Structural Behavior Of Hybrid Timber- Composite Beams
COMPOSITES & POLYCON 2009American Composites Manufacturers AssociationJanuary 15-17, 2009Tampa, FL USAIntroductionEconomic design of large span and / or heavyloaded structures demands the use of high-performancematerials, which means not only high absolute strengthand stiffness but also a high ratio of strength and stiffness to self-weight. Particular advantages associated withfiber-reinforced plastics (FRP) over traditional reinforcing materials such as steel include, besides the lowweight, the ease of handling and the corrosion resistance.This enhancement is very noticeable in flexural bendingbehavior, with FRP working in tension and timber incompression. This type of association brings goodadvantages when used in rehabilitation works and reinforcement of solid timber floors for new construction. Itis a solution that does not overload the existing structurewith additional weight, while significantly increasing itsstrength and stiffness and enhancing the serviceability bycontrolling the natural vibration mode, which is important in wooden structures.Structural Behavior of Hybrid TimberComposite BeamsbyKay-Uwe Schober & Karl Rautenstrauch,Bauhaus-University of Weimar,Department of Timber and Masonry Engineering,Marienstrasse 13A, Weimar, 99423, Germanykarl.rautenstrauch@uni-weimar.deAbstractIt is a well-known feature of timber beams that theyusually fail suddenly due to the breaking of fibers on thetensile face. This behavior may be changed by anadequate strengthening of the tensile face, which maylead to a more ductile failure induced by the gentle localbuckling of the fibers in the compressive face. Suchstrengthening may be achieved by bonding a passive(slack) or active (pre-stressed) reinforcement of FRPwith adhesives to the structural members. The development of a high quality in-service bond between the woodand the FRP laminate is essential to the composite interaction between the two materials. Adhesive bonding is apossible joining technology and it has the advantage ofavoiding the stress concentrations induced by mechanicalfasteners. Ideally, a rigid connection will lead to the mosteffective composite behavior, with the stress and straindistribution governed by the classic theory of strength ofmaterials. This type of behavior needs a full connectionthat does not allow any slippage of the interface and willbe obtained by a thin film of resin.The development of timber-concrete compositestructures has shown that composite systems are a veryefficient solution to increase the load-carrying capacityand overall performance of timber structures. The weakness of these systems is clearly marked by the stiffnessof the bond line, the natural limitations of the timber anddefects in wood.An innovative solution of these problems has beenfound in a new-type composite beam for structural rehabilitation and upgrading, combining polymer concrete inthe compression zone, fiber-reinforced plastics in thetension zone and timber in between. All composite partners have been revised according the material formulation, structural performance of the composites, fractureand delamination behavior. Appropriate mechanicalmodels for the numerical simulation based on a finiteelement approach have been developed. The results ofthe experiments have highlighted the limitations of thecomposite structure as well as the advantages of thevarious combinations. The test results show an increaseof the load-carrying capacity up to 185% for short-termloading,depending of the ratio between polymer concrete andtimber. For long-term loading, the creep influence hasbeen calculated with a rheological model to 1.8-2.5-timesof the deflections compared to the unreinforced beams,depending also on the configuration.Apart from this, each piece of wood differs in theamount of stiffness-reducing defects such as knots, splits,and checks and therefore, it is hard to say at what stresslevel the reinforced beams would have failed if they hadnot been reinforced. The presence of FRP reinforcementarrest the crack opening, confines local rupture andbridges local defects in the timber. Therefore, thestructure can support higher loads before failure. Moreover, as can be observed from experimental studies done,there is an increase of the load-carrying capacity becauseof the quoted crack opening arrest, but the limits aredistinguishable in timber crushing under bending loadsand the quality of the bond line itself.Regarding economical and practical aspects, thepresented composite system describe a good alternativeto conventional solutions for structural upgrading inreconstruction.COMPOSITES & POLYCON 20091
