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Resin-injected dowel joints in glulam and structural timber composites
Construction and Building Materials 15 Ž2001. 157᎐167
Resin-injected dowel joints in glulam and structural timber composites Tim J. DavisU , Peter A. Claisse School of The Built En¨ ironment, Co¨ entry Uni¨ ersity, Priory Street, Co¨ entry CV1-5FB, UK Received 1 February 2000; accepted 27 January 2001
Abstract Structural timber composites offer higher strength and larger sections than solid timber. They are being more widely used in the UK construction industry in more highly stressed timber components and structures. In this programme of research epoxy resin-injected mild-tensile steel dowel joints have been tested in solid timber, glulam and two commercially available structural timber composites, MicrolamTM and ParallamTM. The results suggest that the resin-injection dowel joint is difficult to fabricate and inappropriate for use in Parallam. 䊚 2001 Elsevier Science Ltd. All rights reserved. Keywords: Timber; Dowels; Composites
1. Introduction The UK imports a significant proportion of its construction timber and the availability of suitable highquality construction timber is leading the production of reconstituted wood products that use existing forest resources more efficiently w1x. Existing products manufactured outside the UK include small pieces of wood bonded in a formaldehyde-based resin, known as parallel strand lumber ŽPSL. and, more commonly, thin plies bonded into a laminate-laminated veneer lumber ŽLVL.. These composite materials offer reduced variability and the removal of strength-reducing defects such as knots that produce a higher strength and stiffness for the material w2,3x. A possible cause for concern is the fact that the reconstitution of the wood in some cases may lead to internal voids that will cause stress concentrations. This may lead to increased deformations within the highly stressed areas of a mechanically fastened joint.
2. Research significance
This paper presents results that show the comparative performance of glued laminated timber Žglulam. and two structural timber composites utilising a resininjected doweled connection. The work shows the relative strength and stiffness of the reconstituted wood materials when used with this jointing system. The application of this jointing system and the relative merits of the composites are discussed. The European Timber design code, EC5 w4x, is currently a draft for development within the UK. It requires characteristic material data in order to facilitate timber design. There is currently a lack of available data for the design of joints, particularly with structural timber composites.
3. Literature review U
Corresponding author. Tel.: q44-24-7688-8485; fax: q44-247688-8435. E-mail address: [email protected] ŽT.J. Davis..
A review of the use of structural timber composites,
0950-0618r01r$ - see front matter 䊚 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 1 . 0 0 0 0 2 - 2
158
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
generic doweled joints and timber design w5x highlighted the following: 䢇
䢇
䢇
The reconstitution of wood gives structural timber composites improved mechanical properties relative to solid timber from the same species. The embedment strength of a dowel-type connector is related to the density of the base wood material. Timber design standards, such as EC5 require characteristic test data to facilitate joint design.
Nails, bolts and dowels are all examples of mechanical fasteners that form timber joints through a laterally loaded dowel action. Bolted connections are fabricated with holes in the timber members that are typically 1 or 2 mm larger than the bolt diameter. Transfer of load across the joint is consequently only achieved after an initial slip of the joint that brings the bolt into bearing contact with the wood. The use of a dowel joint, where there is an interference fit between the dowel and wood, eliminates this initial slip. In bolted and doweled joints initial loading causes ‘bedding-in’ as the relatively high contact stresses cause localised crushing of the cut wood surface. Initial slip and bedding-in both lead to relatively large, variable, initial displacements of the joint when used in timber structures. During subsequent loading the shank of the connector is likely to bend, depending on the geometry of the joint w6,7x and cause increased stresses in the wood that reduce the strength and stiffness of the joint. The purpose of injecting a resin into the gap between the connector and the wood is to improve the performance of bolted and doweled joints by: 䢇 䢇 䢇
facilitating full load transfer without initial slip; remove the ‘bedding-in’ displacement; and increase the friction at the dowel-wood interface so that the stress in the wood is not increased so much by the deformation of the dowel.
The application of resin-injection, utilising polyester and epoxy, with standard bolts has been investigated w8,9x. Techniques for injection of the resin have been developed and the application of the embedment strength and wood density principles has been successfully applied to the structural response of the joint w10x in softwood and hardwood species of timber. Comparisons between resin-injected and plain dowels showed significant improvement in load᎐slip performance. The authors found no reports of the testing of resininjected doweled joints in structural timber composites.
