Wood joints and laminated wood beams assembled by mechanically-welded wood dowels

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Wood joints and laminated woodbeams assembled by mechanically-welded wood dowelsJ.-F. Bocquet a , A. Pizzi b , A. Despres c , H. R. Mansouri d , L.Resch e , D. Michel f & F. Letort ga ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, Franceb ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, Francec ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, Franced ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, Francee ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, Francef ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, Franceg ENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc,BP 1041, F-88051 Epinal, FrancePublished online: 02 Apr 2012.

To cite this article: J.-F. Bocquet , A. Pizzi , A. Despres , H. R. Mansouri , L. Resch , D.Michel & F. Letort (2007) Wood joints and laminated wood beams assembled by mechanically-welded wood dowels, Journal of Adhesion Science and Technology, 21:3-4, 301-317, DOI:10.1163/156856107780684585

To link to this article: http://dx.doi.org/10.1163/156856107780684585

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J. Adhesion Sci. Technol., Vol. 21, No. 3–4, pp. 301–317 (2007) VSP 2007.Also available online - www.brill.nl/jast

Wood joints and laminated wood beams assembled bymechanically-welded wood dowels

J.-F. BOCQUET, A. PIZZI ∗, A. DESPRES, H. R. MANSOURI, L. RESCH,D. MICHEL and F. LETORTENSTIB-LERMAB, University of Nancy 1, 27 Rue du Merle Blanc, BP 1041,F-88051 Epinal, France

Received in final form 11 January 2007

Abstract—Dowel welding by high-speed rotation was used to join two wood blocks and strong jointswere obtained. Dowel angle to the surface of the wood blocks to be joined had a marked influenceon the mechanical performance of the joint. When the dowel was inserted at 90◦ to the substrate,the dowel was subjected to and resisted a shear force only. When introduced at an angle such as 30◦or 45◦, the dowel was subjected to and resisted both shear and tension forces, resulting in better jointstrength. The joint almost always failed by dowel fracture. The dowel/substrate interface was almostalways stronger and did not break. Short two-layer beams joined exclusively by a series of weldeddowels were prepared, tested in shear according to structural standards, and their performance wascompared to those of solid wood and of glued laminated beams (glulam) of the same dimensions.The short two-layer beams prepared for testing met the Eurocode 5 standard requirements when theoptimum dowel insertion angle was used. Then 2-m-long two-layer wood beams were prepared, withthe two layers connected exclusively by a series of welded dowels, and tested in bending. Theirmaximum failure strength and stiffness in bending were determined. These beams outperformed bothnailed beams and glued-dowel beams. All the beams had the same length and conformation. Thenumber of nails necessary was double the number of dowels used.

Keywords: Joints; wood welding; dowel welding; welded joints; structural element; welded beams;adhesives.

1. INTRODUCTION

High-speed rotation-induced wood dowel welding, without any adhesive, haspreviously been shown to rapidly yield wood joints of considerable strength [1].This type of welding is effective due to the temperature-induced softening andflowing of some amorphous, cells interconnecting wood material. The consequenceis high densification of the bonded interface [1–3]. Wood species, relative diameter

∗To whom correspondence should be addressed. E-mail: [email protected]

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302 J.-F. Bocquet et al.

difference between the dowel and the receiving hole, and dowel insertion time werethe parameters that yielded significant strength differences [1–3].

Historically, nails have been the most prevalent wood fasteners and they are stillused in current practice in the timber and construction industries. Nailing is quickand does not require sophisticated equipment. Structural wood joining by hammer-inserted wood dowels was the predominant practice before the widespread use oftimber nailing. Nailing became more popular due to the decreased cost of nailsbeginning in the 19th century. Dowel joints are not even recognized by moststandards as working joints. The widespread use of nails has limited dowel joining.Only the furniture, interior joinery, and do-it-yourself (DIY) sectors are still majorusers of joining by wood dowels.

