bolted joints in glulam and structural timber...

Ž .Construction and Building Materials 14 2000 407]417

Bolted joints in glulam and structural timber composites

Tim J. DavisU, Peter A. ClaisseSchool of the Built Enïronment, Coëntry Uni ersity, Coëntry, CV1 5FB, UK

Received 15 November 1999; received in revised form 1 August 2000; accepted 20 August 2000

Abstract

Ž .The widespread adoption of the European design code for timber structures EC5 will facilitate a number of design optionspreviously unsupported by British Standards. This code uses design equations that need characteristic material data, which existsfor solid timber and some sheet materials, but not for the structural timber composites that were evaluated in this research. Inthis programme high-tensile steel black bolts have been used with solid timber, glulam and two commercially available structuraltimber composites } MicrolamTM and ParallamTM. The results suggest that the timber composites offer similar performance tohigh-density timbers in line with EC5 design guidance. Q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Timber; Bolts; Composites

1. Introduction

The UK imports a significant proportion of its con-struction timber but is keen to better utilise its home-grown resources through various industry and govern-ment initiatives. One method that has the potential touse the available resources efficiently is to producereconstituted wood products. Existing commercialproducts include small pieces of wood bonded in aformaldehyde-based resin-known as parallel strand

Ž .lumber PSL , and, more commonly, thin plies bondedŽ .into a laminate-laminated veneer lumber LVL . These

composite materials offer reduced variability and theremoval of strength-reducing defects such as knots. Apossible cause for concern is the fact that the reconsti-tution of the wood may give rise to internal voids whichwill cause stress concentrations and hence increaseddeformations within the highly stressed areas of a me-chanically fastened joint. The results presented in this

U Corresponding author. Tel.: q44-2476-888-485; fax: q44-2476-838-485.

Ž .E-mail address: [email protected] T.J. Davis .

paper are taken from a larger investigation into theperformance of mechanically fastened joints in struc-tural timber composites. As part of this programme theuse of resin-injected dowelled joints has also beenassessed.

2. Research significance

This paper presents results in order to show thecomparative performance of glued laminated timberŽ .glulam and two structural timber composites utilisinga standardised bolted connection. The work shows therelative strength and stiffness of the reconstituted woodmaterials when used with this jointing system. Theapplication and relative merits of the composites arediscussed. The new European design code, EC5, iscurrently a draft for development within the UK. Itrequires characteristic material data in order to facili-tate timber design. There is currently a lack of avail-able data for the design of joints, particularly withstructural timber composites.

0950-0618r00r$ - see front matter Q 2000 Elsevier Science Ltd. All rights reserved.Ž .PII: S 0 9 5 0 - 0 6 1 8 0 0 0 0 0 4 4 - 1

( )T.J. Da¨is, P.A. Claisse r Construction and Building Materials 14 2000 407]417408

Table 1aGrade stresses of structural timber composites, Southern pine and European Whitewood

Property Wood type2Ž .all values in Nrmm Whitewood Southern Parallam Microlam

Ž . Ž . Ž .SS pine SS 2.1 E

bBending parallel to grain 7.5 9.6 16.8 16.2Tension parallel to grain 4.5 5.8 14.8 10.1Compression parallel to grain 7.9 10.2 15.1 14.3

cCompression perpendicular to grain 2.1 2.5 3.6r2.8 4.9r3.0bShear parallel to grain 0.82 0.98 2.2 1.9

Ž .Modulus of elasticity in bending mean 10 500 12 500 12 750 12 400

a Southern pine is the source species for the composites, European Whitewood was used for solid timber and glulam samples in this research.b When loaded as a joist.c Parallelrperpendicular to glue linerwide face of strand.

