A FEASIBILITY STUDY FOR STRENGTHENINGTIMBER BEAMS WITH FIBREGLASS

The flexual theory in structuraldesign has directed attention to thetensile stresses in structural members,since most materials are able to resistlarge compressive forces. This is particularly true for a material such asconcrete which has little fibre

strength. Concrete, however, can bestrengthened by adding materialswhich have high tensile strength andby placing these materials at criticalpositions within the member. Steelreinforcing bars and welded wiremeshes have been used with concrete

for a great many years and reinforcedconcrete has become a recognizedstructural material. The reason for

employing a non - homogenous material is, primarily, to obtain a balanced design in which compressiveand tensile forces are distributed proportionally within the loaded memberand to obtain a member which is

more compact and sometimes moreeconomical.

Materials like steel, aluminum,cement asbestos and plywood maybe shaped to distribute the appliedforces and sections of angles, I beams,channels or other forms and are

chosen by the designer for specificapplication.

Timber is the most common struc

tural material in Canada and is

probably the one which is most abusedby designers. Comparatively, it is auniform material with wood fibres

providing the strength to resist applied forces. It is, usually, not economical to shape timber to distribute thecompressive and tensile stresses norhas it been customary to reinforce timber in any way to give it assistance incarrying tensile loads (figure 1). Indeed, it is recommended by theCanadian Institute of Timber Construction that steel reinforcing shouldnot be used with a timber structuralmember, either plain or laminated.There have been many instanceswhere steel rods have been anchored

in timber on the tensile side and there

have been steel plates embedded inthe lower portion of laminated members, but, in these cases, there hasbeen no joint strength between thesteel and timber and in fact, the breakin the wood fibres has decreased theinitial strength of the timber member.Other methods of reinforcing timber

by

F. H. Theaks+onMember C.S.A.E.

Department of Engineering ScienceUniversity of Guelph

Guelph, Ontario

Figure I. Test Specimen under Load (Failure inTension). No Reinforcement.

beams consist of strapping steel platesto the sides or top and bottom of themember. This is a sound approachstructurally, but is not economicaland does not place the steel in thecritically stressed portion of the beamjust below the neutral axis.

Fibreglass is a material w;hich combines two of the characteristics of bothsteel and timber. It is fibrous and has

a high tensile strength which are attributes desirable for reinforcement

material. The strength to weight performance of this material has indica

ted an advantage over some of themore familiar materials used for rein

forcing, since transportation to aconstruction site is very easy andhandling on the site should reducelabour. Desirable compound curvatures can be readily achieved with thismoldable material which can be form

ed as easily in one shape as anotherwithout the use of forming tools.

In agricultural, or other structures,the designer is often confronted withthe problem of carrying differenttypes of loads even within one member. This can be accomplished in anyportion of the structure, with thesame basic materials, by simply changing the cross-section and orienting thehigh strength fibres in the direction ofstress. Large tension loads can be efficiently supported by using cloth oruni-directional fibres oriented so asto be aligned with the direction ofload (figure 2). An impact resistantarea can be created by increasinglaminate thickness locally with reinforcements selected for high impactstrength and oriented to be equallygood in all directions. Furthermore,with many types of resins available, abond can be obtained between thefibreglass and timber to such a degreethat the glass fibres becomes an in-

CANADIAN AGRICULTURAL ENGINEERING, JAN. 1965

Figura 2. End view of reinforced laminated beam.(Strands not visible).

tegral part of the timber and oftenthe resin is stronger than the woodfibres themselves (figure 3).

Figure 3. End view of reinforced laminated arch.(Strands not visible)

Fibreglass laminates are essentiallya combination of high strength glassfibres bonded together with comparatively low strength resin. The glassfibres are distributed throughout thelaminate and provide the strength tothe combination. Although individualglass filaments can develop tensilestrength between 250,000 and 400,000pounds per square inch, the mechanical distribution of the filaments in alaminate does not permit the combination to develop this strength. In essence, when fibreglass is used to reinforce timber beams, a laminate isformed, because the resin soaksthrough the fiberglass and adheres tothe timber.

