Construction and Building ELSEVIER Construction and Building Materias 1(0)407-417 MATERIALS www.elsevier.com/locate/conbuildmat Bolted joints in glulam and structural timber composites Tim J.Davis*,Peter A.Claisse School of the Built Environment.Coventry University.Coventry.CVI 5FB.UK Received 15 November 1;received in revised Abstract evouta Standard Th oden that nd characersatedat this programme high-ten r similar performance to Keyords Timber:Bolts:Composites 1.Introduction The UK im orts a significant tion of its co sites A of thi use of resin-injected dowelled joints has also been grown resources through various industry and govern- assessed. ment initiati mthod that has the potea products include small pieces of wood bonded in a 2.Research significance umbaldehyde-based resin-known as parallel snded more commo y,t removal of strength-reducing defects such as knots.A standardised bolted conne ction.The work shows the relative strength and stiffness of the reconstituted wood materials when used with this jointing system.The deformations within the highly stressed areas of a me e con chanically fastened joint.The results presented in this currently a draft for developm ment within the UK.It requires characteristic material data in order to facili- 838-485 responding author.Tel +4-247688-485:x+442476 the design tdavis ntry.ac.uk(T.J.Davis). 0Eevier Scne Ld.ll r reed
Construction and Building Materials 14 2000 407 Ž . ]417 Bolted joints in glulam and structural timber composites Tim J. DavisU, Peter A. Claisse School of the Built En¨ironment, Co¨entry Uni¨ersity, Co¨entry, CV1 5FB, UK Received 15 November 1999; received in revised form 1 August 2000; accepted 20 August 2000 Abstract The widespread adoption of the European design code for timber structures EC5 will facilitate a number of design options Ž . previously unsupported by British Standards. This code uses design equations that need characteristic material data, which exists for solid timber and some sheet materials, but not for the structural timber composites that were evaluated in this research. In this programme high-tensile steel black bolts have been used with solid timber, glulam and two commercially available structural timber composites } MicrolamTM and ParallamTM. The results suggest that the timber composites offer similar performance to high-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 construction timber but is keen to better utilise its homegrown resources through various industry and government initiatives. One method that has the potential to use the available resources efficiently is to produce reconstituted wood products. Existing commercial products include small pieces of wood bonded in a formaldehyde-based resin-known as parallel strand lumber PSL , and, more commonly, thin plies bonded Ž . into a laminate-laminated veneer lumber LVL . These Ž . composite materials offer reduced variability and the removal of strength-reducing defects such as knots. A possible cause for concern is the fact that the reconstitution of the wood may give rise to internal voids which will cause stress concentrations and hence increased deformations within the highly stressed areas of a mechanically fastened joint. The results presented in this U Corresponding author. Tel.: q44-2476-888-485; fax: q44-2476- 838-485. E-mail address: t.davis@coventry.ac.uk T.J. Davis . Ž . paper are taken from a larger investigation into the performance of mechanically fastened joints in structural timber composites. As part of this programme the use of resin-injected dowelled joints has also been assessed. 2. Research significance This paper presents results in order to show the comparative performance of glued laminated timber Ž . glulam and two structural timber composites utilising a standardised bolted connection. The work shows the relative strength and stiffness of the reconstituted wood materials when used with this jointing system. The application and relative merits of the composites are discussed. The new European design code, EC5, 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. 0950-0618r00r$ - see front matter Q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 0 0 Ž . 00044-1
40s TJ.Davis,P.A.Claisse/Co and Building)407-417 s.Souther Wood type Parallam pine(SS) ending parallel to grain ion 23159 80 328 30 10500 12750 12400 wood was used for solid timber and glulam samples in this research r to glue line/wide face of strand 3.Literature review 3.1.The use of structural timber composites performed in the 1930s during which the mode of action of a wide range of timber connectors tO P difficult to-date re from the natural forest resource These materials research.of timber joint design in the were introduced to the UK construction industry in the arly and U New jointing ystms structural eith significant ly higher vestiga be seen in Table 1.the benefits of reconstitution nts [ol UK timber design is currently going through a major appro the rigin Bs5268I990G artial co are h on bending strength).The modulus of elasticity the permissible stress approach used by BS5268:Part 2 