sq.ft. (0.71 KN/m2). An additional load of felt and stone chippings included as part of the roof giving a total load of about 1 5 Ib sq, feet.

The results of these tests were quite surprising.

The minimum short term failure load for three units tested without steel strips was 1.89 x design load. The tests with steel strips added showed failure at greater than 2.5 x load. The deflection at 28 days was considerably below the British Standards requirement i.e. 0.003 x span-in this case 22mm.

Figure 11 shows a section of the lower part of a beam with steel strip 20 attached by nails 24.

Claims

1. A method of improving the load bearing capabilities of a beam of the kind comprising two sheets of material spaced apart by top and bottom battens which comprises providing a roller strip of mild steel 1.3 to 1.75 mm thick having close spaced holes, unrolling said strip and applying it to the beam on the outer side of only one sheet, side by side with the lower batten and fixing the strip to the sheet and batten by nails or screws applied through said holes.

2. A method as claimed in claim 1 wherein the strip is approximately the same depth as the lower batten.

3. A method as claimed in claim 1 or 2 wherein the strip is applied to the beam of an existing building.

4. A method as claimed in claim 1, 2 or 3 wherein the sheets are plywood glued to the battens.

5. A method of improving load bearing capabilities of a beam of the kind comprising two sheets of material spaced apart by top and bottom battens substantially as described with reference to the accompanying drawings.

6. A building having a beam strenthened in accordance with the method of claim 1, 2 or 3.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Roofs and Roof Beams This invention relates to roofs and roof beams. One common form of roof beam is of the kind which consists of two sheets of plywood spaced apart by top and bottom battens (or "chords") which in turn are spaced apart by other strips of wood, the plywood being glued to the battens and strips. These beams have been widely used and in general are satisfactory. However, instances have occurred in which beams have failed because of unusual loads and lack of adhesion over the entire glue surfaces. Various possibilities have been considered for improving the load bearing capabilities of such beams both those already in buildings with restricted accessability and for new beams. Nailing or screwing at all glue surfaces is the obvious approach but careful consideration shows that such close spacing of nails or screws would be necessary as might lead to splitting of wood. Another possibility is to clamp pieces of wood on opposite sides of the beam but this is cumbersome, costly and difficult for accessability. Again one could consider the use of various angle pieces but this also involves difficulties. According to the present invention a beam of the kind referred to has applied to it on one side only a strip of mild steel 1.3 to 1.75mm thick which can be rolled into a coil, the steel strip having close spaced holes for nails or screws, the steel strip lying side by side with the lower batten and being nailed or screwed to it. The steel strip may be approximately the same depth as the lower batten. The steel strip coil can be easily transported and moved into position in an existing building and can be progressively unrolled as it is nailed or screwed to the beam. Considerable tests have been necessary to establish the practicability of such a construction. In these tests the steel strip was 1 6 gauge galvanised steel 7.290m long and 1 OOmm wide with 3.2mm diameter holes punched in two rows at 90mm centres offset by 45mm to be transported in coil from approximately 450mm diameter. The strip was nailed by 10 gauge x 50mm long twist nails. The roof tested was as shown in Figures 1, 2 and 3. The roof had pairs of beams 10, 11 and steel strips were attached on the outer surfaces of each pair. The beams were as shown in Figures 4 to 10. Figures 4 is an elevation, Figures 5 a plan, Figure 6 a detail of the end of the beam, and Figures 7 to 10 sections on planes W-W, X-X, Y-Y, Z-Z on Figure 4.The upper and lower battens 14, 1 5 are spaced apart by wood strips 1 6 and plywood sheets 18, 19 were glued to the battens and strips. The steel strips are shown at 20, 21. The beams are connected by cross-members and wood panels 22. The roof was about 25 feet 6 inches overall length and 8 feet four and a half inches wide. The roof was loaded with bricks (674 bricks average weight 4.67 Ibs each) giving a load of 14.74 lb sq.ft. (0.71 KN/m2). An additional load of felt and stone chippings included as part of the roof giving a total load of about 1 5 Ib sq, feet. The results of these tests were quite surprising. The minimum short term failure load for three units tested without steel strips was 1.89 x design load. The tests with steel strips added showed failure at greater than 2.5 x load. The deflection at 28 days was considerably below the British Standards requirement i.e. 0.003 x span-in this case 22mm. Figure 11 shows a section of the lower part of a beam with steel strip 20 attached by nails 24. Claims

1. A method of improving the load bearing capabilities of a beam of the kind comprising two sheets of material spaced apart by top and bottom battens which comprises providing a roller strip of mild steel 1.3 to 1.75 mm thick having close spaced holes, unrolling said strip and applying it to the beam on the outer side of only one sheet, side by side with the lower batten and fixing the strip to the sheet and batten by nails or screws applied through said holes.

2. A method as claimed in claim 1 wherein the strip is approximately the same depth as the lower batten.

3. A method as claimed in claim 1 or 2 wherein the strip is applied to the beam of an existing building.

4. A method as claimed in claim 1, 2 or 3 wherein the sheets are plywood glued to the battens.

5. A method of improving load bearing capabilities of a beam of the kind comprising two sheets of material spaced apart by top and bottom battens substantially as described with reference to the accompanying drawings.

6. A building having a beam strenthened in accordance with the method of claim 1, 2 or 3.