WO2004097138A1

WO2004097138A1 - A composite beam

Info

Publication number: WO2004097138A1
Application number: PCT/AU2004/000535
Authority: WO
Inventors: Gerardus Maria Van Erp
Original assignee: The University Of Southern Queensland
Priority date: 2003-04-30
Filing date: 2004-04-23
Publication date: 2004-11-11
Also published as: EP1618261A4; AU2003902044A0; EP1618261A1; CA2523853A1

Abstract

A composite beam (1) comprising: a) an elongated timber member (10); and b) at least one hybrid structural module (20) having a tubular fibre composite member (30); ii) a filled resin system (40) located within said tubular fibre composite member; and iii) at least one elongated steel member (50) located within the filled resin system, wherein the elongated timber member and the hybrid structural module are joined together.

Description

A COMPOSITE BEAM" FIELD OF THE INVENTION This invention relates to a composite beam. In particular, the invention relates to a composite beam that has improved structural properties.

BACKBROUND OF THE INVENTION Timber has long been used in the construction of buildings, bridges and other infrastructure. There are many applications for timber including bearers, joists, battens, studs, rails, posts and trusses. The type of timber that is used for the various applications depends to a large extent on the load that the timber is likely to be subjected to in use. For example, bearers are normally made from timber that is capable of withstanding large loads. Australian timber is rated according to a Stress (F) Rating. The stress rating is a measure of the modulus of elasticity of timber used for structural purposes. The stress rating has a number of categories, namely F4, F5, F7, F14, F17, F27 and F34, with the higher the grade, the better the strength. Softwoods typically fall within the categories of F5 to F8, whilst hardwoods extend over the entire range of F4 to F34.

The price obtained for timber is dependant on the timber's stress rating. The higher the stress rating, the higher the price obtained for the timber. Therefore, the highest priced and strongest timber is F34 hardwood. The reason that F34 hardwood is expensive is that hardwood trees take a long time to grow. Most hardwood trees take at least fifty years before they reach maturity and are normally located in old natural forests. As the demand for housing and other infrastructure increases, so does the need for good quality hardwood to produce hardwood beams. Unfortunately hardwood is a rapidly diminishing resource, and the demand for high category hardwood is currently outstripping supply. Commercial production of hardwood timber is not economically sustainable due to the length of time hardwood trees take to reach maturity.

Softwood trees can reach maturity in less than half the time of hardwood trees and thus, economical production of softwood is sustainable. However, softwood trees do not have the high stress rating needed for the production of high load beams.

Concrete and steel beams can provide sufficient load characteristics. However, they are normally much stiffer and heavier, and more difficult to work with than timber beams and hence are not a viable option in many instances. OBJECT OF THE INVENTION

It is an object of the invention to overcome or alleviate the abovementioned problems or provide the consumer with a useful or commercial choice.

SUMMARY OF THE INVENTION In one form, although not necessarily the only or broadest form, the invention resides in a composite beam comprising: an elongated timber member; and at least one hybrid structural module having: a tubular fibre composite member; a filled resin system located within said tubular fibre composite member; and at least one elongated steel member located within the filled resin system; wherein the elongated timber member and the hybrid structural module are joined together.

The timber member may be constructed from any type of wood, such as hard wood or soft wood, or any type of wood product, such as fibreboard, chipboard, particleboard or the like material. Preferably, the timber member is made from plywood Preferably, the tubular fibre composite member is a pultruded member. The pultruded member may be substantially square or slightly rectangular in transverse cross-section. The internal void of the tubular member may be square, rectangular or circular.

The tubular fibre composite member may have the majority of its fibres orientated in a longitudinal direction.

The resin in the filled resin system could be a polyester, vinylester, polyurethane, or epoxy resin. Preferably, the filled resin system is a filled epoxy system.

Preferably, the filled resin system has a low shrinkage rate. Preferably, the shrinkage rate is less than 4%. More preferably, the shrinkage rate is less than 2%. Preferably, the filled resin system has high adherence to both the steel and the tubular fibre composite member.

The filled resin system may allow for high filler loadings (up to 45%) to be used without severely affecting the flowability of the filled resin system. Usually, the filler is inert. Preferably, the filler has a compression strength of between 20MPa and 60Mpa.

Preferably, the failure strain of the filled resin system is larger than the serviceability strain of a typical engineering structure. Usually, the failure strain of the filled resin system is between 0.8 -1.0% whilst the serviceability strain is typically between 0.1 - 0.2%. The filled resin system may include a light aggregate and a heavy aggregate. The light aggregate may have a specific gravity less than that of the resin. The light aggregate can be any type of light aggregate, or combination of light aggregates, dependent on the desired structural and corrosion resistant properties of the filled resin system. Usually, the light aggregates have a specific gravity of 0.5 to 0.9. The light aggregates usually make up 20-25% by volume of the filled resin system. Preferably, the light aggregates are centre spheres. Preferably, the filler consists of centre spheres with a specific gravity of approximately 0.7, a nominal particle size range between 20-300 microns, and compression strength of approximately 40MPa. Alternately, hollow glass micro-spheres with a similar specific gravity and volume may be used.