The cost of metal nail connectors is now increasing due to higher steel andpetroleum costs. Furthermore, structural wood processes such as cutting andplaning are not compatible with the presence of metal nails in wood. Wood gluing,an extensively used and an excellent valid alternative, also presents some problems,such as (1) in some cases real or perceived environmental pollution; and (2) depositson glue applicators that would require cleaning. Thus, a novel approach wouldbe to use welded wood dowels to replace metal nails and/or dowels bonded withpoly(vinyl acetate) (PVAc). The final aim would be to make a full-scale prototype ofa welded-dowel structural element to compare with a steel-nailed element and witha PVAc-bonded-dowel element. The ideal outcome would be to obtain structuralperformance from the welded-dowel joint at least equivalent to a nailed joint orPVAc-bonded-dowel joint.

This study compared the structural performance of welded-dowel joints with thatof nailed joints and PVAc-bonded-dowel joints and the performance of 2-m-longbeam assemblies joined by either bonded wood dowels or mechanically weldedwood dowels.

2. EXPERIMENTAL

2.1. Dowel insertion modes

Fluted groove beech dowels of 8 to 10% moisture content and 10 mm diameterwere used. The dowels were either hammer-inserted or PVAc-bonded in the testspecimens according to the configurations shown in Figs 1–3a,b. For each hole,PVAc was added in a sufficient amount to cover the whole dowel and still be enoughfor a considerable squeeze-out of adhesive during insertion. Dowels were insertedat 120◦, 90◦, 75◦, 60◦, 45◦ and 30◦ to determine if transferring the applied stressfrom shear only to shear+tension or shear+compression produced a higher shearstrength. The specimens were tested according to European Norm EN 1380 [4] andEuropean Norm EN 383 [5]. To distinguish angled insertion of the dowels from theperpendicular (90◦) insertion, the former has been named skew-welded.

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High-speed rotation-induced dowels welding 303

Figure 1. Possibilities for nail and dowel test mounting indicating that for symmetrical mountingthere is equilibrium of momenta, while for asymmetrical mounting it is difficult to determine sheardue to momentum. Advantage of symmetrical over asymmetrical mounting. Curved arrows indicatemomenta and straight arrow the force applied.

Figure 2. Schematic representation of the type of force the dowel or nail are subjected to accordingto their angle of insertion in the substrate blocks.

2.2. Short beam assembly and testing according to EN 1380

Quintuplicate 300 × 40 × 30 mm beams were made and tested for each of thefollowing cases:

1. Solid beech wood.

2. Solid spruce wood.

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(a)

(b)

Figure 3. Schematic representation of the type of force nails or dowels are subjected to accordingto their insertion angle in symmetrical mounting: (a) 90◦ shear only; 75◦ and 60◦ shear + tension;(b) 120◦ shear + compression; 45◦ shear + tension.

3. Beech wood laminated beams (glulam) obtained by joining two 300 × 20 ×30 mm pieces of beech wood with a melamine–fortified urea–formaldehyde(MUF) resin at 200 and 350 g/m2 spread.

4. Beech wood laminated beams (glulam) obtained by joining two 300 × 20 ×30 mm pieces of beech wood with a PVAc adhesive at 350 g/m2 spread.

5. Beech wood laminated beams obtained by joining two 300×20×30 mm piecesof beech wood with two series of ten 3.1 × 55 mm nails (for a total of 20 nails)inserted parallel to each other. The spacing between each nail and the next inthe row was 2.8 cm (measured from nail centre to nail centre) and the first andlast nails were 2.4 cm distant from the beam ends.