argely un [11]In anticipation of the introduction of the new gover ed omp te R(TRADA an De [121 of tice for timher ioints and established research data that the design grade is recommended as SC5.This appar ent restriction on design of joint ura design cod The r ne general lack of was of the Structural timber composites are currently used in of EC5 and promoting the use of timber within Europe the UK predominantly to provide the lit more the STEP/EUROFORTECH initiative produced a sig- men he ISA sh 121 and medium span highway bridges [31 Several es tablished sources for r the mechanical properties of solid species The authors tound no pub 3.3.Bolted connections in plain timber he sites othe Nails.bolts and dowels (whether utilising an interfer stresses s in British Board of Agrement (BBA)certifi- ence fit or some form of resin bonding)are all exam cates[6,7刀. ougn a late dowe Iray 3.2.Timber jointing systems and their desigr Several state-of-the-art reviews of mechanically fas- rations.This work formed the basis of the empirical tened jointing systems have been performed,usually as design data for UK permissible stress design codes
408 T.J. Da¨is, P.A. Claisse rConstruction and Building Materials 14 2000 407 ( ) ]417 Table 1 Grade stresses of structural timber composites, Southern pine and European Whitewooda Property Wood type 2 Ž . all values in Nrmm Whitewood Southern Parallam Microlam Ž. Ž. Ž . SS pine SS 2.1 E b Bending parallel to grain 7.5 9.6 16.8 16.2 Tension parallel to grain 4.5 5.8 14.8 10.1 Compression parallel to grain 7.9 10.2 15.1 14.3 c Compression perpendicular to grain 2.1 2.5 3.6r2.8 4.9r3.0 b Shear parallel to grain 0.82 0.98 2.2 1.9 Modulus of elasticity in bending mean 10 500 12 500 12 750 12 400 Ž . 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 the USA in the late 1980s in order to provide high-quality structural timber that was proving difficult to obtain from the natural forest resource 1 . These materials w x were introduced to the UK construction industry in the early 1990s and offered significantly higher grade stresses than either solid softwood timber or glulam. As can be seen in Table 1, the benefits of reconstitution are an increase in permissible stresses of between approximately 50 and 200% on the original solid timber. This results in the reconstituted products being assigned a strength class of SC7 to BS 5268:1990 based Ž on bending strength . The modulus of elasticity is . largely unaffected and since it is deflection that usually governs the design of timber beams, the composites appear to be best utilised in axially loaded structures such as trusses. However, for joints in such structures, the design grade is recommended as SC5. This apparent restriction on the design of joints in structural wood composites was one of the main reasons for initiating this programme of research. Structural timber composites are currently used in the UK predominantly to provide the more highly stressed elements in timber-framed buildings 2 al- w x though their use in the USA has extended into short and medium span highway bridges 3 . Several es- w x tablished sources for the mechanical properties of solid wood species exist 4,5 . The authors found no pub- w x lished source for the mechanical properties of the structural wood composites other than the grade stresses in British Board of Agrement BBA certifi- ´ Ž . cates 6,7 . w x 3.2. Timber jointing systems and their design Several state-of-the-art reviews of mechanically fastened jointing systems have been performed, usually as a result of the introduction of new design codes that rely on existing research data. The first significant review was performed in the 1930s during which the mode of action of a wide range of timber connectors was established. The American Society of Civil Engineers ASCE provided a much needed up-to-date re- Ž . view 8 which details the design rules, and supporting w x research, of timber joint design in the USA, Canada and UK. New jointing systems that utilise structural adhesives are being investigated that show enhanced structural performance over mechanically fastened joints 9 . w x UK timber design is currently going through a major period of change as a result of the introduction of the draft EC5 10 and BS 5268: Part 1, which are both w x partial coefficients limit states design codes rather than the permissible stress approach used by BS 5268: Part 2 w x 11 . In anticipation of the introduction of the new codes the Timber Research and Development Association TRADA conducted a review 12 of design prac- Ž . w x tice for timber joints and established research data that was needed to support joint design to the new EC5 design code. The review highlighted the general lack of research data available, particularly for the new structural timber composites. In support of the introduction of EC5 and promoting the use of timber within Europe the STEPrEUROFORTECH initiative produced a significant review of timber engineering including jointing systems 13 . A similar work is available for US design w x standards 14 . w x 3.3. Bolted connections in plain timber Nails, bolts and dowels whether utilising an interfer- Ž ence fit or some form of resin bonding are all exam- . ples of mechanical fasteners that form timber joints through a laterally loaded dowel action. Trayer 15 w x performed an extensive research programme into bolted joints involving several wood species and joint configurations. This work formed the basis of the empirical design data for UK permissible stress design codes
