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 tim￾ber. 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 appar￾ent 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 fas￾tened 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 Engi￾neers 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 Associa￾tion 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 struc￾tural timber composites. In support of the introduction of EC5 and promoting the use of timber within Europe the STEPrEUROFORTECH initiative produced a sig￾nificant 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 configu￾rations. This work formed the basis of the empirical design data for UK permissible stress design codes