comparison of the jointing system. Solid timber samples were machined from commercially available European WhitewoodrRedwood. Selected, conditioned pieces of this wood were used to manufacture the glulam samples using a resorcinol-formaldehyde resin. The LVL and PSL structural wood composites were both products of the American company TrusJoist MacMillan w11x, ParallamTM PSL Grade 2.0E w12x and MicrolamTM LVL Grade 2.1E w13x. The epoxy resin adopted for this investigation was the Spabond 120 TM system manufactured by SP Systems Ltd. w14x. This resin is commercially used in glass-fibre composite construction. The availability of supporting technical literature enabled the production of a range of resin strength and viscosity for the production of the test connection. A total of 24 samples employing the test connection were formed using 12-mm diameter mild steel bars in a 14-mm diameter hole in the wood sample. Attempts to standardise the drilling rate and the condition of the drilled wood surface were unsuccessful due to the presence of the resin in the composites, particularly the Parallam. The hole was deliberately made oversize in order to facilitate complete encasement of the dowel with resin. The dowels were chemically degreased and lightly sanded with emery paper prior to use. The resin-injection technique involved sealing one end of the joint with a paper washer and mastic sealant Žwhich also kept the dowel centrally located.. The epoxy resin was then injected at the open end from one side of the hole only. This allowed the resin to fill the void between the wood and dowel without the formation of air pockets. The general success of this system was confirmed by the visual inspection of the resin after testing. The testing programme utilised in this research has been fully described previously w5,15x. All joints were in accordance with BS EN 26891 w16x in order to give a load-deformation response. This standard involves a multi-stage loading regime that represents both the initial loading and subsequent in-service loading of the joint. The positioning of the LVDTs used to monitor displacement of the joint is shown in Fig. 1.
5. Results of the testing programme A typical load᎐slip response resulting from the test and the identification of the derived characteristics, are shown in Fig. 2. The following parameters were obtained from the load-slip response: 䢇
4. Laboratory testing programme 䢇
Four base wood materials were employed to enable a
Fmax : the maximum load in kN achieved by the joint and the corresponding slip in mm; f h : the embedment strength in Nrmm2 , defined as Fmaxrprojected area of the fastener;
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
159
Fig. 1. Loading rig and LVDT positions for recording movement of the test joint.
䢇
䢇
K i : the initial stiffness of the joint in kNrmm, determined from a linear regression analysis of the load slip response after any initial slip and 0.4 Fest ; and K s : the stiffness of the joint in kNrmm, determined from a linear regression analysis of the load᎐slip response during the reloading stage 0.1 to 0.4 Fest .
The initial stiffness for the joint, representing ‘bedding-in’ of the connection following joint fabrication, is typically 60᎐70% of the reload Žworking. stiffness of the joint and is used to determine non-recoverable deformation of the structure. Although this behaviour is more characteristic of bolted connections, the loading regime was employed in this programme to ensure consistency and to facilitate a comparison of the struc-
tural response of the connections. The embedment strength is calculated according to the relationship: fh s
Fmax dt
where d is the diameter of the dowel Ž12 mm. and t is the thickness of the test sample Ž44 " 1 mm.. The use of ds 12 mm reflects the fact that although the resin distributes the load from the dowel by increasing the bearing area it has properties more similar to the wood than the steel. Inspection of the failed joints showed extensive cracking of the resin where the dowel had yielded which supports this suggested mode of action. The embedment strength and stiffness results are
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
160
Fig. 2. Typical load᎐slip responce of joint test and identification of calculated test parameters.
Table 1 Summary statistics for resin-injected doweled joints in solid timber a Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
Slip at maximum lad Žnm.
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
S.D. 1 S.D. 2 S.D. 3 S.D. 4 S.D. 5 S.D. 6
36.7 31.3 29.3 23.5 NrA 38.4
38.4 49.8 39.4 34.3 39.4 32.7
24.2 28.7 33.4 31.5 32.7 30.3
3.10 2.42 2.93 3.81 3.37 2.60
45.9 54.3 63.3 59.7 61.9 57.5
500 490 560 550 530 550
Shear Shear Shear Shear Shear Shear
Ave. CoV
31.9 0.19
39.0 0.15
30.11 0.11
3.04 0.17
57.1 0.11
530 0.05
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
Shear Shear Shear Shear Shear Shear
a
Moisture content at test s 8 " 2%.
Table 2 Summary statistics for resin-injected doweled joints in glulama Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
G.D. 1 G.D. 2 G.D. 3 G.D. 4 G.D. 5 G.D. 6
17.9 29.1 32.0 22.0 19.1 14.5
24.9 34.0 28.9 23.8 23.1 22.0
19.5 23.2 24.0 24.9 22.9 21.5
3.01 3.05 3.78 3.58 3.99 3.88
36.9 44.0 45.4 47.1 43.3 40.8
420 450 450 470 450 440
Ave. CoV
22.4 0.30
26.1 0.17
22.7 0.08
3.55 0.12
42.9 0.08
450 0.03
a
Moisture content at test s 10 " 2%.
Slip at maximum lad Žnm.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
161
Table 3 Summary statistics for resin-injected doweled joints in microlama Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
M.D. 1 M.D. 2 M.D. 3 M.D. 4 M.D. 5 M.D. 6
30.4 29.0 14.8 14.1 19.7 15.0
39.3 30.7 26.3 27.2 29.4 30.6
28.5 32.4 28.9 31.7 28.4 28.1
3.95 3.69 4.62 5.48 4.72 4.94
Ave. CoV
20.54 0.36
30.6 0.15
29.7 0.06
4.57 0.14
a
Slip at maximum lad Žnm.