The mechanical properties of the PC have beeninvestigated for dependence on the ambient airtemperature when cast on site and the curing time [1]. Inthe laboratory tests, only a small increase of thecompressive strength with cumulative curing time andstorage temperature could be observed. Lowtemperatures result in an interruption of the chemicalreaction in cement bounded concrete. In epoxy resinbounded PC, the chemical reaction continues when theambient air temperature arises. Higher temperaturesreflect a higher early compressive strength sooner thanby standard climate conditions.A high strength material can be the optimal add-onto restore the load-bearing capacity in the compressionzone. This add-on was found in an epoxy resin-boundedpolymer concrete (PC). The idea of this application wasfocused on combining the favorable characteristics of thepolymer concrete layer, the existing timber beam andFRP reinforcement in a composite structure. The disadvantages of the single components are compensated andthe tension-stressed timber and FRP and the compression-stressed PC layer offering a much better loadbearing behavior.A study of these two reinforcement techniques oftimber floors under bending loads has been carried out atthe Bauhaus-University of Weimar. The experimentaland numerical study consists of two parts: First, the useof epoxy resin-based polymer concrete as strengtheningmaterial, whereby the removal of the suspended ceilingon timber floors is not necessary, and second, the use ofstructural adhesives on the building site, whereby theremoval of the overhanging part of the structure as wellas the inserted ceiling is not necessary. This reinforcingtechnique can be used for structural upgrading andrepair of existing structures or for new constructionswhere the height of the beams is limited.For characterization of the structural performanceof the composite structure, several lab tests have beencarried out for different specimen configuration [2, 3].Since these types of building systems have been realizedas timber-concrete composite beams mainly used forbridges or revitalization of timber floors, steel doweltype connectors were normally used to transfer the shearforces. These steel connectors realize the transmission ofshear stress in the contact area of timber and surfacelayer in timber-concrete composite structures. In thebuilding system described here, only the natural adhesivebond between timber and epoxy PC should be sufficientto transfer the shear- and tension forces in the compoundjoint. The main practical advantages for the chosen system are:Strengthening with polymer concretePolymer concrete is a composite material formedby combining mineral aggregates such as sand or gravelwith a monomer resin. Rapid-setting organic polymersare used in PC as binders. Studies on epoxy polymershave shown that curing method, temperature and strainrate influences the strength and stress-strain relationships. PC is increasingly used as an alternative to cementconcrete in many applications. Today, PC is used forfinishing work in cast-in-place applications, precastproducts, highway pavements, bridge decks and wastewater pipes. PC exhibits a brittle failure. Improving itspost-peak stress-strain behavior is important. TheDevelopment of better PC systems and characterizing thecompressive strength in terms of constituents are essential for the efficient utilization of PC. However, the dataon epoxy PC are rather limited, and there is an increasinginterest in the deformation characteristics under workingconditions in combination with other materials such aswood for composite structures. The section design can be done easily by a timberformwork on the level of the necessary constructionheight. All work can be done from top side; the suspendedceiling will remain unaffected. The floor below the reconstruction work can be usedwithout any restrictions. The full load-carryingcapacity is achieved after one day.As result of the axial bonding and shear tests, thestructural behavior of the compound between timber andepoxy PC can be assumed as rigid. This assumptioncould be confirmed by bending tests where failureoccurred in the timber traction zone without completeplasticization of the compression region in all specimenand test series. The results of the bending test show anincrease of the load carrying capacity of 150-200% for alayer thickness of 2.5 and 3.5 cm (1 and 1.4”).Strengthening with fiber reinforced plasticsThe used polymer concrete for this application consists of the epoxy resin, an accelerator and special gravelwith a grading of 0-4 mm (0.16”). The components weremixed together in a ratio of 14:2.67:83.33 by weight.Comparing the used PC with high-strength concreteC100/115 after curing for seven days on a temperature of20 C, the mean compressive strength is about the same,whereas epoxy PC has a triple value of the bendingstrength of C100/115. A comparison with the usually onbuilding site used concrete C25/30 is shown in Table 1.Over the past years, different investigations on FRPreinforcement, mostly carbon fiber FRP’s (CFRP), havebeen done to study the structural improvement whenusing this technique for timber structures [4, 5]. Thesolid timber beams were reinforced with a continuouscarbon fiber lamella with intermediate modulus fiberswithin the clear span, otherwise over the full lengthembedded in the specimen at different section locations.COMPOSITES & POLYCON 20092
