STRUCTURAL PERFORMANCE OF ACCOYA WOOD UNDER SERVICE CLASS 3 CONDITIONS

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1 STRUCTURAL PERFORMANCE OF ACCOYA WOOD UNDER SERVICE CLASS 3 CONDITIONS Ferry Bongers 1, Julian Marcroft 2, Fernando Perez 3, John Alexander 4, Matt Roberts 5, Ian Harrison 6 ABSTRACT: The benefits from acetylation of wood to enhance resistance against fungal decay and dimensional stability have been known for many years. Since 2007 Accsys Technologies has been commercial producing Accoya wood that is based on acetylation of Radiata pine and more recently Alder and SYP for particular uses. Accoya has shown its potential for many applications, including structural uses. Testing to-date, in qualifying the material at a number of universities and institutes, has been in support of specific design projects and, as is normal for solid timber, has focused on establishing characteristic material data in Service Class 1 (SC1) conditions. Accoya is predominantly specified in Service Class 3 (SC3) situations and new work has been undertaken to evaluate Accoya wood in matched service class 1 and 3 conditions so that we might establish if Eurocode k mod factors for solid timber are appropriate for Accoya. The associated evaluations involve new tests in bending, tension, hanger and nail withdrawal. Results indicate solid wood k mod factors often underestimate Accoya structural performance in Service Class 3 application and more appropriate k mod factors are needed. KEYWORDS: acetylated wood, structural properties, structural design, service class 1, service class 3 1 INTRODUCTION 123 Acetylation of wood to enhance its resistance against wood decaying fungi, as well as improving its dimensional stability under varying moisture conditions, has been studied extensively over the last decade [1]. Accsys Technologies introduced acetylated wood, named Accoya wood ( into the market in Accoya wood is based on the acetylation of radiata pine (Pinus radiata D. Don). Encouraged by the success of the two heavy load-bearing traffic bridges constructed using Accoya wood in Sneek the Netherlands [2-4], there is increasing interest in using acetylated wood for structural applications. Several pedestrian bridges and various other column type structures situated in wet (Service Class 3) conditions have been completed. 1 Ferry Bongers, Accsys Technologies, PO Box 2147, NL-6802 CC Arnhem, The Netherlands. ferry.bongers@accsysplc.com 2 Julian Marcroft, Marcroft Timber Consultancy, UK 3 Fernando Perez, Simpson Strongtie, UK 4 John Alexander, Accsys Technologies, UK 5 Matt Roberts, Accsys Technologies, USA 6 Ian Harrison, Simpson Strongtie, UK With the long-term objective of taking the product forward to an accepted general structural approval, Accsys Technologies is continually undertaking studies which add to the database of structural properties for Accoya wood. In 2012 a Design Guide for Accoya structural wood was published that demonstrates Accoya structural Radiata pine achieves the properties of C24 as given in EN 338. The research behind this manual is shown in [5]. It should be noted that Accoya Structural is based on machine stress grading and is distinct from the regular Accoya wood used for non-load-bearing applications. In the last couple of years additional research has been undertaken primarily at University of Karlsruhe, MPA Stuttgart and Simpson Strong Tie s European test laboratory. A portion of these results is presented in [6-9]. This paper presents test results which investigate the performance of Accoya in both dry (service class 1: SC1) and wet (service class 3:SC3) conditions. This data will be used to augment the Structural Design Guide to Eurocode 5.