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
55.2 62.8 56.1 61.4 55.0 53.2
690 700 700 710 710 700
Shear Splitting Splitting Combined Splitting Shear
57.3 0.07
700 0.01
Moisture content at test s 8 " 1%
Table 4 Summary statistics for resin-injected doweled joints in parallama Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
P.D. 1 P.D. 2 P.D. 3 P.D. 4 P.D. 5 P.D. 6
29.1 30.0 24.4 23.7 22.3 18.8
44.2 42.9 36.4 34.1 36.3 35.3
31.6 36.9 34.0 32.6 30.6 30.9
4.72 6.89 5.19 4.29 4.56 5.20
Ave. CoV
24.7 0.17
38.2 0.11
32.8 0.07
5.14 0.18
a
Slip at maximum lad Žnm.
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
59.9 69.9 64.4 61.8 57.9 58.6
710 710 720 710 700 710
Shear Combined Combined Combined Combined Splitting
62.1 0.07
710 0.01
Moisture content at test s 8 " 1%.
Fig. 3. Embedment strength results Žcolumns represent test samples in sequential order..
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T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
Fig. 4. Joint stiffness results, initial stiffness k i and k s .
summarised in Figs. 3 and 4, respectively. All of the results are shown in order to indicate the spread of the data. Other test results, including modes of failure are listed in Tables 1᎐4 for each of the base wood materials.
The embedment strengths are very consistent within a material group ŽFig. 3. with low coefficients of variation for all samples. The solid timber samples ŽTable 1. exhibited a slightly higher variation in density and embedment strength than the other materials.
Fig. 5. Embedment strength vs. wood density.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
The initial joint stiffness values are more variable but the Žreload. stiffness results are quite consistent ŽFig. 4., generally half as variable apart from the solid timber samples. It was expected that there would be no difference between initial and reload joint stiffness values since the resin was to take up any lack of fit in the oversize holes and remove the ‘bedding-in’ action. This difference is likely to be due to the approximation of the non-linear response of the joints during initial loading that was used to calculate an initial joint stiffness value. This effect can be clearly seen in the load᎐slip curves where the reload of the joint produced a more distinct linear response. Fig. 5 shows the embedment strength of all the samples plotted against their density. These results follow a linear trend consistent with the embedmentdensity relationships given in EC5 for bolts in solid timber: f h ,0,k s 0.082 Ž 1 y 0.01d . k giving f h s 0.072
163
where, f h ,0 ,k is the characteristic embedment strength of the connector when loaded parallel to the grain Ž f h is the average strength., d-is the diameter of the connector and k is the characteristic wood density Ž is the average density.. No comparable equation for the structural timber composites is offered due to the low number of samples not being statistically significant. Similarly, average rather than characteristic values Žbased on a 5% failure criterion. for the joint properties are reported here. This equation does not take into account the presence of the resin but is shown here for the results to be placed in context of similar bolted connections. A comparison of the solid timber and glulam results suggests that the location of the connector on the glueline had no apparent detrimental effect on either joint strength or stiffness. However, the structural wood composites did not perform as well as expected given their relative high density. This is due to problems encountered during joint fabrication. The Microlam
Fig. 6. Load᎐slip graphs for resin-injected doweled joints in solid timber.
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T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
samples were the first to be manufactured and the resin that was used was found to be too viscous. This made it very difficult to ensure complete encasement of the dowel using the chosen fabrication system. The subsequent solid timber and glulam samples were fabricated with resins of a lower viscosity and the resin injection system proved successful. The Parallam samples also proved difficult to form. Parallam contains internal voids that result during its manufacture. The presence of these voids was observed during fabrication of the joint and their extent confirmed by the extra of resin amount Žin some cases 2᎐3 times the average. needed to form the joint due to the presence of internal, inter-connected voids. Until the resin had started to set, it was observed to continue to flow away from the connector. The load᎐slip response of the Microlam and Parallam samples confirms that the resin-injection was unsuccessful in some of the samples.
6. Modes of failure and load–slip responses All of the solid timber and glulam samples exhibited a block shear mode of failure within the wood material. The composite materials exhibited a wider range of failure modes. Some failed in block shear and some by a splitting mode of failure, local crushing of the wood fibres at the bearing interface followed by transverse Žperpendicular to the grain. tension failure. However, the majority exhibited a combined shearrsplitting failure. All of the dowels underwent extensive plastic deformation during failure of the joint. The load᎐slip graphs for the tests are shown in Figs. 6᎐9. On initial loading the joints all exhibited an approximately linear response. In the solid timber samples ŽFig. 6., reloading highlighted some non-recoverable ‘bedding-in’ deformation that was not expected with resin-injection. The cause of this anomaly is not
Fig. 7. Load᎐slip graphs for resin-injected doweled joints in glulam.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
165
Fig. 8. Load᎐slip graphs for resin-injected doweled joints in microlam.
known. The majority of the glulam samples exhibited the expected response ŽFig. 7., within working stress levels the joints gave a linear load᎐slip response but on loading beyond 0.4 Fest , the response was non-linear upto the maximum load, which occurred at a slip of 3᎐4 mm. The failure of the resin-injection in the Microlam samples is clearly evident in the load᎐slip graphs ŽFig. 8.. Samples 1 and 2 showed negligible difference between initial loading and reloading as expected but in the other samples the initial loading produced a joint slip of approximately 1 mm. This is the amount that would be expected if the connection had been just a standard bolt where initial loading would cause the connector Žlocated at the centre of the hole. to move the 1 mm into contact with the wood. A similar load᎐slip response is exhibited by the Parallam samples ŽFig. 9., again indicating failure of the resin-injection.