Different types of interface damage laws have beendeveloped so far for different structural and thermalproblem formulations using linear, bilinear, polynomialor exponential functions. For the combination of theanisotropic properties of wood with FRP reinforcementin an interface damage law, the interaction of shear andnormal stresses have to be considered. This problem canbe solved by using a delamination analysis and an exponential interface damage law. The proposed exponentialinterface law consists of a continuously differentiablestress-crack opening-behavior, robustness regardingnumerical problems and takes both, shear and tensionstiffness, into account [6].The CFRP layer was glued / embedded by meansof a commercially available epoxy resin, consisting ofBisphenol-A-Epichlorhyd, Bisphenol F and Epichlorhydrin. The mean mechanical properties are shown inTable 2. The wood beams reinforced with CFRP stripsrevealed a behavior that is more ductile compared tounreinforced beams. The strength increase of thereinforced specimen was defined as the bending stress atthe deflection in linear range before failure, divided bythe bending stress of the un-reinforced specimen at thesame deflection value. It has been calculated for all testseries between 6% and 12%. The presence of CFRP reinforcement arrested crack opening, confined local ruptureand bridged local defects in the timber, therefore thistechnique is very promising for structural enhancementin tension stressed areas.The cohesive zone model consists of a constitutiverelation between the traction T acting on the interfaceand the corresponding interfacial separation δ (displacement jump across the interface) developed from thesimple and convenient universal binding law furnishedby SMITH and FERRANTE, where e exp(1).Numerical simulation of the bond line behaviorThe bond line behavior between timber and PC hasbeen confirmed as rigid, so no further action in thenumerical model is necessary. Between timber and FRP,debonding and delamination effects have been observedand should be included in an appropriate model.Decohesion along interfaces plays an important role in awide variety of failure processes in structures when usingchemical bonding as the optimal form of combining twosurfaces with each other. The various theories of bondingdeveloped over the past years can only limited explainthe observed effects from laboratory tests and realstructure monitoring. The development of finite elementtechniques, focusing on crack propagation andinterlaminar damage, provides new tools to predict thestructural performance enhancement and the serviceability of fiber-reinforced structures.T e σ c ( δ δ ) exp ( δ δ)(1)The definition of traction and separation depend onthe finite element and the applied material model. It isbased on the local corotational element coordinatesystem. Decohesion response was specified in terms of asurface potential Φ(δ) relating the interface tractions andthe relative tangential and normal displacements δn andδt across the interface [6], [7]. The resulting work of normal separation and tangential separation can be related tothe critical values of the energy release rates. The surfacepotential is defined toΦ (δ ) e σ c δ n ( 1 exp ( n ) ζ )withThe analysis of delamination is commonly dividedinto the study of crack initiation and crack propagation.Delamination initiation analysis is usually based onstresses and interaction criteria of the interlaminarstresses in conjunction with a characteristic distance asfunction of geometry and material properties. Crackpropagation is usually predicted using the FractureMechanics (FM) approach. This approach avoids thedifficulties associated with a stress singularity at thecrack front but requires the presence of a pre-existingdelamination. When used in isolation, neither thestrength-based approach nor the FM approach is adequate for a progressive delamination failure analysis.Both approaches have to be considered together usingspecial interfacial decohesion elements, placed betweencomposites material layers or in the structure wherecracking can occur (Figure 1). These elements aresurface-like and are compatible with general bulk finiteelement discretizations of the solid model. Cohesiveelements, or decohesion elements, bridge nascent surfaces and govern their separation in accordance with thecohesive material law.σcδnδt1 q r 1r q q nexp ( t2 )r 1 ζ ( 1 r n )(2)maximum normal traction at the interfacenormal separation where the maximum normaltraction is attained with δt 0shear separation where the maximum sheartraction is attained at t 1 2 2ratio between normal separation work andshear separation workδ n* value of δn after complete shear separationwith Tn 0 n δ n / δ n , t δ t / δ t , q Φ t Φ n , r δ n* δ n .qIn most of the computations all cohesive surfacesare taken to have identical cohesive properties whichsimplifies the potential from eq. (2) with q 1 and r 0.The potential lead by derivation on the displacements tothe stresses if δ δmax where the traction components Tare coupled to both normal and tangential crack openingCOMPOSITES & POLYCON 20093