3. Literature review

3.1. The use of structural timber composites

Structural timber composites were introduced to theUSA in the late 1980s in order to provide high-qualitystructural timber that was proving difficult to obtain

w xfrom the natural forest resource 1 . These materialswere introduced to the UK construction industry in theearly 1990s and offered significantly higher gradestresses than either solid softwood timber or glulam. Ascan be seen in Table 1, the benefits of reconstitutionare an increase in permissible stresses of betweenapproximately 50 and 200% on the original solid tim-ber. This results in the reconstituted products being

Žassigned a strength class of SC7 to BS 5268:1990 based.on bending strength . The modulus of elasticity is

largely unaffected and since it is deflection that usuallygoverns the design of timber beams, the compositesappear to be best utilised in axially loaded structuressuch as trusses. However, for joints in such structures,the design grade is recommended as SC5. This appar-ent restriction on the design of joints in structuralwood composites was one of the main reasons forinitiating this programme of research.

Structural timber composites are currently used inthe UK predominantly to provide the more highly

w xstressed elements in timber-framed buildings 2 al-though their use in the USA has extended into short

w xand medium span highway bridges 3 . Several es-tablished sources for the mechanical properties of solid

w xwood species exist 4,5 . The authors found no pub-lished source for the mechanical properties of thestructural wood composites other than the grade

Ž .stresses in British Board of Agrement BBA certifi-´w xcates 6,7 .

3.2. Timber jointing systems and their design

Several state-of-the-art reviews of mechanically fas-tened jointing systems have been performed, usually as

a result of the introduction of new design codes thatrely on existing research data. The first significantreview was performed in the 1930s during which themode of action of a wide range of timber connectorswas established. The American Society of Civil Engi-

Ž .neers ASCE provided a much needed up-to-date re-w xview 8 which details the design rules, and supporting

research, of timber joint design in the USA, Canadaand UK. New jointing systems that utilise structuraladhesives are being investigated that show enhancedstructural performance over mechanically fastened

w xjoints 9 .UK timber design is currently going through a major

period of change as a result of the introduction of thew xdraft EC5 10 and BS 5268: Part 1, which are both

partial coefficients limit states design codes rather thanthe permissible stress approach used by BS 5268: Part 2w x11 . In anticipation of the introduction of the newcodes the Timber Research and Development Associa-

Ž . w xtion TRADA conducted a review 12 of design prac-tice for timber joints and established research data thatwas needed to support joint design to the new EC5design code. The review highlighted the general lack ofresearch data available, particularly for the new struc-tural timber composites. In support of the introductionof EC5 and promoting the use of timber within Europethe STEPrEUROFORTECH initiative produced a sig-nificant review of timber engineering including jointing

w xsystems 13 . A similar work is available for US designw xstandards 14 .

3.3. Bolted connections in plain timber

ŽNails, bolts and dowels whether utilising an interfer-.ence fit or some form of resin bonding are all exam-

ples of mechanical fasteners that form timber jointsw xthrough a laterally loaded dowel action. Trayer 15

performed an extensive research programme into boltedjoints involving several wood species and joint configu-rations. This work formed the basis of the empiricaldesign data for UK permissible stress design codes.

( )T.J. Da¨is, P.A. Claisse r Construction and Building Materials 14 2000 407]417 409

w xJohansen 16 supplemented Trayer’s work and devel-oped theoretical equations for predicting the yield loadand ultimate load of doweled joints that now form the

w x w xbasis of design in EC5 17 . Whale and Smith 18extended this work by performing an extensive testingprogramme into the load-embedment response ofdoweled joints. They concluded that there was a goodcorrelation between the embedment response and den-sity of wood. Similar experimental techniques have

w xbeen employed on a range of investigations 19]21that have since been integrated into current testing

w xstandards 22 that were adopted for this research pro-gramme. Wilkinson compared design strengths basedon Johansens’ equations with the American design

w xstandard 23 .

4. Laboratory testing programme

The solid timber was commercially available Euro-Žpean WhitewoodrRedwood, visually graded SS C24 to

.EC5 , obtained from a local timber merchant. Selected,conditioned, pieces of this wood were used to manufac-ture the glulam joint samples using a resorcinol-for-maldehyde resin.