6. Beech wood beams obtained by joining two 300 × 20 × 30 mm pieces ofbeech wood with ten 10-mm-diameter welded dowels (fluted, beech) for testingaccording to EN 383 and EN 1380. These dowels were inserted into substrate

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(c)

Figure 3. (Continued.) (c) Left, example of a 6-mm dowel, hammer-inserted at 90◦, deformed bypure shear in testing; right, example of a 6-mm dowel, hammer-inserted at 45◦, showing much lessdeformation due to shear but block separation due to tension in testing.

holes according to the parameters optimized for the wood-to-wood weldingtechnology as reported previously [1, 3]. The predrilled holes were 8 mm indiameter in the upper wood piece and 7 mm in diameter in the lower woodpiece. This ratio has already been optimized and the reasons are reportedelsewhere [2]. The dowels were inserted at 45◦ to the wood surface. Thedowels traversed completely the upper wood piece and were welded to thelower wood piece to a depth of 20 to 25 mm. Ten beams were prepared withthe dowels inserted at 90◦ to the surface of the wood with the speed of dowelrotation at 900 rpm (five beams) or at 1515 rpm (five beams). Five beams wereprepared with the dowels inserted at 45◦ to the surface of the wood with thespeed of dowel rotation at 1515 rpm. The center of each dowel was 2.8 cmfrom the centers of the preceding and the following dowels.

The welding conditions used were those already optimized for single and double-block samples [1–3]: commercial beech wood fluted groove dowels, dried to 2%moisture content, 10 mm in diameter and 80 mm in length, were inserted usinga high-speed fixed-base, manually operated drill (Syderic SH 32, Syderic-Vernier-Cato, France) in pieces of beech wood with predrilled holes 8 mm and 7 mm indiameter. The dowel was welded to the surfaces of the hole in the substrate to forma bonded joint by a fast rotational movement at 1515 rpm. When fusion and bondingwere achieved (occurring in 2–3 s) the rotation of the dowel was stopped. A handclamp or a mechanical clamp was used to hold the two pieces of wood together toachieve maximum strength.

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All the beams so prepared were aged for 7 days before testing, mainly to assurethat the adhesive-bonded beams were cured completely. The beams were then testedin shear according to EN 1380.

2.3. Assembly and testing of 2-m beams

The 2-m-long beech wood beams were doweled at a 30◦ insertion angle with 5610-mm-diameter beech wood dowels, spaced along the beam at 2-cm intervals(dowel edge to dowel edge) with the central 30 cm of each beam left withoutdoweling (Fig. 6a), and compared to 2-m-long beams joined by the same number of30◦ insertion dowels bonded with PVAc. 2-m-long spruce beams were also preparedunder the same conditions. For comparative purposes double-nailed beech andspruce beams were also prepared and tested. To distinguish angled insertion of thedowels from the perpendicular insertion, the former has been named skew-welded.The beams, their separate component planks before assembly and the two plankscomposing each beam assembled but not joined, before doweling, were tested forYoung’s modulus by vibrational methods.

2.4. Equivalence between dowels and metal nails

Comparative studies with spruce wood have shown that when a dowel is insertedat 45◦ and bonded, it can resist a force of approximately 0.3 kN, with a stiffnessof 2 kN/mm. Eurocode 5 [6] defines the stiffness for metal nails according to thefollowing equation:

Ks = ρ1.5m × d/23,

where Ks – stiffness modulus (N/mm), ρm – wood density (kg/m3), d – nail diameter(3.1 mm).

Taking the dimensions of a standard nail as 3.1×70 mm, and the density of sprucewood as 380 kg/m3, one can calculate its stiffness as:

Ks = 3801.5 × 3.1

23, Ks = 1 kN/mm.

This indicates that two 3.1 × 70 mm nails are necessary to obtain the same stiffness(2 kN/mm) as can be obtained with one dowel. Experimental tests with nails, notreported here, confirm this value of a nail’s stiffness given in Eurocode 5 [6]. Thenail can resist a shear force of 0.040 kN.

2.5. Measurement of Young’s modulus

The longitudinal Young’s modulus was determined by a vibrational method [7]for each piece of timber used to make the 2-m beams. The beams, their separatecomponent planks before assembly and the two planks composing each beamassembled but not joined, before doweling, were tested for Young’s modulus bysuch a vibrational method. This method consists in imparting a controlled shock

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to the piece of timber to be tested. The propagation of a shock wave in the timberinduces a particular type of vibration of the wood piece. The vibration frequencyof the wood piece is related to the longitudinal elastic modulus according to thefollowing expression [7]:

fi = i2π

2L2

√EI

m,

where: f – vibration frequency (Hz), i – number of vibration modes, here i = 1,E – longitudinal Young’s modulus (MPa), I – area moment of inertia (mm4), m –linear mass of the beam (kg/m), L – beam length (m).