The heavy aggregate may have a specific gravity larger than that of the resin. The heavy aggregate may be any type of heavy aggregate, or combination of heavy aggregates, dependent on the desired structural and corrosion resistant properties of the filled resin system. The heavy aggregates usually make up 40-60% by volume of the filled resin system. Preferably, the heavy aggregate is basalt. Usually, the basalt is crushed. The crushed basalt may have a particle size of 1 to 7mm. Preferably, the basalt makes up between 40-50% by volume of the filled resin system. The basalt normally has a specific gravity of approximately 2.8. Alternately, sand that has a similar specific gravity as basalt may be used. Preferably, the sand makes up between 50-60% by volume of the filled resin system.

Preferably, the resin contains a thixotrope to keep the light aggregate in suspension.

The filled resin system of the present invention may also include fibrous reinforcement material to increase the structural properties of the filled resin system. The reinforcement material may be glass, aramid, carbon and/or thermo plastic fibres.

The steel member may be a round or deformed bar, threaded rod or tendon (cable). Preferably, the steel member is a high strength steel member with a yield strain of approximately 0.25% and a failure strain in excess of 2%.

The steel member may be made of plain carbon steel, galvanised steel, or stainless steel.

The steel member may be slighter shorter than the length of the tubular fibre composite member so that the steel is located fully within the tubular member. These ends of the tubular member may be completely filled with the filled resin system in order to create a solid 'block' of corrosion protection for the steel member at both ends of the tubular member.

Alternatively, the steel member may extend outwardly from the tubular member and the resin system to allow the beam to be attached to other building components.

There may be a single steel member or multiple steel members located within the beam. If there are multiple steel members, they may be spaced substantially an equal distance away from each other.

The steel member may be prestressed prior to the hybrid member being formed.

The dimensions of the steel member and the wall thickness of the tubular fibre composite member can be tailored to obtain specific predefined stiffness and/or strength characteristics.

The timber member and the hybrid member are preferably adhered to each other. The adhesive could be a polyester, phenolic, epoxy, or other adhesive system. Preferably, the adhesive is an epoxy system. The timber member may provide a recess in which the hybrid member is located.

The number of hybrid elements may be dependent on the desired bending moment capacity of the beam.

The type of timber member used may be dependant upon the desired shear stress capacity of the beam. A number of strengthening rods may extend through the timber member to increase the transverse strength of beam. The rods may extend substantially laterally with respect to the hybrid member.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of a beam according to a first embodiment of the invention;

FIG. 2 is a front view of a beam according to a second embodiment of the invention; FIG. 3 is a front view of a beam according to a third embodiment of the invention;

FIG. 4 is a front view of a beam according to a fourth embodiment of the invention;

FIG. 5 is a front view of a beam according to a fifth embodiment of the invention; and

FIG. 6 is a graph comparing the "load-displacement" behaviour of standard timber products to the beam shown in FIG. 1. Like numerals will be used to describe like components throughout the detailed description of the preferred embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a composite beam 1 having a timber member 10, and four hybrid structural modules 20.

The timber member 10 is made from plywood and comprises a large number of individual timber sheets that are adhered together using adhesive. Recesses 11 are formed in the timber member by either having shorter different width timber sheets or cutting the timber member once it is formed.

Each hybrid structural module 20 is formed from a tubularfibre reinforced composite member 30, a filled resin system 40 and three steel reinforcement bars 50.

The tubular fibre composite member 30 is a pultruded member that is substantially square in transverse cross-section. The cross-section dimensions of the tubular fibre composite member are 75 mm x 75 mm. The length of the tubular fibre composite member is variable according to the length of the composite beam 1.

The filled resin system 30 fills the void between the steel bars 50 and tubular fibre composite member and adheres to both the steel bars 50 and inside of the tubularfibre composite member to make the steel bars 50 and tubular fibre composite member 40 operate as one structural unit. The filled resin system 40 has a 'custard-like" consistency such that it easily flows in the void between the steel bars 50 and the tubular fibre composite member 30 without creating large air voids.

The filled resin system 40 has very little shrinkage, less than 2%, in order not to create large internal stresses between the steel bars 50 and tubular fibre composite member 30. Further, the low shrinkage of the filled resin system 40 prevents cracking during the production of hybrid structural member 20. Any cracks could allow moisture or other corrosive liquids to reach the steel bar 50, which is undesirable. The filled resin system has high adherence to both the steel bars 50 and the tubular fibre composite member 40.