The LVL and PSL structural wood composites wereboth products of the American company TrusJoist

w x TMMacMillan 24 -Parallam PSL Grade 2.0E and Mi-crolamTM LVL Grade 2.1E. They are both forms ofreconstituted wood: Parallam is made from strands ofDouglas Fir or Southern Pine timber bonded together

Ž .with a phenol-formaldehyde PF adhesive, while Mi-

Fig. 1. Glulam and the family of structural timber composites produced by TrusJoist MacMillan.


crolam is made from laminated veneers of SouthernPine timber bonded, again bonded with a PF adhesive.The lay-up of these materials is shown in Fig. 1.

The testing programme utilised in this research hasw xbeen fully described previously 21 . In summary, all

wood was conditioned for 3 weeks prior to the test,which was performed in accordance with BS EN 26891in order to give a load-deformation response. Thisinvolves a multi-stage loading regime, the key elementsof which are: initial loading to 40% of the estimated

Ž .maximum load F , approximately the working load,estof the joint; removal of load to 10% of F and finallyestloading to failure.

A total of 26 samples were tested using 12-mmdiameter Grade 8.8 bolts in a 13-mm diameter hole. Six

samples were made from solid timber, glulam andParallam and 8 samples from Microlam. The sampleshad a nominal thickness of 44-mm, which resulted in

Ž .an embedment response a Lrd ratio of 3.67 . Theexperimental arrangement, showing the sample dimen-sions and positioning of LVDTs for this series of tests,is reproduced in Fig. 2. The actual loading rig, with theLVDTs removed for clarity, is shown in Fig. 3.

5. Results of the testing programme

A typical load-slip response resulting from the test,and the identification of the derived characteristics, is

Fig. 2. Loading rig and LVDT positions for recording movement of the test joint.


Fig. 3. Loading rig with LVDTs removed for clarity.


Fig. 4. Typical load-slip response of joint test and identification of calculated test parameters.

shown in Fig. 4. The following parameters were ob-tained from the load-slip response:

v F , the maximum load in kN achieved by themaxjoint, and the corresponding slip in mm;

v f , the embedment strength in Nrmm2, defined ashF rprojected area of the fastener;max

v K , the initial stiffness of the joint in kNrmm,idetermined from a linear regression analysis of theload-slip response after any initial slip and 0.4 F ;estand

v K , the stiffness of the joint in kNrmm, determinedsfrom a linear regression analysis of the load-slipresponse during the reloading stage 0.1 to 0.4 F .est

The initial stiffness for the joint, representing ‘bed-ding-in’ following joint fabrication, is 60]70% of the

Ž .reload working stiffness of the joint, and is used todetermine non-recoverable deformation of the joint.The embedment strength is calculated according to therelationship:

Fmaxf sh dt

Ž .where d is the diameter of the bolt 12 mm , and t isŽ .the thickness of the test sample 44"1 mm .

A summary of the embedment strength and stiffnessresults is shown in Figs. 5 and 6, respectively. On thesegraphs all of the samples are shown in order to indicatethe spread of the data. Other test results, includingmodes of failure are listed in Tables 2]5 for each of

the base wood materials. The embedment strengthswere very consistent with low coefficients of variationfor all but the Parallam samples. The coefficient of

Ž .variation CV of a set of results has been defined asŽ .the ratio of its mean to the standard deviation sny1

of the results. The joint stiffness values, however, aremuch more variable-both within a material group, andbetween groups. The glulam and solid timber resultsare comparable although some solid samples exhibited

Ž .extreme ductility in excess of 10 mm at failure . It canbe concluded that the glueline had no apparent detri-mental effect on joint strength or stiffness, in fact thestiffness, and slip at maximum load was much lessvariable in the glulam. The Microlam samples per-formed consistently better in both strength and, espe-cially, stiffness terms. They also exhibited less slip at

Žmaximum load although it was four times more vari-.able . The Parallam samples were particularly disap-

pointing, exhibiting the widest variations in results, butŽthis is due to the presence of internal voids visible

.during fabrication , the relatively large variation insample density, and the influence of the relatively poorresults of samples P-B-1 and P-B-3 which had a lowerthan average density.