The glass filament used as reinforcement material is a lime-alumina boro-silicate E glass of low alkali contentwhich has high chemical stability andmoisture resistance. This is desirablein agricultural applications because ofthe high humidity conditions in mostfarm buildings and because of thepossibility of insect infestation. Sometimes, steel reinforcement is corrodedby acids from milk waste or silage but

17

glass, and the resins used in the combination, will be resistant to acids encountered on the farm.

TYPES OF MATERIAL

Rovings

Rovings consist of straight bundlesof continuous strands resembling aloose untwisted rope. This is used asunidirectional reinforcement and is

the best material available for reinforcing timber, provided the strandscan be kept reasonably taut andparallel during the application.

„

Figure 4. Timber beam reinforced on bottom. Lamination with Epoxy Resin and woven roving. (Failure at

top in Shear).

Woven Roving

This consists of flattened bundlesor rovings of fibreglass filaments i/g"to 14" wide, woven in to a plainsquare pattern (figure 4). This material has application for reinforcingtimber if the fibreglass is soakedthrough with the resin and if theresin has a good contact with thetimber components.

Cloth

Cloths and tapes are woven fromtwisted and plied strands of glass filaments and may be used successfullyfor reinforcing timber components ifproperly saturated with resin (figure5). It is especially good where highstress areas are recognized, but due toa relatively high cost of the material,it is not practical to use for continuous reinforcement.

Figure 5. Timber beam reinforced on bottom lamination with Epoxy Resin and fibreglass cloth.

is

Mat

Mats of fibreglass are available andconsist of chopped strands of fiberglass, randomly deposited to form asheet or layer. This may be used forreinforcing, but does not have thetensile strength of some other typesof construction, since the fibres arelocated in a non-uniform pattern.However, an interesting and practicalapplication of this may be in the useof the Patterson spray gun which consists of a three-way nozzle, whereinresin, chopped strands of fibreglassand a catalyst meet simultaneouslyand are sprayed on the contact surface.

Prc-impregnated

Pre-impregnated reinforcements arereinforcements preloaded with resins(figure 6). It is commonplace to apply

Figure 6. Panite and pre-impregnated joint at bottomlamination (failure through de-lamination).

pressure and heat to create contactbetween the pre-pregnated materialand the contact surface. However,most of the materials in this categoryhave an affinity for water and theadhering qualities are soon lost asmoisture is taken from timber or the

surrounding air. This is an especialdetriment in farm buildings wherehigh moisture conditions are present.

Resins

The resin is an important factor inthe combination for reinforcing timber and must be chosen with care.

Usually, the thermosetting type, suchas, polyesters, epoxies, phenolics andmelamine are used for this purposeand cannot be remolded once cured

to the solid state. Curing may take

Figure 7. Arch 'ith woven roving reinforcement andEpoxy Resin.

place in 30 minutes or less, but in nocase, should brittleness be permittedin beam construction.

Research has been carried out at

the Ontario Agricultural College, todetermine the reinforcing capacitiesof fibreglass and resin where conjoined with timber in a plain or laminated state. Most of the combinationsof fibreglass strands were studied forcomparison reasons regardless oi theultimate cost or difficulties of application, since it is to be expected thatrefinements in production in the plantor in the field will make the method

practical in due course. No othercomparable work is evident in literature review, though the application olfibreglass reinforced plastics havebeen applied for many other purposesand the physical and chemical properties of resins and fibreglass arewell known. The studies w^ere madeby constructing test specimens ofwhite pine and douglas fir with anunsupported length of 24 inches andiy8 inches wide. Five laminations ofi/2 inch material were used lor laminated beams and similar overall

dimensions were used for solid beams.

Three samples of each specimen wereused in each test and the tests werecarried out in pairs, according todirection of grain, since there was awide variation in this factor in the

material used for test. The reinforcingmaterial was placed at the bottomlaminated joint, though in some caseswhere compression failures tended tooccur, studies were made with thereinforcement also placed at the toplaminated joint (figure 7).

The primary tests were made withthe pre - impregnated fibreglass andpanite as the bonding material. Whilethe initial contact was very good andit appeared that a complete bond wasobtained, overnight setting caused adelamination of the test specimen.This was caused by the moisture inthe timber specimen, since the pre-impregnated fibreglass had an affinityfor the water. Panite is a water base

adhesive which gave up more moisture to the reinforcing material. Application of pre-impregnated fibre-glass would be difficult for largescale production, since heat andpressure must be applied for goodresults.