7. Discussion
The embedment strength-density plot for solid timber and glulam ŽFig. 5. suggests that the resin-injected doweled joint does give enhanced performance over a bolted connection of the same diameter. No relationship is presented here because a much larger investigation would need to be performed in order to give a reliable correlation. In glulam, the positioning of a dowel connection on the glueline appears to have no detrimental effect on the performance of the joint. The structural timber composites did not achieve significantly higher embedment strengths or stiffness values, even though they have a significantly higher density than the solid timber and glulam samples. The test results for the structural timber composites
166
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
highlight the difficulty in achieving a reliable yet simple resin-injection method. In the Microlam samples, the higher viscosity of the resin led to a failure to achieve complete encasement of the connector, which in turn eliminated any advantage of the system. The presence of internal voids in Parallam produced a similar failure of the resin-injection. The inability to seal these voids suggests that any resin-injection system is unlikely to be successful. This behaviour is contrary to the expected response that their performance in flexure would suggest and needs further investigation since the embedment strength of wood materials that is used in the design of timber connections to EC5 is largely dependent on wood density alone.
8. Conclusions Resin-injected dowel joints in Parallam appear to be unsuccessful due to the presence of inter-connected
internal voids that allow the resin to flow away from the dowel surface where it is needed. This behaviour is likely to be evident in other forms of parallel strand lumber composites and thus makes the use of a resininjection system problematic in these materials. The presence of these internal voids would not be taken into account under current EC5 design rules for timber connections where the wood density alone is used to give the embedment strength of the connection. In this investigation the Microlam samples also performed below expectations. This is a result of the fabrication method adopted rather than the composite itself. Microlam does not contain internal voids and should behave in a similar manner to the solid timber and glulam materials and give a similar embedment strength-density relationship. Resin-injection is a relatively complicated fabrication method although it does enhance the structural performance of a connection, not least by eliminating initial joint slip due to ‘bedding-in’ of the connector and by
Fig. 9. Load᎐slip graphs for resin-injected doweled joints in parallam.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
potentially offering enhanced strength. The widespread use of resin-injection in timber connections is unlikely to be justified given the difficulties in fabrication and hence cost.
Acknowledgements The authors gratefully acknowledge the support of the EPSRC and its staff in their support of this project. References w1x Moody, R. and Ritter, M. Structural wood products. Proc First Materials Eng Congress Pt. 1, ASCE, Boston, MA, USA, 1990: 41᎐52. w2x Meyer, C.B. Structural wood composites in bridge construction. Proc First Materials Eng. Congress Pt. 1, ASCE, Boston, MA, USA, 1990: 413᎐422. w3x Milner MW, Bainbridge RJ. New opportunities for timber engineering. Struct Eng 1997;75Ž16.:278᎐282. w4x DD ENV 1995-1-1: 1994. Eurocode 5-Design of timber structures: Part 1.1: General rules and rules for buildings. British Standards Institution, London, 1994. w5x Davis, T.J., Claisse, P.A. Bolted connections in glulam and structural timber. Construct. Build. Mater. 2000;14:407᎐417. w6x Trayer, G.W. 1932. The bearing strength of wood under bolts. Technical Bulletin 332, US Department of Agriculture, Washington, D.C., USA.
167
w7x Johansen KW. Theory of timber connections. Zurich, Switzerland: International Association for Bridge and Structural Engineering Publications, 1949:249᎐262. w8x Riberholt, H. 1986. Glued bolts in glulam. Report No. R210, Tech. Uni. of Denmark, Lyngby, Denmark. w9x Rodd PD, Hilson BO, Spriggs RA. Resin injected mechanically fastened timber joints. Proc 2nd Pacific Timber Eng Conf, Auckland, N.Z. 1989;1989:350᎐357. w10x Rodd PD, Spriggs RA, Hilson BO. Prediction of embedment characteristics for laterally loaded resin injected bolts in timber.. Proc Int Timber Eng Conf, London, U.K. 1991;1991: 350᎐357. w11x Trus Joist MacMillan, 4225 Kincaid Street, Burnaby, British Columbia, Canada V5G 4P5. w12x Parallam PSL ŽParallel Strand Lumber.. British Board of Agre´ ment ŽBBA. certificate no. 92r2813 Žsecond issue 1996., BBA, Watford, UK, 1996. w13x Microlam LVL ŽLaminated Veneer Lumber.. British Board of ŽBBA. certificate no. 94r3040 Žsecond issue 1994., Agrement ´ BBA, Watford, UK, 1994. w14x Structural Polymer Systems Limited, St. Cross Business Park, Newport, Isle of Wight, England PO30 5WU w15x Claisse PA, Davis TJ. High performance jointing systems for timber. Construct Build Mater 1998;12:415᎐425. w16x BS EN 26891: 1991. Timber Structures-Joints made with mechanical fasteners-General principles for the determination of strength and deformation characteristics. British Standards Institution, London, 1991.