The filled resin system 40, in this embodiment, is produced using a mixture that is 28% by volume of resin, 22% by volume of light aggregate, and 50% by volume of heavy aggregate. The light aggregate is in the form of centre spheres having a specific gravity of approximately 0.7. The heavy aggregate is formed from crushed basalt having a specific gravity of approximately 2.8 and a particle size of 1-3 mm.

The light aggregate has a specific gravity that is slightly less than that of the resin whilst the heavy aggregate has a specific gravity that is larger than that of the resin.

A thixotrope is added to the resin so that the light aggregate will stay in suspension within the resin. Consequently, the resin together with the lighter aggregate in suspension becomes a flowable filled resin system in its own right. The amount of the lighter aggregate suspended in the resin can be varied as required. To obtain an economical filled resin system, the lighter aggregate is approximately 45% by volume of the flowable filled resin mix.

The hybrid structural member 20 is produced by first abrading the inside of the tubular fibre composite member 30 to increase the adhesion between the filled resin system 40 and the tubular fibre composite member

30. The steel bars 50 are cleaned with a solvent to increase the adhesion between the fibre member and the steel bars 50.

The steel bars 50 are lowered in the tubular fibre composite module and resin is poured in the module to fill the void.

The steel bars 50 provides the stiffness, the tubular fibre composite member 30 provides a corrosion protective shell for the steel bars

50 together with extra strength and stiffness, and the filled resin system binds the steel bars 50 and tubular fibre composite member 30 together and provides an additional thick layer of corrosion protection to the steel bars 50.

The tubularfibre composite member 30 has the large majority of its fibres in longitudinal direction. This results in a thermal coefficient of expansion in the tubular fibre composite member 30 that is comparable to that of the steel bars 50. Furthermore, having the large majority of the fibres in longitudinal direction results in the operating strains under serviceability conditions (generally between 0.16%-0.2%)to be optimal in both the tubular fibre composite member and the steel member.

The steel bars 50 are high strength 16 mm diameter steel bars with a yield strain of approximately 0.25% and a failure strain in excess of 2%. Yielding of the steel bar 50 ensures that the tubular fibre composite member reaches its failure strain (generally between 1.3%-2%) before the steel bars 50 reach their failure strain. The tubular composite member 30 and the steel bars 50 will contribute fully to the ultimate load carrying capacity of the hybrid structural module 20 and thus the composite beam 1.

The composite beam 1 is formed by adhering the hybrid structural modules 20 within the recesses 11 of the timber member 10. Standard glue such as epoxy resin is used for this purpose. Holes are drilled laterally through the timber member 10 and fibre composite rods 60 are glued in these holes. The rods 60 assist in preventing the plywood sheets used to form the timber member from delaminating.

FIG. 2 shows a second embodiment of a composite beam 1. In this embodiment, the timber member 10 is made from softwood such as pine.

The hybrid structural module 20 is constructed similarly to that described previous except that there is only a single steel bar 50. The single steel bar 50 is a high yielding 28mm diameter steel bar. The filled resin system 40 is a filled epoxy system. The filler consists of centre spheres with a nominal particle size range between 20-300 microns and a strength of approximately 40MPa.

The composite beam 1 is produced by routing a recess 11 along opposing sides of the length of the timber member. Hybrid structural modules 20 are adhered within each of these recesses.

FIG. 3 shows a composite 1 beam in which the hybrid structural module 20 is located centrally 17 within the timber member 10. The timber member 10 is produced using fibreboard. The hybrid structural module 20, in this embodiment, is produced using the filled resin system described in FIG. 2 and has four steel members 50.

FIG. 4 shows a composite beam 1 using a timber member 10 formed of particleboard and three hybrid structural modules 20 located along opposite sides of the timber member 10.

The hybrid structural modules 10 are the same as those used in the composite beam 1 described in FIG. 2.

To produce the composite beam 1 of this embodiment, recesses 11 are formed within the timber member 10. The hybrid structural modules 20 are then glued within the recesses 11 to form the composite beam 1.

FIG. 5 shows a composite beam 1 similar to that described in FIG. 1 , however only a single hybrid module 20 is located adjacent a side of the timber member 10.

The timber member 10 is formed from plywood as described in FIG. 1. The hybrid module 20, used in this embodiment, is the same as the hybrid modules 20 shown in FIGS. 2 and 3.

The composite beam 1 is effective due the hybrid structural module 20 providing excellent tensile and compressive strength whilst the timber member 10 provides excellent shear strength.

FIG. 6 shows a graph comparing Load vs Deflection for a F14 Plywood beam, an F27 timber beam, an F34 Timber beam, and a Composite beam. The composite beam is similar to that shown in FIG.1. The composite beam has the follow specifications:

Overall Dimensions: 200 mm x 200 mm x 2700 mm

Sheets: 7 x 28 mm thick plywood sheets glued together using epoxy resin

Hybrid Elements: 4 x 50 mm x 50 mm Hybrid Modules each containing 3 x 16mm steel reinforcing bars. Each hybrid module is filled with 56% epoxy resin and 44% enviro-spheres (by volume).