Three types of 2-m-long beams were prepared: double-row nailed beams, single-row welded-dowel beams, and single-row glued-dowel beams. Each of the threetypes of beam was prepared using two pieces of spruce and then using two piecesof beech. All beams were prepared using fluted groove beech dowels. The finishedbeams (nailed, glued-dowel, and welded-dowel) were tested in four-point bendingat a test rate of 20 s/mm on an Instron 4206 universal testing machine.

3. RESULTS AND DISCUSSION

Previous studies have shown that beech wood pieces can be assembled two by two toform a joint by just welding a dowel through them without any adhesive [2, 8–10].Good results were obtained by maintaining the two pieces of substrate tightly joinedtogether using a clamp [8]. The details of this have been discussed elsewhere[2, 8]. Clamping while inserting the dowel increased considerably the average jointstrength of the two blocks to almost 3900 N [8]. This is a characteristic value thatcan be consistently obtained by dowel welding with a beech substrate. Clampingwhile inserting the dowel was thus the technique used to assemble the beams in thestudy described in this paper.

A steel nail assembly subjected to a force first shows a linear reversible defor-mation. The second part of the deformation is irreversible and corresponds to theplastic behavior of the steel nail. The curve then continues to the assembly breakpoint. In Eurocode 5 [6, 11], the assembly deformation is called service stiffness.The stiffer the assembly, the greater the breaking force necessary, and the lower theplastic deformation. Nailed assemblies are rather stiff in relation to other types ofassemblies.

Glued and nailed joints, in general, subject the bondline or the nail to tensionforces, mainly but also to shear force. The purpose of nail connectors is todistribute a strong force on a large number of small connectors, the nails. InEurocode 5 [6, 11], one admits that the risk = 0 does not exist and one acceptsa probability of failure of 5%, the fifth percentile. This defines that 95% of thetested population is greater than this value and is a characteristic parameter of anassembly under examination. According to European Norm ENV 1991-1 [12], for

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Table 1.Coefficient of variation k(n) as a function of the number of samples (n) for determining thecharacteristic force for the calculation of the 5th percentile, according to ENV 1991-1

n 3 6 12 18 30 Infinite

k(n) 1.89 1.77 1.71 1.7 1.67 1.64

a known coefficient of correlation:

Fk,05 = Fmean − k(n) × σ,

where Fk,05 – characteristic value of F (N), Fmean – average of F (N), k(n) –statistical coefficient of variation (Table 2) [10], n – number of samples, σ –standard deviation (N).

Table 1 shows the coefficient of variation as a function of the number of samplesfor determining the characteristic force for the calculation of the 5th percentile,according to ENV 1991-1 [12].

The first decision needed before testing was whether to use asymmetrical orsymmetrical mountings (Fig. 1). A symmetrical mounting was chosen to equilibratethe moments when determining the performance of dowels in shear. When sheartested in a symmetrical mounting, 6-mm-diameter dowels had a mean maximumforce of Fmean = 1.22 ± 0.07 kN, a stiffness Kmean = 322 + 25 N/mm and a plasticflow moment Mmean = 3434 ± 88 N/mm.

In general, metallic nails are subjected mainly to shear but also to tension stress.Dowels are usually inserted in the wood at a 90◦ angle to its surface. However,other insertion angles also tend to bind together two pieces of timber with a dowel.In principle, three main cases present themselves, as shown in Figs 1–3a,b. In thesecases, the dowel can work in pure shear, or in shear and tension, or in shear andcompression (Fig. 2). Furthermore, in the cases in which the angle of insertion ofthe dowel is lower or higher than 90◦, the relative proportions of shear and tensionwill vary with the angle of insertion of the dowel. Examples of this are shownin Fig. 3c which shows dowels inserted at 90◦ and 45◦ after testing. The dowelinserted at 90◦ is clearly deformed by shear, whereas the dowel inserted at 45◦ ishardly deformed at all, because its insertion angle assures that during testing it issubjected to both shear and tension and, hence, to much lower shear forces.