2 2 DESIGN RULES FOR SC3 CONDITIONS Characterisation of service classes The characterisation of service classes 1-3 in EN is as follows: Service class 1 (SC1) is characterised by a moisture content in the materials corresponding to a temperature of 20 C and the relative humidity of the surrounding air only exceeding 65% for a few weeks per year (average moisture content in most softwoods will not exceed 12%). Service class 2 (SC2) is characterised by a moisture content in the materials corresponding to a temperature of 20 C and the relative humidity of the surrounding air only exceeding 85% for a few weeks per year (average moisture content in most softwoods will not exceed 20%). Service class 3 (SC3) is characterised by climatic conditions leading to higher moisture contents than in service class 2. Implicit in all the above characterisations, as reference is made to upper bound moisture conditions, is the expectation that structural properties decrease in magnitude with increasing moisture content of the timber member. For service class 3 no specific moisture condition is given but, based on the same expectation, lower bound structural properties will be found provided that the moisture content of the timber member exceeds its fibre saturation point. Evaluation of structural properties of timber members in service class 3 conditions In EN k mod values are given to modify strength values for both load-duration and service class. For solid timber the ratio of service class 3 k mod value to service class 1 k mod value ranges from 0.78 to 0.83 depending on which load-duration class is being considered. This is simply a consequence of rounding errors with the intended ratio being 0.8, which is the ratio found by interpolation at the test duration for determination of characteristic values of 5 minutes. EN gives a single set of k mod values for all stress types. This is unlike some National Codes such as the British Standard BS , where the characterisation of service class is the same as EN but where modifications for service class 3 are markedly different ranging from 0.6 to 1.0 depending on the stress type. It also appears to be at odds with EN 384 (2010) where in clause clearly differing adjustments for moisture content are made for different stress types. This may have been a pragmatic decision by the Codewriters, putting ease of use over accuracy for this design aspect, in view of the fact that the majority of structural usage of wood-based materials is in service classes 1 or 2. However whilst such pragmatism might be sensible for solid timber, in view of the fact it is almost invariably used in external applications, for Accoya wood a more accurate conversion of service class 1 characteristic values to service class 3 design values is pursued. 3 BENDING STRENGTHS AND STIFFNESSES UNDER SC1 AND SC3 CONDITIONS Structural pre-selected Radiata pine (435 boards) were graded with a Microtec Viscan, acetylated, and then graded again. In total 100 boards were selected to determine the modulus of elasticity (MOE) and modulus of rupture (MOR) according to edgewise four-point bending described in EN 408 at MPA Stuttgart. Half of the boards (50) were tested in dry (65% RH, 20 C) and the other half in wet (water saturated) conditions to reflect Service Class 1 and 3 conditions. The boards were selected on basis of the Microtec Viscan Dynamic MOE calculations such that both groups reflect the population distribution, but with more focus on the lower boundary (the lower part of the distribution being of especial interest in respect of the grading settings). Due to the limited length of the basin for water immersion the boards had to be cut to slightly shorter length than stipulated by EN 408, and testing was done with 14.6 (dry) and 14.4 (wet) span-depth ratios (EN 408 requires minimum span of 15 times the depth). In Figure 1 the cumulative distribution of the local MOE in dry and wet conditions is given. It can be seen that the MOE values for the tests after water soaking are shifted by about 10% to the left (i.e. to lower values) versus the values tested at dry conditions, roughly in the same manner throughout the whole distribution. The bending strength at mean level is reduced by circa 20% due to the water soaking, but the characteristic values are similar (see Figure 2). Although significantly less than for solid timber, there is a small increase in Accoya dimensions between service classes 1 and 3 due to moisture swelling the wood. Thus the above strength and stiffness service class 1-to-service class 3 reductions are slightly less when considered in the context of the entire bending member itself rather than in terms of unit stresses/moduli.