Fig. 7 shows the embedment strength of all thesamples plotted against their density. These resultsfollow a linear trend consistent with the embedment-density relationships given in EC5 for solid timber andplywood.

Ž .For solid timber f s0.082 1y0.01d rh,0,k kgiving f s0.072 rh


Ž .Fig. 5. Embedment strength results columns represent test samples in sequential order .

Ž .and for plywood f s0.11 1y0.01d rh,0,k kgiving f s0.097r.h

No comparable equation for the structural timbercomposites is offered due to the low number of sam-ples not being statistically significant. Similarly, averagerather than characteristic values for the joint propertiesare reported here.

6. Load-slip responses and modes of failure

All but two samples exhibited a splitting mode ofŽ .failure Fig. 8 . This took the form of local crushing of

the wood fibres at the bearing interface followed byŽ .transverse perpendicular to the grain tension failure,

the remaining two samples exhibited a block-shear

Fig. 6. Joint stiffness results, initial stiffness K and stiffness K .i s


Table 2aSummarised results for bolted joints in solid timber

Sample Initial Stiffness Maximum Slip at Embedment Wood Failurenumber stiffness K K load F maximum strength f density modei s max h

2 3Ž . Ž . Ž . Ž . Ž . Ž .kNrmm kNrmm kN load mm Nrmm kgrm

S-B-1 13.0 17.9 18.3 3.08 34.7 490 SplittingS-B-2 11.3 21.7 17.9 2.79 33.9 490 SplittingS-B-3 11.1 20.7 21.0 3.28 39.8 500 SplittingS-B-4 6.41 10.8 19.3 4.61 36.5 510 SplittingS-B-5 11.7 16.6 21.7 3.83 41.0 510 SplittingS-B-6 8.91 22.4 16.8 2.93 31.9 470 Splitting

Ave. 10.4 18.3 19.2 3.42 36.3 490CoV 0.23 0.24 0.10 0.20 0.10 0.03

a Moisture content at test s11"2%.

mode of failure. The high-tensile steel bolts were gen-erally unaffected, confirming the desired embedmentresponse, although the three strongest Parallam sam-ples did cause noticeable permanent deformation. Noneof the glulam samples failed at the glueline, the trans-verse splitting occurred in the adjacent wood.

Typical load-slip graphs for the tests are shown in

Fig. 9. On initial loading the joints exhibited a non-lin-ear, non-recoverable, ‘bedding-in’ response. Withinworking stress levels, i.e. on reloading, the joints gave alinear load-slip response but on loading beyond 0.4Festthe response was non-linear up to the maximum load,which occurred at a slip of 2]4 mm. Some of the solid

Ž .timber samples exhibited extreme ductility Fig. 9a

Table 3aSummarised results for bolted joints in glulam



G-B-1 11.7 20.0 19.9 3.60 37.6 500 SplittingG-B-2 15.5 24.2 20.0 3.59 37.8 510 SplittingG-B-3 18.4 23.0 22.4 2.88 42.4 550 SplittingG-B-4 8.38 22.1 21.4 3.79 40.5 510 SplittingG-B-5 13.6 21.0 19.4 3.20 36.7 500 SplittingG-B-6 12.0 20.5 22.8 3.80 43.2 510 Splitting

Ave. 13.5 22.1 20.6 3.41 39.0 510CoV 0.28 0.08 0.06 0.11 0.06 0.04

a Moisture content at test s12"1%.