Table 2 shows the results for dowels glued with PVAc or hammer-inserted atdifferent angles as shown in Fig. 3a and 3b, and with different dowel/hole diameters.This was done to test which assembly yielded the best result. Glued dowels yieldedmuch better results than hammer-inserted dowels, except in the 120◦ case. At 120◦,the dowel works in shear and compression. When the insertion angle is higher than90◦ (Fig. 3a,b), the geometries of the connection and dowel cause the connectedparts to separate. Whereas when the insertion angle is lower than 90◦ (Fig. 3a,b)the geometries of the connection and dowel cause the connected parts to press onone another. The results show that the assemblies yielding best stiffness and highest

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Table 2.Stiffness and breaking force values for the doweled joints of the types shown in Figs 2 and 3, accordingto dowel insertion angle and type of insertion (by hammer or by PVAc gluing)

Average Average Remarksstiffness breaking force(kN/mm) (kN)

Dowel 90◦, glued, 6–6 mma,* 2.06 ± 0.71 1.50 ± 0.41 shear force,dowel plasticization

Dowel 75◦, glued, 6–6 mm* 1.54 ± 1.04 2.07 ± 0.27 tension forceDowel 60◦, glued, 6–6 mm* 1.59 ± 0.36 2.21 ± 0.65 tension forceDowel 90◦, hammer inserted, 6–6 mm 0.37 ± 0.08 0.90 ± 0.07Dowel 90◦, hammer inserted, 6–5.5 mm 0.66 ± 0.09 1.08 ± 0.10Dowel 90◦, hammer inserted, 6–5 mm 0.51 ± 0.07 1.26 ± 0.12Dowel 30◦, glued, 6–6 mm* 1.75 ± 0.14 2.38 ± 0.27 tension forceDowel 45◦, glued, 6–6 mm* 2.14 ± 0.47 3.02 ± 0.31 tension forceDowel 120◦, glued, 6–6 mm* 0.43 ± 0.25 0.66 ± 0.08 compression force

a The first mm figure refers to the dowel diameter and the second refers to the substrate holediameter. Fluted groove beech dowels into beech substrate were used.

* As in Figs 2 and 3.

Figure 4. Typical force–displacement curves for five of the 300-mm beams with dowels insertedat 90◦.

maximum failure force are those with dowels inserted at 45◦ followed by those withdowels inserted at 30◦. In both cases, the dowels work in tension. In these cases, thecurves of force as a function of displacement for all the specimens presented a verysimilar trend. An example of these curves for the case of one type of beam is shownin Fig. 4. In short, in chronological order, the following phases can be distinguished:(1) a first shear zone where the dowel starts to undergo plastic deformation (mode f

in Eurocode 5 [6, 11]); (2) a wood break near the glued part of the dowel; (3) slidingon this same side; and (4) breakage of the dowel in tension. These different phasescan be observed on the test curves (Fig. 4). To conclude this study, the best resultsappear to be at 45◦ and 30◦ where the dowel is subjected simultaneously to shear and

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tension (Table 2 and Fig. 3b). The stiffness of the assembly appears to be due to thebinder, allowing the dowel to be subjected to tension forces, while the resistance tobreakage appears to be due to the dowel itself. The results in Table 2 clearly indicatethat when the dowel is subjected simultaneously to shear and compression the jointgives much lower, unacceptable results. The results in Table 2 indicate which ofthe insertion angles tried should be used for testing the short beams according to arelevant standard.