3 The maximum tensile strength referring to the cross section was calculated according to EN 408:2010+A1:2012. For the measurement of the displacement, two displacement sensors with accuracy of 1% were positioned in the way that the effect of distortion was minimized. The deformation was measured over a length of five times of the mean width of the boards. The modulus of elasticity in tension is calculated from the loaddeformation graph within the range of elastic deformation where the square of the correlation coefficient is greater than The tensile strength values were adjusted to a width of 150 mm (k h -factor) according to EN 384:2010. Figure 1: Cumulative distributions of local MOE before (dry condition) and after water immersion. The results for Accoya structural are summarised in Table 1 with the characteristic strengths having been evaluated in accordance with EN Table 1: Summary of results of tension tests on Accoya Structural property Characteristic tension strength parallel to grain Mean modulus of elasticity Tensile properties Ratio (N/mm 2 ) in: SC1 SC3 SC3/SC Figure 2: Cumulative distributions of bending strength (MOR) before and after water immersion. 4 TENSILE STRENGTHS AND STIFFNESSES UNDER SC1 AND SC3 CONDITIONS From the same pool of 435 pre-selected Radiata pine boards, a total of 56 boards were selected to determine the tensile strength by the Holzforschung München at Technische Universität München. Half of the boards (28) were tested in dry (65% RH, 20 C) conditions and the other half in wet (water saturated) conditions to reflect Service Class 1 and 3 conditions. All pieces were tested in tension according to the standard EN 408:2010+A1:2012. The free test length between the testing machine grips was nine times the mean width of the boards. The estimated weakest point was positioned within the test length. The tests specimens were loaded parallel to grain using gripping devices which permit the application of a tensile load whilst minimising any bending in the specimens. The measuring of the load was with accuracy of 0.1% of the maximum applied load. The load was applied such that maximum load should be reached within 300±120 seconds. 5 STRENGTHS OF HANGER JOINTS UNDER SC1 AND SC3 CONDITIONS The downward vertical load capacity of an end grain-toside grain connection formed using a three-dimensional nailing plate (hanger) was tested in accordance with ETAG 015: mm x 150 mm Accoya Radiata pine and Accoya SYP was used in the study. Pieces, selected at random, were cut to the lengths prescribed by the test standard for the joists and headers and then conditioned in a room at 85% RH / 20 C until constant mass was reached. The Accoya members were assembled using the stainless steel hanger SAIX250/38/1.5 fully nailed with stainless steel CNA4,0x35mm. The Accoya SYP members were predrilled with a drill bit of 4 mm diameter since some trials showed that nailing without predrilling can result in splitting the timber. Once they were assembled half of the samples were placed in a room at 65% RH / 20 C and the other half were submerged in water until they reached constant mass. The tests were carried out at Simpson Strong-Tie EU Laboratory in accordance with ETAG 015 (Figure 3). Five test replications were done for both Accoya species. In view of the relatively small number of test replications comparisons of the capacities of the connection under service classes 1 and 3 are made in Table 2 at mean level. The failure mode of the tests was the same for all samples

4 tested. The header member split before the differential displacement of 15 mm was reached (Figure 4). As reference a mean value of 12.5 kn is calculated for C24 timber (Norway spruce) when assuming F k = 0.75 F m in service class 1&2 conditions. Table 1: Summary of hanger tests in Accoya Species Mean downward vertical capacity of hanger connection (kn) in: Ratio SC1 SC3 SC3/SC1 Accoya radiata pine Accoya SYP Figure 4: Typical failure mode in hanger tests. 6 NAIL WITHDRAWAL CAPACITIES UNDER SC1 AND SC3 CONDITIONS Stainless steel annular ring nails CNA4,0x35mm withdrawal capacities were measured in accordance with EN 1382 for both Accoya Radiata pine and Accoya SYP under service class 1 and service class 3 conditions. Ten test replications were done for both Accoya species and comparisons of the nail withdrawal capacities under service classes 1 and 3 are made in Table 3 again at mean level. As reference a mean value of 0.81 kn is calculated for C24 timber (Norway spruce) when assuming F k = 0.75 F m in service class 1&2 conditions. Table 2: Summary of nail withdrawal tests in Accoya Species Mean nail withdrawal capacity (kn) in: Ratio SC1 SC3 SC3/SC1 Accoya radiata pine Accoya SYP Figure 3: ETAG 015 test set up. 7 SUMMARY OF THE RELATIVE STRENGTHS OF ACCOYA WOOD BETWEEN SC1 AND SC3 An overview of mechanical properties of Accoya wood in service class 1 and 3 conditions was given previously [6]. An update including bending and tension properties is shown in Table 4. Service class 3-to-service class 1 ratios as found by test are compared to these ratios given in firstly EN and secondly the British Standard BS Table 3: Service class 3-to-service class 1 ratios for some structural properties of Accoya wood Structural SC3/SC1 SC3/SC1 timber property found for EN BS Accoya MOE (bending)-mpa 0.90 No value 0.8 given MOE (bending)-shr 0.91 No value 0.8 given MOE (tension) 0.93 No value 0.8 given Bending strength Tension strength Compression parallel to grain Compression perpend to grain Bolt embedment parallel to grain Bolt embedment perpend. to grain Shear Tension perpend. to grain (Hanger test) Nail withdrawal strength