Table 4aSummarised results for bolted joints in microlam



M-B-1 19.2 20.7 28.0 3.49 54.2 650 SplittingM-B-2 13.8 20.0 30.5 3.24 59.0 640 SplittingM-B-3 18.4 31.8 26.8 2.08 51.9 650 SplittingM-B-4 9.01 18.6 28.0 4.14 54.3 640 SplittingM-B-5 30.4 36.7 24.8 1.35 48.0 630 SplittingM-B-6 26.1 38.1 25.1 1.35 49.9 690 SplittingM-B-7 24.1 40.8 29.0 1.62 57.5 700 ShearM-B-8 25.6 66.1 26.0 1.96 51.6 660 Splitting

Ave. 18.2 25.5 27.6 2.86 53.5 640Cov 0.44 0.32 0.07 0.39 0.07 0.01

a Moisture content at test 7"1%.


Table 5aSummarised results for bolted joints in parallam



P-B-1 5.28 12.3 21.7 5.09 41 660 SplittingP-B-2 16.9 21.0 38.7 4.29 73 780 SplittingP-B-3 19.2 38.0 21.1 1.84 41 680 SplittingP-B-4 20.0 20.9 36.6 4.64 71 760 SplittingP-B-5 13.0 18.6 32.8 3.77 62 800 SplittingP-B-6 15.7 20.8 42.9 5.26 81 760 Combined

Ave. 14.9 22.2 30.2 3.93 58 740Cov 0.40 0.43 0.28 0.32 0.27 0.09

a Moisture content at test 8"1%.

although the majority behaved in a similar mannerŽ .to the glulam samples Fig. 9b . A number of the

Microlam samples were noticeably brittle in theirŽresponse-failing suddenly at the maximum load Fig.

.9c . Although this behaviour did occur in some of theParallam samples the majority exhibited similar load-

Ž .slip responses Fig. 9d to solid wood and glulam.

7. Discussion

The positioning of a bolted connection on the glue-line appears to have no detrimental effect on theperformance of the joint. The structural timber com-posites give an increased embedment strength relativeto solid timber and glulam. The higher density of thesematerials would suggest a higher strength and this is

confirmed. An actual embedment strength]density re-lationship is not offered but is likely to be similar tothat for bolted joints in plywood. The compositesshowed a wider variation in response than the solidtimber samples, especially given that the actual mate-rial is more homogeneous. This is misleading however,since the natural wood samples used in this experimentwere carefully selected } more so than would be thecase in stress-graded timber for construction.

Microlam gave the best overall performance in termsof strength and stiffness but showed a lower joint slipat maximum load than solid timber and glulam. TheParallam samples were in many ways disappointing.Their results were generally the most variable and gavelower joint stiffness values than their density wouldsuggest. Visual inspection showed that internal voidswere present in the bearing zone of the joint and this is

Fig. 7. Embedment strength plotted against wood density.


Fig. 8. Local crushing and longitudinal splitting failure of a Microlam sample.

the likely explanation for the performance of the sam-ples during testing. This is at odds with Parallam’sperformance in flexure where it offers better strengthand stiffness than Microlam.

8. Conclusions

Bolted connections in structural timber compositesappear to give strength and stiffness properties at leastcomparable to a solid timber of similar density. Thepositioning of a bolted connection on the glueline of aglulam member does not detrimentally affect the joint’sperformance. The internal voids contained within par-

allel strand lumber appear to produce a lower, morevariable, performance in bolted connections than theirperformance in flexure would suggest. This behaviourneeds further investigation since the embedmentstrength of wood materials that is used in the design oftimber connections to EC5 is largely dependent onwood density. The presence of these internal voids, notvisible during fabrication, would not be taken intoaccount under current EC5 design rules.

Acknowledgements

The authors gratefully acknowledge the support ofthe EPSRC and its staff for this project.


Ž . Ž . Ž . Ž .Fig. 9. Typical load-slip graphs for bolted joints in: a solid wood; b glulam; c microlam; and d parallam.