Each beam spanned 2500 mm and was loaded at four points, i.e., at each end and 1000 mm from each end of the span.

As is clearly demonstrated by the graph, the composite beam substantially outperforms the timber beams by providing higher load characteristics and greater deflection before failure. An advantage that the composite beam provides is that it has a yield platform. The wooden beams do not have a yield platform and therefore fail catastrophically which is undesirable. However, as the composite beam has a yield platform, there is an indication that the composite beam is going to fail before failure actually takes place. The composite beam is therefore a safer alternative to wooden beams.

The composite beam is generally price comparable with high stress rated hardwood beams. It is weight comparable and allows standard fastening methods, such as nail, screws, ties and the like to be used in construction. Further, the wood products used in the timber members can be made from softwood that is economically sustainable and environmentally friendly.

It should be appreciated that the timber member may be made from numerous different wood products dependant upon application. For example, marine ply may be used for exterior applications where the composite beam is like to be exposed to water.

It should be appreciated that the location and number of the hybrid members can be changed dependant upon application. For example, for high load requirements, the number and/or size and/or composition of the hybrid modules can be increased. It should be appreciated that various other changes and modifications may be made to the embodiments described without departing from the spirit or scope of the invention.

Claims

CLAIMS:

1. A composite beam comprising: an elongated timber member; and at least one hybrid structural module having: a tubular fibre composite member; a filled resin system located within said tubularfibre composite member; and at least one elongated steel member located within the filled resin system; wherein the elongated timber member and the hybrid structural module are joined together.

2. The composite beam of claim 1 wherein the timber member is made from plywood.

3. The composite beam of claim 2 wherein the plywood timber member is constructed of plywood sheets.

4. The composite beam of claim 1 wherein the tubular fibre composite member is a pultruded member.

5. The composite beam of claim 4 wherein the pultruded member is substantially square in transverse cross-section.

6. The composite beam of claim 1 wherein the tubular fibre composite member has the majority of its fibres orientated in a longitudinal direction.

7. The composite beam of claim 1 wherein resin in the filled resin system is a polyester, vinylester, polyurethane, or epoxy resin.

8. The composite beam of claim 1 wherein the filled resin system is a filled epoxy system.

9. The composite beam of claim 1 wherein the filled resin system has a shrinkage rate of less than 4%.

10. The composite beam of claim 9 wherein the filled resin system has a shrinkage rate of less than 2%.

11. The composite beam of claim 1 wherein the filled resin system has high adherence to both the steel and the tubular fibre composite member.

12. The composite beam of claim 1 wherein the failure strain of the filled resin system is between 0.8 -1.0%

13. The composite beam of claim 1 wherein the serviceability strain is between 0.1 - 0.2%.

14. The composite beam of claim 1 wherein the filled resin system includes a light aggregate and a heavy aggregate.

15. The composite beam of claim 14 wherein the light aggregate has a specific gravity of 0.5 to 0.9.

16. The composite beam of claim 14 wherein the light aggregate is 20- 25% by volume of the filled resin system.

17. The composite beam of claim 14 wherein the light aggregate is centre spheres.

18. The composite beam of claim 14 wherein the heavy aggregate is 40- 60% by volume of the filled resin system.

19. The composite beam of claim 14 wherein the heavy aggregate is basalt.

20. The composite beam of claim 14 wherein the filled resin system contains a thixotrope.

21. The composite beam of claim 1 wherein the steel member is a round or deformed bar, threaded rod, or cable.

22. The composite beam of claim 1 wherein the steel member has a yield strain of approximately 0.25%.

23. The composite beam of claim 1 wherein the steel member has a failure strain in excess of 2%.

24. The composite beam of claim 1 wherein the steel member is made of plain carbon steel, galvanised steel, or stainless steel.

25. The composite beam of claim 1 wherein the steel member is shorter than the length of the tubular fibre composite member.

26. The composite beam of claim 1 wherein ends of the tubular member are completely filled with the filled resin system.

27. The composite beam of claim 1 wherein the steel member extends outwardly from the tubular member and the resin system.

28. The composite beam of claim 1 wherein the steel member is prestressed prior to the hybrid member being formed.

29. The composite beam of claim 1 wherein the timber member and the hybrid member are adhered to each other.

30. The composite beam of claim 29 wherein the adhesive is a polyester, phenolic, epoxy, or other adhesive system.

31. The composite beam of claim 29 wherein the adhesive is an epoxy system.

32. The composite beam of claim 1 wherein at least one strengthening rod extends through the composite beam.

33. The composite beam of claim 32 wherein the rod extends substantially laterally with respect to the hybrid member.