Short beech beams were prepared by joining them with welded beech fluteddowels and then they were tested according to EN 383 [5] and EN 1380 [4]. Thedowels were inserted at 90◦ and at 45◦ to the wood surface. To distinguish angledinsertion of the dowels from the perpendicular insertion, the former has been namedskew-welded dowels. The average results obtained for these beams are shown inTable 3. Figure 5 shows two beams after testing: Fig. 5a shows the dowels insertedat a 45◦ angle and Fig. 5b shows the dowels inserted at a 90◦ angle. The figuresclearly show the following: (1) 90◦ inserted dowels are only subjected to shearforces, due to the very visible deformation and slippage of the body of the doweland (2) 45◦ inserted dowels show much lower deformation of the dowel as theyare subjected to both shear and tension forces, because shear-induced deformationof the dowel is minimized. Figure 5 also shows that the welded dowel/substrateinterface is very strong. In the 90◦ case, failure is mainly caused by double crackingin the dowels (clearly visible) due to the joints being subjected to shear forces. Inthe 45◦ case, some welding near the interface of the two surfaces is affected, butfailure is at least equally due to dowel elongation. In both the 90◦ and 45◦ cases, thewelded interface has clearly not been damaged.

Table 3.Comparison of 300-mm-long beams of solid wood, glued laminated beams, and beams joined by10 welded dowels at different insertion anglesa

Average Characteristic Slippage at Fracture typestress stress (5%) max force(MPa) (MPa) (mm)

Solid spruce wood control 7.13 6.80 3.46 100% woodSolid beech wood control 7.32 7.05 2.07 50% wood/50% epoxyb

Glulam, UF + 4% Melamine,200 g/m2 7.52 7.22 1.89 60% wood/40% epoxyb

Glulam, UF + 4% Melamine,350 g/m2 6.66 6.31 1.93 60% wood/40% MUF joint

Glulam, PVAc, 350 g/m2 9.52 8.20 3.76 80% wood/20% epoxyb

Double-nailed beech wood 3.75 3.49 5.31

Welded dowels, 90◦, 900 rpm 3.65 3.26 3.05 unbroken-ductileWelded dowels, 90◦, 1515 rpm 3.94 3.92 3.91 unbroken-ductile

Welded dowels, 45◦, 1515 rpm 5.38 5.20 2.00 mixed fracture

a Tested in shear according to EN 1380 and EN 383.b The beams were bonded to the test rig with an epoxy resin.

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(a)

(b)

Figure 5. Sections of 300-mm-long (a) 45◦ and (b) 90◦ welded-dowel beams after testing accordingto EN 1380 and EN 383. Note the frequent double cracking of the dowels in the 90◦ (b) case dueto the force being purely in shear. Note the following in the 45◦ (b) case: (1) the absence of dowelcracks; (2) the small dowel deformation due to the shear component of the force on the joint, and thevery short welding breaks at the dowel/substrate interface this causes; and (3) the slight separation ofthe laminae due to the tension component of the force. Note that in both cases most of the welding isstill intact.

In Table 3, the results for the short beams bonded by welded dowels inserted at90◦ and 45◦ (skew-welded) are compared with solid blocks of some softwoods andhardwoods and to beams of the same dimensions bonded with different types ofadhesives: PVAc and melamine-fortified UF resin. For each average value given,the results in Table 3 show the following:

1. Doweled joints always resist stress equally well, or better, than nailed joints.

2. The 45◦ welded-dowel joints, although still having lower maximum failure stresscompared to PVAc-bonded and UF-bonded glulam (not doweled, just surface tosurface glued) have lower slippage than the PVAc-bonded joints. The PVAcbeams in which PVAc was used to bond the whole interface of the two woodpieces had an average maximum failure stress almost double that of the beamsheld together by 45◦ welded dowels. Conversely, however, the slippage of thePVAc laminated beam is almost double that of the best result for the beams heldtogether by just 10 welded dowels inserted at 45◦. These results document thelow stiffness characteristic of a PVAc bondline. This lower stiffness leads towider stress distribution in PVAc-bonded joints [13] and, hence, to higher valuesof maximum failure force. Similarly, the creep of PVAc joints is worse than injoints bonded with more rigid structural adhesives. The results in Table 3 confirmthis interpretation as beams bonded with the rigid MUF adhesive show a lowervalue of maximum failure strength, but a comparatively much lower slippagethan PVAc-bonded ones. MUF is then a more rigid adhesive; hence, the jointpresents a higher stiffness than PVAc joints. The values of failure force and joint

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slippage shown in Table 3 indicate that the behavior of the welded-dowel jointsresembles more that of the MUF-bonded laminated beam, hence of a hardenedadhesive of greater stiffness.