References

w x1 Moody R., Ritter M. Structural wood products. Proceedings ofthe First Materials Engineering Congress Pt. 1, ASCE, Boston,MA, USA,1990: 41]52.

w x2 Milner MW, Bainbridge RJ. New opportunities for timberŽ .engineering. Struct Eng 1997;75 16 :278]282.

w x3 Meyer CB. Structural wood composites in bridge construction.Proceedings of the First Materials Engineering Congress Pt. 1,ASCE, Boston, MA, USA,1990: 413]422.

w x4 Lavers GM. The strength properties of timber. 3rd ed Watford,UK: Building Research Establishment, 1983.

w x5 Bodig J, Jayne BA. Mechanics of wood and wood composites.New York, USA: Van Nostrand Reinhold, 1982:1982.

w x Ž .6 Parallam PSL Parallel Strand Lumber . British Board of Agre-´Ž . Ž .ment BBA certificate no. 92r2813 second issue 1996 , BBA,

Watford, UK, 1996.w x Ž .7 Microlam LVL Laminated Veneer Lumber . British Board of

Ž . Ž .Agrement BBA certificate no. 94r3040 second issue 1994 ,´BBA, Watford, UK, 1994.

w x8 Mechanical connections in wood structures. ASCE Manualsand Reports on Engineering Practice No. 84, ASCE, NewYork, USA, 1996.

w x9 Claisse PA, Davis TJ. High performance jointing systems fortimber. Constr Build Mater 1998;12:415]425.

w x10 DD ENV 1995-1-1: 1994. Eurocode 5-design of timber struc-tures: part 1.1: general rules and rules for buildings. BritishStandards Institution, London, 1994.

w x11 BS 5268: 1991. Structural use of timber: part 2: code of practicefor permissible stress design, materials and workmanship.British Standards Institution, London, 1991.

w x12 Page AV, Mettem CJ. Efficient detailing of mechanical jointsin engineered timber structures. Research Report 2r93, Tim-

ber Research and Development Association, High Wycombe,UK, 1993.

w x13 Timber Engineering STEP 1. Centrum Hout, The Netherlands,1995.

w x14 Wood handbook: wood as an engineering material. ForestProducts Laboratory, US Department of Agriculture, Washing-ton, DC, USA, 1987.

w x15 Trayer GW. The bearing strength of wood under bolts. Techni-cal Bulletin 332, US Department of Agriculture, Washington,DC, USA, 1932.

w x16 Johansen KW. Theory of timber connections, vol. 9. Zurich,Switzerland: Int. Assoc. for Bridge and Structural Eng. Publica-tions, 1949:249]262.

w x17 Hilson BO, Whale LRJ. Developments in the design of timberŽ .joints. Struct Eng 1990;68 8 :148]150.

w x18 Whale LRJ, Smith I. Mechanical timber joints. Research Re-port 18r86, Timber Research and Development Association,High Wycombe, UK, 1986.

w x19 Church JR, Tew BW. Characterization of bearing strengthfactors in pegged timber construction. J Struct Eng 1997;

Ž .123 3 :326]332.w x20 Erki MA. Modelling the load-slip behaviour of timber joints

Ž .with mechanical fasteners. Can J Civil Eng 1991;18 4 :607]616.w x21 Rodd PD, Spriggs RA, Hilson BO. Prediction of embedment

characteristics for laterally loaded resin injected bolts in tim-ber. Proc Int Timber Eng Conf, London, UK, 1991: 350]357.

w x22 BS EN 26891. Timber structures } joints made with mechani-cal fasteners } general principles for the determination ofstrength and deformation characteristics. British Standards In-stitution, London, 1991.

w x23 Wilkinson TL. Bolted connection design values based on Euro-Ž .pean Yield Model. J Struct Eng 1993;119 7 :2169]2186.

w x24 TrusJoist MacMillan, 4225 Kincaid Street, Burnaby, BritishColumbia, Canada V5G 4P5.

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