5 The following observations are made in relation to the service class 3 to service class 1 strength ratios (SC3-to- SC1 ratios) shown in Table 2: 1. A comparison between the SC3-to-SC1 ratios found by test for Accoya and given for solid timber in BS indicate that: The properties with the most marked reductions for solid timber are the same properties with the largest reductions for Accoya. Across all the structural properties tested the SC3- to-sc1 reduction is significantly less for Accoya than for solid timber. 2. The application to Accoya of the same constant ratio between the service class 3 k mod factor and the service class 1 k mod factor used by EN for solid timber would be inefficient for several structural properties of Accoya. 8 CLASSIFICATION OF ACCOYA STRUCTURAL The mean and characteristic bending strength and stiffness values from this research (MPA), together with previous research are shown in Table 5. The results give a similar outcome to the earlier test programmes by SHR and Napier [5]. All three test programmes indicate that the current Viscan setting leads to a characteristic MOR > 30 and a mean MOE > and therefore Accoya structural meets the required bending strength (MOR) and bending stiffness (MOE) for strength class C24 (very comfortably in the case of MOR). Table 4: Bending stiffness and strength results for Accoya structural in dry condition. Data set Mean local MOE Mean MOR Characteristic MOR by ranking Characteristic MOR to EN MPA SHR Napier All ADDITIONAL RESEARCH ACTIVITIES The interest from the market on Accoya Structural and the latest research results have led to the decision to develop a research program with MPA for acceptance by DIBt (General German Building Approval). In the last couple of years various data has been gathered on machine strength grading of untreated Radiata pine and Accoya structural wood. In cooperation with Microtec the data will be further analysed to improve the strength grading model. Based on the latest research results the Accoya Structural Design Guide will be revised in conjunction with Marcroft Timber Consultancy (UK) and ARUP. ACKNOWLEDGEMENT The authors are very grateful of the pleasant cooperation with (in alphabetic order): ARUP (Andrew Lawrence), IEB (Evan Buytendijk), Microtec (Martin Bacher), MPA (Dr. Simon Aicher, Dr. Ing. Gerhard Dill-Langer), Napier University (Dr. Robert Hairstains and David Crawford), Schaffitzel + Miebach GmbH (Jürgen Schaffitzel, Frank Miebach, Jürgen Langer), SHR Timber Research (Prof. Dr. André Jorissen and Wim de Groot), Simpson Strong-Tie (Ian Harrison), Tenon (Wayne Miller), University of Brighton (Dr. Dave Pope), University Karlsruhe (Prof. Dr. Hans Joachim Blaß and Dr. Ing. Matthias Frese), University of München (Prof. Dr. Jan-Willem van der Kuilen, Dipl.-Ing. Frank Hunger) and Van Wessem Houtbewerking (John Honingh). REFERENCES [1] C.A.S. Hill: Wood Modification: Chemical, Thermal and Other Processes. Wiley series of renewable resources, [2] B. Tjeerdsma and F. Bongers: The making of a traffic timber bridge of acetylated Radiata pine. In: Proceedings of the Forth European Conference on Wood Modification, pages 15-22, [3] B. Tjeerdsma, B. Kattenbroek and A. Jorissen: Acetylated wood in exterior and heavy load-bearing constructions. Building of two timber traffic bridges of acetylated radiata pine. In: Proceedings of the Third European Conference on Wood Modification, pages , [4] A. Jorissen and E. Lüning: Wood modification in relation to bridge design in the Netherlands, In: Proceedings of 11th World Conference on Timber Engineering, [5] F. Bongers, J. Alexander, J. Marcroft, D. Crawford and R. Hairstans: Structural design with Accoya wood. International Wood Products Journal 4(3): , [6] J. Marcroft, F. Bongers, F. Perez, J. Alexander and I. Harrison: Structural performance of Accoya wood under service class 3 conditions. In: RILEM Conference. Materials and Joints in Timber Structures Recent Advancement of Technology, [7] F. Bongers, J. Alexander and J. Marcroft: Structural design with Accoya wood update. In: 7 th European Conference on Wood Modification, [8] H.J. Blaß, M. Frese, H. Kunkel and P. Schädle: Brettschichtholz aus acetylierter Radiata Kiefer. Karlsruher Berichte zum Ingenieurholzbau / Karlsruher Institut für Technologie, Holzbau und Baukonstruktionen (Band 25). KIT Scientific Publishing, 2013.

6 [9] M. Frese and H.J. Blaß: Dauerhaftes Brettschichtholz aus acetylierter Radiata Kiefer. Bautechnik 91, Heft 1, [10] R.M. Rowell, B. Kattenbroek, P. Ratering, F. Bongers, F. Leicher and H. Stebbins: Production of dimensionally stable and decay resistant wood components based on acetylation. In International Conference on Durability of Building Material and Components, Istanbul, Turkey, 2008.