COMPOSITE LOAD BEARING MEMBER

Abstract

The invention provides a composite load bearing member, such as a pole, which can be built for any specific design load with the objective of low cost to replace wood support applications for similar loads, typically up to 10 ton yield support load. The member has at least two layers but may also comprise of three layers for applications where UV protection and flame retardancy is paramount. The inside layer consists of a thermoplastic pipe of low cost, preferably HOPE, PET or PVC. The thermoplastic may or may not contain a flame retardant. The next layer consists of a thin fibreglass pipe as shell for the thermoplastic to enable strength. This fibreglass pipe will be tight fitting on the thermoplastic since the thermoplastic can be used as mandrel for the fibreglass pipe which can be manufactured by pultrusion or pullwinding or fibreglass fabric rolling. The fibreglass resin can be phenolic, epoxy, polyester or vinylester.

Claims

1. A composite load bearing member having at least two layers: (i) an inner thermoplastic layer; and (ii) an outer fiberglass layer, wherein the thermoplastic layer is in the form of a pipe having a wall thickness of from 2 to 10 mm, and further wherein the fiberglass layer is in the form of a pipe having a thickness of from 1 to 10 mm.

2. The composite member as claimed in claim 1 having a third UV-protective layer on the outside of the outer fiberglass layer, wherein a fiberglass fibre orientation in the outer fiberglass layer varies between 1-49% radial and the balance of the fibre orientation being longitudinal, and wherein a wall thickness ratio of the inner thermoplastic layer to the outer fiberglass layer is preferably larger than 1:1.

3. The composite member as claimed in claim 1 which is in the form of a pole.

4. The composite member as claimed in claim 1, wherein the inner thermoplastic layer is made of a material selected from HDPE, PET, and PVC.

5. The composite member as claimed in claim 1, wherein the resin of the outer fiberglass layer is selected from a phenolic, epoxy, polyester, and vinyl-ester resin.

6. (canceled)

7. (canceled)

8. The composite member as claimed in claim 1, wherein the wall thickness ratio of thermoplastic to fibreglass can vary between 0.7 and 3.0 going from 20 tons yield load down to 1.5 tons.

9. (canceled)

10. The composite member as claimed in claim 2, wherein the fiberglass fibre orientation in the outer fiberglass layer is 60-80% longitudinal and 20-40% radial.

11. (canceled)

12. The composite member as claimed in claim 2, wherein the third UV-protective layer includes a gel coat.

13. The composite member as claimed in claim 8, wherein the gel coat includes polyester or epoxy.

14. The composite member as claimed in claim 8, wherein the gel coat includes a flame retardant.

15. The composite member as claimed in claim 8, wherein the gel coat has a uniform thickness between 250-500 micron.

16. The composite member as claimed in claim 1, wherein the outer fiberglass layer comprises by itself of a UV-stabilizing filler.

17. The composite member as claimed in claim 1, which is a pole having a length of at least 1 meter.

18. The composite member as claimed in claim 1, wherein the outer fiberglass layer gets manufactured by pultrusion or pullwinding or fibreglass fabric rolling.

19. Use of the composite member as claimed in claim 1, as replacement for wood support members.

Description

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0034] The invention will now be described, by way of non-limiting examples only, with reference to the accompanying representations.

EXAMPLE 1

[0035] Composite pole designed for load of 2.5 tons for vineyard or farm fence support. The thermoplastic pipe (with or without flame retardant) has an inside diameter of 28 mm and outside diameter of 32 mm. The fibreglass shell on the outside of the thermoplastic pipe has an inside diameter of 32 mm and outside diameter of 34.5 mm (therefore a 2.5 mm wall thickness for supplying strength to the thermoplastic for balancing axial load and wind bend moment forces). For this application a phenolic resin will be preferred for flame resistance. A polyester resin can also be used for the fibreglass in the case where the outside gel coat has flame resistant properties. With these dimensions the composite pole will be able to support a load of 2.5 tons before yielding. The fibreglass is then coated with a gel coat in this example for UV protection with a uniform thickness between 250 micron and 500 micron (complying with SABS standard SANS141).

[0036] See FIG. 1 for actual photos after a yield test.

Experiments

[0037] FIGS. 2 and 3 show axial load test results of composite poles described above.

[0038] The Tests were conducted on composite poles as follows:

[0039] Test 1: Fibreglass only with ID=26 mm and OD=34 mm. Not tapered at top.