displacements and leads to separation at n 1, so thatTn 0 for n 1. The traction components in normal andtangential direction are given in eq. 3 and 4.Tn e σ c n exp ( n 2t)Example – Reconstruction of Mansfield CastleAccompanying to the theoretical investigations,both systems were tested for new constructions andupgrading of existing structures independently andtogether under practice-related conditions on site. Firstpractical insights have shown primarily doubts regardingcleanliness and feasibility could not confirmed and thework progress was done accurate and efficient after shortbriefing of the construction worker.(3)Tt 2 e σ c ( δ n δ t ) t ( 1 n ) exp ( n t2 ) (4)The main advantage of the cohesive zone modelingis that, when it is known where fracture may occur apriori, a cohesive zone may be placed anywhere alongelement interfaces in that areas, to take these effects intoaccount. Furthermore, using decohesion elements, bothonset and propagation of delamination can be simulatedwithout previous knowledge of crack location and propagation direction and therefore suitable for structuraldesign and evaluation of timber composite beams.For the new occupancy and floor design of theMansfield Castle, an increase of the existing dead loadsand the life loads for the structural design of the waffleslab above the „Blue Hall” (Figure 3, 4) was required.The main girders got a deflection of 9 cm (3.5“) over thepast years. After removing of the ceiling cover, largelongitudinal and inclined cracks in the girders werevisible. Due to high loading from the secondary ceilinggirders and the specific construction of the waffle slab,the combination of described reinforcement systemswere chosen for the main girder – upgrading of thetension zone with carbon fiber strips (Figure 5) and anadditional PC layer on top (Figure 6).Combined FRP and PC reinforcementWith the obtained knowledge from the experimental tests and numerical investigations, it is now possibleto describe and design any possible combination of thethree materials as a composite structure. For the firstestimation of the structural behavior of the compositestructure, the tested specimen with PC reinforcementwith a section of 12/14 cm (4.7/5.5”) and a span of2.20 m (7.2 ft) have been evaluated. The thickness of thePC layer has been chosen to 2.5 and 3.5 cm (1 and 1.4”)placed on top with an additional CFRP reinforcement onbottom, cross section 50 x 1.4 mm (2 x 0.06”), materialproperties according to Table 2. The compound betweenthe laminate and timber has been determined to be freefrom defects and delamination. Figure 2 shows theresults of the structural analysis as strength increase bycomparison of the bearable bending moments. The mosteconomical configuration here is type (e) with only a thinPC layer on top. The increase of the ultimate bendingmoment mu by 200% evidences the double load-carryingcapacity of this composite system and optimal performance on the building site [8].The structural design was done using the describedfinite-element model with cohesive law for the bond line.The material properties for the numerical model areshown in Table 3. When comparing the stress distribution over the section height (Figure 7) small tensionstresses in the polymer concrete layer are present in areaswhere the main girder section is cut off for the secondarygirder joints. The magnitude of these stress peaks in thebond surface was calculated to 40% of the design valuefcm,d and covered by additional four solid glass fiberrebar’s with a diameter of 15 mm (0.6”) in the PC layer.ConclusionsA new-type composite beam for structuralrehabilitation and upgrading, combining polymerconcrete in the compression zone, fiber reinforcedplastics in the tension zone and timber in the center hasbeen investigated numerically and tested in practice. Theused mechanical model show good agreement withrecent test results and addresses structural nonlinearitiesand FRP debonding. With the proposed composite beam,the load-carrying capacity can be nearly doubledcompared to solid timber structures, where the construction work remains significant low. Regarding economical and practical aspects, the presented compositesystem describe a good alternative to conventionalsolutions for structural upgrading in reconstruction. It issuitable for difficult floor and workspace situations.This approach has been repeated for a solid timberfloor, cross section 22/26 cm (8.7/10”), span 7 m (23 ft)with PC reinforcement of 3 cm (1.2”) and high modulusCFRP reinforcement 100 x 1.4 mm (4 x 0.05”). The bondline has been described with the cohesive zone modeland anisotropic parameters for the timber secti
of the deflections compared to the unreinforced beams, depending also on the configuration. Regarding economical and practical aspects, the presented composite system describe a good alternative to conventional solutions for structural upgrading in reconstruction. Introduction Economic design of large span and / or heavy
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The American Institute of Timber Construction, established in 1952, is a non-profit industry association for the structural glued laminated timber industry. Its members design, manufacture, fabricate, assemble, and erect structural timber systems utilizing both sawn and structural glued laminated timber components.
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