3. While 45◦ welded-dowel beech wood beams have a lower value of failure forcethan both UF-bonded glulam and solid beech wood, this is still very acceptable,as the standard requires an average value of stress �5.0 MPa. Furthermore, the45◦ welded-dowel beam, the solid beech wood, and the MUF-bonded laminatedbeam show approximately the same slippage. These results indicate that welded-dowel beams can be used for some types of structural applications in place ofnailed or glued joints.

The encouraging results on short beams were confirmed by the average 4-pointbending strength of 2-m-long beams (Tables 4 and 5; Fig. 6). Their longitudinal

Table 4.Results of 2-m-long beams in 4-point bending

Average Characteristic Slippage Stiffnessfailure force failure force (5%) (mm) (kN/mm)(kN) (kN)

SpruceDouble-nailed 3.20 2.70 69 0.04Glued dowels, PVAc, dowels at 30◦ 3.21 3.08 49 0.06Welded dowels, dowels at 30◦ 3.25 3.11 42 0.08

BeechDouble-nailed 7.00 6.62 79 0.08Glued dowels, PVAc, dowels at 30◦ 7.06 6.81 60 0.12Welded dowels, dowels at 30◦ 7.20 6.94 49 0.15

Table 5.Values of longitudinal Young’s modulus for the timber components used to prepare 2-m-long beamsjoined with nails (double nailed), glued-dowels, and welded-dowels (single row of dowels)

Piece 1 Piece 2 Superimposed Assembled(MPa) (MPa) without joining beam

Pieces 1 + 2 (MPa)(MPa)

SpruceDouble-nailed 7 630 11 000 9 016 8 530Glued dowels 8 650 9 700 8 918 9 370Welded dowels – – 10 280

BeechDouble-nailed – – – 8 220Glued dowels 12 590 11 961 11 957 9 022Welded dowels 12 175 12 228 11 962 11 449

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(a)

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Figure 6. The 2-m-long double-row nailed and single-row doweled two-layer beams: (a) schematicrepresentations of the assemblies; the nailed beam is on the left and the doweled beam is on the right;(b) photograph of the spruce beams after preparation.

Young’s modulus was determined by a vibrational method for each piece of timberused to make the 2-m beams [7]. The Young’s modulus values obtained for eachpiece are different. The presence of knots and other defects decreases the valueof the modulus, but notwithstanding their difference, the assembled beams do stillcorrespond to a C18 mechanical class in Eurocode 5 [6, 11].

The 2-m-long welded-dowel beams were also compared to nailed beams (Fig. 6).In recognition of the equivalence described earlier in the Experimental section (twonails are necessary to obtain the same stiffness that can be obtained with one dowel)

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(a)

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Figure 7. Force as a function of displacement during the 4-point bending test on the following:(a) two-layer beech 2-m-long beams joined by welded dowels and PVAc-glued dowels, with the flutedgroove dowels inserted at 30◦ in both cases; (b) two-layer 2-m-long spruce beams joined by nails andby PVAc-glued dowels, with both nails and fluted groove dowels inserted at 90◦.

the comparison shown here is between single-row doweled beams and double-row nailed beams. The nailed beam had 112 nails. The doweled beam had only56 welded dowels. The curves in Fig. 7 clearly indicate that the welded-dowel beamperformed better than the PVAc-bonded-dowel beam, both in terms of maximumfailure force and stiffness of the joint (illustrated by a steeper slope of the curve inFig. 7a, which means higher stiffness, see Table 4). The values of strength andslippage for the same assemblies reported in Table 4 also indicate that the welded-dowel 2-m beams have much lower slippage. Thus, they are stiffer assemblies thanthe other two assembly types tested. Figure 7b shows the curves of force as afunction of displacement for the PVAc-glued-dowel and the nailed spruce beams.The values of force and stiffness are lower than what would be expected with beech,

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(a)

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(c)

Figure 8. Sequence of events during the 4-point bending test on a 2-m-long beam joined by weldeddowels: (a) start of test; (b) maximum bending before break; (c) breaking point.

because spruce is a weaker timber species. However, these curves and the results inTables 3 and 4 clearly indicate the better performance of single-row doweled beamscompared to double-row nailed beams.