Epoxy was used as a replacementfor Panite and proved to be muchmore satisfactory, but failed in thebrittle qualities which it gained onset (figure 8).

CANADIAN AGRICULTURAL ENGINEERING, JAN. 1965

TEST RESULTS USING EPOXY WITH FIBREGLASS REINFORCEMENT

Ultimate Load

laminations) 1650 lbslaminations and Fabric cloth) 2150 lbslaminations and Woven Roving) 2460 lbslaminations and Roving) 2700 lbslaminations) 1640 poundslaminations and Woven Roving) 2140 poundslamination and Roving) 2280 pounds

Material

Rectangular Timber (5Section Timber (5(Douglas Timber (5Fir) Timber (5

Arch Timber (6

Construction Timber (6

(White Pine) Timber (6

NOTE: Cloth was not used in this test due to difficulty in fabricating the arch.

The resin and fibreglass used in reinforcing timber beams are materialswhich are resistant to rot, insect infestation, and are not effected by acidsnormally found in and around agricultural structures. They are materialsthat are readily available and easilyapplied.

There should be additional research

carried out in this field, since the potential is excellent in the farm struc

tures area.

DOUGLAS FIR—FIBREGLASS AND RESIN SPRAYED WITH PATTERSON

GUN (DIRECTION PARALLEL TO GRAIN)

Deflection—0.590 inches Ultimate Load—3130 pounds

(DIRECTION PERPENDICULAR TO GRAIN)Deflection—0.830 inches Ultimate Load—3130 pounds

Figure 8. Arch with roving reinforcement and EpoxyResin.

Subsequent tests were carried outon the strength properties providedby wrapping the exterior of a solidtimber beam and applying a load toultimate failure of the beam (figure9). In this case, the ultimate for the

Figure 9. Timber beam wrapped with pre-impregnatedfibreglass.

wrapped beam was 3260 pounds, ascompared with 2000 pounds for anunwrapped specimen. The 39 per centincrease in load carrying ability morethan compensates for the materialused in the wrapping process and,moreover, the beam will still supporta load even though the ultimate hasbeen attained. This factor is onewhich should not be overlooked in

design, since adequate warning willbe given before complete destructionof a structure occurs.

A test was carried out at NaugatukChemicals in Elmira, Ontario, usingthe Patterson Spray Gun to coat oneside of a Douglas Fir member 8 incheswide and y4 inches thick. This member was subsequently used as a partof a laminated beam and tested for

ultimate load and deflection. Thecoatings were applied in the direction

of the grain in one specimen and perpendicular to the grain in anotherspecimen.

The theoretical loading for a beamof this type is 474 pounds with an allowable stress of 1800 psi.

CONCLUSIONS

It is quite practical to reinforcetimber beams with fibreglass and resinwith little increase in the thickness ofthe members. Fibreglass mat, cloth,and sirands can all be used lor rein

forcement, but the stands providethe greatest tensile strength if care istaken when applying the strands tothe surface of the material to which itis to be bonded. Erittleness in theresin must be avoided if completeuniformity in the joint is to be retained. Setting time of the epoxy resin isvariable depending on the type ofresin used, but 30 minutes setting timewill allow adequate time for goodconstruction of the member in mostinstances.

Fibreglass wrapped on the outsideof a solid or laminated timber beam

increases the ultimate strength of themember appreciably. Even when failure occurs, the fibreglass has enoughretentive strength to support loadsand may be an asset from a safetypoint of view.

Pre-impregnated fibreglass does notstand up under test, since materialsused in this program have an affinityfor moisture and would not be useful

in agricultural structures.

CANADIAN AGRICULTURAL ENGINEERING, JAN. 1965

ACKNOWLEDGEMENTS

This paper was made possiblethrough the co-operation of the following who gave freely of their experience in reinforced plastics, andsupplied material for the tests:

Canadian General Electric Company Limited, Guelph, Ontario.

Naugatuk Chemicals CompanyLimited, Elmira, Ontario.

Miss Helen Tucker, Student Assistant, Ontario Agricultural College.

Mr. D. P. Naraine, Student Assistant, Ontario Agricultural College.

GLASSHOUSE DESIGN

continued from page 11

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