TJ.Davis,PA Claisse /Construction and Building Materials 14(00)407-417 Johansen [16]supplemented Trayer's work and devel- 4.Laboratory testing programme oped theoretical equations for predicting the yield load The solid timber was commercially available Euro- me into the load-embedment response ne doweledonThey d that thereood ture the glulam joint samples using a resorcinol-fo correlation between the embedment response and den- maldehyde resin. sity of wood. tec The LVL and PSL structural wood composites wer that have been integrated into current testing standards [22]that were adopted for this research pro crolamTM LVL Grade 2.1E.They are both forms of gramme.Wilkinson compared design strengths based reconstituted wood:Parallam is made from strands of the American design while Mi Intrallam Parallam Glulam Microlam Fig.1.Glulam and the family of structural timber composites produced by TrusJoist MacMillan
T.J. Da¨is, P.A. Claisse rConstruction and Building Materials 14 2000 407 ( ) ]417 409 Johansen 16 supplemented Trayer’s work and devel- w x oped theoretical equations for predicting the yield load and ultimate load of doweled joints that now form the basis of design in EC5 17 . Whale and Smith 18 wx wx extended this work by performing an extensive testing programme into the load-embedment response of doweled joints. They concluded that there was a good correlation between the embedment response and density of wood. Similar experimental techniques have been employed on a range of investigations 19 w x ]21 that have since been integrated into current testing standards 22 that were adopted for this research pro- w x gramme. Wilkinson compared design strengths based on Johansens’ equations with the American design standard 23 . w x 4. Laboratory testing programme The solid timber was commercially available European WhitewoodrRedwood, visually graded SS C24 to Ž EC5 , obtained from a local timber merchant. Selected, . conditioned, pieces of this wood were used to manufacture the glulam joint samples using a resorcinol-formaldehyde resin. The LVL and PSL structural wood composites were both products of the American company TrusJoist w x MacMillan 24 -Parallam PSL Grade 2.0E and Mi- TM crolamTM LVL Grade 2.1E. They are both forms of reconstituted wood: Parallam is made from strands of Douglas Fir or Southern Pine timber bonded together with a phenol-formaldehyde PF adhesive, while Mi- Ž . Fig. 1. Glulam and the family of structural timber composites produced by TrusJoist MacMillan
uction and Building Materials1 000)407-417 crolam is made from laminated veneers of Southern samples were made from solid timber,glulam and dhesive. Theay-up o n Fig. ess in thi ch hae a Ld ratio of 3.67.The been fully described previously 21 In summary,all experimental arrangement,showing the sample dimen sions and positioning of LVDTs for this series of tests n order to remov of which are:initial loading to 40%of the estimated maximum load(F),approximately the working load. 5.Results of the testing programme of the tested using ot sample an09n289 est connector LVDT position to o I test nt of 44 mm 100mm HT teel bolts) Section A-A Fig.2.Loading rig and LVDT positions for recording movement of the test joint
410 T.J. Da¨is, P.A. Claisse rConstruction and Building Materials 14 2000 407 ( ) ]417 crolam is made from laminated veneers of Southern Pine 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 has been fully described previously 21 . In summary, all w x wood was conditioned for 3 weeks prior to the test, which was performed in accordance with BS EN 26891 in order to give a load-deformation response. This involves a multi-stage loading regime, the key elements of which are: initial loading to 40% of the estimated maximum load Ž . F , approximately the working load, est of the joint; removal of load to 10% of F and finally est loading to failure. A total of 26 samples were tested using 12-mm diameter Grade 8.8 bolts in a 13-mm diameter hole. Six samples were made from solid timber, glulam and Parallam and 8 samples from Microlam. The samples had a nominal thickness of 44-mm, which resulted in an embedment response a L Ž . rd ratio of 3.67 . The experimental arrangement, showing the sample dimensions and positioning of LVDTs for this series of tests, is reproduced in Fig. 2. The actual loading rig, with the LVDTs 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
TJ.Davis,PA Claisse /Construction and Building Materials 14(00)407-417 Fig.3.Loading rig with LVDTs removed for clarity
T.J. Da¨is, P.A. Claisse rConstruction and Building Materials 14 2000 407 ( ) ]417 411 Fig. 3. Loading rig with LVDTs removed for clarity