Resin-injected dowel joints in glulam and structural timber composites Tim J. DavisU , Peter A. Claisse School of The Built En¨ ironment, Co¨ entry Uni¨ ersity, Priory Street, Co¨ entry CV1-5FB, UK Received 1 February 2000; accepted 27 January 2001
Abstract Structural timber composites offer higher strength and larger sections than solid timber. They are being more widely used in the UK construction industry in more highly stressed timber components and structures. In this programme of research epoxy resin-injected mild-tensile steel dowel joints have been tested in solid timber, glulam and two commercially available structural timber composites, MicrolamTM and ParallamTM. The results suggest that the resin-injection dowel joint is difficult to fabricate and inappropriate for use in Parallam. 䊚 2001 Elsevier Science Ltd. All rights reserved. Keywords: Timber; Dowels; Composites
1. Introduction The UK imports a significant proportion of its construction timber and the availability of suitable highquality construction timber is leading the production of reconstituted wood products that use existing forest resources more efficiently w1x. Existing products manufactured outside the UK include small pieces of wood bonded in a formaldehyde-based resin, known as parallel strand lumber ŽPSL. and, more commonly, thin plies bonded into a laminate-laminated veneer lumber ŽLVL.. These composite materials offer reduced variability and the removal of strength-reducing defects such as knots that produce a higher strength and stiffness for the material w2,3x. A possible cause for concern is the fact that the reconstitution of the wood in some cases may lead to internal voids that will cause stress concentrations. This may lead to increased deformations within the highly stressed areas of a mechanically fastened joint.
2. Research significance
This paper presents results that show the comparative performance of glued laminated timber Žglulam. and two structural timber composites utilising a resininjected doweled connection. The work shows the relative strength and stiffness of the reconstituted wood materials when used with this jointing system. The application of this jointing system and the relative merits of the composites are discussed. The European Timber design code, EC5 w4x, is currently a draft for development within the UK. It requires characteristic material data in order to facilitate timber design. There is currently a lack of available data for the design of joints, particularly with structural timber composites.
3. Literature review U
Corresponding author. Tel.: q44-24-7688-8485; fax: q44-247688-8435. E-mail address: [email protected] ŽT.J. Davis..
A review of the use of structural timber composites,
0950-0618r01r$ - see front matter 䊚 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 1 . 0 0 0 0 2 - 2
158
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
generic doweled joints and timber design w5x highlighted the following: 䢇
䢇
䢇
The reconstitution of wood gives structural timber composites improved mechanical properties relative to solid timber from the same species. The embedment strength of a dowel-type connector is related to the density of the base wood material. Timber design standards, such as EC5 require characteristic test data to facilitate joint design.
Nails, bolts and dowels are all examples of mechanical fasteners that form timber joints through a laterally loaded dowel action. Bolted connections are fabricated with holes in the timber members that are typically 1 or 2 mm larger than the bolt diameter. Transfer of load across the joint is consequently only achieved after an initial slip of the joint that brings the bolt into bearing contact with the wood. The use of a dowel joint, where there is an interference fit between the dowel and wood, eliminates this initial slip. In bolted and doweled joints initial loading causes ‘bedding-in’ as the relatively high contact stresses cause localised crushing of the cut wood surface. Initial slip and bedding-in both lead to relatively large, variable, initial displacements of the joint when used in timber structures. During subsequent loading the shank of the connector is likely to bend, depending on the geometry of the joint w6,7x and cause increased stresses in the wood that reduce the strength and stiffness of the joint. The purpose of injecting a resin into the gap between the connector and the wood is to improve the performance of bolted and doweled joints by: 䢇 䢇 䢇
facilitating full load transfer without initial slip; remove the ‘bedding-in’ displacement; and increase the friction at the dowel-wood interface so that the stress in the wood is not increased so much by the deformation of the dowel.
The application of resin-injection, utilising polyester and epoxy, with standard bolts has been investigated w8,9x. Techniques for injection of the resin have been developed and the application of the embedment strength and wood density principles has been successfully applied to the structural response of the joint w10x in softwood and hardwood species of timber. Comparisons between resin-injected and plain dowels showed significant improvement in load᎐slip performance. The authors found no reports of the testing of resininjected doweled joints in structural timber composites.
comparison of the jointing system. Solid timber samples were machined from commercially available European WhitewoodrRedwood. Selected, conditioned pieces of this wood were used to manufacture the glulam samples using a resorcinol-formaldehyde resin. The LVL and PSL structural wood composites were both products of the American company TrusJoist MacMillan w11x, ParallamTM PSL Grade 2.0E w12x and MicrolamTM LVL Grade 2.1E w13x. The epoxy resin adopted for this investigation was the Spabond 120 TM system manufactured by SP Systems Ltd. w14x. This resin is commercially used in glass-fibre composite construction. The availability of supporting technical literature enabled the production of a range of resin strength and viscosity for the production of the test connection. A total of 24 samples employing the test connection were formed using 12-mm diameter mild steel bars in a 14-mm diameter hole in the wood sample. Attempts to standardise the drilling rate and the condition of the drilled wood surface were unsuccessful due to the presence of the resin in the composites, particularly the Parallam. The hole was deliberately made oversize in order to facilitate complete encasement of the dowel with resin. The dowels were chemically degreased and lightly sanded with emery paper prior to use. The resin-injection technique involved sealing one end of the joint with a paper washer and mastic sealant Žwhich also kept the dowel centrally located.. The epoxy resin was then injected at the open end from one side of the hole only. This allowed the resin to fill the void between the wood and dowel without the formation of air pockets. The general success of this system was confirmed by the visual inspection of the resin after testing. The testing programme utilised in this research has been fully described previously w5,15x. All joints were in accordance with BS EN 26891 w16x in order to give a load-deformation response. This standard involves a multi-stage loading regime that represents both the initial loading and subsequent in-service loading of the joint. The positioning of the LVDTs used to monitor displacement of the joint is shown in Fig. 1.