[0040] Test 2: HDPE pipe with ID=26 mm and OD=32 mm, with Fibreglass shell with ID=32 mm and OD=34 mm. Fibreglass not tapered at top.

[0041] Test 3: Fibreglass only with ID=26 mm and OD=34 mm. Not tapered at top. Repeat of test 1.

[0042] Test 4: HDPE pipe with ID=26 mm and OD=32 mm, with Fibreglass shell with ID=32 mm and OD=34 mm. Fibreglass tapered at top to enable slow yielding mechanism.

[0043] It can also be noted that test no. 4 had a taper and test no. 2 had no taper. The effect of gradual deformation on test no. 4 is clearly visible.

[0044] In FIG. 2 there is seen the load deformation graphs for test no. 2 and test no. 4 which shows a yield load of 2.6 tons and 2.3 tons respectively

[0045] All tests shown in FIG. 2 were done with a longitudinal (tensile strength) fibre lay-up of 80% with the balance of 20% in the radial (hoop strength) direction. Further tests were done varying the tensile versus hoop strength for optimising the wall thickness of the fibreglass sleeve for lowest cost. FIGS. 4 and 5 show these test results.

[0046] FIG. 4 shows two test results from Table 2 below:

TABLE-US-00002 TABLE 2 Fibreglass HDPE HDPE Fibreglass Fibreglass fibre tensile ID (mm) OD (mm) ID (mm) OD (mm) to hoop ratio Test 1 84 90 90 98 50:50 Test 2 84 90 90 98 20:80

[0047] FIG. 5 show test results for the following tests in Table 3:

TABLE-US-00003 TABLE 3 Fibreglass HDPE HDPE Fibreglass Fibreglass fibre tensile ID (mm) OD (mm) ID (mm) OD (mm) to hoop ratio Test 1 84 90 90 98 80:20

[0048] The test results show that the maximum yield load (18 tons) is achieved with a tensile to hoop ratio of 80:20. This ratio gives the optimum lowest cost for balancing tensile vs hoop strength. The hoop strength is necessary for handling buckling forces.

[0049] The only positive result from the low hoop strength test (FIG. 4, test 2, tensile vs hoop ratio of 20:80) was the fact that a slow yielding mechanism was enabled. But this yielding mechanism can also be obtained by tapering the fibreglass pole at the top (reference patent by same inventor ZA2012/05524). Lower hoop strength application might be considered for specific applications where major non-axial forces could be expected.

[0050] FIG. 4 shows the effect of varying fibreglass tensile vs hoop fibre ratio for the same ID and OD HDPE and fibreglass pole. Test 1 has tensile to hoop ratio of 50:50 and test 2 has tensile to hoop ratio of 20:80

[0051] FIG. 5 shows Tensile to hoop ratio of 80:20 for same ID and OD pole as shown in FIG. 4 This is optimal for the lowest cost with best balance between tensile and hoop strength.

[0052] Another design constraint for the fibreglass wall thickness and fibre tensile vs hoop ratio is wind load. The American Association of State Highways and Transportation Officials (AASHTO, 1985) standard for wind loads on signs and luminaires was used to indicate acceptable design tolerances for composite poles. According to this specification the wind load force for a 112 km/h wind will be 365 Pa for a lifetime exposure of 25 years. The maximum allowed deflection for an exposed pole length of 2 m is 200 mm on the tip (10% deflection allowed on length).

[0053] Table 4 below shows the results for deflection as calculated for a wind load force of 112 km/h (365 Pa). As can be seen from the table, all designs are within the specification of 10% deflection of total length above ground. The typical installation height shown is for vineyard support poles. As soon as the tensile to hoop ratio goes above 80:20 the ability of the pole to withstand side impact forces deteriorates and buckling can occur.

TABLE-US-00004 TABLE 4 Wind load deflection results for typical vineyard support applications. Max allowed Height deflection Yield HDPE HDPE Fibreglass Fibreglass above Wind Deflection at (10% of Safety load ID (mm) OD (mm) ID (mm) OD (mm) ground (m) Force (N) tip (mm) length) factor 1.5 ton 21 25 25 27 2 19.7 91 200 2.2 2.5 ton 28 32 32 34.5 2 25.2 43 200 4.7 3 ton 35 40 40 42.2 2 30.8 32 200 6.3 7.5 ton 71 75 75 78.4 2 57.3 7 200 28.6 10 ton 86 90 90 93.9 2 68.6 4 200 50.0