The sequence of events of the 4-point bending test of the welded-dowel beechbeam is shown in Fig. 8. Note in Fig. 8b the lack of displacement of the twopieces of timber in relation to each other, indicating a certain stiffness of the joint.Figure 8c shows the same beam as it breaks. Figure 8d shows the broken interface.

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(d)

Figure 8. (d) nature of the break. One can see dowels that have broken in the open gap due to thetension component of the force. The arrows indicate where two of these dowels have broken.

4. CONCLUSIONS

These experiments were conducted to determine if dowel-welding could be used asan alternative joining technique for structural joints. The results obtained show thatwelded-dowel structural joints are capable of both satisfying the relevant standardsand of outperforming equivalent nailed joints and glued-dowel joints.

The angle at which the dowel is inserted in the substrate for welding was shownto have a marked influence on the mechanical performance of the joint. When thedowel is inserted at 90◦ to the substrate, the dowel is only subjected to shear forces.When the dowel is inserted at an angle such as 30◦ or 45◦, the dowel is subjectedto shear plus tension forces, resulting in better joint strength. In both cases, thefailure mode was almost always by dowel fracture. The welded interface wasalways stronger and did not generally break. The short two-layer beams preparedfor standard testing met the European Norms and Eurocode 5 standard requirements

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once the best dowel insertion angle among those tested was used. The 2-m-long,two-layer doweled beams outperformed nailed beams of the same length, eventhough twice as many nails were used as dowels. They also outperformed glued-dowel beams of the same length and conformation. This was true for both maximumbreaking strength and stiffness.

REFERENCES

1. A. Pizzi, J.-M. Leban, H. F. Kanazawa, M. Properzi and F. Pichelin, J. Adhesion Sci. Technol.18, 1263 (2004).

2. C. Ganne-Chedeville, A. Pizzi, A. Thomas, J.-M. Leban, J.-F. Bocquet, A. Despres and H. R.Mansouri, J. Adhesion Sci. Technol. 19, 1157 (2005).

3. H. F. Kanazawa, A. Pizzi, M. Properzi, L. Delmotte and F. Pichelin, J. Adhesion Sci. Technol.19, 1025 (2005).

4. EN 1380, Timber structures, test methods: load-bearing nailed joints (1999).5. EN 383, Determination of assemblies’ local bearing performance (1993).6. Eurocode 5, DD ENV 1995-1-1, Design of timber structures: general rules and rules for building,

European Standardisation Committee, Brussels (1995).7. G. Pissarenko, A. Yakovlev and V. Matveev, in: Elastic Oscillation. chap. 19, pp. 652–719. MIR

Press, Moscow (1975).8. A. Pizzi, A. Despres, H. R. Mansouri, J.-M. Leban and S. Rigolet, J. Adhesion Sci. Technol. 20,

427 (2006).9. M. Boonstra, A. Pizzi, C. Ganne-Chedeville, M. Properzi, J.-M. Leban and F. Pichelin,

J. Adhesion Sci. Technol. 20, 359 (2006).10. A. Pizzi, J. Adhesion Sci. Technol. 20, 829 (2006).11. Eurocode 5, EN 1995-2, Conception and design of timber structures (2005).12. ENV 1991-1, annex D, Test assisted conception (1994).13. A. J. Kinloch, Adhesion and Adhesives. Chapman and Hall, London (1987).

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Documents

Wood joints and laminated wood beams assembled by mechanically-welded wood dowels