5. Results of the testing programme A typical load᎐slip response resulting from the test and the identification of the derived characteristics, are shown in Fig. 2. The following parameters were obtained from the load-slip response: 䢇
4. Laboratory testing programme 䢇
Four base wood materials were employed to enable a
Fmax : the maximum load in kN achieved by the joint and the corresponding slip in mm; f h : the embedment strength in Nrmm2 , defined as Fmaxrprojected area of the fastener;
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
159
Fig. 1. Loading rig and LVDT positions for recording movement of the test joint.
䢇
䢇
K i : the initial stiffness of the joint in kNrmm, determined from a linear regression analysis of the load slip response after any initial slip and 0.4 Fest ; and K s : the stiffness of the joint in kNrmm, determined from a linear regression analysis of the load᎐slip response during the reloading stage 0.1 to 0.4 Fest .
The initial stiffness for the joint, representing ‘bedding-in’ of the connection following joint fabrication, is typically 60᎐70% of the reload Žworking. stiffness of the joint and is used to determine non-recoverable deformation of the structure. Although this behaviour is more characteristic of bolted connections, the loading regime was employed in this programme to ensure consistency and to facilitate a comparison of the struc-
tural response of the connections. The embedment strength is calculated according to the relationship: fh s
Fmax dt
where d is the diameter of the dowel Ž12 mm. and t is the thickness of the test sample Ž44 " 1 mm.. The use of ds 12 mm reflects the fact that although the resin distributes the load from the dowel by increasing the bearing area it has properties more similar to the wood than the steel. Inspection of the failed joints showed extensive cracking of the resin where the dowel had yielded which supports this suggested mode of action. The embedment strength and stiffness results are
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
160
Fig. 2. Typical load᎐slip responce of joint test and identification of calculated test parameters.
Table 1 Summary statistics for resin-injected doweled joints in solid timber a Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
Slip at maximum lad Žnm.
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
S.D. 1 S.D. 2 S.D. 3 S.D. 4 S.D. 5 S.D. 6
36.7 31.3 29.3 23.5 NrA 38.4
38.4 49.8 39.4 34.3 39.4 32.7
24.2 28.7 33.4 31.5 32.7 30.3
3.10 2.42 2.93 3.81 3.37 2.60
45.9 54.3 63.3 59.7 61.9 57.5
500 490 560 550 530 550
Shear Shear Shear Shear Shear Shear
Ave. CoV
31.9 0.19
39.0 0.15
30.11 0.11
3.04 0.17
57.1 0.11
530 0.05
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
Shear Shear Shear Shear Shear Shear
a
Moisture content at test s 8 " 2%.
Table 2 Summary statistics for resin-injected doweled joints in glulama Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
G.D. 1 G.D. 2 G.D. 3 G.D. 4 G.D. 5 G.D. 6
17.9 29.1 32.0 22.0 19.1 14.5
24.9 34.0 28.9 23.8 23.1 22.0
19.5 23.2 24.0 24.9 22.9 21.5
3.01 3.05 3.78 3.58 3.99 3.88
36.9 44.0 45.4 47.1 43.3 40.8
420 450 450 470 450 440
Ave. CoV
22.4 0.30
26.1 0.17
22.7 0.08
3.55 0.12
42.9 0.08
450 0.03
a
Moisture content at test s 10 " 2%.
Slip at maximum lad Žnm.
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Table 3 Summary statistics for resin-injected doweled joints in microlama Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
M.D. 1 M.D. 2 M.D. 3 M.D. 4 M.D. 5 M.D. 6
30.4 29.0 14.8 14.1 19.7 15.0
39.3 30.7 26.3 27.2 29.4 30.6
28.5 32.4 28.9 31.7 28.4 28.1
3.95 3.69 4.62 5.48 4.72 4.94
Ave. CoV
20.54 0.36
30.6 0.15
29.7 0.06
4.57 0.14
a
Slip at maximum lad Žnm.
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
55.2 62.8 56.1 61.4 55.0 53.2
690 700 700 710 710 700
Shear Splitting Splitting Combined Splitting Shear
57.3 0.07
700 0.01
Moisture content at test s 8 " 1%
Table 4 Summary statistics for resin-injected doweled joints in parallama Initial stiffness Ki ŽkNrmm.
Stiffness Ks ŽkNrmm.
Maximum load Fmax ŽkN.
P.D. 1 P.D. 2 P.D. 3 P.D. 4 P.D. 5 P.D. 6
29.1 30.0 24.4 23.7 22.3 18.8
44.2 42.9 36.4 34.1 36.3 35.3
31.6 36.9 34.0 32.6 30.6 30.9
4.72 6.89 5.19 4.29 4.56 5.20
Ave. CoV
24.7 0.17
38.2 0.11
32.8 0.07
5.14 0.18
a
Slip at maximum lad Žnm.
Embedment strength ŽNrmm2 .
Wood density Žkgrm3 .
Failure mode
59.9 69.9 64.4 61.8 57.9 58.6
710 710 720 710 700 710
Shear Combined Combined Combined Combined Splitting
62.1 0.07
710 0.01
Moisture content at test s 8 " 1%.
Fig. 3. Embedment strength results Žcolumns represent test samples in sequential order..
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T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
Fig. 4. Joint stiffness results, initial stiffness k i and k s .
summarised in Figs. 3 and 4, respectively. All of the results are shown in order to indicate the spread of the data. Other test results, including modes of failure are listed in Tables 1᎐4 for each of the base wood materials.
The embedment strengths are very consistent within a material group ŽFig. 3. with low coefficients of variation for all samples. The solid timber samples ŽTable 1. exhibited a slightly higher variation in density and embedment strength than the other materials.
Fig. 5. Embedment strength vs. wood density.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
The initial joint stiffness values are more variable but the Žreload. stiffness results are quite consistent ŽFig. 4., generally half as variable apart from the solid timber samples. It was expected that there would be no difference between initial and reload joint stiffness values since the resin was to take up any lack of fit in the oversize holes and remove the ‘bedding-in’ action. This difference is likely to be due to the approximation of the non-linear response of the joints during initial loading that was used to calculate an initial joint stiffness value. This effect can be clearly seen in the load᎐slip curves where the reload of the joint produced a more distinct linear response. Fig. 5 shows the embedment strength of all the samples plotted against their density. These results follow a linear trend consistent with the embedmentdensity relationships given in EC5 for bolts in solid timber: f h ,0,k s 0.082 Ž 1 y 0.01d . k giving f h s 0.072
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where, f h ,0 ,k is the characteristic embedment strength of the connector when loaded parallel to the grain Ž f h is the average strength., d-is the diameter of the connector and k is the characteristic wood density Ž is the average density.. No comparable equation for the structural timber composites is offered due to the low number of samples not being statistically significant. Similarly, average rather than characteristic values Žbased on a 5% failure criterion. for the joint properties are reported here. This equation does not take into account the presence of the resin but is shown here for the results to be placed in context of similar bolted connections. A comparison of the solid timber and glulam results suggests that the location of the connector on the glueline had no apparent detrimental effect on either joint strength or stiffness. However, the structural wood composites did not perform as well as expected given their relative high density. This is due to problems encountered during joint fabrication. The Microlam
Fig. 6. Load᎐slip graphs for resin-injected doweled joints in solid timber.
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T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
samples were the first to be manufactured and the resin that was used was found to be too viscous. This made it very difficult to ensure complete encasement of the dowel using the chosen fabrication system. The subsequent solid timber and glulam samples were fabricated with resins of a lower viscosity and the resin injection system proved successful. The Parallam samples also proved difficult to form. Parallam contains internal voids that result during its manufacture. The presence of these voids was observed during fabrication of the joint and their extent confirmed by the extra of resin amount Žin some cases 2᎐3 times the average. needed to form the joint due to the presence of internal, inter-connected voids. Until the resin had started to set, it was observed to continue to flow away from the connector. The load᎐slip response of the Microlam and Parallam samples confirms that the resin-injection was unsuccessful in some of the samples.
6. Modes of failure and load–slip responses All of the solid timber and glulam samples exhibited a block shear mode of failure within the wood material. The composite materials exhibited a wider range of failure modes. Some failed in block shear and some by a splitting mode of failure, local crushing of the wood fibres at the bearing interface followed by transverse Žperpendicular to the grain. tension failure. However, the majority exhibited a combined shearrsplitting failure. All of the dowels underwent extensive plastic deformation during failure of the joint. The load᎐slip graphs for the tests are shown in Figs. 6᎐9. On initial loading the joints all exhibited an approximately linear response. In the solid timber samples ŽFig. 6., reloading highlighted some non-recoverable ‘bedding-in’ deformation that was not expected with resin-injection. The cause of this anomaly is not
Fig. 7. Load᎐slip graphs for resin-injected doweled joints in glulam.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
165
Fig. 8. Load᎐slip graphs for resin-injected doweled joints in microlam.
known. The majority of the glulam samples exhibited the expected response ŽFig. 7., within working stress levels the joints gave a linear load᎐slip response but on loading beyond 0.4 Fest , the response was non-linear upto the maximum load, which occurred at a slip of 3᎐4 mm. The failure of the resin-injection in the Microlam samples is clearly evident in the load᎐slip graphs ŽFig. 8.. Samples 1 and 2 showed negligible difference between initial loading and reloading as expected but in the other samples the initial loading produced a joint slip of approximately 1 mm. This is the amount that would be expected if the connection had been just a standard bolt where initial loading would cause the connector Žlocated at the centre of the hole. to move the 1 mm into contact with the wood. A similar load᎐slip response is exhibited by the Parallam samples ŽFig. 9., again indicating failure of the resin-injection.
7. Discussion
The embedment strength-density plot for solid timber and glulam ŽFig. 5. suggests that the resin-injected doweled joint does give enhanced performance over a bolted connection of the same diameter. No relationship is presented here because a much larger investigation would need to be performed in order to give a reliable correlation. In glulam, the positioning of a dowel connection on the glueline appears to have no detrimental effect on the performance of the joint. The structural timber composites did not achieve significantly higher embedment strengths or stiffness values, even though they have a significantly higher density than the solid timber and glulam samples. The test results for the structural timber composites
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T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
highlight the difficulty in achieving a reliable yet simple resin-injection method. In the Microlam samples, the higher viscosity of the resin led to a failure to achieve complete encasement of the connector, which in turn eliminated any advantage of the system. The presence of internal voids in Parallam produced a similar failure of the resin-injection. The inability to seal these voids suggests that any resin-injection system is unlikely to be successful. This behaviour is contrary to the expected response that their performance in flexure would suggest and needs further investigation since the embedment strength of wood materials that is used in the design of timber connections to EC5 is largely dependent on wood density alone.
8. Conclusions Resin-injected dowel joints in Parallam appear to be unsuccessful due to the presence of inter-connected
internal voids that allow the resin to flow away from the dowel surface where it is needed. This behaviour is likely to be evident in other forms of parallel strand lumber composites and thus makes the use of a resininjection system problematic in these materials. The presence of these internal voids would not be taken into account under current EC5 design rules for timber connections where the wood density alone is used to give the embedment strength of the connection. In this investigation the Microlam samples also performed below expectations. This is a result of the fabrication method adopted rather than the composite itself. Microlam does not contain internal voids and should behave in a similar manner to the solid timber and glulam materials and give a similar embedment strength-density relationship. Resin-injection is a relatively complicated fabrication method although it does enhance the structural performance of a connection, not least by eliminating initial joint slip due to ‘bedding-in’ of the connector and by
Fig. 9. Load᎐slip graphs for resin-injected doweled joints in parallam.
T.J. Da¨ is, P.A. Claisse r Construction and Building Materials 15 (2001) 157᎐167
potentially offering enhanced strength. The widespread use of resin-injection in timber connections is unlikely to be justified given the difficulties in fabrication and hence cost.
Acknowledgements The authors gratefully acknowledge the support of the EPSRC and its staff in their support of this project. References w1x Moody, R. and Ritter, M. Structural wood products. Proc First Materials Eng Congress Pt. 1, ASCE, Boston, MA, USA, 1990: 41᎐52. w2x Meyer, C.B. Structural wood composites in bridge construction. Proc First Materials Eng. Congress Pt. 1, ASCE, Boston, MA, USA, 1990: 413᎐422. w3x Milner MW, Bainbridge RJ. New opportunities for timber engineering. Struct Eng 1997;75Ž16.:278᎐282. w4x DD ENV 1995-1-1: 1994. Eurocode 5-Design of timber structures: Part 1.1: General rules and rules for buildings. British Standards Institution, London, 1994. w5x Davis, T.J., Claisse, P.A. Bolted connections in glulam and structural timber. Construct. Build. Mater. 2000;14:407᎐417. w6x Trayer, G.W. 1932. The bearing strength of wood under bolts. Technical Bulletin 332, US Department of Agriculture, Washington, D.C., USA.
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w7x Johansen KW. Theory of timber connections. Zurich, Switzerland: International Association for Bridge and Structural Engineering Publications, 1949:249᎐262. w8x Riberholt, H. 1986. Glued bolts in glulam. Report No. R210, Tech. Uni. of Denmark, Lyngby, Denmark. w9x Rodd PD, Hilson BO, Spriggs RA. Resin injected mechanically fastened timber joints. Proc 2nd Pacific Timber Eng Conf, Auckland, N.Z. 1989;1989:350᎐357. w10x Rodd PD, Spriggs RA, Hilson BO. Prediction of embedment characteristics for laterally loaded resin injected bolts in timber.. Proc Int Timber Eng Conf, London, U.K. 1991;1991: 350᎐357. w11x Trus Joist MacMillan, 4225 Kincaid Street, Burnaby, British Columbia, Canada V5G 4P5. w12x Parallam PSL ŽParallel Strand Lumber.. British Board of Agre´ ment ŽBBA. certificate no. 92r2813 Žsecond issue 1996., BBA, Watford, UK, 1996. w13x Microlam LVL ŽLaminated Veneer Lumber.. British Board of ŽBBA. certificate no. 94r3040 Žsecond issue 1994., Agrement ´ BBA, Watford, UK, 1994. w14x Structural Polymer Systems Limited, St. Cross Business Park, Newport, Isle of Wight, England PO30 5WU w15x Claisse PA, Davis TJ. High performance jointing systems for timber. Construct Build Mater 1998;12:415᎐425. w16x BS EN 26891: 1991. Timber Structures-Joints made with mechanical fasteners-General principles for the determination of strength and deformation characteristics. British Standards Institution, London, 1991.